Smart Materials and Structures

Energy harvesting technologies have been explored by researchers for more than two decades as an alternative to conventional power sources (e.g. batteries) for small-sized and low-power electronic devices. The limited life-time and necessity for periodic recharging or replacement of batteries has been a consistent issue in portable, remote, and implantable devices. Ambient energy can usually be found in the form of solar energy, thermal energy, and vibration energy. Amongst these energy sources, vibration energy presents a persistent presence in nature and manmade structures. Various materials and transduction mechanisms have the ability to convert vibratory energy to useful electrical energy, such as piezoelectric, electromagnetic, and electrostatic generators. Piezoelectric transducers, with their inherent electromechanical coupling and high power density compared to electromagnetic and electrostatic transducers, have been widely explored to generate power from vibration energy sources. A topical review of piezoelectric energy harvesting methods was carried out and published in this journal by the authors in 2007. Since 2007, countless researchers have introduced novel materials, transduction mechanisms, electrical circuits, and analytical models to improve various aspects of piezoelectric energy harvesting devices. Additionally, many researchers have also reported novel applications of piezoelectric energy harvesting technology in the past decade. While the body of literature in the field of piezoelectric energy harvesting has grown significantly since 2007, this paper presents an update to the authors’ previous review paper by summarizing the notable developments in the field of piezoelectric energy harvesting through the past decade.

Cement-based materials possess an inherent autogenous self-healing capability allowing them to seal, and potentially heal, microcracks. This can be improved through the addition of microencapsulated healing agents for autonomic self-healing. The fundamental principle of this self-healing mechanism is that when cracks propagate in the cementitious matrix, they rupture the dispersed capsules and their content (cargo material) is released into the crack volume. Various healing agents have been explored in the literature for their efficacy to recover mechanical and durability properties in cementitious materials. In these materials, the healing agents are most commonly encapsulated in macrocontainers (e.g. glass tubes or capsules) and placed into the material. In this work, microencapsulated sodium silicate in both liquid and solid form was added to cement specimens. Sodium silicate reacts with the calcium hydroxide in hydrated cement paste to form calcium-silicate-hydrate gel that fills cracks. The effect of microcapsule addition on rheological and mechanical properties of cement is reported. It is observed that the microcapsule addition inhibits compressive strength development in cement and this is observed through a plateau in strength between 28 and 56 days. The improvement in crack-sealing for microcapsule-containing specimens is quantified through sorptivity measurements over a 28 day healing period. After just seven days, the addition of 4% microcapsules resulted in a reduction in sorptivity of up to 45% when compared to specimens without any microcapsule addition. A qualitative description of the reaction between the cargo material and the cementitious matrix is also provided using x-ray diffraction analysis.

In this paper, a novel boundary element plate formulation is proposed to model ultrasonic Lamb waves in both pristine and cracked plates for structural health monitoring (SHM) applications. Lamb waves are generated and sensed by piezoelectric discs. An equivalent pin-force model is newly proposed to represent the actuation effect of piezoelectric discs, which is more accurate than the classical pin-force model. The boundary element formulation is presented in the Laplace-transform domain based on plate theories, which allows three-dimensional analysis of Lamb wave behaviours, such as propagation and interaction with cracks, in thin-walled structures. A damage detection algorithm is used for crack localization alongside the BEM-simulated data. The BEM solutions show excellent agreement with both 3D finite element simulation and experimental results.

This review contains a comparative study of reported fabrication techniques of gallium based liquid metal alloys embedded in elastomers such as polydimethylsiloxane or other rubbers as well as the primary challenges associated with their use. The eutectic gallium–indium binary alloy (EGaIn) and gallium–indium–tin ternary alloy (galinstan) are the most common non-toxic liquid metals in use today. Due to their deformability, non-toxicity and superior electrical conductivity, these alloys have become very popular among researchers for flexible and reconfigurable electronics applications. All the available manufacturing techniques have been grouped into four major classes. Among them, casting by needle injection is the most widely used technique as it is capable of producing features as small as 150 nm width by high-pressure infiltration. One particular fabrication challenge with gallium based liquid metals is that an oxide skin is rapidly formed on the entire exposed surface. This oxide skin increases wettability on many surfaces, which is excellent for keeping patterned metal in position, but is a drawback in applications like reconfigurable circuits, where the position of liquid metal needs to be altered and controlled accurately. The major challenges involved in many applications of liquid metal alloys have also been discussed thoroughly in this article.

