Table of contents

37^th INTERNATIONAL DEEP-DRAWING RESEARCH GROUP CONFERENCE - "FORMING OF HIGH PERFORMANCE SHEET MATERIALS AND COMPONENTS" – IDDRG 2018

The 36th IDDRG conference was held June 3-7, 2018 in Waterloo, Canada and was hosted by the University of Waterloo. The conference chairs were Professors Michael Worswick and Cliff Butcher of the University of Waterloo and Professor Alexander Bardelcik of the University of Guelph. The conference theme was "Forming of high performance sheet materials and components".

International Deep Drawing Research Group

IDDRG was started in the late 1950s as an organization of national groups devoted to the study of sheet metal forming including forming processes, materials, formability issues, tooling, tribology and many other interesting aspects of sheet metal forming research and industrial practices.

List of Conference Topics, Participants, Acknowledgement, Keynote speakers, Scientific Committee, Local Organizing Committee (University of Waterloo), Sponsors and exhibitors and Editors.

All papers published in this volume of IOP Conference Series: Materials Science and Engineering have been peer reviewed through processes administered by the proceedings Editors. Reviews were conducted by expert referees to the professional and scientific standards expected of a proceedings journal published by IOP Publishing.

Advanced high strength steels, especially dual and multiphase steels, exhibit interesting dynamic strain aging behavior when subjected to isothermal tests at varying strain rates. In this study, uniaxial tensile testing results are presented for two selected steels over a temperature range from 25 to 200 °C at strain rates from 5 × 10^-4 to 5 × 10^-1 s^-1. At different temperature and strain rates, some specimens exhibit substantial dynamic strain aging (DSA) effects including negative rate sensitivity and formation of pronounced Portevin Le-Châtelier (PLC) bands resulting in inhomogeneous material deformation. Implications of DSA on performance consequences are addressed in the paper.

Significant effort has been invested into the development and research of Advanced High Strength Steel. In particular, transformation induced plasticity (TRIP) steel has shown promise as a candidate for lightweighting of vehicles because of their enhanced formability. However, predictive modeling of TRIP steel in formability is difficult due the combination of dislocation and transformation mechanisms occurring during deformation for various strain paths. This work presents a crystal plasticity constitutive model to simulate the behaviour of deformation induced γ → α' transformation. A physics based transformation model is implemented into the crystal plasticity framework to fully capture the behavior of TRIP assisted steels. Simulations are calibrated and compared to experimental measurements of Duplex Stainless Steel. The calibrated model is then used to predict the forming limit diagram using the Marciniak-Kuczynski approach. The results highlight the importance of a physics based used in simulation to fully utilize TRIP assisted steels.

High strength steels with good press formability and high fatigue strength were developed by NSSMC. They are hot-rolled steels of tensile strength grade of 590-780 MPa. They have high elongation and fatigue property as DP steels, and have higher hole expansion ratio than the DP steels and conventional HSLA steels. In this study, press formed automotive parts made from the developed steels were compared with those by using the DP steels and the conventional HSLA steels. To compare the press formability, the steels were pressed into lower-arms by bend flange with a pad method. Height of flange was changed to control the effort required for the press forming. In the case of flange height of 14.5mm, the developed steels were able to be formed without cracks. However, there were some cracks when the DP steels and the conventional HSLA steels were used. Since the cracks were initiated at the region where stretch-flangeability was needed, the developed steels are thought to be applicable to the parts which require high stretch-flangeability. To compare the fatigue strength, plane bending fatigue tests and coaxial low cycle fatigue tests were conducted. The plane bending fatigue tests showed that fatigue strength of the DP steels and the developed steels were superior to those of the conventional HSLA steels between 10⁴ to 10⁷ cycles. Low cycle fatigue tests indicated that although the conventional HSLA steels exhibited fatigue softening, the DP steels and the developed steels showed fatigue hardening.

For this study, three 1.2GPa Gen3 AHSS grades with different steel designs were selected for an industrial stamping trial of a B Pillar part. Formability analyses were performed using an Argus system, the material work hardening was evaluated using sub-size tensile specimens sampled from different locations of the formed part and the TRIP effect was quantified using X-Ray diffraction. It was found that the strain distribution and strain localization are strongly related with the microstructural phase composition of the steel and with the mechanical stability of the retained austenite. The increase of the mechanical properties of the formed part represents a combined effect of strain hardening and transformation strengthening. The paint baking contribution to the increase of the yield strength of the formed panels decreases above certain percentage of induced effective strains. The remaining austenite after forming shows high thermal stability at the paint curing temperature.

This paper presents the results of intentional Aluminum addition on formability of commercial DP1180 steel grades produced at ArcelorMittal. Formability was evaluated using a suite of tests such as bending under tension test, the plane strain forming limit, crack propagation resistance using compact tension test, hole expansion, bendability, etc. Results of these formability tests demonstrated that the local formability, especially the resistance to edge fracture of the product with higher Al addition was improved in comparison with the nominal product. In addition, more detailed microstructural characterization and mechanical behavior of the products was conducted to further the understanding of the effect of Al on the performance of Dual Phase microstructures.

The hot deformation behaviors of two Fe-Mn-Al lightweight steels were investigated by hot compression tests on a Gleeble-3500 thermal simulation machine over a practical range of temperatures and strain rates. Optical microscope (OM) and electron backscattered diffraction (EBSD) were carried out to observe the microstructural mo rphologies of samples after hot deformation. Different from the single austenite steel, the hot compression tests of the duplex steel were actually carried out on an austenite/δ-ferrite duplex matrix with various phase compositions. At the commencement of deformation, strain was intensively distributed in δ- ferrite due to its higher stacking fault energy (SFE), and the dynamic softening behavior occurred earlier in δ-ferrite than in austenite during the whole hot deformation period. As a consequence, the prior dynamic recovery (DRV) in δ-ferrite caused the yield-like behavior, and the strain partitioning between δ-ferrite and austenite resulted to a visible serration on the flow curves. The dominant continuous dynamic recrystallization (CDRX) in δ-ferrite at high temperature caused a typical dynamic recovery characteristic, and the dominant discontinuous dynamic recrystallization (DDRX) in austenite at low temperature resulted in a typical dynamic recrystallization characteristic of the flow curves.

Hot-stamped components are an important part in steel-intensive automobile lightweight design in which the use of the heat-treatable steel 22MnB5 has been established. However, the field of application is limited by the low elongation at break in the hardened state. In order to improve ductility and consequently the crash-performance, the formation of martensite can be locally suppressed. This process known as tailored tempering is accompanied by decreasing tensile strength. Regarding its lightweight potential, 22MnB5 is reaching its limits and new materials come into focus. Promising potential to fulfil the mentioned demands is offered by carbon-martensitic chromium steels. Besides improved tensile strength and elongation at break, this material series has further advantages. Due to its low critical cooling rate, the formation of martensite is achieved in the hot stamping process as well as by cooling at ambient temperature. Moreover, the low martensite-start-temperature allows the use of thin material sheets. However, the process management required to achieve the respective demands for automobile applications is not trivial. Considering the materials X20Cr13 and X46Cr13, this study investigates the influence of varying process parameters on mechanical properties. In order to comprehend the relationship between solution annealing and tempering parameters, a design of experiments has been performed by means of tensile tests. In addition, miniature tensile tests were conducted to obtain flow curves at varying forming temperatures and to examine the effects of varying cooling and strain rates.

Hot stamping of boron alloyed steel has become a standard in the automotive industry for safety relevant chassis components. Hot stamping of ultra-high strength steel allows the design of complex geometries with superior mechanical properties. In the present work, a laboratory scale test component is followed up from blank to fractured component. The production process starts with a pre-cut blank, which then is austenitized, transferred to the press hardening tool, formed and quenched and ends with post-cooling to room temperature. These components are tested under tensile deformation until fracture, where force, elongation and the strain field on the components surface are measured. The strain field measurements are performed by using digital image correlation (DIC). The laboratory scale test component is evaluated using finite element modelling. The production process is modelled starting with a pre-cut austenitized blank, subsequent transfer and forming operation, and ends with post-cooling. Furthermore, the deformation and fracture under tension/bending is studied using the OPTUS damage model. The as-produced component is measured using a three dimensional scanning system. Shape deviation and thickness change are compared to in the forming simulation predicted geometry after post-cooling. A finite element investigation on the deformation and fracture under tensional/bending loading is conducted applying shape and thickness deviations in the model. The majority of industrial components undergo paint curing before they are included in an assembly. Paint baking is a heat treatment at relatively low temperatures and causes relaxation in a martensitic microstructure. The effect of paint baking on the mechanical response of the laboratory scale test component is investigated. In the present work the reliability of modelling tools from blank to fractured component is shown. The possibility is shown to predict the failure of the component, with the specific phase composition after the hot stamping process obtained from simulations. Furthermore, the influence of the paint baking process on the mechanical properties is presented.

Effects of Cu addition on microstructure and tensile property of a hot rolled austenite-based high specific strength (i.e. yield strength-to-mass density ratio) steel (HSSS) are investigated. The addition of Cu promotes the precipitation of intermetallic compounds (B2), resulting in higher volume fraction of nanoscale B2 particles in the steel. Addition of 2.8wt% Cu improves both the yield strength (by 53MPa) and tensile strength (by 138MPa) of the steel, and maintains high ductility. The yield strength enhancement is predominantly attributed to nanoscale B2 intermetallic precipitation. A high number density of ultrafine (2-10nm) B2 and Cu uniformly precipitates in the austenite martrixwith three different types of ultrafine nanoscale particles.

Austenitization models for ultrahigh strength steels like 22MnB5 are needed to: optimize existing furnace-based processes for hot forming die quenching (HFDQ); explore the potential of intercritical annealing to obtain softer, more ductile parts; and develop new HFDQ technologies such as direct-contact heating. This paper evaluates three such models using austenite phase fractions derived from Vickers hardness measurements made on Al-Si coated 22MnB5 coupons heated within lab-scale muffle furnace and then water-quenched. Overall, predicted trends are in good agreement with the experimental results at different soak temperatures, but experimental trends with increasing soak times are less obvious. The experimental results also highlight a deficiency in a recently-proposed physics based austenitization model at higher soak temperatures.

Advances in materials and manufacturing processes help in reduction of strength to weight ratio of automotive components which is a significant driver for achieving a reduction in CO₂ emissions and minimize fuel consumption. High strength materials, such as Advanced High Strength Steels have limited formability and exhibit high spring back leading to lower dimensional accuracy. Hot stamping is a thermomechanical forming process which also involves phase transformation. Due to the interaction of thermal, mechanical and metallurgical phenomena during the process, accurate simulation is extremely complex and computationally intensive. The objective of this study is to predict formability of 22MnB5 steel under hot stamping conditions. A series of experiments were carried out on a thermo-mechanical simulator to obtain the constitutive behavior of the material as a function of temperature and strain rate. The Nakazima Test was simulated to obtain Forming Limit Curves at different strain rates and temperatures. As a part of this study, different necking criteria were studied for prediction of limit strain under high-temperature deformation which demonstrated that formability increased with increasing temperature and strain rate.

The mechanical properties of hot-stamped parts strongly depend on the tool's cooling performance. The cooling rate hinges on sufficient temperature gradients and on an excellent contact conductance between tool and workpiece. A uniform distribution of the contact pressure is the key to an even cooling behaviour, and hence, a homogenous micro structure of the formed part. Elastic deformations of machine and tool components under load are a major influence on the pressure distribution between tool surface and hot-stamped part. This applies to hot and cold forming tools. An additional difficulty in hot stamping are superimposed thermal expansions and contractions of the tool, which also affect the part's mechanical properties due to their influence on the normal contact pressure. Manual die spotting needs to compensate for all these undesired effects and makes tool try-out a large time and money-consuming factor in the development of hot forming tools. This paper presents methods to transform the spotting of hot forming tools into a virtual production reality in order to reduce manual labour and lower costs. It gives details on the numerical compensation of tool surfaces for elastic tool and machine deformations and for temperature-induced tool expansions and contractions. The authors critically analyse necessary and achievable accuracies of computed surfaces and point out required improvements for the future implementation of virtual try-outs into tool development and manufacturing processes.

The mechanism of the cooling, including its intensity and distribution, of hot stamped components is the most important factor that determines not only the cost effectiveness of the process, but also the formability of the new ultra-high strength sheets and of more complex component geometries. The parameters that determine the cooling mechanism are: contact condition between the tool and stamped parts, initial tool temperature, thermal properties of the tool material and hence the tool, and the cooling system configuration and performance. Once the blank-tool contact conditions have been optimized in terms of applied pressures, contact surface adjustments, roughness and eventually interstitial material, the remaining most important parameters to be optimized are the tool material's thermal properties, cooling strategy and initial tool temperature. These parameters are strongly related as the heat must be extracted through the tool material before it is evacuated through the cooling channels, and because the cooling rate determines the initial tool temperature. The distance and the diameters of the cooling lines from the surface also depend on the tool material strength. The thermal properties of tool steels may decrease when the strength of the same increase. However, the cooling channels cannot be brought too close to the active tool surface because of the risk of premature tool cracking, nor can the cooling lines be adapted to conform with the tool cavity due to conventional machining limitations. This work explains the increase of the cooling capacity of tools by using the new cost effective high thermal conductivity tool materials FASTCOOL-50 and HTCS-230, which differ from other high thermal conductivity grades by easy machining and hardening processes, as well as a homogeneous distribution of material properties along the different tool sections. Also, the usage of the high thermal conductivity tool steel powder for cavity conformal cooling channels, especially for complex geometries, will be presented by semi industrial pilot tests.

In order to improve the set-up of industrial press hardening processes, companies and researchers are spending considerable efforts in the design and positioning of cooling channels. Following the increasing demands for modelling options, FEA software vendors have implemented corresponding features in their codes. Modeling cooling channels in FEA simulation facilitates a more accurate prediction of the location and dimensions of hot spots on the tool surface.

The present simulation case study is based on a complete tool set up for a typical part. The FE-model of tool set-up, including a suitable cooling channel design, has been utilized for a study on press hardening of 22MnB5. The effectiveness of cooling channel design is determined by the flow conditions of the cooling media - here water. The flow rate and cooling media temperatures are the parameters with most influence on the thermal behavior of the tool. The impact of variations in these parameters on the final surface temperature distribution on tools, and the ramp-up of this distribution to thermal steady state, are the subject of this study. Beyond tool temperature distribution, the impact of the above parameters on the quality of produced parts is assessed by evaluating typical part quality metrics: hardness, martensite volume fraction in the part's microstructure, and thermal distortion.

It is observed that due to the complexity of the press hardening process, the details of cooling channel flow conditions do not carry a direct and proportional impact on part quality outcomes. This observation clearly indicates that engineers can derive descriptions of the cooling channel flow conditions from easy-to-use analysis tools that are fully integrated with the simulation of the forming process, rather than resort to extended, time consuming "offline" flow computations. An analytical method for calculation of cooling channel flow conditions is also introduced by the authors.

Microstructural evolution of Al-10wt.%Si-(2-3)wt.%Fe coating during heat-treatment processing of the press hardenable steel Usibor^® 1500 was studied experimentally. Two coating thicknesses, AS150 (60-100 g/m2) and AS80 (35-65 g/m2), were subjected to the same heat-treatment schedule. The interrupted experiments were performed at heating and soaking stages in order to track the microstructural evolution. The effects of heating rates (and steel substrate thickness) on evolution of the coating were researched as well. Furthermore, the experimental results of coating development were then compared with the results of analytical modelling.

In most hot forming die quenching (HFDQ) processes, roller hearth furnace parameters, including roller speed, blank layout, and zone temperatures, are adjusted by trial-and-error. The resulting solution is always suboptimal and often leads to incomplete austenitization. This paper presents an optimization procedure for an industrial roller hearth furnace for minimizing cycle time, structured around a thermo-metallurgical model used to determine the total heat load, dropout temperature, and austenite phase fraction. Imposed constraints include minimum fraction austenite and burner capacity, and the optimization problem is solved using nonlinear programing. The outcome highlights the potential of improving process efficiency through design optimization, and the importance of high-fidelity metallurgical modelling.

