Table of contents

Digital twins (DTs) have witnessed a paramount increase in applications in multidisciplinary engineering systems. With advancements in structural health monitoring (SHM) methods and implementations, DT-based maintenance and operation stages have been implemented significantly during the life cycle of civil infrastructure. Recent literature has started laying the building blocks for incorporating the concept of DTs with SHM of large-scale civil infrastructure. This paper undertakes a systematic literature review of studies on DT-related applications for SHM of civil structures. It classifies the articles based on thematic case studies: transportation infrastructure (i.e. bridges, tunnels, roads, and pavements), buildings, off-shore marine infrastructure and wind turbines, and other civil engineering systems. The proposed review is further uniquely sub-classified using diverse modeling approaches such as building information modeling, finite element modeling, 3D representation, and surrogate and hybrid modeling used in DT implementations. This paper is solely focused on applications relating DTs to SHM practices for various civil engineering infrastructures, hence highlighting its novelty over previous reviews. Gaps and limitations emerging from the systematic review are presented, followed by articulating future research directions and key conclusions.

In the last two decades, magnetorheological (MR) fluids have attracted extensive attention since they can rapidly and continuously control their rheological characteristics by adjusting an external magnetic field. Because of this feature, MR fluids have been applied to various engineering systems. This paper specifically investigates the application of MR fluids in shock mitigation control systems from the aspects of three key technical components: the basic structural design of MR fluid-based energy absorbers (MREAs), the analytical and dynamical model of MREAs, and the control method of adaptive MR shock mitigation control systems. The current status of MR technology in shock mitigation control is presented and analyzed. Firstly, the fundamental mechanical analysis of MREAs is carried out, followed by the introduction of typical MREA configurations. Based on mechanical analysis of MREAs, the structural optimization of MREAs used in shock mitigation control is discussed. The optimization methods are given from perspectives of the design of piston structures, the layout of electromagnetic coil, and the MR fluid gap. Secondly, the methods of damper modeling for MREAs are presented with and without consideration of the inertia effect. Then both the modeling methods and their characteristics are introduced for representative parametric dynamic models, semi-empirical dynamic models, and non-parametric dynamic models. Finally, the control objectives and requirements of the shock mitigation control systems are analyzed, and the current competitive methods for the ideal 'soft-landing' control objectives are reviewed. The typical control methods of MR shock mitigation control systems are discussed, and based on this the evaluation indicators of the control performance are summarized.

Application of magnetorheological gel (MRG) is a promising tool for high performance mitigation due to its outstanding energy absorption and dissipation properties. However, the lack of recognition on micromorphological variation for MRG and its magneto-mechanical coupling mechanism limits its extensive application. Herein, combined with the magnetic sensitivity nature of MRG, we develop a magneto-controlled microfluidic system for flexible simulation toward ms-level impact conditions. Microstructural changes of MRG, prepared with solid–liquid composite method, are characterized from variable magnet-field setups and gradual velocities. Experiments reveal that the increasing magnetic flux density can effectively enhance the stability of chains in as-fabricated MRG, while the chains can support excessive velocities up to 4.5 m s⁻¹ before breaking. Meanwhile, under the preset velocity range, the maximum change rates of the average and standard deviation for inclinations are 183.71% and 40.06%, respectively. Successively, an experiment-conducted microdynamic model is developed for numerical simulation of the MRG mechanical behaviors. During that, high-velocity MRG behaviors are explored with a tubular rather than regular flat-structure boundary condition setups, to pursue more trustable results. Simulation readouts meet nicely with those from experiments in revealing the magneto-mechanical coupling mechanism of MRG under multiphysics. The interaction between magnetic force, repulsive force and viscous resistance is mainly illustrated. This work provides a reliable observation basis for micromorphological variation of MRG, also suggests a new method for the mechanism of magneto-mechanical coupling at extreme velocities.

With the development of ultrasonic transducers, spherical piezoelectric transducers have attracted tremendous attention in a variety of application fields due to their ability to resist higher pressures and produce omnidirectional radiation. However, the wall thickness of piezoceramic spherical shells is usually thin due to the limitations of polarization technology and operating voltage, leading to the limited vibration performance and power capacity of the spherical transducer. We present a piezoceramic shell-stacked spherical transducer (PSST) capable of addressing the problem of difficult excitation caused by the thick wall of the piezoceramic shell. The resulting device consists of a two-layered piezoceramic shell interposed between the inner and outer concentric spherical metal shells. By removing the equivalent mechanical transformers, a novel electromechanical equivalent circuit of the PSST is established to simplify the theoretical analysis of the designed PSST. The electromechanical characteristics of the resulting device are experimentally verified, and the measured results are in good agreement with the theoretical predictions and simulation results. Our design opens up possibilities for designing spherical transducers with high-vibration performance and may offer potential for a wide range of applications such as underwater detection and structural health monitoring.

Polyvinyl alcohol (PVA) has good biocompatibility, a simple fabrication process, and environmental protection, which is very suitable for the production of triboelectric nanogenerator (TENG) applied to smart home control. However, the output performance of the TENG composed of PVA and PDMS films is not high. Previous research has explored the enhancement of PVA-based TENG performance by doping with conductive materials to modify the dielectric properties of PVA composite films. Nevertheless, this approach is associated with issues of high production costs and energy consumption. This work prepared a mullite/PVA composite material TENG (MP-TENG), the introduction of mullite induced interfacial polarization in the composite film. This effect resulted in the appearance of polarization centers, thereby enhancing the charge-sensing capability of the composite film. Consequently, the triboelectric output performance of the MP-TENG was improved. MP-TENGs with different amounts of mullite fiber doping were prepared, and the maximum output performance was obtained when the doping level reached 3 wt%. At this concentration, the composite film exhibited an open-circuit voltage of 70.89 V and a short-circuit current of 2.45 μA. An enhancement of 1.78 and 1.71 times was achieved with respect to the pure PVA-TENG, respectively. In addition, MP-TENG exhibited excellent sensing characteristics, a smart home control system was designed in conjunction with a hardware circuit, which captured hand motions and encoded them to generate binary codes to control the on/off state of the indoor home.

Soft robots have significant advantages in flexibility and adaptability and have potential applications in the field of engineering. Unlike traditional manufacturing methods, three-dimensional (3D) printing provides a fast way to fabricate customized and multi-functional robots. However, the fabrication of soft robots requires multimaterial printers and the high-accuracy multi-step assembly process. Among them, fused deposition modeling (FDM) technology has taken the lead compared to other 3D printing methods due to its ease of use, accuracy, and repeatability. However, the FDM multimaterial printing has not been thoroughly explored. Here, we proposed a rigid and flexible material integrated printing approach based on FDM 3D printing technology and reported a cable-driven flexible pipe robot based on Yoshimura origami crease patterns. The implementations show that the robot can realize four-direction bending effectively by the corresponding drive control, which indicates the feasibility of our design and manufacturing method. The proposed approach paves an effective way to design and fabricate the rigid-flexible robot and other devices in the future.