In the study at hand the Joule heating effect of electrically conductive cementitious nanocomposites filled with different loadings of multi-walled carbon nanotubes (MWCNTs) is investigated. Nanofiller dispersions were initially prepared via ultrasonication in deionized water (d-H ₂O) utilising a commercial superplasticizer as surfactant. Electrically percolated nanocomposites were fabricated via shear mixing and subsequent casting into moulds. Storing the prepared samples under different humid conditions enabled explanation of the role of water content as well as cement age on Joule heating performance. All prepared specimens were investigated at ages of 3 d, 7 d and 28 d by applying two different DC bias voltages. Infrared-thermography (IR-T) images were recorded after 1 min, 5 min and 10 min in order to visualize the differences in the Joule heating effect as a function of time, keeping contact with the DC bias voltage. The observed results showed a significant dependency of the Joule heating effect on water content as well as on filler concentration. Moreover, increasing cement age provided more effective electrical heating. This work elucidates the complexity of the electrical heating phenomena occurring in cementitious/MWCNT nanocomposites via the well-known Joule heating effect because it contributes to the understanding of the underlying mechanism. The main parameters used and the corresponding results are envisaged to be applicable for large-scale, heatable concrete structures in future respecting buildings temperature, aerial control, de-icing, thermal management, and better energy efficiency, etc.

Flexible and wearable sensor based on nanocomposite hydrogels has been proposed for monitoring the human large-scale, small-scale movements and several physiological signals. The nanocomposite hydrogel, prepared from graphene oxide (GO), polyvinyl alcohol (PVA) and polydopamine (PDA), exhibits excellent mechanical and electrical properties with tensile stress of 146.5 KPa, fracture strain of 2580%, fracture energy of 2390.86 KJ m ⁻³, and the conductivity of 5 mS cm ⁻¹. In addition, it possesses other merits including good self-healing with the electrical self-healing efficiency of 98% of its original resistance within 10 s, and strong self-adhesion onto a variety of surfaces of materials. This self-adhesive, self-healing, graphene-based conductive hydrogel can further assembled as wearable sensors to accurate and real-time detect the signals of human large-scale motions (including bending and stretching fingers joints, wrists joints, elbows joints, neck joints and knees joints) and small-scale motions (including swallowing, breathing and pulsing) through fracturing and recombination of reduced graphene oxide (rGO) electrical pathways in porous structures of hydrogel networks. Furthermore, the hydrogel can also be used as self-adhesive surface electrodes to detect human electrophysiological (ECG) signals. Therefore, the hydrogel-based wearable sensor is expected to be used for long-term and continuous monitoring human body motion and detecting physiological parameters.

Materials and structures with negative Poisson’s ratio exhibit a counter-intuitive behaviour. Under uniaxial compression (tension), these materials and structures contract (expand) transversely. The materials and structures that possess this feature are also termed as ‘auxetics’. Many desirable properties resulting from this uncommon behaviour are reported. These superior properties offer auxetics broad potential applications in the fields of smart filters, sensors, medical devices and protective equipment. However, there are still challenging problems which impede a wider application of auxetic materials. This review paper mainly focuses on the relationships among structures, materials, properties and applications of auxetic metamaterials and structures. The previous works of auxetics are extensively reviewed, including different auxetic cellular models, naturally observed auxetic behaviour, different desirable properties of auxetics, and potential applications. In particular, metallic auxetic materials and a methodology for generating 3D metallic auxetic materials are reviewed in details. Although most of the literature mentions that auxetic materials possess superior properties, very few types of auxetic materials have been fabricated and implemented for practical applications. Here, the challenges and future work on the topic of auxetics are also presented to inspire prospective research work. This review article covers the most recent progress of auxetic metamaterials and auxetic structures. More importantly, several drawbacks of auxetics are also presented to caution researchers in the future study.

The vibration responses of structures under controlled or ambient excitation can be used to detect structural damage by correlating changes in structural dynamic properties extracted from responses with damage. Typical dynamic properties refer to modal parameters: natural frequencies, mode shapes, and damping. Among these parameters, natural frequencies and mode shapes have been investigated extensively for their use in damage characterization by associating damage with reduction in local stiffness of structures. In contrast, the use of damping as a dynamic property to represent structural damage has not been comprehensively elucidated, primarily due to the complexities of damping measurement and analysis. With advances in measurement technologies and analysis tools, the use of damping to identify damage is becoming a focus of increasing attention in the damage detection community. Recently, a number of studies have demonstrated that damping has greater sensitivity for characterizing damage than natural frequencies and mode shapes in various applications, but damping-based damage identification is still a research direction ‘in progress’ and is not yet well resolved. This situation calls for an overall survey of the state-of-the-art and the state-of-the-practice of using damping to detect structural damage. To this end, this study aims to provide a comprehensive survey of uses and features of applying damping in structural damage detection. First, we present various methods for damping estimation in different domains including the time domain, the frequency domain, and the time-frequency domain. Second, we investigate the features and applications of damping-based damage detection methods on the basis of two predominant infrastructure elements, reinforced concrete structures and fiber-reinforced composites. Third, we clarify the influential factors that can impair the capability of damping to characterize damage. Finally, we recommend future research directions for advancing damping-based damage detection. This work holds the promise of (a) helping researchers identify crucial components in damping-based damage detection theories, methods, and technologies, and (b) leading practitioners to better implement damping-based structural damage identification.

