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.

Sports concussions are a public health concern. Improving helmet performance to reduce concussion risk is a key part of the research and development community response. Direct and oblique head impacts with compliant surfaces that cause long-duration moderate or high linear and rotational accelerations are associated with a high rate of clinical diagnoses of concussion. As engineered structures with unusual combinations of properties, mechanical metamaterials are being applied to sports helmets, with the goal of improving impact performance and reducing brain injury risk. Replacing established helmet material (i.e. foam) selection with a metamaterial design approach (structuring material to obtain desired properties) allows the development of near-optimal properties. Objective functions based on an up-to-date understanding of concussion, and helmet testing that is representative of actual sporting collisions and falls, could be applied to topology optimisation regimes, when designing mechanical metamaterials for helmets. Such regimes balance computational efficiency with predictive accuracy, both of which could be improved under high strains and strain rates to allow helmet modifications as knowledge of concussion develops. Researchers could also share mechanical metamaterial data, topologies, and computational models in open, homogenised repositories, to improve the efficiency of their development.

Hard-magnetic soft materials (hMSMs) are smart composites that consist of a mechanically soft polymer matrix impregnated with mechanically hard magnetic filler particles. This dual-phase composition renders them with exceptional magneto-mechanical properties that allow them to undergo large reversible deformations under the influence of external magnetic fields. Over the last decade, hMSMs have found extensive applications in soft robotics, adaptive structures, and biomedical devices. However, despite their widespread utility, they pose considerable challenges in fabrication and magneto-mechanical characterization owing to their multi-phase nature, miniature length scales, and nonlinear material behavior. Although noteworthy attempts have been made to understand their coupled nature, the rudimentary concepts of inter-phase interactions that give rise to their mechanical nonlinearity remain insufficiently understood, and this impedes their further advancements. This holistic review addresses these standalone concepts and bridges the gaps by providing a thorough examination of their myriad fabrication techniques, applications, and experimental, and modeling approaches. Specifically, the review presents a wide spectrum of fabrication techniques, ranging from traditional molding to cutting-edge four-dimensional printing, and their unbounded prospects in diverse fields of research. The review covers various modeling approaches, including continuum mechanical frameworks encompassing phenomenological and homogenization models, as well as microstructural models. Additionally, it addresses emerging techniques like machine learning-based modeling in the context of hMSMs. Finally, the expansive landscape of these promising material systems is provided for a better understanding and prospective research.

Soft actuators can be classified into five categories: tendon-driven actuators, electroactive polymers, shape-memory materials, soft fluidic actuators (SFAs), and hybrid actuators. The characteristics and potential challenges of each class are explained at the beginning of this review. Furthermore, recent advances especially focusing on SFAs are illustrated. There are already some impressive SFA designs to be found in the literature, constituting a fundamental basis for design and inspiration. The goal of this review is to address the latest innovative designs for SFAs and their challenges and improvements with respect to previous generations, and to help researchers to select appropriate materials for their application. We suggest seven influential designs: pneumatic artificial muscle, PneuNet, continuum arm, universal granular gripper, origami soft structure, vacuum-actuated muscle-inspired pneumatic, and hydraulically amplified self-healing electrostatic. The hybrid design of SFAs for improved functionality and shape controllability is also considered. Modeling SFAs, based on previous research, can be classified into three main groups: analytical methods, numerical methods, and model-free methods. We demonstrate the latest advances and potential challenges in each category. Regarding the fact that the performance of soft actuators is dependent on material selection, we then focus on the behaviors and mechanical properties of the various types of silicone that can be found in the SFA literature. For a better comparison of the different constitutive models of silicone materials proposed and tested in the literature, ABAQUS software is here employed to generate the engineering and true strain-stress data from the constitutive models, and compare them with standard uniaxial tensile test data based on ASTM412. Although the figures presented show that in a small range of stress–strain data, most of these models can predict the material model acceptably, few of them predict it accurately for large strain-stress values. Sensor technology integrated into SFAs is also being developed, and has the potential to increase controllability and observability by detecting a wide variety of data such as curvature, tactile contacts, produced force, and pressure values.