Use of press hardened parts in BIW (Body in White) structures has evolved in recent years to encompass wide range of part complexity, size and mechanical properties. In addition, the number of components per vehicle has also increased pushing demand for more capital investments. Suppliers of press hardened parts need to accommodate these changes while staying competitive. Advanced design of heat treatment furnace has to offer a unique furnace design that provides flexibility to handle future part sizes minimizes down time to increase line utilization and offers a unique solution to produce tailor tempered parts for crash performance. This paper presents advanced innovative design of continuous roller furnace. These types of furnaces are generally used in hot forming lines. Design is focused on optimal heating layout, modern drives of rollers, new design and other items respecting the optimal technological and technical aspects. Also the technological functions like the dew point temperature regulation, oxygen rate regulation. Based on the long-time experiences with manufacturing and development of the machinery for the automotive industry, new roller furnaces were designed using modern methods including the FEM analyses for numerical simulations of heating processes and heating power distribution. The numerical solution of many mathematical problems involves the combination of external and internal conditions and different technological processes.

Currently, it is state of the art to use precipitation hardening 6000-series aluminum alloys to manufacture high-strength aluminum automotive parts by extrusion or in a cold forming process. Alternatively, it is also possible to produce such parts by the use of non-precipitation hardening 5000-series aluminum alloys in a work-hardened condition. Therefore, BENTELER Automobiltechnik GmbH developed a special sheet forming process, henceforth referred to as "flash forming process". The application of the flash forming process, consisting of a rapid heat treatment and a subsequent cold die stamping, increases the forming capability of the work-hardened 5000-series aluminum sheets and results in high-strength parts with a very good ductility and weldability. In addition, this thermal assisted forming process allows a cost-saving production of such high-strength aluminum parts due to lower material costs of 5000-series aluminum alloys than those of a 6000-series material. Furthermore, the weight-saving effects of "flash formed" parts can be higher compared to extruded or cold formed 6000-series aluminum alloys. The suitability of the process is evaluated by forming a commercial AW-5182 H18 aluminum sheet to a crash-relevant automotive part. However, to accurately simulate the flash forming process itself, a temperature dependent fracture model is necessary. Investigations on a coupon basis also showed that the effect of adiabatic heating due to plastic work cannot be neglected. In cooperation with Paderborn University, a detailed mechanical testing, aided by digital image correlation (DIC) and thermal imaging, is carried out to characterize the yield, hardening and fracture behavior at elevated temperatures. The experimental tests are followed by the calibration of a FLD and an incremental stress state dependent fracture model in LS-DYNA. Finally, the simulation models are validated on a cross die deep drawn cup.

The 7000-series of aluminium alloys are an attractive material for anti-intrusion components in the car body-in-white due to their high specific strength and lower density. The limited room temperature formability of these alloys can be overcome through elevated temperature forming while controlling heat exposure to prevent changes to the microstructure such as over-aging. The present work details a comprehensive characterization of a developmental alloy, 7xxx-T76, for non-isothermal warm forming of a cylindrical deep drawn cup. Material anisotropy and a non-associated constitutive model were developed with the hardening response as a function of temperature and strain-rate. Friction was characterized at elevated temperature using a Warm Twist Compression Test apparatus developed at the University of Waterloo. Significant material anisotropy was observed in the tensile characterization results and during warm forming with the formation of eight ears in the deep drawn cups for a drawing ratio of 2.25. The predictions of the forming model were evaluated in terms of the earring profile, punch force, and surface strains from optical strain measurements using ARGUS.

Heat treatable aluminum alloys are very attractive materials to replace traditional steels in automotive body-in-white construction. However, their reduced formability and high springback limits their application in sheet metal forming operations. Moreover, these alloys are prone to natural aging that causes variability in forming operations and increases the amount of scrap. Warm forming can be a very interesting solution to improve formability and reduce springback. However, depending on the processing time, precipitation hardening may occur during the warm forming of these heat treatable alloys. Thus, punch speed has a major influence on the success of sheet metal forming operations. The present study addresses the influence of natural aging and punch speed in the warm forming of an Al-Mg-Si alloy: EN AW 6016-T4. The thermo-mechanical behavior was studied in function of temperature (from 22 to 300 °C), punch speed (from 0.1 to 10 mm·s⁻¹) and storage time (from 1 to 18 months), using uniaxial tensile tests, cylindrical cup tests, and split ring (springback) tests. Warm temperatures between 200 and 250 °C reduce the flow stress, minimizing the effects of natural aging on the variability of the mechanical properties. A high punch speed is advantageous since it minimizes the occurrence of precipitation hardening which contributes to increase the springback. Thus, warm conditions at high punch speed can be used as an effective solution to minimize the variability caused by the natural aging in forming operations of heat treatable aluminum alloys.

The usual application of AA6000 series aluminium alloys in the automotive industry consists of forming the material in a soft T4 temper and applying a post-forming heat treatment, artificially ageing the material to achieve a higher final component strength. The strengthening response of a given AA6000 alloy has been found to depend on the heat treatment parameters as well as the amount of plastic deformation the material has undergone during the forming process. As a consequence, an inhomogeneous distribution of mechanical properties occurs over the finished component. In this work, a method of modelling the pre-strain dependant age hardening response of the material to be used in finite element simulations on the component level is presented. A microstructural model of the precipitation process has been applied to predict the material properties after age hardening as a function of the thermal exposure as well as the accumulated plastic deformation in the material. The results of the microstructural model are then implemented into a finite element analysis to determine the behaviour of components produced in this manner under high levels of plastic deformation (e.g. automotive crash simulation). Initial validation of the method is carried out on the example of a uniaxial tensile test; final validation is conducted using top-hat type tubes under axial crushing.

Lightweight design has become increasingly in focus for the manufacturing industry. Global environmental challenges, goals and legislations imply that lighter and sustainable products are imperative to remain competitive. One example is stamped products made of aluminum alloys which are of interest to the automotive industry, where lightweight designs are essential. In order to increase formability and to produce more complex geometries in stamped aluminum components there is a need to develop hot forming techniques. The Finite Element Method (FEM) has enabled important advances in the study and design of competitive manufacturing procedures for metal parts. Predicting the final geometry of a component is a complex task, especially if the forming procedure occurs at elevated temperatures. This work presents selected results from thermo-mechanical material testing procedures, FE-analyses and forming validation tests in AA6016 material. The material tests are used to determine the thermo-mechanical anisotropic properties, strain rate sensitivity and formability (Forming Limit Curves, FLC) at temperatures up to 490°C. Stretch-bending tests are performed to compare predicted results with experimental observations such as punch force, strain levels, thinning, forming temperatures, springback and failure. It was found that the heat-treatment and forming at elevated temperatures substantially increased formability and that measured responses could in general be predicted if care was taken to model the initial blank temperatures, heat transfer and thermo-mechanical material properties. The room temperature case confirms the importance of considering anisotropy.

Hot stamping of sheet metal materials provides advantages such as application for mass production lines and net shape manufacturing at economical cost. Recent regulations by governments world-wide on environment, emission controls and safety determine the need to provide complete lightweight solutions in the transport industry particularly for the automotive sector. These regulations are influencing multi-material-design concepts in automotive body engineering and the use of high strength aluminium alloy materials: 6xxx and 7xxx series in the development of electric and non-electric vehicles. The use of high strength aluminium alloy materials to manufacture car body parts is increasing in the lightweight solutions mainly due to weight-strength ratio advantage. Due to different material behaviours such as formability and strengthening mechanism, compared with high strength steel alloys, an industrial proven redefined hot forming process route, HForm™ will be needed to ensure high productivity and return on investment in the manufacture of parts using the aluminium alloys sheets. The produced parts will also satisfy functional requirements in the automotive industry such as surface quality, tight tolerances, strength, crashworthiness, corrosion resistance as well as ease-to join in the body-in-white (BiW) structure and e-coat. However to implement the HForm™ process and manufacture functional automotive body parts requires dedicated furnace for aluminium alloy materials, intermediate cooling station, handling/transport systems, a servo hydraulic press, and coated dies with integrated lubricant metering system. Presented in this paper is a complete solution to enable paradigm-shift in the car body engineering.

Today's automotive industry is increasingly adopting aluminum alloys. The main benefit of aluminum is a higher ratio of strength density compared to common steel alloys. Replacing steel with aluminum can give a weight reduction without loss of frame rigidity or crash safety. However, stamping aluminum is more difficult than steel because of its limited formability and the lack of experience with aluminum alloys for design and manufacturing requirements. Warm forming aluminum can help improve formability. Finite element analysis is a useful tool for predicting stamping failures. With accurate predictions stamping countermeasures can be made before tooling is cut and production has begun. Simulation was used to predict splits in a sidebar part formed at room and elevated temperatures. Forming limit curves were developed and used to test the formability using forming limit diagrams. Both forming limit diagrams and thinning were insufficient to predict the splits seen in the parts. This led to the use of additional fracture criteria that were successful in predicting the severity and locations of the splits.

Although the demand of aluminum alloys has been increasing to reduce vehicle's weight in automotive industries, the low formability of aluminum alloys has made a limit on industrial applications. This work presents an infrared ray (IR) local heat treatment in the purpose of improving formability of aluminum alloys. The concept of the IR local heat treatment is focusing IR rays on target areas where the level of plastic deformation reaches the fracture level. The local heat treatment employs the intermediate heat treatment scheme that is conducted between two stages, 1^st forming and 2^nd forming. In the 1^st forming, the blank is deformed until a part of the blank reaches 15% strain. Then, the part is locally heat treated. After the heat treatment, the 2^nd forming makes the heat treated blank match the target shape. Authors recently conducted this heat treatment with an IR local heating method to resolve the formability problem of an AA5083 alloy [1]. However, in the paper, the IR heat treatment was applied to only a linear heating shape. Neither the heat treatment for a curved shape nor local heating effect in temperature distribution was discussed in the paper. In this work, the IR heat treatment is studied in three points of view. The first is to shortly introduce the IR heat treatment in the purpose of improving formability with the AA5083 alloy. The second is studying the local heating effects in temperature distribution and application of a curved shape. Finally, an industrial application of tailgate is discussed with reduction of heating energy. The results show that this focused IR heating can improve formability with reducing heating energy for industrial applications.

The hot forming of 7000 series aluminum alloys, or die quenching (DQ), consists of solutionizing the blank in a furnace and subsequently stamping it in a chilled die set. Being a relatively new technology, adopting the DQ process into automotive assembly lines involved several challenges. One of these challenges is the constitutive modelling, which is being addressed in literature. Another challenge is determining the proper formability conditions for DQ to work. To this end, the work presented herein investigated different forming parameters required to successfully deep draw AA7075 discs. Two disc sizes were selected: 177.8 and 203.3 mm. Parameters that were investigated were: binder load and lubricant applied. The deep draw experiments were also modelled in the LS-Dyna finite element code, using the constitutive data and a Barlat-2000 yield surface developed in other work by the authors. The predicted deep draw curves were found to match well the experimental data. The earring profiles also matched well.

In recent years, the automobile industry has been calling for new lightweight solutions to fulfill increasing ecologically requirements. A weight reduction of automobile body constructions is necessary to reduce fuel consumption and CO₂ emissions. In order to reach this aim, new materials and novel forming processes are required. Also, the importance of aluminum alloys in the automobile industry is constantly increasing. Nowadays, mainly alloys of the 5000 and 6000 aluminum series are used for structure parts or shell parts, respectively. Another alloy series are the 7000 aluminum materials. These alloys offer a great lightweight potential due to their high specific strength combined with a moderate ultimate elongation. Nevertheless, these alloys are not yet widely used in the automobile industry. The reason is the limited formability of 7000 aluminum at room temperature in high-strength heat treatment condition. There are two approaches to increase the formability based on elevated temperatures, specifically the two processes of warm and hot forming at temperatures lower than the recrystallization temperature or above it, respectively. This paper deals with the investigation of the influence of forming conditions, especially the forming temperature. Trapezoidal parts were deep-drawn at different forming temperatures and subsequently investigated by determining the deformation with an optical measurement system as well as the springback of the material. In addition, the influence of the forming temperature on the flange feed was investigated as well as the influence of the paint bake process on the artificial aging step. Results show, that the formability increases with increasing forming temperature in the warm forming process route. Also, the artificial aging time can be decreased by a combined aging with paint bake heat treatment.

Optimised manufacturing rates offer enormous cost saving benefits to industry. FAST (Fast light Alloys Stamping Technology) has been recently developed to rapidly and economically manufacture high-strength panel components from sheet metal alloys. For heat treatable aluminium alloys, artificial ageing is subsequently employed to strengthen the formed components. The diffusion-controlled precipitation response is dependent on the cooling rate. The temperature evolution during FAST quenching significantly affects the final strength. In the present research, the AA6082 specimens were heated to the target temperature at an ultra-fast rate and cooled by either air (providing different quenching rates) or water, followed by artificial ageing at 180°C. Hardness measurements were conducted to track the strength evolution of the specimens during thermal cycles. Transmission electron microscopy was also performed to characterise the microstructures under different cooling conditions. Based on the experimental results, quench sensitivity during FAST has been analysed in depth and modelled. This detailed quenching sub-model was incorporated in the post-form strength prediction model, for simulating strength of the components. A great agreement between experimental data and modelled results has been achieved with the deviation less than 7%. By identifying undesirable quenching methods, optimisation of the forming process is thus possible, improving the final strength of the formed parts.

The automotive industry has significant interests and material applications involving multiple aluminum alloys (5xxx, 6xxx, and 7xxx) for light weighting. However, the industry experiences a rather lengthy development time for utilizing aluminum alloys and the need for standard approaches. This paper compares the drawability of various aluminum 5xxx, 6xxx, and 7xxx alloys among cold forming and warm forming processes. A warm forming (WF) test cell was established to conduct warm forming tests with real-time monitoring and controlling the heating and forming temperatures of aluminum blanks during heating, part transferring and stamping. For cold forming trials, a 300-Ton servo press was used for obtaining the maximum drawability. A cross-form die was used to compare the aluminum drawability between cold and warm forming processes. Increased drawability varied with different aluminum 6xxx alloys. The warm-forming process window was determined for different aluminum alloys. This paper also compares finite element (FE) prediction results of the forming process between advanced material yield function model, Barlat 2000, and a conventional model, Hill 48. The Barlat 2000 model gave a superior correlation to both cold and warm forming experimental results compared with the Hill 48 model.

The warm forming response of ZEK100 sheet was studied between 150 °C and 250 °C using hydrostatic bulging coupled with full-field displacement mapping. Due to the strain-rate sensitivity of magnesium alloys at elevated temperature, it was important to ensure that the strain-rate remained reasonably constant during the bulge test. Various gas pressure versus time profiles were used to achieve strain-rates in the range of 0.01 s⁻¹ to 0.1 s⁻¹. The results from equi-biaxial bulge testing will be detailed in addition to bulge testing performed using elliptical dies with various aspect ratios to generate data under different biaxial stretching conditions. Supplementary characterization was provided using shear specimen data and tension test data from the rolling and transverse directions of the sheet. The biaxial, tensile, and shear data was used to calibrate a linear transformation-based anisotropic yield function at 200 °C. The yield function coefficients were optimized by matching the surface displacement data between experiments and finite element simulations of the bulge test. Agreement of the surface displacement data for a given applied pressure ensured that both the stress and strain in the bulged dome were accurately captured. Finite element simulations of the bulge tests were compared to experimental data to confirm accuracy of the calculated yield function.

Cylindrical cup deep drawing experiments are performed on as-received magnesium rare-earth alloyed ZEK100 rolled sheet between room temperature and 250 °C. All tests are performed using warm tooling comprising a 101.6 mm (4") diameter cylindrical punch and smooth die and blank holder. Both isothermal and non-isothermal experiments are performed with a draw ratio of 2.25. Warm temperature formability of ZEK100 is investigated through determination of draw depths of cylindrical cups under isothermal and non-isothermal conditions. The effect of sheet anisotropy during deep drawing operation is investigated by measuring the earring profiles by means of digital imaging as well as sheet thickness from the center to the outer diameter in the rolling direction. It is found that ZEK100 sheets exhibit significantly better warm temperature drawing performance over commercial wrought magnesium alloy sheet – e.g. a full draw of 203.2 mm (8") blanks of ZEKIOO-O was achieved with tool temperature of 150 °C compared to a full draw for AZ31B-O with a tool temperature of 225 °C. Temperature process windows are used to present a direct comparison of forming behavior between ZEK100 and AZ31B. The increased forming performance of ZEK100 at elevated temperatures is attributed to the weakened texture resulting from alloying with Zr and Nd, respectively, allowing for elevated slip activity at lower temperatures.