Current soft pneumatic grippers cannot robustly grasp flat materials and flexible objects on curved surfaces without distorting them. Current electroadhesive grippers, on the other hand, are difficult to actively deform to complex shapes to pick up free-form surfaces or objects. An easy-to-implement PneuEA gripper is proposed by the integration of an electroadhesive gripper and a two-fingered soft pneumatic gripper. The electroadhesive gripper was fabricated by segmenting a soft conductive silicon sheet into a two-part electrode design and embedding it in a soft dielectric elastomer. The two-fingered soft pneumatic gripper was manufactured using a standard soft lithography approach. This novel integration has combined the benefits of both the electroadhesive and soft pneumatic grippers. As a result, the proposed PneuEA gripper was not only able to pick-and-place flat and flexible materials such as a porous cloth but also delicate objects such as a light bulb. By combining two soft touch sensors with the electroadhesive, an intelligent and shape-adaptive PneuEA material handling system has been developed. This work is expected to widen the applications of both soft gripper and electroadhesion technologies.

Soft and smart robotic end effectors with integrated sensing, actuation, and gripping capabilities are important for autonomous and intelligent grasping and manipulation of difficult-to-handle and delicate materials. Grasping and actuation are challenging to achieve if using only one opto-mechanical tactile sensor. It is highly desirable to equip these useful sensors with multimodal actuation and gripping functionalities. Current electroadhesive (EA) grippers, however, cannot differentiate object size and shape, nor can they grasp concave or convex objects. In this paper, we present TacEA, an integration of a pneumatically actuated visio-tactile (TacTip) sensor and a stretchable EA pad, resulting in a monolithic soft-smart robotic end effector with concomitant sensing, actuation, and gripping capabilities. This soft composite-materials device delivers the first soft tactile sensor with actuation and gripping capability and the first EA end effector that can sort different 2D object sizes and shapes with one touch, and which can actively grasp flat, concave and convex objects. The soft-smart TacEA is expected to widen the capabilities of current tactile sensors and increase EA end effector use in material handling and in processing and assembly lines.

Energy harvesting technologies have been explored by researchers for more than two decades as an alternative to conventional power sources (e.g. batteries) for small-sized and low-power electronic devices. The limited life-time and necessity for periodic recharging or replacement of batteries has been a consistent issue in portable, remote, and implantable devices. Ambient energy can usually be found in the form of solar energy, thermal energy, and vibration energy. Amongst these energy sources, vibration energy presents a persistent presence in nature and manmade structures. Various materials and transduction mechanisms have the ability to convert vibratory energy to useful electrical energy, such as piezoelectric, electromagnetic, and electrostatic generators. Piezoelectric transducers, with their inherent electromechanical coupling and high power density compared to electromagnetic and electrostatic transducers, have been widely explored to generate power from vibration energy sources. A topical review of piezoelectric energy harvesting methods was carried out and published in this journal by the authors in 2007. Since 2007, countless researchers have introduced novel materials, transduction mechanisms, electrical circuits, and analytical models to improve various aspects of piezoelectric energy harvesting devices. Additionally, many researchers have also reported novel applications of piezoelectric energy harvesting technology in the past decade. While the body of literature in the field of piezoelectric energy harvesting has grown significantly since 2007, this paper presents an update to the authors’ previous review paper by summarizing the notable developments in the field of piezoelectric energy harvesting through the past decade.

This study proposes a cross-coupled dual-beam structure for energy harvesting from vortex-induced vibrations (VIV) induced by wind flows in different directions. A series of wind tunnel tests are conducted to investigate the performance of the proposed energy harvester subjected to the wind load with various speeds and directions. The upper and bottom piezoelectric beams can generate a maximum power output of 6.77 μW and 56.64 μW, respectively. The dominant operation frequencies in different directions are different which indicates a potential broadband capability. A parametric study is performed to reveal the effect of the dimension of the bluff body on the performance of the proposed energy harvester.

The aim of this paper is to introduce tunable continuous-stable metamaterials with reversible thermo-mechanical memory operations by four-dimensional (4D) printing technology. They are developed based on an understanding on glassy-rubbery behaviors of shape memory polymers and hot/cold programming derived from experiments and theory. Fused decomposition modeling as a well-known 3D printing technology is implemented to fabricate mechanical metamaterials. They are experimentally tested revealing elastic-plastic and hyper-elastic behaviors in low and high temperatures at a large deformation range. A computational design tool is developed by implementing a 3D phenomenological constitutive model coupled with a geometrically nonlinear finite element method. Governing equations are then solved by an elastic-predictor plastic-corrector return map procedure along with the Newton-Raphson and Riks techniques to trace nonlinear equilibrium path. A tunable reversible mechanical metamaterial unit with bi-stable memory operations is printed and tested experimentally and numerically. By a combination of cold and hot programming, the unit shows potential applications in mimicking electronic memory devices like tactile displays and designing surface adaptive structures. Another design of the unit shows potentials to serve in designing self-deployable bio-medical stents. Experiments are also conducted to demonstrate potential applications of cold programming for introducing recoverable rolling-up chiral metamaterials and load-resistance supportive auxetics.