In this paper, the propagation of mechanical waves in flexoelectric solids with the consideration of both the direct and converse flexoelectric effects is studied via a collocation mixed finite element method (MFEM). The dynamic effects associated with mechanical waves propagation are accounted by introducing the kinetic energy in the Hamilton's principle. In the proposed collocation MFEM, a quadratic polynomial is independently assumed for each component of the mechanical strain and electric field. The independently assumed mechanical strain and electric field are collocated with their counterparts computed from the displacement and electric potential at 9 Gaussian quadrature points. Thus, except for the fundamental field variables, no additional degrees of freedom (DOFs) are introduced. By performing the numerical experiments using the collocation MFEM, it is found that due to the direct flexoelectric effect, the propagation of mechanical waves can result in electric polarization in materials. Besides, the converse flexoelectric effect can induce mechanical waves when there are non-uniform transient electric field applied to the material. Numerical results indicate that by increasing the loading speed of the time varying mechanical displacement load, the direct flexoelectric effect associated with the mechanical strain gradient could be significantly enhanced.

High-performance three-dimensional force (3D-force) tactile sensors with the capability of distinguishing normal and tangential forces in sync play a vital role in emerging wearable devices and smart electronics. And there is an urgent need for 3D-force tactile sensors with fast response and high flexibility. Herein, we design a capacitive 3D-force tactile sensors inspired by the U-shaped river valley surface morphology, which has satisfactory performance in terms of rapid response/recovery time (∼36 ms/∼ 36 ms), low hysteresis (4.2%), and high sensitivity (0.487 N⁻¹). A theoretical model of general value for congener sensors is also proposed, obtaining a higher sensitivity through optimizing parameters. To verify the application potential of our device in actual scenarios, the robustness testing and gripping gamepad application were carried out. And it can recognize different motions in humans. Furthermore, principal component analysis is also conducted to demonstrate the distinct classification of different motions. Therefore, our work is eligible for the applications in wearable electronics, human–machine interaction, and soft intelligent robots.

4D printing has garnered significant attention within the field of engineering due to its capacity to introduce novel functionalities in printed structures through shape-morphing. Nevertheless, there persist challenges in the design and fabrication of intricate structures, primarily stemming from the intricate task of controlling variables that impact morphing characteristics. In order to surmount these hurdles, the approach of multi-material 4D printing is employed, underpinned by parametric studies, to actualize complex structures through a two-phase morphing process. This study specifically investigates the utilization of acrylonitrile butadiene styrene (ABS) and polycarbonate/ABS. The distinction in glass transition temperatures within these materials enables the realization of two distinct morphing phases. The research delves into the impact of structural parameters on morphing properties. Finite element analyses are subsequently conducted, leveraging the insights gained from parametric studies, to facilitate the accurate prediction of a diverse array of shape alterations in response to temperature fluctuations. Several structural models are both simulated and fabricated to experimentally validate the precise forecasting of desired morphing phases. The culmination of this study manifests in the design and fabrication of multiple multi-material structures, exemplifying both their functionality and intricate geometric complexity.

Within microelectromechanical system sensors, the establishment of a vacuum environment is a prerequisite for the control of specific residual gas molecules. At the wafer-level package stage, the interior of the sensor can be easily converted into a vacuum environment. However, after packaging, degassing occurs due to the accumulation of fumes with additional processing, resulting in a significant reduction in sensor reliability. To counteract this, non-evaporable getter (NEG) film is commonly packaged together with the sensor to absorb the outgassing gas molecules and maintain a vacuum environment within the sensor. Most NEG films require an activation process to migrate the adsorbed gas molecules from the surface to the bulk by thermal annealing. Recently, NEG films have been considered to reduce the activation temperature and time to avoid heat damage. Depositing an anti-oxidant layer on NEG film or alloying the NEG film with metallic materials through co-sputtering to create a distinct valence state during activation was found to prevent further oxidation of NEG film. However, these methods require expensive materials and fabrication equipment. In this study, we demonstrate that a much lower activation temperature (T = 350 °C) and time (t = 10 min) for Ti NEG film can be achieved by controlling the surface morphology depending on the deposition method and condition, without requiring further treatment such as the deposition of a capping layer or co-sputtering. Increasing the grain size of the Ti NEG film results in a larger surface area, which enables more efficient adsorption of gas molecules. Additionally, higher porosity in the film increases the diffusion of gas molecules, thus enhancing the overall gas adsorption capacity. Our experiments show that the Ti NEG film, which was deposited at 7.8 Å s⁻¹ using a sputtering method, exhibited a grain size of 411 nm and a surface roughness of 59.185 nm. Furthermore, after an activation process at 350 °C for 10 min, the atomic ratio of the adsorbed gas molecules was 23.14%.

Pneumatic soft robots have become a popular research area for future robots because of their lightweight, high efficiency, non-pollution, and high reliability. However, the pneumatic pump, which is the 'heart' of these robots, is large in size, heavy in weight, noisy in operation, and must be separated from the robot body, which seriously affects the portability and autonomy of the robot. Portable soft pumps fabricated using smart materials provide a viable solution to the above challenges. We present a segmented electrode pump (SEP) driven by electrostatic forces, which combines the advantages of an electro-pneumatic pump (EPP) and an enhanced electrode structure. Compared with the EPP, the developed SEP showed improved characteristics in terms of higher specific flow rate output (1.67 ml sg⁻¹), higher specific pressure output (0.483 kPa g⁻¹) and lower power consumption (24 mW). The proposed SEP is expected to provide new solutions to the challenges of embedding air sources and facilitating air supply flexibility, and opens up new opportunities for fully flexible robots.

This work demonstrates the reliability-aware analysis of the Junctionless negative capacitance (NC) FinFET employed as a hydrogen (H₂) gas sensor. Gate stacking of the ferroelectric (FE) layer induces internal voltage amplification owing to the NC property, thus, improving the sensitivity of the baseline junctionless FinFET. A well-calibrated TCAD model is used to investigate the sensing characteristics of the proposed FinFET-based H₂ sensor by employing the palladium (Pd) metallic gate as a sensing element. The mechanism involves the transduction of H₂ gas molecules over the metal gate; due to the diffusion process, some atomic hydrogen diffuses into the metal. The H₂ gas absorption at the metal surface causes a dipole layer formation at the gate and oxide interface, which changes the metal gate work function. As a result, this change in the work function can be used as a sensing parameter of the proposed gas sensor. Further, the threshold voltage and other electrical characteristics, such as output conductance, transconductance, and drain current are examined for sensitivity analysis for both NC and without NC JL FinFET at different pressure ranges, keeping the temperature constant (i.e. 300 K). The device variation, i.e. Fin thickness, Fin height, doping and thickness of HfO₂ ferroelectric layer, etc, on sensor sensitivity has been evaluated through extensive simulation. This paper also presents a detailed investigation of the sensor's reliability in terms of work function variation, random dopant fluctuation, trap charges, and device aging, i.e. end of a lifetime. At last, the acquired results are compared with earlier reported data, which justifies the profound significance of the proposed junctionless negative capacitance FinFET-based H₂ gas sensor.