Effect of strain path change on formability and microstructural characteristics of AZ31B automotive magnesium sheet material is studied at 300°C. The standard forming tests are carried out using Nakazima tests within the environmental chamber. The strain path change tests are performed by in-plane uniaxial, plane strain and balanced biaxial pre-strained specimens in the first stage and Nakazima test in the second stage. The enhancement in the activity of prismatic and pyramidal <c + a> slip as well as a reduction in material anisotropy with increase in temperature is responsible for improvements in formability. Compared to the standard FLD, all uniaxial, plane strain, and balanced biaxial pre-strained materials exhibit higher limit strains. This rise in FLD also incorporates the effect of additional annealing during temperature rise period prior to testing which cannot be avoided with the present experimental set-up and test methodology. The pre-strain effect is reduced by additional annealing effect caused by elevated temperature soaking time. At 300°C, the pre-strained material is almost fully recovered and behaves as the as-received material. Also, significant dynamic recrystallization is observed at 300°C.

In recent decades, there are many requirements stated against car manufacturing both from the customers' side and also as legal requirements to reduce harmful emissions to provide increased environment protection, as well as to achieve increased safety, higher comfort and more economical vehicles. To meet these often contradictory requirements, application of light weight design principles is one of the most widely applied solutions. There are two main trends for producing lightweight automotive structures with low cost manufacturing which are particularly valid for car body elements produced by sheet metal forming. Application of high strength steels is one of the main possibilities. Various generations of high strength steels (e.g. DP-steels, TRIP steels, XHSS and UHSS advanced high strength steels) were developed in the last decades which are already successfully applied in the automotive industry. The application of lightweight alloy materials – particularly various aluminum alloys – is regarded as the other possible solution to meet the requirements of lightweight car body constructions. Aluminum has even higher weight reduction potential than steel materials, but aluminium has lower formability than steel; replacing steel with lighter materials such as aluminum can be costly and is not simply straightforward. Aluminum sheet, in particular, has lower formability at room temperature than typical sheet steels. This is one of the main reasons that recently the hot forming of aluminum alloys came to the forefront of research activities. In this paper, some recent results obtained within a joint European project entitled Low Cost Materials Processing Technologies for Mass Production of Lightweight Vehicles will be introduced.

This paper presents an overview of the elevated temperature characterization of AA7075 aluminium sheet and calibration of a custom material model implemented in LS-DYNA. The plasticity characterization was performed by isothermal tensile testing while formability characterization was performed via isothermal Nakazima testing in a wide range of temperatures and strain rates. A special heat treatment path representative of the hot stamping was developed and applied in both series of tests. The material model included Hill'48 yield with a non-associated flow rule and phenomenological damage model similar to GISSMO but extended to cover non-isothermal conditions.

This work examines the nature of the strain distributions and limit stains during die quenching of a 7000-series aluminum alloy sheet. Forming limit experiments using limiting dome height (LDH) specimens were performed under plane-strain loading conditions using a 100 mm hemispherical Nakazima punch. Strains were measured using in situ stereoscopic digital image correlation (DIC). Two forming processing routes were examined: (i) an intermediate quench and form (IQF) processing route in which the LDH coupons were solutionized, quenched to a preset temperature, and isothermally formed and (ii) a die-quench (DQ) process where the LDH coupons were solutionized, and quenched and formed simultaneously with room temperature (RT) tooling under non-isothermal conditions. The DQ processing route was devised to understand the formability of the alloy under practical die-quenching conditions, while the IQF route was meant to understand the influence of temperature on the formability of the material. The DQ processing route exhibited the best formability from a localization standpoint; however, it was found that at deformations in excess of 0.5 major true strain, an orange-peel defect was present. The IQF process with room temperature tooling may be comparable to a W-temper forming operation. For these conditions, significant Portevin-Le-Chatelier (PLC) bands were present and the formability was approximately 75% less than for the DQ process. All formability results are summarized and a discussion of the interpretation of forming limits under the diffuse necking conditions associated with elevated temperature forming is presented.

Vehicle weight reduction has been identified as one of the most effective ways of achieving reduced energy consumption and CO₂ emissions in the automotive industry. Aluminium has been used as a lightweight replacement to steel in the automotive industries for many years. In addition to the high specific strength, the formability of high-strength sheet aluminium is increased significantly at elevated temperatures. New manufacturing technologies that allow forming of high and ultra-high strength aluminium alloys have emerged recently. One such technology is HFQ^® - solution heat treatment, forming and in-die quenching which combines material tempering with mechanical deformation. In this article, firstly, HFQ^® Technology is used when forming a uniform thickness blank and, secondly, further benefits are shown when combining the HFQ^® technology with friction stir welding. The friction stir welded AA6082 tailor welded blanks (TWBs), with gauge of 2.0-3.0 mm have been prepared and successfully formed into automotive components using the HFQ^® process. The recent advances in the FE analysis and the implementation of a novel CDM model in industrial applications of the HFQ^® process has been described. This paper presents the use of CDM, integrated into FEA package Pam-Stamp, to accurately predict the forming of an automotive tailor welded cross member panel.

A methodology for stress-based forming limit analysis has been developed for advanced high strength steel (AHSS). It was proposed that localized necking occurs when a critical normal stress condition is met. Using a basic, isotropic material model (von Mises, power law hardening), the criterion was applied to various 980 Class AHSS. In most cases the simplified criterion adequately described the experimental strain-based forming limit curve (FLC). For AHSS with substantial volume fractions of metastable austenite, a more sophisticated material model and/or an adapted failure criterion will be required. A strong linear relationship was found between the critical normal stress and the measured true stress at maximum load in tension. This empirical functionality applies over a large range of strength levels and may form the basis for a methodology by which FLCs may be estimated from standard tension tests. Finally, in the context of the proposed failure criterion, the effects of work hardening behavior on the "shape" of the strain-based FLC are explored.

In dualphase steels damage initiation can be triggered by interface debonding between ferrite and ferrite or ferrite and martensite, by martensite breaking, or by strain localization in the ferritic matrix. The roughness of sheet materials strongly influences the damage initiation and accumulation of dualphase steel, because roughness provokes local strain concentrations which accelerate the evolution of ductile damage. The presented study quantifies the effect of surface roughness on strain localization and ductile damage evolution in a numerical simulation framework established on different scales. On the macro-scale, an uncoupled phenomenological ductile damage mechanics model is applied to ductile material failure. On the microscale, sub-models are investigated that contain information on surface roughness profiles. Two different conditions are investigated: i.) samples that have been only grinded, ii.) samples that have been grinded and polished. In both cases, the surface roughness is characterized by white light confocal microscopy, and the raw data of these experimental measurements are used as input data for a numerical algorithm that reproduces surfaces with the same statistical roughness profiles. The numerical studies reveal the pronounced influence of surface roughness on material resistance against ductile failure.

The forming limit curve (FLC) has been used to represent the stretchability of sheet metal under various forming modes. However, the formability window in stamping depends not only on the material's forming limit, but also on its strain distribution ability. In the present work, a total forming capacity (TFC) index is introduced to describe the global formability of sheet metals, which accounts for the contributions of both forming limit and strain distribution ability in the material during forming. Essentially, this index is constructed by integrating the instantaneous n-value from zero to the effective strain limit of the material. The usefulness of this new index is demonstrated through a comparison of the overall global formability between a cold-rolled (CR) 590DP steel and a CR980GEN3 steel via a dome test coupled with digital image correlation (DIC). Contradicting of its lower forming limit, the 980GEN3 can be formed to a higher limiting dome height (LDH) than the 590DP under a near plane strain condition. This superior overall global formability of 980GEN3 is attributed to its capability to dissipate strain uniformly during forming, resulting in lower strain gradient and delayed strain concentration. While revealing the limitation of using the FLC alone, findings in this study evince the strength and effectiveness of this new index in assessing the overall global formability of sheet metals. The higher index is successfully correlated to the higher LDH of the 980GEN3. Further, the TFC concept is applied to evaluate the global formability of a multitude of high strength steels at different thicknesses. An explicit relationship is established between the calculated index and experimental LDH results. The development and application of the TFC indices promise a straightforward way to assess the overall global formability of a material. It also serves as a more precise yet simple tool for material comparisons and selections.

Nowadays, FLC is widely used to determine the feasibility of stamped parts. A FLC is usually determined by the experimental Nakajima trials. However, operating conditions can influence the value of the FLC. Experimental trials tend to suffer from result variability (scatters, errors ...)and it is rather difficult to interpret the results and find the root causes of such or such phenomenon.

The goal of this study is to investigate the influence of operating parameters of the Nakajima test on the FLC by using finite elements simulation. This paper is dedicated to the first phase of this study: define a rather reasonable model that is able to capture the main phenomena. Our goal is not to study and compare very complex plasticity models, that is a very important topic, but also an endless job. We want to build a robust, easy to understand, representative enough model. For that a solid element mesh as fine as CPU time allows was used (0.2 mm) in order to well represent the necking. The procedure used to identify the FLC is the same that for experimental determination. A virtual grid of 1mm on the upper skin is used to compute the strains and the inverse parabola fitting methods of the ISO 12004-2 standard is used to determine the FLC points. Damage was also considered in the model, as it allowed to get closer to analytical and experimental prediction. The damage parameters were a combination of data found in literature and of fitting with an experimental FLC. With this model the effect of thickness was highlighted. Globally it was consistent with what we can observe in a real Nakajima test but we noted a strange behaviour around the uniaxial tension path that need further work to be explained.

For meeting time, cost and quality targets in industrial sheet metal forming processes, correct formability prediction in a very early project stage has become a crucial factor. It is well known that the conventional Forming Limit Curve (FLC), which is most commonly used for this purpose, is only valid for linear strain paths. Yet, in most industrial sheet metal components, the occurring strain paths are nonlinear. Due to this fact, most users apply a safety margin of 10 to 20% when using the Forming Limit Diagram for formability prediction resulting in higher process robustness, but also increasing component weight and material costs in production. Reasons for the broad application of the FLC in industry is the simple experimental determination of the curve as well as the easy implementation in finite element post-processing. In this paper, an advanced failure criterion for nonlinear strain paths is presented and applied to a deep drawn door inner part. The investigated deep drawing specimen was manufactured using the aluminium alloy AA6014 T4 and the dual phase steel DP600. The sheet thickness was chosen to be closely to 1.0 mm. The calibration procedure of the criterion for arbitrary sheet materials is based purely on uniaxial tensile test data. Experiments for the calibration of the model as well as the application of the criterion to an experimental stamping part will be explained in the paper. Finally, a comparison of the newly presented model with conventional formability evaluation using standard FLC will be given.

There has been a longtime and steady interest for the influence of strain path changes on the forming and formability of metallic sheets. First from an experimental point of view, leading to a classification of strain path changes according to their severity, related to the microstructural changes at the grain scale and stress stagnation or even decrease at the macroscopic scale. And secondly, from a modeling point of view, with the development of dedicated constitutive equations to represent various behaviors ranging from strain path reversal to abrupt (or orthogonal) strain path change. However, apart from path reversal, there is still a need, out of validation's sake, to design forming tests at the laboratory scale that are sensitive to strain path changes, and particularly to orthogonal ones. Reverse redrawing of cylindrical cups is considered in this study. Indeed, during the second stage, strain path changes are expected to occur and the load prediction and the residual stresses should be dependent on the mechanical model. Though the study is purely numerical in a first step, a dual phase steel DP 980 is considered, and its sensitivity to strain path change made of tension followed by simple shear is first checked. The occurrence of strain paths occurring during the two stages of reverse redrawing are then investigated numerically and a comparison of two indicators is performed.

There continues to be a desire to incorporate advanced high strength steels (AHSS) into automotive and other applications in order to reduce the overall weight of the final product. However, these materials exhibit limited ductility prior to fracture and require high forming forces. One means to address these concerns, at least in a laboratory setting, is by subjecting the material to continuous-bending-under-tension (CBT). In this procedure, a set of three rollers reciprocate over the gauge length of the specimen while the material is continuously pulled in tension. The adjustable parameters during CBT testing are the roller depth, the crosshead velocity (which applies the continuous, tensile force to the specimen), and the carriage speed for the reciprocating rollers (which is typically set to the highest value possible while maintaining a safe process). The formability improvements obtained through CBT processing are decreased applied force and increased elongation to fracture. The latter is achieved by assuring that during the CBT process the entire gauge length elongates to the maximum possible value before fracture occurs. Past research has shown that the concentrated deformation in the fracture location of a tensile specimen is similar to the deformation over the entire gauge length of a CBT processed specimen. In this paper, a parameter space study of the DP 1180 AHSS is conducted in order to determine the optimal roller depth and crosshead velocity to achieve the maximum formability improvements for this material. The results demonstrate a promising means to improve displacement (approximately four times over that in simple tension) prior to fracture for this AHSS when subjected to CBT processing.

A mathematical model is presented for a forming limit for non-proportional loading under plane stress condition. The model results in an approach to reduce the number of experiments needed for the Generalized Forming Limit Concept (GFLC) and presents a numerical approach to calculate the linearized FLC from real Nakajima measurements. The mathematical model has been analyzed in comparison to Polar Effective Plastic Strain (PEPS) diagram and enhanced Modified Maximum Force Criterion (eMMFC), discussing both consistency with plasticity modeling and industrial applicability. An experimental setup based on Nakajima specimens is presented and DIC measurements are used to capture loading paths. The measured loading paths are used to validate predictions made by PEPS, eMMFC and the presented mathematical approach. The latter model shows promising results for prediction of failure for non-proportional loading.

A three-dimensional stress state was implemented in a modified version of the Marciniak and Kuczynski model to predict Forming Limit Curves (FLC) with different through thickness stress values, and the sensitivity of the predicted FLC to the applied out-of-plane stress component was analyzed. Furthermore, the effect of normal stress on the formability of sheet metals under non-proportional loading was investigated. It was previously found that the influence of normal stress on the formability of sheet metals decreases with increased pre-strain, when the same normal stress is applied during both stages of deformation. For multistage metal forming processes, such as tube bending and hydroforming, the first forming stage can be performed without considering the normal stress, whereas the normal stress must be considered in the second forming stage. Alternately, different levels of normal stress may be applied in each forming stage. The present work aims to understand the influence of the normal stress applied in different stages of deformation on the sheet forming limits. Different normal stresses were applied during either the first or second forming stage, and the predicted final stress forming limits were compared in order to determine the influence of the normal stress for non-linear strain paths.

In order to quickly obtain an appropriate (semi-)product shape that prevents edge fracture in stretch flanging, the non-uniform stretch flanging theory proposed by Nagai was applied to the prediction of strain distribution observed in sheet metal forming of a product with a flat top and a concave wall. The strains calculated by the finite difference method along the flange edge are comparable to those obtained by finite element analysis. Furthermore, flange height h, corner radius R, and shape angle θ were selected as significant factors on circumferential strain ε_c along the flange edge. The investigation on the effect of those factors shows that ε_c increases as R decreases, h increases, and as θ increases. A further increase in θ, however, reduces the increasing rate of ε_c.

The material characterization is the first step for an accurate numerical design of forming operations. Since its first definition in the 1960s, the forming limit curve (FLC), usually evaluated with Nakajima or Marciniak tests, is still one of the most used criteria in the industrial and scientific field for the determination of the forming limits under different stress conditions. Despite the several signs of progress in terms of measurement techniques, thanks to the introduction of optical measurement systems, the European standard method for the evaluation of the FLC remains the cross-section-method of 2008. This method is basically suitable for materials with a pronounced necking evolution. For materials with an abrupt localization, like modern high-strength materials, the standard evaluation shows weaknesses. Investigations in 2015 based on pattern recognition on Nakajima tests show that the pattern evolution of the strain distribution during the test can be used for the prediction of the material failure. However, a definition of the pattern development is needed. In order to investigate new possibilities for the determination of the FLC, the knowledge about the failure mechanisms during Nakajima tests for these materials has to be increased. The aim of the present work is an analysis of the changes on the surface and the microstructure during the Nakajima tests at different stress states and drawing depths. The correlation between material modifications and failure behavior is conducted on the dual-phase steel DP800. For the analysis of the surface and the thickness, the scanning electron microscope (SEM) is employed. Moreover, considerations about the forming mechanisms of the DP800 at different stress conditions are given and compared with the forming limit prediction of the FLC.