Monitoring the looseness of bolted connection, especially at the inception or early stage, is important to ensure its integrity and the wellbeing of structures. In the past decades, significant progress, including several state-of-the-art structural health monitoring approaches, has been made to characterize health status of bolted connections. One of these approaches is the active sensing method with merits such as low cost and easy-to-operate; however, its current damage index (DI) may saturate at high levels of bolt preload, which will impede its application in detecting early looseness of bolted connections. As the main contribution of this paper, we apply the multiscale permutation entropy algorithm to develop a new entropy-based DI that eliminates the saturation phenomenon of current DI (i.e. the signal energy), enhancing the performance of the piezoceramic-enabled active sensing method in monitoring bolt early looseness. Multiple experiments were conducted on a multi-bolt connection with two surface-bonded piezoceramic transducers to verify the effectiveness of the proposed method. Finally, after processing the received signals through the new DI, we demonstrated that the proposed method effectively eliminated the saturation problem, which is significant in detecting early looseness of bolt connections.

Shape memory polymers (SMPs) are a group of smart materials that, by applying an external stimulation such as the temperature, retrieve their permanent shape from a temporary one. SMP nanocomposites have been developed to improve the mechanical, thermal, electrical, and magnetic properties of SMPs for potential applications in e.g. medical equipment, sensors, actuators, and drug delivery systems. In this research, SMP is reinforced with Coiled carbon nanotubes (CCNT) due to its geometric properties which let material tolerate higher strains and improve thermomechanical properties of SMP. In this paper, the effect of addition of CCNT on thermomechanical response of SMP under large deformations is numerically investigated. Employing a thermo-visco-hyperelastic constitutive model for SMP, a cubic representative volume element is realized using Monte Carlo algorithm. The effect of inclusion’s geometry (e.g. spring length or aspect ratio, pitch or number of coils of CCNT), volume fraction, as well as their distribution on the thermomechanical properties of SMP/CCNT composite in two stress- and shape recovery processes in different heating rates and pre-strains is studied using Finite Element technique. Results reveal that increasing the volume fraction up to 0.6%, leads to a 15% rise in the effective stress in the nanocomposite. Increasing the spring length of the CCNT, the strain recovery of the nanocomposite increases about 8%. It is shown that when the mechanical loading is parallel to the CCNTs orientation, the stress is about 25% larger than when the loading is perpendicular to the unidirectional CCNTs. But for the strain recovery, the orientation does not play an important role in the strain recovery.

This study combined three-dimensional (3D) printing and composite multiscale cross-sample entropy (CMSCE) in structural health monitoring (SHM) and explored a quantification criterion for single-story structural damage index (DI). By quantifying the DI, the study established a SHM system suitable for real-world structures. A numerical model of a seven-story 3D printed structure was first created. Through the establishment of various bracing conditions as failure modes, damage to the structure was simulated properly, and CMSCE was used to effectively indicate the location of damage. Moreover, the DI was used to shorten the assessment time and improve system accuracy. The DI quantification facilitated observation of the effects of various degrees of damage on the analysis results. Based on the results, an experiment involving a 3D-printed structure was conceived. First, an experiment involving a seven-story structure with severe, moderate, and marginal single-story damage was conducted. The signals obtained from these structures were used to perform CMSCE analysis. Structural damage was detected using entropy curves and DI figures to determine the location and degree of damage as well as to quantify the DI. Thus, the study developed a reliable method by combining emerging 3D printing technology with the CMSCE DI to explore the feasibility of practical application.

In recent years, with the rapid increase of railway freight, various types of self-powered sensors have been widely used in intelligent electronic monitoring equipment and on-board electric equipment in order to improve the safety and transportation efficiency of railway freight during operation. In this paper, a type of energy regeneration shock absorber based on twin slider-crank mechanisms is developed to install on the auxiliary suspensions of railway cars parallel to the coil springs. The energy regeneration shock absorber is divided into four components, as follows: a suspension vibration energy input module, a transmission module, a generator module, and an energy storage module. The suspension vibration energy input module captures the vibration energy generated by the railway car suspension. The transmission module converts the linear motion of the shock absorber into the one-way rotation motion of the generator input shaft to generate electrical energy. The energy storage module stores the electricity in the super capacitor to supply energy to self-powered sensors in railway cars. According to the bench test of the MTS^® fatigue testing system, the peak efficiency of the shock absorber is 56.4%, and the average efficiency is 42.9%. Under harmonic excitation with a frequency of 3 Hz and an amplitude of 12.5 mm, the peak phase power is 24.6 W and the average power is 4.8 W, showing that the proposed new energy regeneration shock absorber is effective and practical to apply to self-powered sensors in railway cars.