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.

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.

Mechanical vibrations from heavy machines, building structures, or the human body can be harvested and directly converted into electrical energy. In this paper, the potential to effectively harvest mechanical vibrations and locally generate electrical energy using a novel piezoelectric-rubber composite structure is explored. Piezoelectric lead zirconate titanate is bonded to silicone rubber to form a cylindrical composite-like energy harvesting device which has the potential to structurally dampen high acceleration forces and generate electrical power. The device was experimentally load tested and an advanced dynamic model was verified against experimental data. While an experimental output power of 57 μW cm⁻³ was obtained, the advanced model further optimises the device geometry. The proposed energy harvesting device generates sufficient electrical power for structural health monitoring and remote sensing applications, while also providing structural damping for low frequency mechanical vibrations.

The twisted and coiled polymer actuator (TCPA) has a complex multi-scale structure consisting of crystalline micro-fibrils and an amorphous matrix at the micro-scale, which are organized into a macro-scale fiber. When the polymer fiber undergoes twisting and coiling, its mechanical and thermal properties become variable. In this study, we developed a multi-layer modeling framework capable of accurately predicting the effective mechanical and thermal properties, as well as the thermo-mechanical responses of the TCPA. Our numerical results demonstrate that the effective mechanical and thermal properties of the TCPA are influenced by the radius and twisting angle of the polymer fiber. By analyzing the precise mechanical and thermal properties, the numerical calculated driving responses exhibit good agreement with experimental data. We also examined the influence of initial helical radius, helical pitch and fiber radius on the driving responses of the TCPA. The proposed numerical model can be further utilized to optimize the driving responses of the TCPA by adjusting geometric parameters and the twisting angle of the polymer fiber.

The investigation of the conversion of vibrational energy into electrical power has become a major field of research. In recent years, bistable energy harvesting devices have attracted significant attention due to some of their unique features. Through a snap-through action, bistable systems transition from one stable state to the other, which could cause large amplitude motion and dramatically increase power generation. Due to their nonlinear characteristics, such devices may be effective across a broad-frequency bandwidth. Consequently, a rapid engagement of research has been undertaken to understand bistable electromechanical dynamics and to utilize the insight for the development of improved designs. This paper reviews, consolidates, and reports on the major efforts and findings documented in the literature. A common analytical framework for bistable electromechanical dynamics is presented, the principal results are provided, the wide variety of bistable energy harvesters are described, and some remaining challenges and proposed solutions are summarized.

Advanced model-based control techniques hold great promise for the precise control of pneumatic soft bending actuators (PSBAs) with strong nonlinearities. However, most previous controllers were designed in a cumbersome nonlinear form. Considering the simplicity of linear system theory, this paper presents a novel perspective on using model-based linear control to handle nonlinear PSBAs, and for the first time, summarizes two methodologies, global linearization and pseudo-linear construction. Derived from them, Koopman-based and hysteresis-based linear control approaches are proposed, respectively. For the former, a novel fusion prediction equation is developed to build a high-fidelity Koopman model, realizing global linearization, and then the linear model predictive control (MPC) is deployed. For the latter, the inverse of the generalized Prandtl–Ishlinskii (GPI) model cascades with the PSBA to construct a pseudo-linear system, eliminating the asymmetric hysteresis, which activates the linear proportional-integral-derivative (PID) control. It is worth noting that the above two are based on data-driven models adapted to various PSBAs with material and structural customization. Finally, the two model-based linear control approaches are verified and compared through a series of experiments. The results show that the proposed linear controls, with more concise designs, achieve comparable or even superior performance than nonlinear controls.