In order to extend the understanding of damage evolution in sheet metal forming, standardized Nakajima tests are carried out on a DP800 dual phase steel. Sample geometries for characteristic stress-strain states are drawn in incrementing stages and their damage states are analyzed using light and scanning electron microscopy as well as micro hardness measurements. Numerically analyzed load paths are correlated with the respective damage states to allow prediction of damage evolution in deep drawing processes. The influence of anisotropy is investigated by testing samples cut at various angles to the rolling direction of the sheet material. The result of the conducted research is the understanding of interactions between load paths and damage evolution in sheet forming. These results will later be used to optimize load paths in a deep drawing process, taking into account Lode parameter and stress triaxiality to produce damage controlled parts.

A new concept of the forming limit curve testing method has been developed for evaluating sheet metal formability. This new method complies with the current ISO 12004-2 standard, and has been verified by the numerical analysis in terms of specimen deformation, strain path characterization and lubrication effect. It combines the advantages from both the standardized Marciniak and Nakajima tests, deforming the sheet metal materials under a complicated deformation mode following the linear strain path without using a carry blank. By comparing the simulation results with the experimental measurements, it is proven that this testing method works with the thinner and thicker sheet metals for a variety of blank geometries.

Spatio-temporal characteristics of plastic instabilities in the aluminium alloy sheet AA5754-O were investigated under three different stress states: uniaxial tension, plane strain and equi-biaxial tension. Tensile tests including conventional uniaxial tensile specimens as well as optimized ISO 16842 cruciform specimens for plane strain and equi-biaxial tensile deformation conditions were carried out at room temperature. Arm strength of the cruciform specimens was enhanced by laser deposition of thickening layers using Al-Mg alloy wire material in order to delay fracture and extend the equivalent plastic strain in the gauge area of the cruciform specimens from ~0.03 to ~0.13 under plane strain condition and to ~0.07 under equi-biaxial tension. The differences in serrated flow behaviour and spatio-temporal evolution of Portevin–Le Châtelier (PLC) bands under these three stress states were investigated using digital image correlation (DIC) techniques. The originality of this paper is to present different PLC phenomena under plane strain condition and equi-biaxial tension, which requires improved models or modifications of existing theoretical frameworks for uniaxial tension to account for different characteristics of PLC phenomena under plane strain and equi-biaxial tension of Al-Mg alloy sheets.

The accuracy of the forming limit diagram (FLD) determined through Nakazima test is often influenced by non-linear strain path (NLSP) and through-thickness stress. The influence of these factors on measurement accuracy can be weakened or avoided in Marciniak test. But meanwhile, Marciniak test is difficult in operation and possesses poor repeatability. Therefore, the Nakazima test eliminating the influencing factors of accuracy is still an attractive method due to the easy operability and good repeatability. In the present research, two FLDs of aluminium alloy 6014-T4 that were established through Nakazima and Marciniak tests were compared. In order to obtain an accurate FLD in plane-stress state and linear strain paths via Nakazima test, a modified compensation method was applied in Nakazima test to eliminate the interferences of NLSP and through-thickness stress on measuring forming limits. The ultimate strains measured in Nakazima test were transformed to stress-based forming limits through constitutive models to remove the influence of through-thickness stress. Then, the corrected ultimate stresses were transformed back to the limit strains along linear strain paths. Compensated results of Nakazima test showed a good agreement with the experimental results from Marciniak test, proving the validity of this method. Thus, through the easy-operate Nakazima test, an FLD with comparable accuracy to that based on Marciniak test was established.

The Forming Limit Curve (FLC) is a line that is consisted of the major and minor strain pairs for different kinds of strain paths. It is widely adopted for the quantitative description of the sheet metal formability. In this paper, a stand-alone servo-motor apparatus associated with the high-accuracy measurement system was developed, based on which the FLC of AA5754-O with the thickness of 3.0mm was investigated. Firstly, the material properties of metal sheet were explored and the anisotropic coefficients in the constitutive law were calculated to describe the yield behaviour. Secondly, in order to increase the strain level of cruciform specimens, various simulations with different shapes of specimens were performed with ABAQUS/Explicit. Finally, the biaxial tensile tests with the optimized shape of cruciform specimens were performed under different loading ratio conditions. The experimental FLC of AA5754-O was obtained. The fracture zone of each specimen was occurred in the central region without premature arm fracture, which verified the feasibility of the cruciform specimen purposed.

In body design, Finite Element Analysis becomes an unavoidable step in optimizing forming processes to ensure the feasibility of a specific designed shape. Different failure criteria exist but the Forming Limit Diagram remains the most used criterion. It can be built in a wide variety of forms but the most usual one is composed only of a Forming Limit Curve (FLC) which represents the onset of localized necking limit of sheet metal. FLC is determined experimentally by standardized Nakajima or Marciniak tests. However, both present lots of roadblocks in the accurate determination of product formability limits due to the use of counter-blanks, no linear strain paths and because they are not adapted for high ductility steels. Tensile tests were performed in the past to determine the left hand side of the FLCs. They were not included into the ISO 12004-2 standard because of technical reasons although they present lots of advantages (frictionless, no curvature effect and planar configuration). Now, thanks to the current advanced technologies and tools, these issues are overcome. In this paper, the advantages of tensile tests compared to Nakajima or Marciniak ones are briefly discussed. The design and conceptualization of specific jaws to perform plane strain tensile tests on AHSS are presented. A wide range of AHSS was characterized through plane strain tensile tests and results were compared to formability limits determined by the usual practice using Nakajima tests. Different evaluation strategies were used to determine the maximum formability: the position dependent method, the time dependent one and close to fracture.

The present research was aimed to evaluate the formability of aluminium alloy 5083-O sheet. Two-dimensional DIC method was used to measure the strain field of uniaxial tensile specimens. The flow stress equation was obtained on the base of these measured strain and stress data. The influence of virtual gauge on strain measurement was analysed. The hemispherical punch tests were conducted, where three-dimensional DIC was used to measure the strains on the forming regions of aluminium sheets. The FLD of aluminium alloy 5083-O was established using these measured strain data. The data derived from the tests were applied in FE simulations of the inner panel of a car body. Comparison between the results of the experiments and the simulations showed that the necking and fracture of the aluminium alloy component during stamping could be accurately predicted.

The objective of present work is to get insight into the formability behavior of mill annealed 1mm thick Inconel 718 (IN718) sheets under different elevated temperatures and deformation speeds. The preliminary study consisted of uniaxial tensile testing within a temperature range of 500°C – 700°C at an interval of 50°C and crosshead velocities of 0.03,0.3, 3, and 30mm/s. It was observed that the total elongation improved by approximately 18% with the increase in temperature from 500°C to 700°C. However, a considerable reduction in total elongation with increase in flow stress was found with the increase in deformation speed at elevated temperatures. Based on the true stress-strain responses, the Johnson-Cook (JC) constitutive equation was developed to capture the material flow behavior. The correlation coefficient (R), average absolute error ( Δ ) and standard deviation (SDA) were found to be 0.97, 10.8 and 6.8 respectively. Further, stretch forming behavior at isothermal elevated temperatures of 500°C and 650°C were analyzed by performing laboratory scale limiting dome height (LDH) tests. A 20mm width Hasek specimen was deformed at punch speeds of 0.3mm/s and 30mm/s using a sub-sized hemispherical punch of ϕ50 mm. The LDH was found to be higher at 650°C, and the maximum LDH was achieved at lower punch speed of 0.3mm/s. The higher deformation speed decreased the LDH by 7.4% and 12% at 500°C and 650°C respectively. The thermo-mechanical finite element modeling of isothermal LDH tests was developed successfully by incorporating JC model. The predicted punch load, thickness and surface strain distributions were validated with experimental data. It was established that the predicted peak loads were within nominal 8.6% error highlighting the suitability of JC constitutive equation in FE model.

Automotive manufacturers have been trying to improve fuel efficiency without compromising the structural integrity and one of the ways to resolve the issue is to expand usage of tailor welded blanks (TWBs) in car body structure. Friction stir welding (FSW) is a joining process, which can be well fitted to obtaining aluminum tailored blanks when compared to other conventional joining processes. This paper presents an experimental and numerical study on TWBs produced by FSW with dissimilar aluminum 5083-H32 and 6061-T6 alloy sheets. The quality of the friction stir welded dissimilar joints was evaluated in terms of metallographic observations, hardness studies and tensile tests. Moreover, the local property changes in the weld regions were observed through digital image correlation (DIC) method. Formability of friction stir welded blanks was evaluated in the biaxial stretch forming mode using the limiting dome height (LDH) test. The failure location and the LDH values of the formed blanks were correlated to the hardness and local properties across the welds. The FE simulation of the LDH tests was also conducted incorporating Yld2000-2d anisotropy constitutive properties of the parent metal and further considering the properties of the non-homogeneous welded zone. The Marciniak-Kuczynski model was applied to predict the failure successfully in the FE model, and the results were validated with experimental data.

In recent years, implementation of aluminum alloys, the advanced high strength steels and ultra high strength steels (UHSS) is quickly increasing in automotive industry. However, these materials are often sensitive to sheared edge cracking if stretching along the sheared edge occurs in such processes as drawing of panels with blanked windows, stretch flanging and stretch hemming of edges of the panel. This study is dedicated to development of experimental techniques necessary to account for sheared edge condition on material formability as well as reporting the experimental results and general trends. Analysis of the hole punching process indicated that uniformity of the cutting clearance is rather difficult to maintain, especially for UHSS material where cutting forces are substantially higher than for mild steels or aluminum alloys, and stiffness of the tool starts playing critical role. Therefore, the majority of experimental studies were performed as tensile tests of samples sheared along a straight line in a dedicated trim tool where special measures were taken to achieve consistency of the die clearance. Experimental results on sheared edge stretchability of aluminum alloys similar to 6111-T4 and UHSS steel DP980 are reported. The mechanism of fracture propagation in trimming and hole punching processes is discussed in conjunction with sheared edge stretchability. The rather unique mechanism of fracture observed for trimming of UHSS DP980 steel leads to burr breaking at the final stage of the shearing process.

The high potentials of utilizing high strength steels in the automotive industry have been proved. However, there are still some unsolved challenges. Forming of a component that has been produced by blanking is one of these. Blanking is commonly used in sheet metal forming as the initial cutting process. Yet, it introduces damage into the blanked edges that in subsequent forming steps may lead to crack formation. This problem arises in particular in modern multi-phase steels and can currently not be avoided thoroughly due to a lack of understanding of the relevant influences. In the present work, the effects of the blanking process on an HCT980XD sheet were therefore numerically investigated. To achieve this, the Edge-Fracture-Tensile-Test method was simulated. By using this method, a conventional uniaxial tensile specimen is manufactured with one milled side and one blanked side. This allows to highlight the effect of blanking in a subsequent tensile test. In order to investigate the damage process, the coupled Modified-Bai-Wierzbicki model was applied to simulate first the blanking process and then the tensile test. The results revealed that during the blanking process, the failure initiated from the surface elements near the punch and propagated through the thickness. Meanwhile, another crack initiated from the opposite side of the sheet. The elements of these cracks experienced near pure-shear condition at both damage initiation and fracture moments. At the end of this step, the remaining damage, which is considered as the predamage of the next step, was higher in the middle of thickness. During the subsequent uniaxial tension, the crack in the specimen initiated at a shear cut edge and crossed the width. The possible detailed resolution of loading paths and crack formation shows that the damage mechanics simulation provides researchers with a powerful tool for assessing limit states in forming processes.

Software dedicated to simulating the hole-flanging process has been developed for the sheet metal forming profession. This software innovates by providing companies in this sector with an alternative, which is easier and less expensive than existing, more general, software. The hole-flanging process can be applied in various ways. The software uses the most known method in the profession to manufacture a flanged hole, which is based on expanding a hole with or without ironing. This study focuses on evaluating the feasibility of the hole-flanging operation. Three fracture criteria have been used for this purpose: Rice & Tracey, Latham & Cockroft and the Hole Expansion Ratio (HER). The first two criteria apply an uncoupled approach and the values of the criteria are compared with the corresponding thresholds post-process. The third criterion is used either as input data or as a test to determine thresholds for the first two criteria. The fracture thresholds are determined from experimental trials on press and from hole-flanging simulations based on the same configurations. Various hole-flanging trials with or without ironing have been carried out. Comparing the results of experimental trials and simulations highlights similar flange geometries and forming forces. The simulation shows that the locations of fracture areas on the flange are accurately modelled. However, differences appear regarding the sensitivity of the criteria to the process parameters. In particular, the fracture thresholds for the criteria may vary according to whether the flanging is carried out with clearance or with ironing. For this reason, the software allows the user to view the two criteria and choose the best one for their configuration. The HER test is used to determine thresholds for the fracture criteria and seems to be suitable for subsequently evaluating the feasibility of new flanging configurations.

There is an increasing interest in the steelmaking and automotive industries to evaluate the edge cracking sensitivity of Advanced High Strength Steel sheets used in car body manufacturing. Currently, the Hole Expansion test is the only test procedure that is defined by norms (JFS and ISO). This test is increasingly used to assess the formability of cut-edges on punched sheets because it is relatively simple. However, it has been already shown that there can be large differences in Hole Expansion Ratio (HER) values generated by different testing facilities. Among the main sources of variability: punched hole quality and hole expansion termination point. Hole punching operation has a detrimental effect on cut-edge quality and HER values. However, current standards do not give any recommendation. No technical specification is given for the tooling, quality control system and punching speed. It has been stated that the press speed difference in the hole punching operation are significantly different from one laboratory to another one and some steel microstructures sensitive to it. However, very few papers and data are available. As a result, an experimental study was conducted to examine this important issue. A specific 4-post assembly tool was designed to guarantee the best punched hole quality. Different punching speeds, from 0.2mm/s to 367mm/s were tested on different steel sheets (mild steel, AHSS including TWIP and 3^rd Gen steel) to emphasize or not the influence on final HER values.

For the evaluation of the forming behaviour of cut edges of AHSS the Hole Expansion Test (HET) standardized in the ISO 16630 is generally used. However, the observed Hole Expansion Ratio (HER) is prone to significant scatter. One reason for this scatter is the subjective observation of the through thickness crack by the machine operator. Additionally, the ISO is not very specific in its description of the test setup and material preparation, actually allowing great variations in the cutting process. Although the nominal cutting clearance is specified, a low stiffness of the punching machine or tool can cause a non-uniform clearance along the circumference. Additionally, an oblique position of the punch can cause an angled or eccentric sheared hole, resulting in additional crack initiation sites on the shear cut surface. FE simulations are used to investigate the stiffness effects of a C-frame press design. A comparative round robin analysis based on ISO16630 was performed by cross-testing both cutting and hole expansion setups for a high strength hot rolled steel grade. Significant differences were registered in the HER values, whereby the cutting process was identified to have the largest influence on HER variation. The homogeneity of the burnished zone along the circumference was observed to give valuable information about the cut edge quality and subsequent level of HER values.

Tooling durability for advance high-strength steels has been investigated by Auto/Steel Partnership at an industrial site using AISI D2 die for trimming DP980 up to 100,000 hits. The topological characterization of the trimmed workpiece edges were measured by a laser confocal microscope that provided 3D surface geometries, and further analysed. It was found that, despite of initial portion of edge roughening and edge quality deterioration from the new trim die, in the majority portion of the trimming operation up to 100,000 hits the edge quality of the trimmed sheet metal edges show combined roughening and smoothing cycles, or called a "self-reconditioning" effect, with the edge quality to be within a stable roughness window. The non-even wear between two pieces of trim die pair and along the die cutting edge is observed and analysed. This finding leads to the need of better redefinition of trim die failure or trim die reconditioning criterion.