Compared with other components, actuator fault has a higher probability of occurrence in semi-active suspension with magneto-rheological (MR) damper, which will lead to the safety and reliability of the system. Hence, the fault diagnosis and fault-tolerant methods of semi-active vehicle suspension system with MR damper are investigated in this paper to deal with the fault of MR damper. Firstly, the quarter-vehicle suspension system model is established. Secondly, an unknown input observer (UIO) with strong robustness and simple structure is employed to detect the fault of MR damper; meanwhile, the correlation coefficient method based on the system residuals is used to isolate the fault of MR damper. Lastly, the skyhook fault-tolerant controller (FTC) is designed to compensate the system with fault application. The simulation results under sine excitation, random excitation and bump excitation show that the performance of the proposed FTC always outweigh that of without fault-tolerant when MR damper occurs fault.

Soft robotics is a rapidly evolving field offering novel solutions in the development of wearable technologies. Soft pneumatic artificial muscles in particular, have seen widespread use in the development of human scale rehabilitative and assistive wearables. However, these soft actuators have not yet been adapted to address the complex dynamic regime of active (essential tremor) and resting (Parkinson's disease) hand tremor, the most common movement disorder affecting humans. Current solutions to address hand tremor involve expensive medication and surgical interventions, as well as wearable assistive devices that fall short of providing an effective compact design for the suppression of hand tremor. This study focuses on the design of a novel lightweight, compact, bending actuator that will be capable of actively suppressing hand tremor when adapted into an assistive wearable device. The proposed fiber-reinforced bending pneumatic artificial muscle (BPAM), including its design specifications, fabrication process, theoretical modeling, and experimental characterization, are detailed. The developed actuator was capable of producing sinusoidal trajectories with peak-to-peak amplitudes of 40° and a bandwidth of 8 Hz, the dynamic regime of pathological hand tremor. The ability of the fiber-reinforced BPAM to act within the dynamic regime of hand tremor demonstrates its potential to be further developed into a system capable of the active suppression of hand tremor.

Soft elastic composite materials can serve as actuators when they transform changes in external fields into mechanical deformation. Here, we theoretically address the corresponding deformational behavior in model systems of magnetic gels and elastomers exposed to external magnetic fields. In reality, such materials consist of magnetizable colloidal particles in a soft polymeric matrix. Since many practical realizations of such materials involve particulate inclusions of polydisperse size distributions, we concentrate on the effect that mixed particle sizes have on the overall deformational response. To perform a systematic study, our focus is on binary size distributions. We systematically vary the fraction of larger particles relative to smaller ones and characterize the resulting magnetostrictive behavior. The consequences for systems of various different spatial particle arrangements and different degrees of compressibility of the elastic matrix are evaluated. In parts, we observe a qualitative change in the overall response for selected systems of mixed particle sizes. Specifically, overall changes in volume and relative elongations or contractions in response to an induced magnetization can be reversed into the opposite types of behavior. Our results should apply to the characteristics of other soft elastic composite materials like electrorheological gels and elastomers when exposed to external electric fields as well. Overall, we hope to stimulate corresponding experimental realizations and the further investigation on the purposeful use of mixed particle sizes as a means to design tailored requested material behavior.

Nitinol (NiTi) shape memory alloy (SMA) heat engine is a very promising candidate application of SMA since its invention, but yet to be put as commercial due to its low efficiency and performance. In this paper, a review of different types of NiTi SMA heat engine is presented. The review is done based on the following headings: conceptual design, constitutive driving model, the performance of the engine, and engine limitations. Factors like temperature, cooling rate, size of NiTi SMA element, and the stretch ratio of NiTi SMA spring were identified as determinants of final output power and efficiency of SMA heat engines. It is found that a crankshaft NiTi SMA heat engine produced the highest power output value of 4 watt, an unsynchronized pulley NiTi SMA heat engine with 11.3% of engine efficiency has the highest engine performance so far recorded. Some engine’s drawbacks like an improper driving model, drag loss, backsliding were identified as the major problem affecting the engine performance, and which, if solved, will increase the overall performance and efficiency for future development toward its commercialization.