The evaluation of damage tolerance in composite materials is essential for ensuring the safety of aircraft structures. One of the most challenging aspects of applying probability modeling-based methods to evaluate damage tolerance is determining the actual damage size distributions for in-service aircraft structures. Although existing nondeterministic approaches have been used to optimize inspection intervals of composite structures, few studies have investigated the effects of updates on the actual damage size distribution and its impact on both the probability of structural failure and inspection intervals. This paper proposes a dynamic optimization method for inspection intervals of composite structures based on Bayesian updating. The damage size distribution of the composite structure is characterized by a general stochastic distribution. A Bayesian updating methodology is presented to iteratively update the actual damage size distribution whenever new data becomes available. Based on the constructed probability model, the inspection intervals of composite structures are determined under the objectives of optimal safety and economy for civil aircraft using a Monte Carlo approach. Compared to prior distribution models, the proposed method achieves higher safety for structures during a single inspection, reduces the failure probability of structures throughout their entire service life, and incurs lower maintenance costs. It also enables maintenance personnel to flexibly adjust inspection intervals while facilitating quantitative evaluation of both failure probabilities and maintenance costs associated with these intervals. These findings suggest that the proposed method holds great potential in enabling maintenance personnel to make informed decisions regarding inspection intervals for improved safety and economic performance.

This study proposes a new magneto-rheological aircraft main landing gear (MRAMLG) system and comprehensively treats from a mathematical modelling to drop testing for the evaluation of the landing efficiency. A mathematical model is formulated based on specifications and requirements of existing small aircraft Oleo type landing gear system. To ensure structural stability of the landing gear components such as main trust, column and trunnion, a transient structural analysis is carried out using the finite element method (FEM) and a fatigue life is analysed based on empirical formulas and the rainflow-counting (RC) algorithm. Subsequently, a novel controller is formulated to enhance the landing efficiency by integrating the time delay model and 3-stage hysteresis regulator. In the synthesis of the controller, a desired force model based on the energy law is added to accurately track the desired yield stress needed for the desired field-dependent force. In this work, the proposed control algorithm is named as the model-based force-tracking (MBFT) controller. To evaluate the landing efficiency or shock struct efficiency (SSE) of the proposed MRAMLG, an experimental apparatus for drop test is designed and manufactured by considering a dummy (sprung) mass of 640–720 kg at the maximum sink speed of 3 m s⁻¹. The sink speed represents the rate of descent of the MRAMLG's tire just before it touches the ground. It is demonstrated from simulation and experiment that the SSE with the MBFT controller is higher than 83% across various landing conditions with different sprung mass and sink speed, while the conventional controllers, proportional-integral and skyhook, compared to MBFT, do not show consistent performance depending on the sprung mass and sink speed.

A McKibben artificial muscle is a fluid-driven soft actuator comprising sleeve fibres and rubber tube. However, as typical bulky and rigid displacement sensors are unsuitable as sensor elements in soft actuators, displacement sensing is challenging for the McKibben artificial muscle. Therefore, we propose an optical fibre-based smart artificial muscle (OSAM) to estimate self-displacement from the bending loss of the optical fibre used as the sleeve fibre. The optical fibre can be effortlessly integrated into the OSAM sleeve using a braiding machine, which is generally used for manufacturing strings, easing the mass production process. The radius of curvature of the optical fibre changed when the OSAM was driven. The displacement of the artificial muscle was estimated based on the sensor output. To demonstrate the usefulness of OSAM, displacement feedback control experiments were conducted using the optical fibre sensor integrated into OSAM. From the results, OSAM's displacement showed a good response to the target displacement. Therefore, the developed artificial muscle can facilitate displacement feedback control without requiring external sensors, which in turn can improve the performance of rehabilitation and wearable devices.

The quality of aircraft assembly is mainly guaranteed by toolings which are vital to the geometrical accuracy and service performance of aviation products. In this research, a real-time monitoring system that determines the structural deformation and clamping force of reconfigurable toolings using strain data is developed to perceive the service state of the toolings. By laying fiber Bragg gratings on positioning beam and baseplate of the reconfigurable tooling, strain data of the tooling structures are gauged and transferred to curvatures. The beam and baseplate are modeled as one and two-dimensional objects respectively and shape reconstruction algorithms are established to obtain their deflection curve and surface using curvature information. Distribution of fibers is optimized to minimize the conversion error from strain to curvature. An estimation that reveals the mathematical relationship between the shape reconstruction error and measurement interval is implemented, and a mapping model from strains to clamping force of the beam is established. These algorithms are integrated into the self-developed monitoring software and undergo simulating and experimental tests. The maximum relative errors of deformation and force are 4.53% and 4.12% respectively in simulation, and 9.21% and 7.29% individually in experiment, which validates the efficiencies of the method. Tests of the monitoring system suggest that it can provide a timely and accurate sensing of the deformation and force of the tooling.