The adoption of hot rolled AHSS(Advanced High-Strength Steel) has been increased in accordance with the weight reduction of the chassis. The problem of tool life in punching the hole for high-strength steels is more important. Using the high-grade die material is one of the method to improve tool life. Another method for increasing the tool life is reducing the maximum punching load. We investigated the effect of tool shape on reducing the punching load. Conventional, shear-angled and humped tool geometry are considered for punching the flat sheets with 780MPa strength. In flat sheet, the maximum punching load is compared for each case of puch types. Maximum punching load is reduced by 40% of conventional punching process. For investigating the effect of punch shape in a real part, the punching process for the vent hole of the conventional wheel disc is modeled with FEM. The maximum load is efficeintly reduced by using the shear-angled punch.

It is well known that the condition of the edge, whether sheared or milled, can affect the results in edge formability tests (i.e. a hole expansion test). In the current investigation, two experimental studies were performed to evaluate the effects of edge condition on the results of tensile tests and a laboratory formability test for sheet steels. The first study evaluated the tensile properties of a QP980 steel with samples prepared by four different machining methods: wire electro-discharge machine (EDM); mechanical milling; laser cutting; and waterjet cutting. It was found that the EDM and milled samples produced statistically similar values for yield strength, ultimate tensile strength, uniform elongation and total elongation. The yield strength and tensile strength from samples prepared by laser cutting were statistically different. Water jet cutting produced statistically different values for almost all four properties when compared to the EDM and milled samples. In the second study a DP600 steel was subjected to an angular stretch bend formability test with samples prepared by either shearing or milling. It was found that the sheared samples required higher forces but smaller displacements to reach the formability limit. Fracture for the sheared samples initiated on the edge of the sample whereas for the milled sample necking and fracture initiated in the center of the sample, close to the plane-strain condition. Results from both of these studies clearly show the importance of the edge condition on reported mechanical properties and on the formability of the sheet.

Edge quality is an important driver of formability of stamped components, especially during stretch flange operations. During these operations the cut edge is directly exposed to primarily tensile loading; therefore, a poor quality edge can lead to significant reductions in formability due to edge cracking. Machining and laser cutting of sheared edges is a costly process. In the present work, the effect of edge quality on formability is investigated with an AA6xxx aluminum alloy. Tensile specimens prepared using various manufacturing methods are compared. Edges are analyzed to determine surface roughness and damage, and these metrics are compared to common tensile testing results to gauge the effect of edge quality on formability.

In order to increase the use of advanced high strength steel (AHSS) for automotive light-weighting, the significant trimming-induced wear damage that AHSS sheets cause to the trim dies should be reduced so as to decrease die maintenance costs and deterioration of the quality of sheared edge. In this research, the wear characteristics in the upper AISI D2 die inserts used for trimming DP980-type AHSS sheets and the wear-induced plastic deformation at the sheared edges of DP980 were studied. Observation of wear profiles at the edge of the upper trim die revealed that material abrasion was the main damage feature. The trimmed edge quality of DP980 deteriorated with the number of strokes, as indicated by an increase in burr width. Plastic strains near the surface of the sheared edge estimated using the displacements of martensite plates, increased from 2.9 at 2.5 μm below the trimmed edge after 40,000^th cycles, to 55 after 80,000^th cycles. Micro-hardness tests performed to estimate the local flow stresses in the shear affected zone (SAZ) indicated that the flow stress at a distance of 4.5 μm below the trimmed edge increased with the number of trimming cycles to 1.73 GPa in the 80,000^th trimmed part.

To understand better on edge cracking in sheet metal forming, the edge preparation methods, edge topological characteristics, and its evolution in uniaxial tension are investigated. Tensile specimens were prepared by waterjet cutting, milling, and EDM methods. The as-machined and fractured edges were measured with a Laser Scanning Confocal Microscope (LSCM (Keyence VK-9710), which provides statistical roughness parameters and raw data of surface 3D surface geometry at up to 1nm z-direction resolution, allowing topological analysis The edge evolution during tensile straining was reported achieved by interrupted tensile tests at different strains, and replication of the edge surfaces without unloading using a metallurgical replica technique, for post-measurement after the tests. The quality of the three edge preparation methods and the effect on edge cracking in tension are investigated.

Sheet metal blanking is an industrial process widely used in automotive, electronics, aerospace and several other industrial applications. Burrs are the hard and sharp-raised edges formed at the cut edge of the blank during blanking process. Presence of a burr reduces the quality, accuracy and usability of the blank. So, it has to be removed by further processing like machining or deburring, which further increases the cost of production. Choosing proper process and geometric parameters can reduce the extent of burr. In this paper, finite element simulations of the blanking process for AA6082-T6 were carried out using the ABAQUS-CAE software package with various process and geometric parameters. Design of experiments (DoE) methodology was applied to understand the effects of the process and geometric parameters on the burr formation during blanking of metal sheets. This effort will not only help to optimize the blanking process, but also to increase the product quality and reduce the cost of production. Typical pattern of shear strain field evolved at various punch penetrations of sheet metal blanking is being studied.

The trimming behavior of AZ31 and ZEK100 automotive magnesium sheet materials was investigated using lab-based experiments. The effects of trimming process parameters; trimming speed, clearance and tool setup configuration, on quality of trimmed edge were analyzed. Experimental results indicated strong dependence of trimmed edge quality on trimming process conditions. Clearance between punch and die had the most significant influence on the trimming behavior of AZ31 and ZEK100, both the punch load peak and quality of trimmed edge decreased with increase of clearance. The larger is the clearance, the later the crack initiates. ZEK100 was more sensitive to smaller clearance compared to AZ31. Trimmed edge quality of AZ31 and ZEK100 Mg sheets improved with increase in trimming speeds up to 5 mm/sec. The tool setup configuration with cushion consistently resulted in better trimmed edge quality and especially help with burr height reduction which is a key parameter for assessing the quality of the trimmed edge.

Residual formability and edge crack sensitivity of shear cut edges depend largely on the manufacturing technique of the edges. In previous experiments, two-stage shear cutting processes have proven to reduce negative effects on the shear affected zone, which increased the forming capacity and thus also improved edge crack sensitivity significantly.

The aim of this study was to determine the optimal settings for the parameters die clearance and cutting offset in order to maximize the forming capacity of pre-milled edges in a single-stage punching process. The performance of milled reference samples without additional punching determined the optimal outcome. In general, an open cutting line achieves a better residual formability in shear cutting processes than a closed cutting line. But by choosing a closed cutting line with specific punching parameters, the resulting edge conditions can achieve the ones of a process with an open cutting line closely.

The geometry of the shear cut edge, the depth and degree of work hardening in the shear affected zone, as well as the surface roughness can be adjusted by varying the shear cutting parameters die clearance and cutting offset. The use of the collar forming test not only enabled an evaluation of the sample's edge crack sensitivity, but also resulted in the identification of the optimal combination of those shear cutting parameters. These findings allowed for an assessment of the significance of the influencing factors geometry, work hardening, and surface roughness on edge crack sensitivity and residual formability of the high-strength multi-phase steel CP-W 800. The results of this research presented a basis for another research project at the utg regarding a multistage shear cutting process for high-strength steels.

Previous investigations showed that a two-stage punching process reduced the edge crack sensitivity on high-strength multi-phase steels significantly compared to a single-stage process. This is caused by an alteration of the state of stress in the shear-affected zone during the second stage, which results in not only higher expansion ratios, but also higher collars during the collar forming test. Another preceding research project at the Chair of Metal Forming and Casting, Technical University of Munich, investigated the optimized values for cutting offset, and die clearance for both pre-cutting and re-cutting steps for a single-stage punching process.

The aim of this research project was to combine those previous findings by increasing the number of stages in a punching process with each using the optimized parameters. The behavior of milled edges served as a reference for optimum performance. The number of stages influenced material properties, such as surface characteristics, material deformation in the shear-affected zone, and roughness of the cutting surface. The quality of the cutting surface increases drastically due to various factors, such as a changed surface hardness, which affects fatigue resistance and, therefore, the component's lifetime. The validation of this punching strategy not only achieves an even higher expansion ratio in common sheet metals, but also makes highly edge-crack sensitive materials available for industrial applications.

With the increased application of advanced high strength steel (AHSS), the development of a reliable methodology for evaluating and predicting edge cracking is in high demand. In this study, different shear edge conditions of three different AHSS materials, TRIP 780, DP 980, and DP 1180, were evaluated. The edge cracking is evaluated in four steps: shear test, sheared edge characterization, HSDT, and prediction of edge cracking using FEA. The shear edges were prepared with five different shear clearances between 5 and 25% of the material thickness to obtain variable shear quality. In the HSDT, the strain and thinning distribution is captured using a digital image correlation system as the edge cracking limit and failure criteria in FEA. The preferred shear clearance is characterized by the largest stroke in HSDT and highest thinning value of the onset of edge cracking. FEA showed good correlations with the experiment comparing strain and load-displacement curves. The optimized shear clearance for TRIP 780 is 15% material thickness which for DP 980 and DP 1180 is 20%. By comparing test results from shear test, HSDT and FEA simulation, the peak shear load and burr height from simple experiment observation were found to be important indicators of edge condition, which can be monitored in production.

Deep drawn parts often have complex designs and must therefore be trimmed or punched in a subsequent stage. Due to the complex part geometry, most punching areas have a critical slant angle (angle between part surface and ram movement direction) which differs from the vertical direction. Such piercing within the critical range of the slant angle may lead to severe damage of the cutting tool due to bending effects on the cutting punch. Consequently, expensive cam units are required to transform the ram movement direction in order to perform the piercing process perpendicularly to the local part surface. However, the critical angle of attack described above has not yet been sufficiently investigated for modern sheet metal materials. The purpose of this study is to investigate the influencing factors and their effect on lateral punch deflection during piercing of high strength steel DP600 with slant angles using the FEM Software DEFORM 3D and to create a simulation model that enables a previously sophisticated simulation of the shearing process at different cutting angles. The aim of the study was to measure the lateral deviation of the punch when cutting with a slant angle. The calculated results show that the horizontal deviation is mainly influenced by the slant angle of the workpiece surface, the cutting line and the length of the punch.

Enhanced formability mapping, based on parameters derived from uniaxial tensile tests, has been proposed in order to distinguish between local versus global formability of AHSS sheets. The assessment of fracture surfaces represents the basis of local formability indices, i.e. local fracture strains. Different specimen geometries as well as evaluation methods can be found in uniaxial tensile test standards. It is not yet clear whether the choice of test specimen or evaluation method influences the measures of local fracture strain. This paper aims at contributing to the ongoing discussion on these issues during enhanced formability mapping of AHSS grades. In this study two AHSS grades have been investigated in two thicknesses (t = 1.5mm & 3.0mm). Besides standardized test specimens, the widths w of tensile test specimens were deliberately varied to gain custom tensile test specimens, exhibiting widths of 3.0mm, 6.0mm, 15.0mm and 30.0mm. The resulting measures of local fracture strain were related to the respective width-to-thickness ratio w/t as well as to the respective evaluation method. Distinct trends of all local fracture strains with respect to the w/t ratio could be detected. However, these trends tend to level off for certain w/t ratios, depending on the respective strain hardening behaviour of the investigated AHSS grade. For thicknesses up to 2mm standardized geometries with w larger than 20mm may be used indifferently. For higher thicknesses, e.g. for hot rolled AHSS grades, care has to be taken to choose the tensile specimen accordingly. Local fracture strain measures based on the actual fracture surface tend to show less variability and less dependence on the assessment method as compared to measures representing fracture thickness only. The overall trends as well as outliers depicted in the variability of results were discussed with respect to the fracture surface assessment methods.

In the present paper, a comprehensive investigation was conducted into the fracture response of a rare-earth magnesium alloy sheet, ZEK100-O, under quasi-static conditions. Various types of specimen geometries were fabricated in different orientations with respect to the rolling direction of the sheet to reveal the anisotropic fracture response of the alloy under proportional loading conditions. To visualize directional dependency of the fracture response, experimental fracture loci for different orientations were constructed. Furthermore, non-proportional tests were performed by abrupt changes of stress states to study the role of the loading history on fracture behaviour of the alloy. The non-proportional tests entailed pre-straining the material in uniaxial and equi-biaxial tension up to a prescribed deformation level, followed by extreme changes of stress states to plane-strain tension or shear. Based on the results of the proportional and non-proportional tests, the validity of employing "damage indicator" approaches commonly utilized in phenomenological modelling of fracture is assessed.

V-bending tests realized according to VDA 238-100 standard allow determining the bending angles (before and after springback) of a steel sheet of thickness th₀ and the fracture strain in plane strain. The aim of the present study is to propose a correction law for bending angle before springback allowing to predict the angle corresponding to another sheet thickness th (the bending angle increases with the decreasing thickness). This correction allows comparing steels bendability even if experiments are performed with different sheet thicknesses. Another interest is to be able to predict bending angles for sheet thickness mentioned in customer products specifications.

Understanding fracture in the bending of metal sheet is important but difficult especially for ductile materials were fracture is hard to quantify. New bend tests allow fracture analysis at higher bending strains, however, most of them are impractical for industrial application. This work investigates the link between material failure in bending of brittle and highly ductile materials and local tensile ductility which can be measured in a simple tensile test. For this, a 3-point bend test is developed based on the widely accepted VDA238-100, enabling the fracture of highly ductile aluminium alloys. The failure strain of brittle and ductile aluminium sheets in bending is determined. Uni-axial tensile tests are performed in combination with a digital image correlation (DIC) strain measurement system. Using the surface strain data, obtained from the DIC system during tensile testing and bend testing, a correlation between the local ductility in tensile testing and the fracture strain in bending was identified.

A numerical investigation of thin sheets under plane strain tension and bending is presented. Bending simulations are based on a VDA plane-strain bending test while tensile simulations depict a notched plane-strain tensile coupon. A comparative analysis of the stress and strain distributions inside the localized zone of deformation is performed for two investigated modes of behavior. The analyses suggests significant stress state differences in bending and tension driven primarily by the through-thickness response. The results offer insight into fundamental differences in tensile and bending fracture behaviors of thin sheets, as evidenced by discrepancies in strain at fracture experimentally measured using notched tensile coupons comparing to VDA bend tests. The consequences of these differences for material characterization and model calibration for large-scale crash simulations are also briefly discussed.

The proper numerical treatment of strain and stress distributions are an indispensable prerequisite of predicting the fracture initiation in deep drawing operations. The formability of sheet metal is generally limited by the so-called Forming Limit Curve (FLC) that is broadly accepted in forming community. However, fracture actually occurs at higher strains than the membrane instability predicted by the FLC. In the present paper, the results of the well-known Nakazima experiments (which are traditionally used to determine a FLC) are explored beyond the necking limit to determine the multi-axial fracture response. Indeed, the strains at the fracture initiation have been determined using a hybrid experimental-numerical approach, i.e. through the numerical simulation of each experiment. To increase the robustness of the hybrid experimental-numerical technique, we also measured the post-mortem thickness of all failed specimens. Moreover, in view of characterizing the local plasticity, the non-quadratic full stress constitutive model along a recently Hosford-Coulomb fracture criterion are employed. The candidate tests to determine the fracture parameters are the Nakazima test and a special configuration of the cup drawing test, which fails under the out-of-plane shear loading range. The obtained complex constitute model are then applied to predict the fracture in a new designed triangle shape part for deep drawing. This complex new triangular deep drawing shape is fabricated to validate the constitutive model and predict a crack initiation.

This paper documents two different methods to characterize the fracture strains of advanced high strength steels (AHSS) under varied stress states to construct the fracture locus. One experimental method is through the strain measurement using Digital Image Correlation, the other method being via thickness reduction measurement. Two AHSS were characterized to determine the fracture loci using the two different methods. The advantages and limitations for both methods are discussed by the analysis of the measured fracture data. Meanwhile, LS-DYNA MAT_224 (*MAT_TABULATED_JOHNSON_COOK) material cards were determined based on the obtained fracture loci and optimized internal parameters, and the cards were applied to predict the fracture behavior of 3-point bending crash tests on the hat-section beam made of the two AHSS. Built upon the discussions on the testing and simulation results, a practice is recommended to improve the accuracy of fracture characterization and prediction.