In this study, we designed and fabricated a stretchable energy harvesting device. This device operates by inducing buckling in the Lead Zirconate Titanate film through tension applied to the wavy base, resulting in voltage generation. Both simulations and experiments demonstrate that the aspect ratio between the pitch and curve radius of the symmetric wavy base influences the energy conversion efficiency of the piezoelectric device. An in-depth analysis revealed that increasing the resolution of the device leads to a proportional increase in energy conversion efficiency. This finding aligns with the mathematical modeling proposed in our study. Consequently, our study demonstrates the potential of miniaturized wavy piezoelectric devices in diverse applications, including soft robotics, wearable devices, and highly sensitive stretchable sensors. These devices hold promise for enhancing the efficiency of flexible devices by harnessing energy from mechanical movement.

Dynamic control of the reflection from an object is much of importance in microwave engineering. In the past the tunable absorbers are usually employed to realize this goal and have been widely discussed. In this work, we propose a metasurface to offer a more flexible solution to dynamically control the reflection property. The proposed metasurface can be independently modulated by three distinct physical mechanisms. Apart from the absorption mechanism as the conventional tunable absorbers, the reflectivity of the proposed metasurface can be dynamically controlled by modulating the phase distribution profiles in the space as well as the time domains. Such flexibly dynamic reflection controlling ability is demonstrated by a series of experimental measurements. Results indicate that the tuning level of the reflectivity is more than 10 dB in a broad frequency band for the three distinct mechanisms. The proposal may find promising application in various fields, such as wireless communications and the stealth technology.

Owing to the wide modulation capability of their magneto-induced modulus, smart structure-based magnetorheological elastomers (MREs) show great promise for vibration control in numerous engineering applications. In conventional smart structure-based MREs, however, vibration absorption or isolation is mainly used for discrete structural systems, and the requirement for vibration control in continuous structures can limit the application of vibration absorbers and isolators. Therefore, it is necessary to resolve the dynamic properties of the continuous structure to obtain modal information. In this paper, different types of smart beams with adaptive elastic support (AES)-based MREs, containing dual-end elastic support, single-end elastic support, and the combination of a fixed end and an AES at an arbitrary location, are developed to tactically influence the dynamics through magneto-mechanical coupling. A dynamic model of thin-walled beams with AES was established by using the improved Fourier series method (IFSM). The numerical results confirm that the effective suppression bandwidth of the beam with MRE-AES can be shifted as a result of the modal modulation-induced energy transfer from low to high frequencies, which requires a decreasing trend of modal amplitude at the response location as the elastic support stiffness increases. According to the modal analysis, the beams with single-end AES and dual-end AES have a decreasing trend of modal amplitude in the global location as stiffness increases. However, the modal amplitude trend of the beam with a fixed end and an AES is not monotonic at certain locations. The experimental results demonstrate that MRE-AES can effectively attenuate the acceleration responses of the beams with single-end AES and with a fixed end and an AES under harmonic excitation. The resonance peaks in the transmission remarkably shift to higher frequencies with increasing magnetic flux.

Utilizing shape memory alloy (SMA) cables to constrain frictional isolated bridges is considered an efficient approach to limit bearing displacement and prevent serious earthquake damage. Accurate seismic fragility assessments of this kind of structure are crucial for aseismic decision making. However, traditional assessment methods cannot quantitatively describe the impact of the pulse effect on pulse-type seismic motions, which may lead to inaccurate assessment results. Therefore, this study deduced a novel equation for seismic fragility assessment that considers the pulse effect. Firstly, the impact of the pulse effect is quantified. Then, a multivariable probabilistic seismic demand model (MV-PSDM) is developed that is conditioned on the pulse period, peak ground velocity, structural period, maximum friction coefficient and SMA consumption. Based on the MV-PSDM, an effective approach for predicting structural seismic vulnerability is recommended, which does not require finite element modeling or nonlinear time-history analysis. Finally, a novel equation for calculating the intensity measure corresponding to 50% damage probability is deduced. The results indicate that increased friction coefficients and SMA consumption can enhance structural seismic safety under pulse-type ground motions. However, when the ratio of pulse period to structural period is too small, increased friction coefficients or SMA consumption have no meaningful effect on the seismic fragility of the structure.

Pneu-net soft actuators are widely used in the soft robotics society owing to their light weight, high deformation, and fast response. This paper presents a novel theoretical framework to model the static analysis and contact mechanics of pneumatic soft actuators undergoing large deformations. While most soft robots exhibit complex material behaviors, we show that their mechanics can be accurately captured through the fundamental principles of elasticity and contact theories. The core contribution is an inclined membrane contact model that elegantly transforms the complex three-dimensional contact between angled surfaces into an equivalent problem of horizontal contact, enabling the use of established contact solutions. This model is integrated with an energy-based solution for elastic deformation to fully characterize soft actuator bending. The generalizable modeling approach is applied to the example of a pneumatic net actuator, with comprehensive validation against finite element analysis and experiments. This work advances a fundamental understanding of soft machine statics and contact mechanics while providing an analytical tool for the design and control of deformable actuators. The flexible theoretical framework presented can be extended to diverse interdisciplinary problems involving moving surfaces in contact.

We propose a reconfigurable phononic crystal (PnC) for detecting the concentration of solutes in liquids. The designed PnC consists of liquid-filled hollow pillars and connecting bars. The finite element method is used to calculate the transmission spectra and band structures of PnC filled with various liquids. We fabricate 3D printed samples and conduct corresponding experiments. The results show that sound velocity is the key parameter affecting the frequency of the passing band. As the sound velocity increases, the resonance frequency shifts down. For both NaCl solution and ethanol solution, good linear relationships between the resonance frequency and liquid concentration are established. Experimental results show good agreement with simulations, and stable detection capabilities are maintained in the presence of interference. The impact of fabrication tolerances on sensor performance has also been discussed, with a greater impact on sensitivity and a smaller impact on Q-factor. The reconfigurability also shows the potential of the design of multi-liquid PnC sensors.

Nonlinear guided waves exhibit high sensitivity to material microstructural changes, thus attracting increasing attention for incipient damage monitoring applications. However, conventional nonlinear guided-wave-based methods suffer from two major deficiencies which hinder their applications: (1) mostly relying on the first arrivals of wave signals, they apply to limited inspection areas in simple structures in order to avoid wave reflections from structural discontinuities or boundaries; (2) they are prone to numerous deceptive nonlinear sources in the measurement system which might overwhelm damage-induced signal components. To tackle these challenges, we propose a metamaterial-assisted coda wave interferometry (CWI) method using second harmonic Lamb waves, applicable to the monitoring of local incipient damage in complex structures. Embracing the metamaterial concept, a so-called meta-screen is designed, whose geometry and layout can be flexibly tailored to target specific inspection zones in a structure. Capitalizing on its customized bandgap features, the proposed meta-screen allows for the passing of fundamental waves while preventing the second harmonic components generated by deceptive nonlinear sources from penetrating into the inspection area. Through numerical analyses on a plate with a rib stiffener, the efficacy of the meta-screen and the influence of occasional disturbance and regular pollution are evaluated. Experimental validations on an adhesive structure also confirm the superior sensitivity of the nonlinear coda waves to incipient damage, which is further enhanced by the deployment of the meta-screen alongside improved robustness against deceptive nonlinear sources outside the inspection area. The proposed metamaterial-assisted CWI method with second harmonic Lamb waves holds great promise for local incipient damage monitoring of complex structures.