The need for precision positioning applications has enormously influenced the research and development towards the growth of precision actuators. Over the years, piezoelectric actuators have significantly satisfied the requirement of precision positioning to a greater extent with the capability of broad working stroke, high-accuracy, and resolution (micro/nano range) coupled with the advantage of faster response, higher stiffness, and actuation force. The present review intends to bring out the latest advancement in the field of piezoelectric actuator technology. This review brings out the specifics associated with the development of materials/actuators, the working principles with different actuation modes, and classifications of the piezoelectric actuators and their applications. The present article throws light on the design, geometrical features, and the performance parameters of various piezoelectric actuators right from unimorph, bimorph, and multilayer to the large displacement range actuators such as amplified actuators, stepping actuators with relevant schematic representations and the quantitative data. A comparative study has been presented to evaluate the pros and cons of different piezoelectric actuators along with quantitative graphical comparisons. An attempt is also made to highlight the application domains, commercial and future prospects of technology development towards piezoelectric actuators for precision motion applications. The organization of the paper also assists in understanding the piezoelectric materials applicable to precision actuators. Furthermore, this paper is of great assistance for determining the appropriate design, application domains and future directions of piezoelectric actuator technology.

Much recent work has been devoted to utilizing changes in either the inherent or imparted electrical conductivity of self-sensing materials as an indicator of damage, deformation or strain, pressure, or general condition (e.g. moisture content, UV exposure, pH, etc). The scope of self-sensing materials is quite large and includes polymeric composites, cementitious and ceramic materials, specialized fabrics, and sensing skins or paints applied to materials which otherwise cannot be made to be self-sensing. Beyond just utilizing conductivity changes for mere detection, spatial localization and artifact shaping is highly desirable. For this, electrical impedance tomography (EIT) has received much attention (also referred to as electrical resistance tomography, ERT). Because of the diversity of materials which exhibit conductivity-based self-sensing, EIT has broad, far-reaching potential in areas such as structural health monitoring (SHM) and nondestructive evaluation (NDE), human-to-robot interactions and interfacing, and even biomedical implants, prosthetic devices, and human health monitoring via wearable sensor technology. Historically, EIT was developed for biomedical imaging and geophysical prospection—vast literature exists in these areas. However, new applications of EIT in conjunction with stimulus-responsive conductivity in engineered materials poses new challenges and new opportunities not seen in biomedical and geophysical applications. Therefore, a comprehensive review of EIT as it has been applied for damage, deformation or strain, pressure, and condition monitoring is herein presented in order to provide the reader with a perspective of results to date, best practices, and a road map for future development of this exciting technology.

Magnetorheological elastomers (MREs) are one of the categories of smart materials, whose modulus increases considerably in the presence of a magnetic field. These elastomers are prepared by dispersing magnetic micro-sized particles into a soft solid carrier medium. The main feature of these elastomers is that they change their elastic and damping properties quickly in the presence of a magnetic field. The change in properties, also known as the magnetorheological (MR) effect of MREs are dependent on various parameters such as type of matrix material, distribution of magnetic particles, additives, working mode, and strength of the applied magnetic field. Various studies have been conducted to improve the MR effect and seek the possibility to implement the MREs in different applications including but not limited to vibration absorbers, isolators, soft actuators, and sensors. The focus of this review is to present the recent progress of MREs including materials used, fabrication strategies, MR effect, and potential applications.

Simplicity, volume, dimensions, and often mass are important factors in mechanical systems; engineers are looking to smart materials, in particular shape memory alloys (SMAs), to provide actuation and dampening capabilities in highly compact packages. SMAs have long been studied for their potential to impact a number of disciplines, especially the aerospace, biomedical, and automotive industries. However, while axial SMA actuation (e.g. in the form of wires) is highly common and has been extensively reviewed, no comprehensive compilation of studies in SMA torsional actuation is provided in existing literature. It is one of the most compact possible approaches to supplying rotational motion under load. This paper reviews the torsional applications of SMAs and their properties in that regard. First, a brief overview of SMA behavior is given. A review of applications past and present incorporating torsional SMAs is reported, and a summary of experimental data provided in prior literature, essential for understanding real-world capabilities of SMA torsional actuators, is presented. A modeling approach is outlined, and theoretical torque tube performance is demonstrated as compared to prior experimental data. The work concludes with a discussion of future developments required to enable commercial applications of SMA torque tubes.