There is increasing interest in establishing fracture limits for advanced high strength steels where there is significant likelihood of fracture intervening during crash of structural components. Fracture strain data obtained from quasi-static tensile tests conducted at 10⁻³/s are commonly extended to predict crash events in design. In this study, true fracture strain measurements based on DIC as well as microscopy are presented for several high strength steels with ultimate tensile strength ranging from 1000-1200 MPa under uniaxial tension conditions over a range of strain rates from 10⁻³/s to 10⁻¹/s and in different sheet orientations. Implications of orientation and strain rate effects on fracture strain will be discussed in the context of local versus global formability considerations used in material selection as well as usage of fracture strain data in crash simulation.

To increase the ductility of hot-stamped structural components, the use of tailor-welded blanks (TWBs) of both high-strength and ductile steels has recently drawn attention. In this study, the failure behavior of the hot-stamped tailor-welded blanks (TWBs) comprising steel sheets of Ductibor®500-AS laser-welded to Usibor®1500-AS was investigated. TWBs with different combinations of 1.2 mm- and 1.6 mm-thick parent metals were hot-stamped and then tested under uniaxial-tension and equi-biaxial-tension loading conditions. Under longitudinal uniaxial-tension loading, all of the TWB types failed in the Usibor®1500-AS whereas in transverse uniaxial-tension loading, the failure occurred in the Ductibor®500-AS. Furthermore, in the equi-biaxial-tension tests, the 1.2 mm-1.6 mm TWBs presented the highest strains prior to failure, while fracturing in the Ductibor®500-AS sheet.

United States Department of Transportation fuel economy and safety standards have led to the development of economically viable steels with excellent combinations of strength and ductility, better known as 3^rd generation advanced high strength steels (AHSS). Quench and partitioned (QP) steels are of interest due to their excellent mechanical properties, though forming may be a challenge since stamped parts with a sheared edge may experience cracking at low strains. Hole expansion testing (HET) was performed on intercritically annealed QP 980 and QP 1180 steel sheets with two punch geometries (conical and flat bottom), edge conditions (sheared and machined), and complementary microstructural analysis was performed to better understand the effects on hole expansion ratio (HER). X-ray diffraction and electron backscatter diffraction experiments were performed to better understand factors affecting retained austenite (RA) stability as a function of strain. In this study, the QP 980 and QP 1180 steel grades had similar HERs for a majority of the testing conditions. Conical and flat bottom punches resulted in similar HERs despite varying edge conditions for a majority of the testing conditions, and the machined hole samples resulted in a higher HER than sheared hole samples regardless of punch geometry. The RA in QP 980 transformed at a faster rate than QP 1180 as a function of tensile strain. The relative stability of RA in QP 1180 was attributed to both RA morphology and the surrounding microstructure.

The increasing application of lightweight metals in combination with a growth in the complexity of components provides new challenges to the numerical modeling of sheet materials. The choice of the material model and the associated mapping of the hardening behavior are of substantial importance for a realistic process prediction and the following spring back calculation in particular. The implementation of the Bauschinger-effect via isotropic-kinematic hardening laws can lead to a substantial improvement of the prediction quality. It is commonly known, that an accurate prognosis of sheet metal forming processes requires the consideration of the semi-finished products anisotropy. However, the identification of the Bauschinger-effect, which describes the reduction of the yield stress after a load reversal, and corresponding numerical models, is usually done under a specific stress state and in one direction of the sheet. Considering the vast variety of stress states and loading directions occurring in a forming operation, the anisotropic behavior of the Bauschinger-effect under uniaxial stress and its evolution during shearing is analyzed. Using a miniaturized tension-compression test, the cyclic hardening of the mild steel DX56 and the high strength steel DP600 and the aluminum alloy AA6016 is characterized in 0°, 45° and 90° to the rolling direction. By the identification of an isotropic-kinematic hardening law in combination with the anisotropic flow criterion Yld2000-2d the model's ability to extrapolate the hardening behavior is evaluated. In the last step, the transferability of parameters from the uniaxial stress state to results from a modified shear test is analyzed.

For many metals, a transient variation of the yield stress can be observed when changing the orientation of a load-path. Such behavior affects the manufacturing process itself, e.g. by increasing forming forces, altered material properties or springback of the manufactured components. Hence, the aim of this work is to develop a novel experimental setup to characterize hardening effects due to flow-induced anisotropy for sheet metals. The proposed experiment consists of two subsequent forming operations. Initially, a hydraulic bulge test is conducted, followed by torsion of the hemispherical preformed sheet. Such approach captures the effects of flow-induced anisotropy like cross hardening as could be proved for the example of the conventional deep-drawing steel DC04. The benefits of the presented setup are (i) high plastic strains in the pre-loading step and (ii) determination of several combinations of pre- and subsequent loading.

Zinc alloys are used in a wide range of application such as electronics, automotive and building construction. Their various shapes are generally obtained by metal forming operation such as stamping. Therefore, it is important to characterize the material with adequate characterization tests. Sheet Bulging Test (SBT) is well recognized in the metal forming community. Different theoretical models of the literature for the evaluation of thickness and radius of the deformed sheet in SBT have been studied in order to get the hardening curve of different materials. These theoretical models present the advantage that the experimental procedure is very simple. But Koç et al. showed their limitation, since the combination of thickness and radius evaluations depend on the material. As Zinc alloys are strongly anisotropic with a special crystalline structure, a procedure is adopted for characterizing the hardening curve of a Zinc alloy. The anisotropy is first studied with tensile test, and SBT with elliptical dies is also investigated. Parallel to this, Digital Image Correlation (DIC) measures are carried out. The results obtained from theoretical models and DIC measures are compared. Measures done on post-mortem specimens complete the comparisons. Finally, DIC measures give better results and the resulting hardening curve of the studied zinc alloy is provided.

The use of advanced high strength steel (AHSS) is increasing in the automotive industry due to their remarkable strength-to-weight ratio and formability. In recent years, there has been a keen interest to employ high-energy rate forming processes such as electromagnetic and electrohydraulic forming because they can significantly improve the formability of these materials. However, simulating these forming processes requires reliable hardening functions that can accurately predict their flow behaviour in a wide range of strains and strain rates. One of the limitations of uniaxial tension tests is that the maximum uniform strain is not sufficient to calibrate a hardening function at high strain levels. In this work, a new numerical method is proposed to generate the extended flow curves of DP600 and TRIP780 from uniaxial tension data obtained at strain rates ranging from 0.001s⁻¹ to 1000s⁻¹ and from balanced biaxial tension data obtained under quasi-static conditions. Then, a 7-parameter strain-rate dependent Voce hardening function, which accounts for stage IV hardening, was fitted to the true stress-strain curves thus generated. Finally, statistical analysis was used to evaluate the goodness of the fit of predicted results.

Anisotropy plays an important role when forming aluminum alloys. Measurement of anisotropy using traditional extensometers can be challenging when working with AA3104 alloys due to the presence of Piobert-Lüder's banding and the Portevin-Le Chatelier effect. Piobert-Lüder's bands creates large variations in r-value necessitating alternative analysis techniques. In the present work r-values obtained from digital image correlation are compared to viscoplastic self-consistent (VPSC) model predictions. The VPSC predictions are computed based on experimental texture measurements. Good correlation was obtained between the digital image correlation measured and crystal plasticity predicted r-values. The r-value results are assessed based on contribution of different crystallographic texture components.

The continual demand for vehicle weight reduction, improved fuel efficiency and crashworthiness has driven the automotive industry to increasingly fabricate automotive body parts from advanced high strength steel (AHSS) sheet, such as dual phase (DP) and transformation induced plasticity (TRIP) steels. It is therefore essential to carefully investigate the forming behaviour of these sheet materials under various forming conditions. In this work, the quasi-static tensile flow behaviour of DP600 and TRIP780 sheet specimens was obtained in three orientations (RD, DD, and TD) with respect to the sheet rolling direction. A 3-parameter Voce hardening function was then fitted to each flow curve in order to determine true stress and true strain based on constant amount of plastic work per unit volume to calculate the normalized yield stress as well as the r-value for each material orientation. Yoshida's 6th-order polynomial anisotropic yield function, expressed as a function of the second and third invariants of the deviatoric stress tensor (J₂ and J₃, respectively), was used to predict the mechanical response of these two sheet materials. A new optimization method based on the Markov chain Monte Carlo (MCMC) MetropolisHastings (MH) algorithm was employed to calibrate the anisotropic yield function and determine the anisotropic coefficients. The yield loci for both materials were then derived as a function of ${J}_{2}(\sigma =f({\mathop{J}\limits^{\sim }}_{2}))$ only, and also as a function of both J₂ and ${J}_{3}(\sigma =f({\mathop{J}\limits^{\sim }}_{2},{\mathop{J}\limits^{\sim }}_{3}))$. The performance of each function is evaluated and validated by comparing the numerical predictions of r-value and flow stress directionality with the experimental results. And the effects of J₂ and J₃ in predicting the shape of the yield locus of DP600 and TRIP780 are also discussed.

Biaxial stress tests of a commercially pure titanium sheet (JIS #1) using the cruciform specimen and the servo-controlled biaxial testing machine have been carried out in order to elucidate its anisotropic plastic deformation behavior. The geometry of the cruciform specimen is identical to that regulated by the ISO 16842. Nine linear stress paths, σ_x (rolling direction): σ_y (transverse direction) = 1:0, 4:1, 2:1, 4:3, 1:1, 3:4, 1:2, 1:4, and 0:1 in the first quadrant of the principal stress space are applied to the cruciform specimens. Contours of plastic work in the principal stress space and the directions of plastic strain rates at selected levels of plastic work have been precisely measured. The range of the equivalent plastic strain applied to the specimens is 0.002 ≤ ${\varepsilon }_{0}^{{\rm{p}}}$ ≤ 0.01. The shapes of the work contours significantly change with increasing ${\varepsilon }_{0}^{{\rm{p}}}$ the test material exhibits differential hardening (DH). Using the data of the work contours and the directions of plastic strain rates, the applicability of selected anisotropic yield functions to the accurate prediction of the plastic deformation behavior of the test material is examined.

Drawbeads are valuable tools to control the material flow in stamping processes. Its accurate design is necessary so that the part can be formed with a minimal, and well-distributed plastic strain, and so that defects such as fractures or wrinkling can be avoided. This paper analyzes the use of a segmented drawbead to improve the results of a stamping process. Simulations, based on the Finite Elements Method, were conducted for a dual-phase 500 steel, 1.2 mm thick blank, formed using no drawbead, uniform drawbeads, and a segmented drawbead. The influence in the results of using different remesh criteria was also evaluated, and the computational time of each approach compared, the use of different criteria on the region of the blank in contact with the drawbead and the region in contact with the die and punch achieved realistic results with relatively less computational time. By adopting a segmented drawbead, it was possible to improve the results of the simulation, reducing wrinkling and fracture, based on the Forming Limit Diagram.

The competitiveness of the automotive sector has led to a high demand of accuracy and reduction in lead-time of the deep drawing tool making process. In that regard, the numerical simulation of the deep drawing process has become a key method for the correct die design. Even though the accuracy of these simulations reached some high quality levels in terms of formability and defects, the material holding force remains an open issue among the die maker companies. This inaccuracy is related with the inability of shell elements to correctly reproduce the behavior of the material around the drawbeads. In order to overcome this problem, commercial stamping software used an analytical model to predict the drawbead holding forces. Nevertheless, most of these models are based on an experimental methodology developed in the 70's that do not exactly represent the industrial drawbead configuration. In order to be able to experimentally analyze the necessary up-lift force of each drawbead, in this work a new experimental procedure is presented. A wide range of automotive sector materials, ranging from mild steels up to high strength steels, have been tested and new values, compared with previous experiments, have been found. In that regard, the force distribution on the drawbead is also studied stressing the importance of the flat surfaces around the drawbead more than the drawbead punch itself.

The quality of sheet metal formed parts is strongly dependent on the tribology and friction conditions that are acting in the actual forming process. These friction conditions are then dependent on the tribology system, i.e. the applied sheet material, coating and tooling material, the lubrication and process conditions. Although friction is of key importance, it is currently not considered in detail in sheet metal forming simulations. The current industrial standard is to use a constant (Coulomb) coefficient of friction, which limits the overall simulation accuracy. Since a few years back there is an ongoing collaboration on friction modelling between Volvo Cars, Tata Steel, TriboForm Engineering, AutoForm Engineering and the University of Twente. In previous papers by the authors, results from lab scale studies and studies of a door-inner part in Volvo Cars production have been presented. This paper focuses on the tribology conditions during early tryout of dies for new car models with an emphasis on the effect of the usage of new steel material coatings and lubricants on forming results. The motivation for the study is that the majority of the forming simulations at Volvo Cars are performed to secure the die tryout, i.e. solve as many problems as possible in forming simulations before the final design of the die and milling of the casting. In the current study, three closure parts for the new Volvo V60 model have been analysed with both Coulomb and TriboForm friction models. The simulation results from the different friction models are compared using thickness measurements of real parts, and 3D geometry scanning data of the parts. Results show the improved prediction accuracy of forming simulations when using the TriboForm friction model, demonstrating the ability to account for the effect of new sheet metal coatings and lubricants in sheet metal forming simulations.

It is well known that coating microstructure can affect the frictional behavior and the formability of galvannealed steels. Several studies are available in literature describing the influence of different coating phases, such as, gamma, delta and zeta phases, on press formability. In the present study, the authors have investigated the effect of the surface morphology, specifically of the delta phase of galvannealed coating on press formability. Formability tests including conditions of (a) complete metal lockdown and (b) significant metal movement in the dies were conducted. It was found that cubic delta coatings seem to perform better (reduced splitting) in forming operations which are accompanied by significant sliding in the blankholder area than rod delta coatings. This paper presents the results of the formability evaluation for samples with different delta phase morphology and correlation with traditionally used measures for evaluation of surface behavior such as the frictional behavior and surface roughness.

The application of surface texturing on sheet metal is a widely used approach to improve lubrication and control friction in deep drawing applications. However, it has been shown that current texturing processes are not robust to produce uniform textures on the sheet due to rapid and severe wear on texture-rolls. Furthermore, in multi-stage forming processes, deterioration of the sheet texture even at the first stage of forming makes texturing of the sheet metal surface ineffective. Tool surface texturing is a new method to control friction and tool wear in metal forming industry. In the current study, a multi-scale friction model is adopted to investigate the effect of tool texturing on the evolution of friction during sheet metal forming operations. The multi-scale friction model accounts for surface topography changes due to deformation of asperities and ploughing of tool asperities on the sheet metal surface, mixed lubrication regime and furthermore the tool micro-texture effects on lubricant distribution at tool-sheet metal interface. The model is validated with respect to strip-draw experiments using different tool textures. The model is later applied to the simulation of a U-bend forming process. The results show that using textured tools, it is possible to reduce friction and punch force in sheet metal forming processes. The model can be used to tailor and optimize textures on stamping tools for complex parts.

Deep drawing is commonly applied in the automotive industry to produce high quality automotive panels. The deformations applied to the sheet metal in these processes are complex. Accurate modelling of the sheet metal behaviour and its interaction with the forming tools is crucial to quantitatively predict these forming processes. Thus, an accurate material constitutive model and an accurate tribological model are required. In this paper, the current state-of-the-art of sheet metal forming models are used to investigate the model predictions, i.e. the Tata Steel constitutive material model and the friction model implemented by TriboForm. A cross-die experiment on an uncoated automotive sheet steel is performed as a base of reference to assess the prediction quality. The strain evolution in the cross-die formed samples is compared to the numerically predicted evolution. The results show a dependency on the applied deformation mode and the accuracy of the numerical model on the accuracy of the predictions.