Braille displays are a class of human–computer interaction electromechanical devices that display dynamic braille through an array of actuators. However, existing actuators for braille displays suffer from issues such as bulky size, heavy weight, and small tactile displacement, leading to difficulties in improving their resolution and readability. To address the above issues, we developed a novel electroactive artificial muscle actuator and applied it to braille displays. The novel actuator consists of a surface-structured PVC gel and planar electrodes. To investigate the effect of surface structure on the performance of novel PVC gel actuators, four types of surface-structured PVC gels were fabricated by a casting process, and their actuation performance was tested. The results show that the conical and frustum conical array structures are more favorable for improving the displacement of novel PVC gel actuators, while the cylindrical and quadrangular array structures are more favorable for improving their recovery forces. We observed both surface structure and dynamic electrical actuation, suggesting that the actuation of the novel actuator is mainly caused by the deformation of the surface structure of the array. We also analyzed electrowetting effects in PVC gels using the Lippmann–Young equation, to explain the differences in the performance of surface-structured PVC gels with different contact angles. Moreover, six multilayer actuators composed of PVC gels with a conical surface array structure are applied to the braille display unit to display the braille digits from 0 to 9. It has been shown that the novel PVC gel actuator has excellent mechanical properties, which makes it an ideal solution for braille displays.

Shape memory alloys (SMAs) are adaptive materials that exhibit complex thermomechanical behaviors due to multiphysics coupling. The thermomechanical modeling of SMAs is a complex task due to several phenomena involved, and the Preisach model is an interesting alternative to describe the SMA hysteretic behavior based on experimental data. This paper deals with the description of the thermomechanical behavior of SMA using the Preisach model. Experimental tests are performed considering NiTi pseudoelastic wires subjected to different load conditions, establishing reference cases. Afterward, the Preisach model is employed to describe the SMA behavior. Numerical simulations are carried out and compared with the experimental data showing a good agreement. Other experimental data available in the literature are employed to investigate different macroscopic behaviors related to SMAs, including strain-temperature relations of wires and force-displacement relations of springs. Results show that the model is able to describe the thermomechanical behavior of SMAs, being in close agreement with experimental data. Preisach model has advantages such as a simple numerical implementation when compared to phenomenological and thermodynamic-based models, being an interesting approach useful for a wide range of applications that include different macroscopic behaviors.

Herein, we propose a piezoelectric energy harvester (PEH) capable of vibrating in multi-degrees-of-freedom. The resonant frequency, working bandwidth, and output power of the PEH were improved by introducing an auxetic structure (AS). The proposed PEH exhibited a symmetric serpentine structure with a doubly clamped configuration comprising several proof masses at the junctions. Finite element method (FEM) simulation was conducted to investigate the characteristics of an AS PEH and a plain-structure PEH. Prototypes of the PEHs were manufactured by three-dimensional (3D) printing technology, and their performance was evaluated through vibrational energy-harvesting experimental tests. The results showed that introducing the AS reduced the first and second resonant frequencies by 49% and 44%, respectively, considerably improved the output power in the first mode (up to 2548%) and narrowed the frequency bandgap between the first two resonance modes by 29%. The proposed multimode AS PEH can operate in a low-frequency environment of less than 20 Hz. Finally, we discussed several ways of optimizing the AS. It has been found that the PEH performance could be further improved by selecting a reasonable thickness for the AS, increasing the number of periodic unit cells, and using an AS with a variable cross-section unit cell.

In this study, optical frequency domain reflectometry (OFDR) was used to monitor the thermoforming processes of carbon fiber reinforced thermoplastics (CFRTPs) to address the limitations of conventional sensors including large size and low spatial resolution. A bare single-mode fiber with a polyimide coating and a fiber encapsulated by a long metal capillary were cascaded and embedded into composite laminates to withstand the high pressure and temperature during thermoforming, and then connected to the OFDR for monitoring. A fiber encapsulated by a 2 cm short metal capillary was also embedded to demonstrate that a 1 mm resolution of the OFDR is beneficial for reflecting the local change in the composite. After processing by wavelet denoising, signal extraction, and decoupling, the frequency shift along the optical fiber sensor was successfully converted to strain and temperature. In two repeated thermoforming experiments that involved cooling from 340 °C, the average temperature difference measured by the OFDR and reference thermocouple was only 4.64 °C. The strain measured by the OFDR and reference fiber Bragg grating (FBG) decreases in the cooling stage, and has a clear knee point of 250 °C when correlated with the temperature and strain. This knee point is consistent with the liquid–liquid transition temperature of the polyetherimide and indicates the beginning of consolidation when the composite changes its properties significantly. The average strain difference measured by OFDR and the reference FBG was 69 μ when the total strain is approximately 1820 μ if only considering the consolidation process from 250 °C. The results of 1 mm spatial resolution and high accuracy demonstrate that OFDR is a promising high-resolution sensing solution for the in-situ temperature and strain monitoring of the thermoforming of CFRTPs.

The bistable dielectric elastomer actuator (BDEA) possesses two stable positions which offers notable advantages of stable-state self-maintenance, fast response, and threshold snap-through characteristic in comparison with conventional dielectric elastomers. However, the strong nonlinearity induced by the coupling among materials, structure, and electrostatic fields greatly affect the dynamic response and gives rise to stability issues. Hence, a novel BDEA is proposed by introducing DEA film centrally connected with one mass block and linear spring, and the bistability can be adjusted by applying external voltage. A nonlinear dynamical model considering the electro-mechanical coupling effects is established using the Euler-Lagrange method, with which the snap-through procedure is theoretically analyzed and validated through the analytic method and finite element method. The influences of the electric actuation and structural parameters on the number of stable states and natural frequency are analyzed. Additionally, the supercritical pitchfork bifurcation and saddle-node bifurcation are investigated through dynamic analysis under forced vibration. Furthermore, the ranges of electrical actuation parameters can be determined for preventing the bifurcation phenomena under parametric excitations. Moreover, an active morphing strategy for achieving nonlinear dynamic morphing between steady states of BDEA using drive voltage is obtained, thereby enhancing the versatility of conical BDEA.