Magnetorheological elastomers (MREs) consist of micron-sized magnetizable particles embedded in a rubber matrix. Properties of these magneto-sensitive materials are changed reversibly upon application of external magnetic fields. They exhibit highly non-linear magneto-mechanical response which allows developing new devices and applications. However, the coupled magneto-mechanical behavior makes mathematical modeling of MREs quite complicated. So development of a reliable constitutive framework is essential for further understanding of this coupled behavior as well as simulation of the systems that utilize MREs. In this paper, a finite strain continuum model is developed for MREs where the effect of magnetization on material stiffness is directly introduced in the material shear modulus. It is shown that this approach simplifies the constitutive models and also perceives the magnetic saturation of MREs. Moreover, the coupled effects of magnetization, deformation and particle-chains orientation on the mechanical response are also taken into account in the introduced parameter. This reduces the number of material parameters, the required experimental tests for parameters identification and also simplifies the mathematical formulation of the developed constitutive equations which is beneficial for numerical formulations. A systematic two-step method is then introduced for material parameters identification which assures uniqueness of the parameters set. The predictive capabilities of the proposed model are examined via available mechanical and magneto-mechanical experimental data on both isotropic and anisotropic MRE samples at different configurations of magnetic field and loading with respect to the preferred direction of the samples. It is shown that the model can well predict the magneto-mechanical response of MREs at different deformation modes and magnetic fields.

In the study at hand the Joule heating effect of electrically conductive cementitious nanocomposites filled with different loadings of multi-walled carbon nanotubes (MWCNTs) is investigated. Nanofiller dispersions were initially prepared via ultrasonication in deionized water (d-H ₂O) utilising a commercial superplasticizer as surfactant. Electrically percolated nanocomposites were fabricated via shear mixing and subsequent casting into moulds. Storing the prepared samples under different humid conditions enabled explanation of the role of water content as well as cement age on Joule heating performance. All prepared specimens were investigated at ages of 3 d, 7 d and 28 d by applying two different DC bias voltages. Infrared-thermography (IR-T) images were recorded after 1 min, 5 min and 10 min in order to visualize the differences in the Joule heating effect as a function of time, keeping contact with the DC bias voltage. The observed results showed a significant dependency of the Joule heating effect on water content as well as on filler concentration. Moreover, increasing cement age provided more effective electrical heating. This work elucidates the complexity of the electrical heating phenomena occurring in cementitious/MWCNT nanocomposites via the well-known Joule heating effect because it contributes to the understanding of the underlying mechanism. The main parameters used and the corresponding results are envisaged to be applicable for large-scale, heatable concrete structures in future respecting buildings temperature, aerial control, de-icing, thermal management, and better energy efficiency, etc.

The implementation of efficient maintenance strategies of thin-walled structural components require reliable damage detection and localization techniques. In particular, guided ultrasonic waves technology represent an auspicious approach when implemented in a structural health monitoring system. The method is usually based on distributed sensing with piezoelectric elements that act in turn as ultrasound transmitter and receiver. This work aims at a unifying framework for damage localization considering algorithms from different scientific disciplines, e.g. originated from radar and geophysics. Here, we systematically express those algorithms in matrix form and compare the respective damage localization performance with experimental measurements considering an isotropic specimen with a single and also multiple simultaneous defects. In addition, we evaluate the algorithms’ point spread function and propose performance metrics to quantitatively compare the imaging success.

During its lifetime, an aircraft structure is subjected to various impacts from various sources such as tool drops, hail, ground service equipment, etc. In modern composite structures, these impacts have a significant chance of generating barely visible damage (BVID) which may lead to catastrophic failure of a structure if left undetected to grow. However, BVID is difficult to detect during routine visual inspection without specialised non-destructive inspection and thus there is large interest in developing monitoring systems for estimating the location and severity of impact events. Currently, most systems and methods have been developed for controlled lab conditions and do not consider the wide range of impact parameters in real life operation (environmental conditions, vibration, impactor stiffness, angle, etc) which may severely compromise the accuracy of these methods. In this study we have explored two methods for maximum impact force estimation, deconvolution and a novel gradient method, for the purpose of reliable severity assessment in composite aircraft structures under simulated environmental and operational conditions. It is shown that both methods allow accurate and robust estimation of the maximum impact force from various cases of impacts (variation of impact energy, mass, stiffness, angle, temperature, source) using minimum initial data from a single impact case. From further testing it is demonstrated that the gradient method is robust towards the effects of impact localisation errors and noise. The gradient method also has much less computational and storage requirements and is thus more feasible to integrate with current data acquisition systems being developed for structural health monitoring. Thus, we conclude that the proposed gradient method is suitable for impact force monitoring and severity assessment in composite aircraft structures in the simulated environmental and operational conditions.

Interdigitated electrodes (IDEs) on dielectric films is an important electrode design in electrical components such as transducers and sensors. Further development and use of IDEs for characterization of the in-plane properties of dielectric films requires models for the capacitance, particularly when used in a multilayer stack. Previous models for the capacitance have permitted erroneous boundary conditions between layers with associated limitations to accuracy. In this work we present a new model based on fulfilling the boundary conditions between layers with different dielectric constant. We further demonstrate how the model can be used to calculate the in-plane dielectric constant and polarization of BaTiO ₃ films. The model is shown to outperform previous models using both the experimental data from BaTiO ₃ films on SrTiO ₃ substrates and finite element method simulations of the corresponding case. One important advantage compared to previous work is that the new model provides good results regardless of film thickness.