Friction is an important parameter in sheet metal forming since it influences the flow of material in the process. Consequently, it is also an important parameter in the design process of new stamping dies when numerical simulations are utilized. Today, the most commonly used friction model in forming simulations is Coulomb's friction which is a strong simplification of the tribological system conditions and a contributory cause of discrepancy between simulation and physical experiments. There are micromechanical models available but with an inherent complexity that results in limited transparency for users. The objective in this study was to design a phenomenological friction model with a natural level of complexity when Coulomb's friction is inadequate. The local friction model considers implicit properties of tool and sheet surface topography, lubricant viscosity, sheet metal hardness and strain, and process parameters such as sliding speed and contact pressure. The model was calibrated in a Bending-Under-Tension test (BUT) and the performance was evaluated in a cross shaped geometry (X-die). The results show a significant improvement of the simulation precision and provide the user a transparent tribological system.

Stamping tools are prone to wear due to the increased use of advanced high strength steels in the automotive industry. For active monitoring of the wear state of stamping tools using acoustic emission, it is important to establish a correlation between specific wear mechanisms and the acoustic emission signals. An adhesive wear mode (galling), which is commonly observed on the stamping tool, can occur in combination with multiple abrasive wear modes on the workpiece, such as ploughing and cutting. This study will establish a correlation between the sources of the acoustic emission signal to the specific surface wear mechanism observed in the stamping process. Therefore, to investigate the source of acoustic emission signal, sheet metal stamping wear tests were conducted using un-worn and worn tool steel dies (AISI D2) and advanced high strength steel sheet (DP780). Accelerated tribology tests were also conducted using a scratch tester with the same material combination, where galling, cutting and ploughing wear mechanisms were observed. By correlating the acoustic emission features, such as power spectral density from the stamping test and the scratch test, it was observed that the change in the acoustic emission signal observed in the stamping process could be attributed to the galling wear mechanismsThis study contributes to the fundamental understanding of different wear mechanisms in sheet metal forming process, the resulting acoustic emissions, and how these can be utilized to develop active monitoring of the tools in the future.

The pollution of blanked parts with particles emitted by the blanking operation itself can be encountered in different cases of blanking, in particular aluminum blanking. The consequences of this pollution are a degradation of the quality of the parts produced (appearance, function) and, possibly, a degradation of the behavior of the blanking tool. In this context, we studied the effect of the wear of the punch in the case of AW5754 H111 blanking. Press tests were conducted with a complex part shape and with different press test configurations. The results highlighted two types of wear. The first one is called "run-in" (punch surface accommodation) and leads to a reduction of the emission of particles. On the contrary, the second type of wear, called "pick-up" (material transfer from the sheet), leads to increase particle emission. The mechanisms corresponding to the two particle emission modes are studied. The first mechanism is well known and results from the transition from a blanked edge geometry with secondary shear zone to a more conventional geometry without this secondary shear zone. The second mechanism is new, with sheet material being picked up by the punch and then the picked up material being scraped off, eventually leading to particles being deposited on the die and the part. The study also focuses on the effect of lubrication, clearance and punch shape on these mechanisms. The two mechanisms differ by the influence of lubrication: The second mechanism becomes preponderant as soon as lubrication becomes insufficient, and a solution must be found rapidly because of the larger particle emission and the risk of galling of the punch.

The scratch phenomenon of paint coated sheet metal in multi-stage deep drawing has been investigated. The tensile tests at a quasi-static strain rate of sheet metals coated with two different types of paint were conducted along the rolling, diagonal, and transverse directions of the sheet metal in order to consider strain hardening and anisotropy. It has been found that the types of paint are negligible for plasticity behavior. Finite element simulations of the multi-stage deep drawing process was performed using the obtained material properties and the simulation results were compared with the actual testing. Through the simulation results, three approaches, which include the contact pressure, the accumulated slip distance, and the accumulated friction work were implemented to investigate the scratch phenomenon and they were compared for the scratched and non-scratched regions. It was found that the accumulated slip distance at the scratched region was larger than that of the non-scratched region with a good correlation from experimental observation, whereas either the contact pressure or accumulated friction work approach did not predict the experiments. Therefore, it is concluded that the accumulated slip distance is a good method to identify the scratch on the paint coated sheet metal during deep drawing process.

Spring-back issues are critical in stamping procedures for advanced high strength steel. The spring-back can be comprehensively controlled by applying a newly designed hybrid bead to effectuate a post-stretching process in a U-channel part forming. Finite element forming simulation is applied in a cross-section to evaluate and optimize the hybrid bead performance, using DP980. A specific post-stretching die with the optimized hybrid bead was manufactured and applied to demonstrate the performances. Excellent spring-back control and material restricting effect were observed in the stamping results. Both effects of clamping force and post-stretching amount are systematically studied, using 1.2mm DP980 and CP1180 AHSS sheets. Significant tonnage reduction was achieved, comparing to the original stinger bead design. An analytical solution was also applied to predict the same process with good correlation.

Nonlinear elastic behavior and degradation of the E-modulus with increasing plastic strain in advanced high strength steels makes springback prediction more challenging. The conventional method for determining the E-modulus degradation with plastic strain is the loading-unloading-loading tensile test. This paper proposes a new methodology to determine E-modulus variation using a wipe bending operation. During wipe bending, the sheet material experiences simultaneous tension and compression loading through the sheet thickness, so the test conditions closely emulates actual metal forming conditions. Wipe bending tests for 1.2 mm MP980 steel sheet samples were conducted using different bending angles and springback was measured for each sample. A finite element model of the bending process was also developed. A constant apparent E-modulus was determined for each bending angle by comparing the springback predicted by the finite element model with the springback measured during the wipe bending test. Average effective strain was also calculated for each bending angle using FE simulations. A curve relating the E-modulus variation to effective strain was developed by correlating the apparent E-modulus and the average effective strain at each bending angle. Inputting this curve into the FE simulation revealed that springback prediction improved significantly compared to the case of using a constant E-modulus.

In this article, we perform a parametric study of the springback phenomenon. The effect of material parameters related to the work hardening and the thickness is studied by using computer experiments and statistical methods. First, a sensitivity analysis is performed using a fractional factorial design and a linear regression model. After determining the important factors, a Taguchi analysis is performed to estimate the optimum value of the parameters for robustness against springback. Next, we create a Gaussian process meta-model trained with the data generated via Latin hypercube sampling. This meta-model is used to better understand the nonlinearity of the response and the effect of parameter interactions. Finally, by using a Monte Carlo simulation on the meta-model we determine how the variability of the input parameters propagate to the response (springback). The pipeline explained in this work can help with establishing an effective strategy for the springback compensation.

Springback prediction and control of complex automotive parts is a critical issue in industrial practical application. In this study, a method is proposed for characterization, prediction and control of springback based on the springback energy, which is the driving energy causing springback after stamping. A theoretical model was established and experimentally verified. A case study was conducted on the basis of an automotive part formed using aluminium alloy AA6016 sheet, and finite element method was used to obtain the necessary variables of the theoretical model. Springback energy was calculated using the theoretical model and visualized using Matlab. Results show that the magnitude of springback distortion increases with the release of springback energy. It further indicates that in addition to the average level, the distribution of springback energy has an obvious effect on the form and amount of springback. Both the reduction and homogenization of springback energy can effectively reduce springback of the part. This study facilitates the understanding, prediction, and control of springback using a new approach with springback energy that is especially applicable to complex automotive parts.

Many automotive parts are manufactured by means of sheet metal forming technology. The process limits in sheet metal forming are highly dependent on the material flow and friction forces. In most cases, it is beneficial to reduce the friction between tool and workpiece to expand the process limits and increase the lifetime of tools. Macro structuring of tools is a novel approach to reduce the amount of friction forces in the deep drawing process [1]. The induced alternating bending in sheet metal during the deep drawing with macro-structured tools leads to an increase of its geometrical moment of inertia and consequently stabilizes the sheet against wrinkling. An increase in the peak-to-valley height difference, which is directly associated with a greater restraining force in flange area, leads directly to improved dimensional accuracy in terms of springback behavior of workpiece. This positive effect of the increased restraining force is known from the literature [2], but in lubricant free deep drawing process it is not applied by means of conventional blankholder force but also geometrically through macro structuring of tools. Within the scope of this paper, the influence of alternating bending on the springback behavior of parts in deep drawing process with macro-structured tools is studied. Since the springback in a workpiece depends on its material, two industry relevant materials DC04 and Al5182 are chosen for numerical and experimental tests. For this purpose conventional deep drawing process is compared with lubricant free deep drawing process. To investigate the effect of alternating bending on springback behavior of the components in lubricant free deep drawing with macro-structured tools, the ring splitting method is used. In this test, a ring from each cylindrical deep drawn cups formed by standard and macro-structured tools will be cut out and subsequently split to open. The opening gap of split ring indicate the amount of tangential residual stresses in rotationally symmetric deep drawing parts which is an indicator for springback behavior. The results of this paper show that springback behavior of sheet metal parts can be reduced by increasing the peak-to-valley height difference during lubricant free forming process with macro-structured tools.

Especially due to utilization of high strength steels in automotive parts, accurate prediction and reduction of springback have gained more importance in the recent years. One of the widely used approaches to reduce the springback in deep drawn parts especially in and around the bent regions is coining (restrike operation). In this method, the sheet material in the springback-relevant regions is compressed between two rigid tools having clearences less than the sheet thickness. By this way, compressive stresses and strains are superimposed and therefore the bending effects are reduced. During coining, plastic deformation occurs in the thickness direction. In order to predict this behavior in process planning numerical simulations should be capable of considering the effects of through-thickness compression. In order to analyze this effect, axisymmetrical deep drawing tests were performed. In a further step, the bottom surface or the radius region of the geometry was compressed between the punch and additional tools. To analyze the springback behaviour, drawn geometries were cut along the rolling direction, so that the residual stresses are released. The same procedure was also simulated with shell elements having enhanced formulation to account for through-thickness deformation. Experimental springback results show that the coining process influences the springback and this can be captured by the enhancements in the shell formulation.

The need for car body structures of higher strength and at the same time lower weight results in serious challenges for the stamping process. In order to produce design conform parts the stamping dies must be adjusted by the amount of the springback in the opposite direction. This approach can only be successful, if the springback only ranges in a certain bandwidth in series production. To do so, it must be fulfilled that for a common spread of input process parameters first of all the draw-in of the flange edge of the drawshell may not reach the drawbeads, secondly material failure by locally increasing strain may not occur at any possible parameter set and last but not least the location of the part in the respective follow-up operation may not vary in such a way that the springback is being affected considerably. After the three basic requirements have been fulfilled the effect of the integral of the elastic strain energy in bending areas has to be minimized by increasing the tensile stress to a maximum. As a result of that the scatter of the bending springback is being reduced.

To precisely predict springback, the stress and strain state has to be described correctly during the forming process and after load removal. In this work, different yield locus descriptions are considered for springback prediction of a CR700Y980T-DP steel with a thickness of 1.5 mm. Experiments and Finite Element Analyses (FEA) with LS-DYNA of a U-bending validation tool were performed and springback predictions compared. In the FEA Hill48, Barlat89 and Barlat's YLD2000 for isotropic hardening, Yoshida-Uemori as a kinematic hardening model and the homogeneous anisotropic hardening (HAH) model are chosen. Monotonic and two-step tensile tests with a directional change of stress are performed and cyclic tension-compression tests are used to determine the material parameters of the investigated hardening models. It is seen that both Yoshida-Uemori and HAH models provide a significant improvement in the springback prediction. The results, however greatly depend on the identification procedure as well as on the modelling of cross-loading contraction effects.

The sheet metal forming process is characterized by elasto-plastic strains. Therefore, after removing the stamped part from the dies, it presents the springback phenomena. The present paper analyzes the adoption of a springback reduction methods in a hat bending forming process, i.e, the use of variable blank holder force (VBHF) and stake bead (SB). Based on a Finite Element Method (FEM) stress state analysis, springback magnitudes were correlated to the stress moment in the hat shape side wall. Both springback reduction methods, VBHF and SB, were well described and its practical and economic use in the industry discussed. The FEM stress state anlaysis showed a great correlation to the springback phenomena.

The quality of deep drawn parts is affected by many different influences. These can be separated into material related, temperature related and tool related influences. In the case of the stainless steel used in kitchen sink production, the temperature not only influences the friction but also the material properties. Therefore, a continuous monitoring and adjustment of the process is needed. The present paper shows a possibility of monitoring the current state of the part in combination with a control system for the adjustment of the press to compensate for non-measurable influences. The proposed monitoring system consists of an optical measuring device for the part state, as well as a non-destructive material testing system for the determination of material properties. The control system uses the blank holder forces as actuators. Also, the performance of the proposed system in the production line is shown.

The ability to control quality of a part is gaining increased importance with desires to achieve zero-defect manufacturing. Two significant factors affecting process robustness in production of deep drawn automotive parts are variations in material properties of the blanks and the tribology conditions of the process. It is imperative to understand how these factors influence the forming process in order to control the quality of a formed part. This paper presents a preliminary investigation on the front door inner of a Volvo XC90 using a simulation-based approach. The simulations investigate how variation of material and lubrication properties affect the numerical predictions of part quality. To create a realistic lubrication profile in simulations, data of pre-lube lubrication amount, which is measured from the blanking line, is used. Friction models with localized friction conditions are created using TriboForm and is incorporated into the simulations. Finally, the Autoform-Sigma^plus software module is used to create and vary parameters related to material and lubrication properties within a user defined range. On comparing and analysing the numerical investigation results, it is observed that a correlation between the lubrication profile and the predicted part quality exists. However, variation in material properties seems to have a low influence on the predicted part quality. The paper concludes by discussing the relevance of such investigations for improved part quality and proposing suggestions for future work.

Modern deep drawn parts have complex designs and are driven to the limits of the material formability in order to reduce costs. This leads to small process windows and unstable forming processes with high scrap rates. Especially at the beginning of a batch, when the tools are warming up, high scrap rates can occur due to the changing friction behaviour of the system tool – lubricant – metal sheet. To make processes independent from user experience and know-how, process control systems that can compensate for the transient behaviour of the process are desired. In this work, a process control system that is based on the numerical simulation of the friction behaviour of the deep drawing process is presented. The system makes use of numerical simulations of the transient behaviour during warming up of the tools. These simulations are used to generate metamodels of the process, which are used to design and optimize the control algorithm. The control system is tested with an automotive part from Opel. The control system itself consists of two parts: a feed forward controller and a feedback loop. In the feedforward loop the in-line acquired temperature will be used as an indicator for the friction conditions. It will make use of metamodels generated based on numerical simulations in order to depict the process behaviour. The feedback loop will use the in-line measured draw-in as a state variable in order to account for all other process influences. Simulation results, the generation of metamodels, as well as the first off-line tests of the process control are shown in this contribution.

In production of deep drawn sheet metal parts, it is often challenging to achieve a robust process. Especially in the production of kitchen sinks made out of stainless steel, the fluctuation of the process and material properties often lead to robustness problems. Therefore, numerical simulations are used to detect critical regions. To keep a constant product quality, process control is realised based on metamodels, which are computed by means of a series of finite element simulations. In order to enhance the forecast capability of the simulation model and to increase the reliability of quality features, the yield curve, the yield locus and the forming limit curve (FLC) of different suppliers are measured with tensile, bulge and Nakazima experiments. Because of the large deformation capability of stainless steels, large drawing depths can be achieved. However, the classic Nakazima geometries are not distributed homogenously in the forming limit diagram (FLD). To overcome this shortcoming, a shape optimization of the Nakazima specimen has been performed. Furthermore the influence of alloying elements on the hardening behaviour and on the FLC are analysed. In addition, the measured FLC-T (temperature dependent FLC) conducted with heated Nakazima tests is compared with the computed FLC-T with the modified maximum force criterion (MMFC).

Recently, aluminium (Al) alloys are increasingly used in the automobile and aerospace industries because of their light weight and corrosion resistance properties. However, the applications of these materials are limited due to their poor formability especially at room temperature. Although hydroforming technology is one of the approaches used to improve the formability of Al alloys, it still cannot fulfil the requirements of designers and manufacturers to form high-quality complex shaped components. It has been found that high-speed forming is able to improve the formability of low plasticity metals at room temperature. Thus, impact hydroforming (IHF) technology is proposed by combining the advantages of high-speed forming and hydroforming, and it was used to address the issues of quasi-static hydroforming. Accordingly, the aim of this study is to investigate the effect of IHF on the formability of AA5A06 sheet metal. An IHF bulge experiment setup was developed by making use of a light gas gun which can accelerate the projectile up to 300m/s. The results show that the formability of the AA5A06 increased under most of the impact velocities, and the equi-biaxial strain before failure of the IHF is 50% higher than that of quasi-static hydroforming at strain rate of 2 × 10³ s⁻¹. It is concluded that the IHF technology is an appropriate method to improve the formability of Al alloys and form complex shape components.