There is a critical demand for magnetorheological fluids (MRFs) with high particle loading, long-term stability, and high magneto-viscous properties to be used in industrial MRF devices. Bidisperse MRFs composed of highly magnetizable micron-sized carbonyl iron particles and poly(acrylic acid) coated superparamagnetic iron oxide nanoparticles (SPIONs-PAA) that can chemically interact are proposed to achieve such MRFs, here. Coating bare, commercial CI with lauric acid (LA) enhanced its dispersibility in a hydrophobic carrier fluid, allowed high magnetic loading and significantly prevented the sedimentation of the particles when mixed with 9–12 wt% SPION. Different carrier fluids (mineral oil, paraffin, and hydraulic oil) were tested, and hydraulic oil was determined as the best for this particle combination. The most stable bidisperse MRF was achieved at 83%–84% magnetic content with 12 wt-%SPION-PAA, LA-coated-CI and 3 wt% polyvinyl alcohol. Such MRFs outperformed the commercial benchmark, 140-CG^® from Lord Corp., in long-term stability (4 months) and stability under dynamic loading. Bidisperse MRFs were stable between 20 °C and 60 °C. Most importantly, the excellent performance of the bidisperse MRFs in dampers designed for washing machines suggests that these MRFs may provide comparable damping forces with much better stability, ensuring longer shelf-life and longer lifetime in use.

Soft gripper robots provide superior safety, adaptability, and compliance compared to rigid robots. However, soft grippers must address inadequate stiffness and interference resistance. Soft pneumatic electroadhesion (EA) grippers with variable stiffness are potential options for addressing these difficulties. In this paper, we present a soft bionic gripper (SOBG) that resembles human finger movements, such as bending and deflection, employing pneumatic actuation, and whose stiffness is effectively decoupled from its position through a layer jamming-induced variable stiffness structure. By applying electroadhesive forces, the SOBG can perform complex motion tasks that would typically require a wrist joint, making them simpler to perform than with conventional flexible grippers. In addition, the SOBG can perform one-finger object manipulation to grasp flat, concave, and convex objects. To show the potential for more complex robotic applications, we evaluated each function independently by presenting a demonstration of cap-screwing, a material handling system, and an anti-interference research. The SOBG concept and solution proposed in this study may pave the way for the easy integration of EA into soft robotic systems and promote the wider use of EA technology.

An optical fibre-based sensor is developed for measuring the dynamics of the back face deformation of soft body armor. The measurement system consists of embedding an optical fibre into a thin silicone mat to increase survivability. The silicone sensor mat is placed between the soft body armor and the backing material. The optical fibre experiences times of sticking and slipping. The portions of the impact with the optical fibre stuck are reconstructed into slipping-equivalent strain using exponential extrapolation from adjacent slipping portions. The strain on the optical fibre is related to the projectile acceleration when the optical fibre is slipping. The strain is measured with the optical fibre sensor using a fibre Bragg grating. The system is characterized using a gas gun in combination with high-speed imaging. The system is experimentally demonstrated at the Army Test Center in Aberdeen, MD. Of the 23 shots 17 had an error less than 10%.

Real-time monitoring of wellbore status information can effectively ensure the structural safety of the wellbore and improve the drilling efficiency. It is especially important to recognize the wellbore fractures and identify their parameters, which motivates us to propose a wellbore fracture recognition and parameter identification method using piezoelectric ultrasonic and machine learning. To realize a self-model emission detection, we innovatively utilize a single transducer to act as both an actuator and a sensor, allowing for the efficient acquisition of ultrasonic echo signals of the wellbore. For fracture recognition, we use the wavelet packet transform to extract features from the ultrasonic echo signal, while constructing a convolutional neural network model for fracture recognition. Then, we establish the relationships between the fracture width-depth parameter and the echo signal, including the peak value as well as the arrival time difference. The experimental results show that the proposed method effectively recognizes the fractures from the ultrasonic echo signal of the wellbore. At the same time, the established function truly reflects the relationship between the fracture parameters and the echo signal. Therefore, the proposed method can provide an identification function for quantitative monitoring of wellbore fracture parameters. Moreover, the functions can be used as a reference for other structural health monitoring, which has good application prospects.

The shape memory polymer based adhesives have demonstrated excellent programmable switchable adhesion, in which the material properties play an important role. Here, the viscoelasticity, stiffness gradient, and their effects on adhesion of an epoxy shape memory polymer (ESMP) were studied. The ESMP sample and polydimethylsiloxane control sample were fabricated firstly by mold casting and curing techniques. The sample was fixed on an electric heating plate with double-sided tape to conduct the adhesion measurements against a hemispherical glass indenter under different temperature and displacement conditions by using an adhesion tester and the temperature measurements on the double-sided tape and ESMP sample surfaces by using a digital thermodetector. Based on the measured force–displacement–time data, the hysteresis behavior and internal dissipation were evaluated; the peak force relaxation was studied; the reduced modulus was calculated and the stiffness gradient features were evaluated. The results show that the ESMP sample exhibits strong viscoelasticity and significantly enhanced adhesion that strongly depends on the compressive displacement in the glass-rubber transition zone near it is glass transition temperature (${T_{\text{g}}}$), while weaker viscoelasticity and lower adhesion in the soft rubbery state. The reduced modulus of the ESMP sample obviously decreases with the increasing compressive displacement at a preset temperature below 70 °C. It is found that there exists a linear relation between the pull-off force, effective work of adhesion, reduced modulus, and contact area. This study helps deeply understand the material properties and adhesion mechanism of the ESMP, which can guide the design of switchable adhesives.

Ion chelator can enhance the self-healing of cracks and pores in concrete. To further improve the self-healing capability of cement-based materials, different industrial wastes (i.e. fly ash, limestone powder and blast furnace slag (BFS)) and ion chelator were mixed into mortar. The crack closure index, water permeability, water absorption, impermeability, compressive strength recovery and healing products of mortar were studied. The results showed that the mortar mixed with BFS and ion chelator possessed the best repair ability on cracks, the crack with a maximum width of 0.55 mm can be closed within 14 d. Meanwhile, the water permeability and water absorption of mortar incorporating ion chelator and BFS were obviously smaller than that of control mortar. After curing for 56 d, the chloride diffusion coefficient of mortar containing ion chelator and BFS was reduced by 73.7% compared with control mortar. The compressive strength recovery ratio of mortar containing ion chelator and BFS was 63.7% greater than that of control mortar after pre-loading 80% of the failure strength. In addition, scanning electron microscopy and energy dispersive spectroscopy displayed a large number of calcites at the edge of crack section in mortar containing ion chelator and BFS, the inside of crack was mainly repaired by the combined effect of calcium carbonate precipitation and hydration product.

This work relates to piezoelectric vibrating energy harvesters, that are constructed from a unimorph cantilever with a massive edge block. The dynamic response of the cantilever is considered when it is excited into vibrations at its natural frequency, where its deformation amplitude is maximal. The optimal response of such a harvester is achieved when the amplitude of the axial strain in the piezoelectric layer, is uniform. Practical technological considerations dictate the thickness of the unimorph, but its planform geometry (i.e. the vareation of the width along the cantilever) is a design choice. The optimal planform of such a unimorph cantilever has been the focus of many previous studies, which included extensive simulations and experimental investigations. In these previous studies it was concluded that the optimal planform is a trapeze, where the cantilever tapers from its clamped edge towards the edge block. However, to date, no model with explicit predictive capabilities was proposed. In the present study we derive an analytic expression of the planform, that ensures a uniform axial strain over the top surface of a cantilever unimorph with an edge block. Our analysis provides a rational explanation why a trapeze planform is optimal, and provides an explicit functional form of the optimal geometrical parameters of this planform. The predictive capabilities of our model are validated by comparison to finite element simulations.