Electromechanical impedance (EMI) based techniques have been proposed for structural health monitoring due to their sensitivity to low levels of damage. Most of the work in the EMI technique depends on the change in the admittance signature of the structure in the healthy and damaged state. Several metrics have been proposed to quantify this difference in the signature. Most common being root-mean square difference (RMSD), mean absolute percentage deviation, correlation coefficient etc. As the admittance signatures has several troughs and peaks, the statistical metrics are not robust and often show false detection due to ambient changes and measurement noise.

Thus, this paper proposes a novel index for the damage detection using the EMI technique based on the cumulative electrical power. The frequency v/s resistance or conductance plot is used for calculating the normalized cumulative electrical power (NCP) of the system. The NCP curve is a monotonically increasing function and hence robust for statistical comparison. The cumulative power curve is then used to develop three different indices comparing the amplitude difference (RMSD of the NCP curves), difference in the area under the NCP curve as well as the modified Frechet distance between the NCP curves. The performance of these indices are compared with the RMSD index which has been commonly used. The comparison is carried out on four different structures and show very encouraging results. In addition to the experimental validation, sensitivity studies have been carried out on an analytical signal. It is seen that the Frechet distance based index is a robust indicator for damage detection and minimizes the false detection under variety of conditions affecting the EMI signature.

To improve the clinical outcome of total hip replacements (THRs), instrumented implants with sensory functions for implant monitoring and diagnostics or actuators for therapeutic measures are a promising approach. Therefore, an adequate energy source is needed. Batteries and external power supplies bring shortcomings e.g. limited lifetime or dependency on external equipment. Energy harvesting has the clear benefit of providing continuous and independent power for fully autonomous implants. Our present study evaluates by means of finite element analysis (FEA) the capabilities of a concept of a piezoelectric energy harvesting system (ring shaped multilayer piezoelectric element of 5 mm diameter and 2.5 mm height) integrated in a femoral hip stem. The deformations from a modified load-bearing implant are used to generate electric power for various instrumentation purposes. Besides the expected amount of converted energy, the influence on the stress distribution of the instrumented implant is analysed. The results show that the local stress increase for the modified implant geometry does not exceed the stress of the original reference model. The maximum generated open circuit voltage of 11.9 V can be processed in standard energy harvesting circuitry whereas an average power output amounts up to 8.1 µW. In order to increase the electric power in an upcoming design optimization, a sensitivity analysis is performed to identify the most important influencing parameters with regard to power output and implant safety.

In this paper, a novel boundary element plate formulation is proposed to model ultrasonic Lamb waves in both pristine and cracked plates for structural health monitoring (SHM) applications. Lamb waves are generated and sensed by piezoelectric discs. An equivalent pin-force model is newly proposed to represent the actuation effect of piezoelectric discs, which is more accurate than the classical pin-force model. The boundary element formulation is presented in the Laplace-transform domain based on plate theories, which allows three-dimensional analysis of Lamb wave behaviours, such as propagation and interaction with cracks, in thin-walled structures. A damage detection algorithm is used for crack localization alongside the BEM-simulated data. The BEM solutions show excellent agreement with both 3D finite element simulation and experimental results.

Welding is one of the forms of nonautomated, heavy labor used in the shipbuilding industry. In the shipbuilding process, the hull is first welded by an industrial automatic welding machine, then irregular welded part correction and welding in narrow places that machines cannot reach are performed by welding workers. Welding operators are required to weld indeterminate objects in narrow working spaces. In addition, it is difficult to perform high-quality welding because the ability to accurately trace a welding line at a constant speed requires a long training period. In this research, a welding training device that assists trainees in skill acquisition was developed. The proposed device, which comprises two magnetorheological (MR) brakes, renders a welding path and restricts welding speed. The MR brakes have a response speed comparable to that of powder brakes, have a high output-to-weight ratio, and are also considered to be suitable for use in welding training devices. A prototype and control strategy were developed, and the performance of the device in actual welding work was evaluated. Results shows that path rendering strategy is possible to present routes with an accuracy of 1 mm or better and speed control strategy can regulate the speed required for welding.

Dielectric Elastomer Actuators (DEAs) are elastic parallel plate capacitors consisting of polymers as dielectric layer and compliant electrodes’. In dynamic applications with high frequencies DEAs heat up. In this work the heating is studied by experimental investigations as well as by modeling of the visco-elastic behavior of the polymer and the specific electric resistance of the electrode and of the polymer. The experimental investigations have also been performed, in order to obtain parameters for the material model. A partially coupled thermo-electro-mechanical material model is presented and used for the numerical simulations. The experimental and numerical results for the time-dependent thermal behavior show an excellent qualitative correlation. This confirms the quality of the developed multi-field model.