The electromagnetic forming (EMF) process is a typical high speed forming technique using Lorentz forces generated by interaction between the magnetic field and induced electric current. Recently, researchers have discussed form-fit joining using tube compression by EMF without additional connection elements for application in the industry. This paper provides analysis into the form-fit joining behaviour achieved by tube expansion using EMF. The aluminum 6063 tube was inserted into an aluminum 7075 sheet with hole of the same size as the diameter of the tube, and they were fixed on the coil. The flanging and bulging of the tube were simultaneously conducted by the magnetic force with given charging voltage, and then the form-fit joining was achieved between the tube and sheet. To investigate the effect of the input voltages, the experiments were performed with various charging voltages from 5.7 kV up to 11.1 kV. In addition, the connection strength of the joined specimen was measured by the pull-out test between the tube and sheet. As a result, the connection strength was stronger than the yield strength and lower than the tensile strength of the aluminum 6063 tube.

In deep drawing of automotive components, the thickness distribution of the workpiece is a major concern regarding the feasibility of the process. A major geometrical parameter regarding the thickness distribution is the clearance between the punch and the die. The general industrial convention regarding this clearance is to use the initial blank thickness combined with a tolerance. This approach assures that the sheet is not compressed in the thickness direction which leads to undesired or undefined deformations and also high forces. Nevertheless, deformation in the thickness direction during stamping operations can be desired or advantageous in some cases. For instance, in flanging operations, ironing is used to reduce springback. There are already applications using solid elements for modelling such operations but these simulations are industrially not relevant due to high computation times. Currently, there is a need for an efficient simulation solution using shell elements. In order to address this problem, the shell element formulation was enhanced by taking the through-thickness deformation into account. In order to verify the new formulation, cup drawing experiments were performed using three different degrees of ironing. The material was a DC04- SUPERMOD with 1.75 mm thickness that was characterized by tensile tests in three directions, a bulge test and a layer compression test. Numerical and experimental results were compared in terms of the thickness distribution along the cross-section and the cup heights. Results show that the ironing process, which occurs in automotive stamping parts, can be simulated with enhanced shell elements without significant additional computational cost.

Sheet hydroforming technology has been developed and utilized for decades by Amino to mainly produce class A body panels for the automotive industry. Extra deep draw depth, more even material thickness distribution and better surface quality can be achieved through this technology. Several automotive manufacturers are using panels made with this technology for its high degree of formability and lower tooling cost. Other industries such as commercial truck and bus have also developed interest in sheet hydroformed panels in their vehicle architecture.

This paper presents efforts to improve the technology via simulation and new tooling techniques. Warm forming is shown to greatly improve the formability of high strength aluminum alloys. However, surface quality is always a concern which limits its utilization in class A panels. In addition, a new potential forming technology, Electro-Hydraulic Forming which can combine available technologies mentioned above to produce more aggressively styled parts with good surface quality using high strength aluminum alloys.

Finally, this paper will look at current development of sheet hydroforming technology and look at some actual industrial applications including the instillation of a new press for low volume production and future R&D, some review of warm forming experiments recently completed and future developmental programs with warm forming and sheet hydroforming in the near future are discussed.

As the automotive industry becomes increasingly competitive, manufacturers are striving to design the lightest, most fuel-efficient vehicles to capture the market. To achieve this mission, the material processing techniques used in vehicle manufacturing must be investigated further. One such critical technique is sheet metal forming, which creates parts that account for a large percentage of the total vehicle weight. Many of these sheet metal parts make up the structure of the vehicle and are constructed from thin-walled tubes. To manufacture these thin-walled tubes into the desired shape, one of the main techniques is tube forming which can expand or reduce the tubes' diameters to the desired dimension. One specific tube forming technique is the flaring process which is typically performed at the tube end. In this paper, the tool rotation at its flaring axis was considered. The analysis of the expansion ratio, strain path, and failure limit was also performed along with experimentation. Frictional effects were considered. It was observed that varying the rotational speed of the flaring tool influenced the ability of the tube to flare. The expansion ratio can be maximized with a decreased amount of friction, a lower rotational speed of the flaring tool, and with an increase in tool velocity.

Currently the standard for forming tube is to use a variety of shaped mandrels and hydrostatic pressures. Tube forming is the expansion of a circular cross-sectional area into a variety of different shapes. This paper is focused on square tube forming. Traditionally to create a tube with a square cross-section area, a square mandrel must pass through the circular tube in small increments until the circular cross-sectional is completely transformed into a square cross-section. A novel process named Reuleaux forming to flare the tube in to a square section is introduced and the results were compared with conventional forming. In this new process, a triangular shaped bit flares circular cross-sectional tubes to square tube. While experimenting with the Reuleaux triangular bit, the force required to form the tube, profile generated and thickness distribution were analyzed. Furthermore, the effect of friction and punch speed on deformation was studied. It was found that much lower forces were required to form the shape using the Reuleaux triangle as compared to conventional forming. It was also observed that the friction and speed does not affect much during the new process.

Folding is the process of forming cellular structures out of a flat raw material. This technology makes it possible to create folded structures with high depths and multiple bending axes. Folding is part of technical origami and therefore is defined by:

1. its geometry, which is defined through folding and requires a folding algorithm,

2. the initial material, which has a negligible thickness,

3. the fact that there is no global strain or general plastic deformation in the workpiece.

Current research work focuses on applying this technique to metallic materials with non-negligible thicknesses. Such structures can be used for optical appealing cladding, heat exchangers or core materials of numerous sandwich panel designs. In previous projects with very thin raw materials, it was shown that it is necessary to pre-crease the bending axes on the flat raw material to achieve a defined folding shape. The folding was usually achieved manually. For sheet metal, embossing of grooves in particular appears as a promising approach for pre-creasing. General challenges for folding of sheet metals are the forming of radii and hardening effects on the bending axes as well as appropriate tool concepts for the process.

This paper deals with the development of a forming tool to form a complex Miuri-structure made of sheet metal having a thickness of 0.5 mm or more. In a first step a tool is designed and manufactured that is able to fold a simple zigzag structure with one single bending-line orientation. The folding with this tool is investigated experimentally and based on the observations the zigzag tool concept is enhanced to a structure with a tree bending-line orientated Miuri-structure. This paper shows that a complex bending of sheet metals with non-negligible thicknesses is possible if a proper tooling design is taken into account.

Multi-component gearwheels consist of a gear ring, that carries the teeth and a wheel body, that connects gear ring and shaft. Both parts are joined by a press fit after the gear ring's heat treatment, what offers the possibility to choose a lightweight design for the wheel body independently from the gear ring. This paper focuses on a wheel body made by fine blanked sheet metal layers, which are stacked on top of each other until the gear ring width is reached. The manufacturing method influences the gearwheels overall behavior, as fine blanked edges are not rectangular. Lightweight design of the wheel body is a main objective. Again, this influences the gearwheel behavior, as the stiffness of the wheel body changes. To meet the lightweight objective while still fulfilling the mechanical needs of a gearwheel, a holistic development approach is used. In every step of that development process, it is checked, if a design proposal matches the mentioned requirements. The experiments show promising results for the further development of multi-component gearwheels.

The development of new technologies for forming lightweight sheet metals into vehicle components has resulted in a rapid growth of advanced predictive models to analyze such processes. Simulations of the warm and hot forming of aluminium alloys in particular have been conducted on numerous finite element (FE) software packages with various specific phenomena being captured through the implementation of subroutines, to ensure that parts formed are free of defects and meet the required post-form strength. However, access to such models is lacking, with each having to be adapted to the software being used, and are computationally expensive to run, limiting the capabilities of simulations and increasing the challenges of utilizing new forming technologies effectively. Smart Forming is a knowledge-based cloud platform that was developed to overcome these challenges. It is composed of functional modules based on predictive models made accessible on the cloud that can be run individually or simultaneously. A conventional forming simulation is first run locally in any FE software of choice, and the required results uploaded to the modules on the platform for remote computation to investigate the phenomena of interest. In this work, simulation data from two different software packages, PAM-STAMP and Autoform, were processed for the same U-shaped component in the Smart Forming modules 'Formability' and 'Tailor', to demonstrate the multi-objective simulation and software agnostic capabilities of the platform.

Adiabatic softening has an important impact on material behavior under dynamic loading. Temperature rise depends on the amount of plastic work converted into heat, as well as on the quantity of heat dissipation influenced by several parameters. In order to avoid complex thermomechanical coupling in simulation, a pseudo thermomechanical model based on an analytical approach of Dixon and Parry [1] was applied in in the past [2, 3, 4, 5] considering the influence of strain rate on temperature rise. In this work the additional influence of the stress state and the size of the localized zone on temperature rise is investigated for advanced high-strength steels (AHSS). Therefore high speed tests were performed for different multiaxial stress states at loading rates ranging from isothermal to adiabatic conditions. Local strain fields were measured by high-speed video recording, and evaluated by digital image correlation (DIC), and temperature fields were recorded by high-speed infrared (IR) measurement. For shear loading, the results show a significantly larger amount of local plastic work dissipated by heat transfer until failure emergence compared to tensile loading at comparable strain rates. Hence, an extended model "adiabatic tension-shear model (ATS)" is proposed considering adiabatic softening under shear-dominated loading conditions in simulations.

In recent years, one has seen a tremendous progress in methods for the simulation of production processes, especially in the automotive industry. Besides sheet metal forming, casting of alloys, moulding of polymers and various additive techniques are the key methods in manufacturing. In this list, sheet metal forming is unarguably the most mature virtual discipline to predict part producibility and the local properties. However, when it comes to transferring results from sheet metal forming simulation to further disciplines, like stiffness, NVH or crashworthiness simulation, a number of incompatibilities between the models need to be resolved. This is particularly pronounced when locally varying part properties are relevant. For situations in which the discrepancies in the constitutive models are not too dominating, this has been done successfully in the past by simply transferring thickness, plastic strain and possibly stresses, using shell elements in both disciplines. But since local effects, like extreme thinning, sharp bending or the onset of instability may dominate the fracture process in crashworthiness, especially when modern high strength alloys are regarded, these effects need to be investigated in more detail. In particular, their accurate evaluation may require modelling with 3D solid elements. On the one hand, the incompatibilities of the models become clearly obvious from the spatial discretization, while on the other the demand w.r.t. accuracy in crashworthiness is ever increasing. The present contribution focuses on the ability to capture demanding deformation states with classical and advanced shell formulations, which is seen as a first step in order to close the corresponding gap in the simulation process chain in a more general sense.

In order to satisfy continuously increasing regulations, further improvement of crash performance proves to be one of the major technological challenges of modern automotive engineering. It is crucial to have detailed specifications of material properties available that allow strategic material selection for various applications in future car body design. Within the scope of this work, local ductility of three 6xxx-aluminium sheet alloys is investigated based on evaluation of the localised necking behaviour in post-uniform phases of tensile tests (e.g. true fracture strain). The considered ductility criteria are gained by optical, analytical and fracture surface measurement methods. In addition to that, fracture propagation investigations are carried out to refine the ductility characterisation. The potential of local ductility characterisation methods is validated with results of the Edge-Compression Test (ECT) which allows quantification of material ductility at load conditions that occur in actual crash events.

Axial crush is an important mechanism used in automotive front end structures to absorb impact energy. In this work, numerical simulation is used to investigate the crush response of a Usibor® 1500-AS axial crush rail with tailored properties achieved using in-die heated (IDH) hot stamping. In this case, the targeted properties in the softened zone correspond to a tensile strength of 1000 MPa. A finite element model is utilized to predict the crash performance of the tailored in-die heated 1000 MPa crush tip within a Usibor® 1500-AS rail. A numerical parametric study is presented, comparing the crush response based on fracture loci determined using two different plane-strain strain experiments, one the plane-strain Nakazima dome test and the other the VDA-238 V-bending test. In addition a number of different mesh regularization treatments are considered. The predictions exhibit a strong dependency of the onset of fracture upon the plane strain fracture strain level and degree of mesh regularization.

In this study, an experimental investigation was performed to evaluate the potential of thermoplastic composites to be used for the fabrication of an automotive intrusion beam using the composite stamping process. To assess the potential of this technology, mechanical properties of several high performance thermoplastic composite materials were first evaluated on coupons. Due to their higher load and displacement at failure, the most promising materials for the fabrication of a composite intrusion beam were found to be glass fibre/polypropylene (GF/PP) and carbon fibre/polyamide 66 (CF/PA66) laminates. Then, stamped intrusion beams were manufactured from these composites using three different stacking sequences. The beams were later tested under three-point flexural loading. For all components, failure occurred at the attachment points. However, higher properties were obtained for ± 45 laminates for which shear-out failure could be avoided. From the load-displacement curves obtained, it can nevertheless be concluded that stamping of thermoplastic composites is promising for the fabrication of intrusion beams, but that further development will be required to avoid failure at the attachment points.

Hybrid parts are strongly moving into the focus for lightweight applications. Unfortunately, the accurate, simulative design, which comprises the accurate prediction of final part geometry, is still a challenging task. In the scope of this paper, an approach to improve the accuracy of appropriate finite element simulations is presented. To this end, the manufacturing history of the hybrid part is considered within the simulation of the part behavior. To create a finite element model of the considered hybrid composite, the intrinsic manufacturing process is modelled first. This includes the modelling of the thermoforming process of a fiber reinforced polymer as well as the sheet metal forming process for the fabrication of form fit elements. Then, the geometry of the hybrid part is deduced from the geometries of the single components. Afterwards, the material properties, including the local fiber volume content as well as the local fiber orientation, are mapped to the finite elements. Consequently, a workflow to create a finite element model which considers manufacturing history is developed and successfully tested.

High strength vibration damping sheet (VDS) consists of two high strength steel sheets and a viscoelastic polymer core with high vibrational damping characteristics. In this study, high strength VDSs with were fabricated with dual phase (DP) 590 steel sheets with 0.7mm of thickness as outer skins and polymer core with very thin thickness. To investigate the effect of different characteristics of the polymer core on delamination behaviour during press bending operation, three types of polymer cores were considered, which are polyethylene, epoxy and acrylic based adhesives and the V-bending tests were performed using the manufactured VDS. In addition, finite element analysis (FEA) was carried out to analyze the influence of the normal/shear directional characteristics of the polymer core on delamination behaviour in the V-bending test. As a result, it could be shown that different delamination behaviours were caused by the three types of the polymer core having different normal/shear directional characteristics.

Moving deeper into the twenty-first century, lightweight construction has become a central principle in component design in industry-wide efforts towards increasing vehicle fuel economy to maintain adherence to tighter government environmental standards. To achieve new levels of weight reduction in components, simplistic materials are being replaced with compound materials and composites such as tailored blanks and multi-layered materials or 'hybrid' components when dissimilar materials are used together (metal and plastic polymer, for example). Usage of these new composite materials has been observed to yield lower component weights as well as the same or higher performance as conventional materials. To investigate this further, conical flaring of a hybrid, bilayer tube comprising an interior metal tube surrounded by an exterior polymer tube is considered. For experimentation, a steel inner tube was used with a PVC exterior tube. In testing, the formability of the steel tube was observed to have increased with the implementation of the exterior PVC layer in comparison to single layer tubes comprised of steel alone. Observation and analysis of this behavior pointed towards the contact stress of the two materials increasing the formability and delaying the failure. Beyond the scope of observing the flare, another property of the bilayer tube was that the addition of the PVC layer reduced the collapse of the steel tube adjacent to the flared region, which remained undeformed. The results of experimentation confirm that the hybrid component outperforms its conventional counterpart by exhibiting higher formability, lower stress in the flared region, and better overall structural integrity of the specimens after being flared.