To reuse the energy dissipated by vehicle suspension, a semi-active suspension with a self-powered magneto-rheological damper is proposed. An electromechanical coupling model of self-powered semi-active suspension is established. The energy conversion efficiency is defined and investigated by changing the electrical parameters. By considering unmodeled dynamics and perturbation values, an adaptive optimal fault-tolerant control algorithm is proposed to ensure the vibration-isolation performance. The robust index of the adaptive optimal fault-tolerant control algorithm is constructed using the Lyapunov equation and evaluated by changing the key parameters. The sensitivity of the key parameters to the damping force is investigated using a grey relation analysis approach. Furthermore, multi-objective optimization between the vibration-isolation capability and energy harvesting is conducted. Via analysis, the proposed suspension can harvest more energy near the second resonance range. Compared to passive control and self-powered mode, the adaptive optimal control algorithm mitigates vibration more significantly in the time and frequency domains, respectively, under stochastic excitation. The robust index is most sensitive to inductance and the diameter of the magnetism cylinder. The length of the damping channel and the diameter of the magnetism cylinder influence the sensitivity of key parameters to the damping force most obviously.

Shape memory alloy hybrid composite (SMAHC) tubes have the potential to actively control the torsional stiffness of structures. In this work, shape memory alloy (SMA) wires orthogonally braided with basalt fiber were heated by electricity to generate the torsional torque of the SMAHC tube, and then the torsional stiffness of the tube was measured. The macroscopic finite element model of the SMAHC tube was established to assist in the analysis of the torsional stiffness variation before and after heating. According to the experimental results, the actuating torsional ability was affected by the phase transformation recovery force of SMA. The active control of torsional stiffness depends on the change of SMA modulus, the magnitude and direction of SMA wires' recovery force, and the thermodynamic properties of matrix at different temperatures. Then an evaluation model for the torsional stiffness control effect was proposed. These research results can be used for the adaptive control of the stiffness and vibration characteristics of rotating composite structures, and provide a novel and effective method for the control of structural torsional characteristics with lightweight effects.

A cluster magnetorheological (MR) electro-Fenton composite polishing technique was proposed in this work, which can realize high efficiency, ultra-smooth and damage-free of GaN wafer by the synergistic effect of electro-Fenton reaction and flexible mechanical removal of MR polishing. The key parameters of electro-Fenton were optimized through methyl orange degradation experiments based on BBD experimental method. The results showed that the decolorization rate had a strong dependence on H₂O₂ concentration, Fe–C concentration and pH value, where the decolorization rate had the maximum value when the H₂O₂ concentration of 5 wt%, Fe–C concentration of 3 wt% and pH value of 3. Compared with the Fenton reaction, the decolorization and REDOX potential of methyl orange solution were significantly improved in the electro-Fenton reaction. Furthermore, the process parameters of the cluster MR electro-Fenton composite polishing were optimized to obtain the best polishing result, which was realized under the conditions of 3 wt% diamond (grain size: 0.5 µm), a polishing gap of 0.9 mm and a polishing time of 60 min. The novel method achieved a material removal rate of 10.79 μm h⁻¹, which was much higher than that of the conventional technique. In addition, an ultra-smooth and damage-free surface with a roughness of 1.29 nm Ra was obtained.

Bolt monitoring plays a vital role in ensuring the safe operation of engineering structures. The utilization of piezoelectric (PZT) active sensing and analysis of ultrasonic energy transmitted through the interface of bolted connections has demonstrated high feasibility for monitoring bolt looseness. However, the ultrasonic energy saturation effect (i.e. the response signal energy changes slightly as the bolt preload variation) near the rated bolt preload restricts its applicability in early detection of bolt loosening. In this paper, for the energy saturation in the PZT active sensing method, a new bolt looseness indicator with the energy of leading waves (i.e. the first several wave packets) in the response signal is proposed for bolt loosening monitoring, especially for the early bolt loosening monitoring since the energy of the leading wave packets has the linear relationship with bolt preload. The experimental results show that the energy of the first several wave packets in the response signals can be as a looseness indicator of bolt preload. Within the entire range of bolt preload, the indicator exhibits a linear relationship with the bolt preload. Moreover, this method successfully resolves the challenge of energy saturation, providing an effective approach for monitoring bolt preload.

This paper proposes an innovative dual-functional aeroelastic metastructure that effectively suppresses wind-induced structural vibrations under either pure aerodynamic galloping or concurrent galloping and base excitations, while simultaneously harnessing the vibratory energy to potentially allow for self-powered onboard low-power sensing applications. Two configurations are theoretically and experimentally analysed and compared, one consisting of simply regular locally resonating masses subjected to no external forces, while the other comprising locally resonating bluff bodies which experience additional aerodynamic galloping forces. Numerical investigation is conducted based on an established aero-electro-mechanically coupled model. Wind tunnel wind tunnel and base vibration experiments are carried out using a fabricated aeroelastic metastructure prototype to characterize the energy transfer mechanisms and validate the numerical results. The mutual effects of key system parameters, including the frequency ratio, mass ratio, load resistance and electromechanical coupling strength, on the dual-functional capabilities are examined, providing a comprehensive design guideline for efficiently enhancing the energy transfer and conversion. Experimentally, the galloping displacement of the primary structure is attenuated by 78% with a measured power output of 2.63 mW from a single auxiliary oscillator at a wind speed of 8 m s⁻¹. This research opens new possibilities for designing novel metastructures in practical scenarios where both wind-induced vibration suppression and energy harvesting are crucial.

In order to address the issue of reduced damping force dynamic range in magnetorheological (MR) damper caused by the high zero-field viscosity of MR grease, known for its sedimentation stability, this paper introduces a novel dual-channel independent-coil MR damper (DCICMRD). Firstly, the dual-channel configuration and the magnetic circuit structure of independent coils were meticulously designed, and a genetic algorithm was employed to conduct multi-objective optimization of the structural parameters for DCICMRD. The optimization results indicate that all performance metrics of the damper post-optimization exhibit improvements exceeding 15%. Then, the theocratical model of the damping force for DCICMRD under three operational modes are established, and the output damping force of various input currents for different operating mode is obtained. Finally, the DCICMRD was manufactured and subjected to dynamic performance testing. The results revealed that the damping force dynamic range in Mode III, i.e. dual-channel structure, can achieve approximately 15 times, whereas Mode I, i.e. traditional single-channel structure, only attains approximately 9 times. The aforementioned research holds significant implications for expanding the further engineering applications of MR dampers.

Wireless sensor networks that enable advanced internet of things (IoT) applications have experienced significant development. However, low-power electronics are limited by battery lifetime. Energy harvesting presents a solution for self-powered technologies. Vibration-based energy harvesting technology is one of the effective approaches to convert ambient mechanical energy into electrical energy. Various dynamic oscillating systems have been proposed to investigate the effectiveness of energizing low-power electronic sensor devices for supporting various IoT applications across engineering disciplines. Phononic crystal structures have been implemented in vibrational energy harvesters due to their unique bandgap and wave propagation properties. This work proposes a Rubik's cube-inspired defective-state locally resonant three-dimensional (3D) phononic crystal with a 5 × 5 × 5 perfect supercell that contains 3D piezoelectric energy harvesting units. The advantage of defect-induced energy localization is utilized to harness vibrational energy. The 3D piezoelectric energy harvesting units are constructed by the buckling-driven assembling principle. Adapting to the low-frequency and broadband characteristics of ambient vibration sources, soft silicone gel is used to encapsulate the buckled 3D piezoelectric units, which are embedded in the 3D cubic phononic crystal to assemble an entire system. The energy harvesting performance of various defective layouts and their defect modes is discussed. The results demonstrate that the harvester functions well under multidirectional, multimodal, and low-frequency conditions. The proposed methodology also offers a new perspective on vibrational energy harvesters for defective phononic crystals with superior working performance.

The integration of magnetorheological (MR) semi-active suspension systems in all-terrain vehicles (ATV) has garnered significant attention due to their ability to enhance damping performance and off-road capabilities. However, traditional control strategies result in poor control accuracy and limited vibration reduction effects when facing complex road excitations and impact disturbances. With technological advancements, enhanced vehicle environmental perception and road sensing capabilities have made it possible to implement model predictive control (MPC) for vehicle suspensions. Nevertheless, traditional MPC is limited in vehicle suspension applications due to its high computational complexity. To address these issues, this study introduces an explicit model predictive control based on road preview (EMPC-P). Firstly, road data obtained through a non-contact measurement method enables the system to perceive road excitation information in advance. Subsequently, a 7 Degree-of-Freedom (7-DOF) suspension model incorporating road excitations is constructed. By adhering to system constraints and employing a multiparameter optimization method, the control problem based on rolling optimization is transformed into an explicit polyhedral system. The offline precomputation of control state relations enhances the computational efficiency of the control system. Through this approach, the designed EMPC allows the vehicle suspension system to make optimal control decisions quickly and accurately in response to complex driving conditions, thus improving the damping effect of the system. Through a combined approach of simulation and experimental validation, the designed EMPC-P controller is compared with the Skyhook controller under preview and non-preview states, respectively. Empirical testing confirms that the EMPC-P exhibits superior damping effects, significantly improving vehicle ride comfort and handling stability.

Four-dimensional printing technology empowers 3D-printed structures to change shapes upon external stimulation. However, most studies did not consider recovery under loaded conditions. This paper introduces a mechanistic prediction model for forecasting recovery angles in 4D printing utilizing shape memory polymer under various loads. The model integrates Neo–Hookean model to describe the non-linear stress–strain relationship with experimentally determined force density data to characterize polymer restoration properties under various loads. Validation was demonstrated by the recovery experiment of a 3D-printed polylactic acid-thermoplastic polyurethane composite structure loaded by means of a cord and pulley mechanism. The predictive outcomes exhibited reasonable agreement with experimental results, demonstrating a trend of more accurate forecasts as the applied load increased. The model can accommodate various active materials provided that the pertaining force density data is accessible. The predictive model supports the design, optimization and material selection for 4D-printed structures to meet specific performance requirements.

Corrosion of steel in concrete is one of the major problems with respect to the durability of reinforced concrete (RC) structures. Thus, monitoring the corrosion in real-time is essential to prevent structural damage. However, one of the main challenges is to simulate the real-time development of corrosion in the RC structure. In recent years, smart aggregates, also called embedded piezo sensors (EPS), have become increasingly popular for monitoring localized and corrosion damage in RC structures using electro-mechanical impedance (EMI). This paper presents the experimental and numerical investigation of corrosion in RC structures subjected to the chloride-laden environment using EPS via the EMI technique. To fulfil this objective, the study has been carried out in two stages such as; in the first stage, the experiments are conducted on the RC specimen, and the EMI response was obtained both in a pristine state and when accelerated corrosion progressed. In the second step, a numerical model of the RC specimen has been developed based on the experimental data in the COMSOL software, and the effect of corrosion in the form of varying mass loss percentages has been simulated. Based on the results, it is concluded that the experimental and numerical conductance signatures before and after corrosion are matched. The deterioration in terms of stiffness loss in the RC specimen was 18.20% at 30% mass loss.

The crack damage monitoring of aircraft structures is very significant for ensuring aircraft safety, reducing maintenance costs and extending service life. Due to the extreme service environment, the attachment lug is prone to initiate crack damage at the hole edge, which leads to crack propagation and fracture failure. Structural health monitoring technology based on piezoelectric guided wave has been widely studied, promoting the development of crack monitoring. However, at present, research on hole-edge crack damage monitoring of attachment lugs still needs to be further carried out. It is difficult to monitor small cracks at the initial stage of crack propagation, and the accuracy of crack monitoring needs to be improved. By focusing on the accuracy of the crack monitoring in the attachment lug, a crack damage monitoring method based on the circular piezoelectric sensor array is proposed in this paper. Combined with damage alarming and localization imaging, this method comprehensively evaluates the hole-edge crack damage monitoring situation and improves the monitoring effect. The method is verified by an experiment in attachment lug, and this verification includes small crack monitoring and crack propagation monitoring. The experimental results demonstrate that this method can achieve correct damage alarming results, and the maximum localization error of crack damage is only 3.02 mm, which provides a research idea for the accurate monitoring of crack damage at the hole edge.

The MRD with continuously adjustable damping, small compression, and large extension for asymmetric output may improve all-terrain vehicle impact resistance and vibration reduction performance in a variety of conditions. A novel conical flow channel asymmetric MRD (CFC-MRD) is proposed to solve the structure complexity stroke sacrifice, and lack of failure protection concerns in currently studied asymmetric MRD structures. In the design, the non-parallel plate magnetic circuit characteristics of CFC-MRD are investigated, including theoretical analysis and finite element modeling, and the correctness of the model is proved by testing. Considerations in multi-objective optimization include special performance imposing extra restrictions, and making the work more complicated and prone to local optima. To address this, the Nelder–Mead approach is utilized, which decreases the complexity of the optimization model while simultaneously managing performance conflicts. And a collaborative optimization strategy employing Comsol and Matlab tools is applied to improve optimization efficiency. The greatest difference between theoretical optimized values and real values is less than 6.77% in the experiments, showing the efficiency of the CFC-MRD structure design and optimization process.

The extensive application of piezo actuators is attributed to their high responsiveness and ability to achieve nanoscale steps. However, the accuracy and stability of motion are seriously affected by hysteresis caused by nonlinear characteristics. In this paper, a pigeon-inspired optimization (PIO) algorithm based on dynamic opposite learning (DOL) is proposed to address the issue of nonlinear modeling accuracy in piezo actuators by integrating the sparse identification of nonlinear dynamics (SINDy) method. Firstly, the DOL strategy is employed to introduce reverse pigeon flock into the PIO algorithm, thereby enhancing population diversity and optimization performance. Secondly, through combining the DOLPIO algorithm with the SINDy algorithm, sparse optimization for the penalty process in SINDy algorithm is conducted and the sparse coefficient is optimized based on modeling accuracy. Thirdly, the DOLPIO algorithm is utilized again to optimize the framework of optimized sparse penalty model in order to improve overall modeling accuracy. Finally, experiments are conducted on an established platform to validate the effectiveness of this algorithm.

This paper presents an electromagnetic energy harvester based on a unique nonlinear Kresling origami-inspired structure. By introducing the equilibrium shift phenomenon, reversible nonlinearity (i.e. mixed softening-hardening behavior) empowers the proposed harvester to work in a broad frequency band, confirmed by both simulation using a dynamic model and experimentation. The prototyped device can produce the open-circuit root mean square (RMS) voltage from 0.09 V to 0.20 V in the reversibly nonlinear response region in (6.19 Hz, 9.63 Hz) and a maximum output power of 0.4956 mW at an optimum load of 18.1 Ω under the excitation of 1.1 g. Moreover, detailed research further reveals that the design parameters of Kresling origami-inspired structure and electrical and mechanical loads influence reversible nonlinearity. Increasing the tip mass and γ₀ in the M2 region of the design map strengthens the softening behavior, and enlarging the electrical load enhances the hardening behavior. The findings from this work deepen the understanding of the nonlinear behavior of Kresling origami, unveils the great potential of origami structure in energy harvesting and offers a new method to realize broadband vibration energy harvesters.

Traditional upper limb rehabilitation robots have several disadvantageous. For example, they can only conduct rehabilitation training along predetermined trajectories, their safety systems are unreliable, and they lack the ability to adjust or train the affected limb based on the expected torque of the human body. To overcome these limitations, this study proposes a flexible safety system for joint rehabilitation utilising magnetorheological (MR) fluid. MR damper inverters offer significant advantages, including high torque, rapid response, controllable flexibility, and safety assurance. The range of motion trajectories can be adjusted using a four-lever hinge mechanism. The necessary driving force is provided by the motor actuator, and the MR damper imparts flexibility and variable damping characteristics to the output torque. The system uses a force/position impedance safety-control method, and using an internal position closed-loop controller, the MR upper limb rehabilitation flexible joint guides the affected limb to a safe position. A simulation is performed to verify the accuracy of the system's motion torque and position. Extensive research has been conducted on the safe rehabilitation outcomes of the upper limb rehabilitation system under three working conditions (step, incremental, and equation) involving the interaction moment of the affected limb. Simulation and experimental results demonstrate that the MR damper effectively controls the upper limb rehabilitation system to achieve the desired results, even when subjected to incremental and abrupt interaction forces from the patient. The tracking accuracy error remains within the range of 3%–7% for a certain period, confirming the safety and feasibility of the MR-based upper limb rehabilitation robot design.

A traditional wind energy harvester based on galloping can only harvest wind energy from one specific direction, which fails to work efficiently in a natural erratic environment. In this study, we propose a galloping-based piezoelectric energy harvester that can collect energy from wind flow in a wide range of incident directions with multiple vibrational modes being excited. The proposed harvester is composed of a tri-section beam with bonded piezoelectric transducers and a square bluff body with splitters. Finite element analysis of the tri-section beam structure is first performed and confirms the clustered natural frequencies that ease the excitation of different modes. Then, the aerodynamic characteristics of various bluff bodies is conducted through computational fluid dynamics to compare the capability of galloping. Finally, the wind tunnel experiment is carried out to test the wind energy harvesting performance by utilizing the harvester's multi-modal characteristics. The results of this study demonstrate that the proposed harvester can harvest wind energy in multiple directions with the capability of galloping in multiple vibrational modes, and superior performance is achieved when the second bending mode is triggered. The novel design of the harvester from this work provides a viable solution for harvesting wind energy in a natural environment with varying wind conditions.

This study is an attempt to analyze the torsion buckling of a structure consisting of a cylindrical sandwich shell with two isotropic face sheets that surround a magnetorheological fluid (MRF) core layer. In this analysis, the simply supported boundary conditions were considered for the edges of the face sheets and the core layer. The components of displacement were calculated using the first-order shear deformation theory, and the governing equations were derived using Hamilton's principle and were solved drawing upon the Galerkin method. The parameters of interest were magnetic field, buckling analysis, torsional buckling convergence, h/L ratio, ht/h ratio, and rt/L ratio. The equations obtained from MATLAB were verified using ABAQUS owing to the absence of any similar study in the existing literature. A good agreement was observed in terms of torsional buckling, indicating the robustness of the proposed structure. As smart sandwich structures are broadly used in robotics and aerospace, this structure can be a good choice thanks to its lightness (resulting from the thinness of the face sheets and hollowness) and strength and resistance (contributed by MRF core layer), which can be modified with the application of different magnetic fields.

This study proposes an active vibration control technique for pipe structures using dielectric elastomer actuators (DEAs). Vibrations in pipe structures must be eliminated to improve their mechanical reliability, and active vibration control techniques can be applied for effective vibration suppression. Soft actuators, which can completely fit pipe structures with complex-shaped surfaces, are required to transfer their vibration reduction forces to the target. DEA is suitable for this kind of target structure because DEA is characterized by high stretchability, flexibility, large deformation, and fast response. By applying the DEA, the effectiveness of vibration control for the pipe structure was experimentally demonstrated. A stacked DEA was fabricated and attached to the target structure. Its shape and placement were determined based on a modal analysis of the target structure. A control system, in which the controller for the active vibration control was designed based on ${H_\infty }$ control theory, was composed. The vibration control experiment was conducted using the controller with a digital control system, and the vibration reduction effect was evaluated based on the frequency response of the target pipe structure.

This study evaluates the performance of composite structures with embedded conductive yarns during shock loads to create a multifunctional system for immediate failure detection. The scalable sensing yarns were made by braiding Kevlar fibers with Nitinol fibers and then integrating them into a carbon/epoxy prepreg. The multifunctional structure was subjected to a Mach 2 air blast load using a shock tube apparatus. The embedded sensor yarns were used to record their electrical performance, while Digital Image Correlation captured full-field displacements, velocities, and strains. In addition, pressure transducers measured shock event pressures. The results revealed that through-thickness failure of the laminated composite occurred at approximately 2.5% strain, which was visually observable. However, the embedded sensor exhibited out-of-range electrical measurements at around 1.5% strain, even though no visible structural damage was present. This demonstrates the embedded sensing yarns' ability to detect delamination-type failures by responding to interlaminate damage, highlighting their advantages over conventional external sensors. Similarly, the gauge factor for the fiber system was determined to be 1.89 ± 0.07. This multifunctional system shows great potential for enhancing composite structure safety and performance in high-performance aerospace applications and offering real-time structural health assessment.