Four-dimensionally (4D) printing technology effectively demonstrates the advantages of portability, foldability, and improved mechanical properties in origami structures. This study highlights the significance of printing process parameters and origami hinge structures in ensuring rapid and complete deployment of origami structures. Polylactic acid based semicrystalline shape memory polymer was utilized to produce 4D printed origami structures. Through response surface analysis methods and bending tests, optimal printing parameters were determined, including layer thickness of 0.31 mm, extrusion speed of 17 mm s⁻¹, and infill percentage of 71%. Additionally, the study designed a U-shaped hinge and analyzed its recovery mechanism through micromorphology of bending hinges and recovery experiments.

In this review article, different structural types of the magnetic core required for activation of magnetorheological elastomer (MRE) and magnetorheological fluid (MRF) are introduced in terms of design feature, magnetic flux analysis and performance, installation with primary structure and close relationship to material types. As a first step, dynamic functions related to the chosen models are summarized and discussed according to the magnetic field variations including the field-dependent damping force and torque of the application systems. To address on the practical feasibility, main issues of design process are also pointed out and are discussed stating the manufacturing feasibility and the scaled factors of dynamic variables. Then, after analysing the featured models and dynamic functions, the derivation approaches to establish mathematical models of the magnetic circuit core (MCC) are provided and compared as a valuable reference for checking both simplicity and accuracy. In this stage, the chosen symbolized magnetic circuit models are clearly described about linear or/and nonlinear behaviours of the input (current) and output (magnetic field). In addition, a couple of commercial software to design the magnetic circuit model is introduced since they can be effectively adopted to analyse the MCCs of many application systems utilizing MRE and MRF without any difficulty.

Structures inspired by the Kresling origami pattern have recently emerged as a foundation for building functional engineering systems with versatile characteristics that target niche applications spanning different technological fields. Their light weight, deployability, modularity, and customizability are a few of the key characteristics that continue to drive their implementation in robotics, aerospace structures, metamaterial and sensor design, switching, actuation, energy harvesting and absorption, and wireless communications, among many other examples. This work aims to perform a systematic review of the literature to assess the potential of the Kresling origami springs as a structural component for engineering design keeping three objectives in mind: (i) facilitating future research by summarizing and categorizing the current literature, (ii) identifying the current shortcomings and voids, and (iii) proposing directions for future research to fill those voids.

Hard-magnetic soft materials (hMSMs) are smart composites that consist of a mechanically soft polymer matrix impregnated with mechanically hard magnetic filler particles. This dual-phase composition renders them with exceptional magneto-mechanical properties that allow them to undergo large reversible deformations under the influence of external magnetic fields. Over the last decade, hMSMs have found extensive applications in soft robotics, adaptive structures, and biomedical devices. However, despite their widespread utility, they pose considerable challenges in fabrication and magneto-mechanical characterization owing to their multi-phase nature, miniature length scales, and nonlinear material behavior. Although noteworthy attempts have been made to understand their coupled nature, the rudimentary concepts of inter-phase interactions that give rise to their mechanical nonlinearity remain insufficiently understood, and this impedes their further advancements. This holistic review addresses these standalone concepts and bridges the gaps by providing a thorough examination of their myriad fabrication techniques, applications, and experimental, and modeling approaches. Specifically, the review presents a wide spectrum of fabrication techniques, ranging from traditional molding to cutting-edge four-dimensional printing, and their unbounded prospects in diverse fields of research. The review covers various modeling approaches, including continuum mechanical frameworks encompassing phenomenological and homogenization models, as well as microstructural models. Additionally, it addresses emerging techniques like machine learning-based modeling in the context of hMSMs. Finally, the expansive landscape of these promising material systems is provided for a better understanding and prospective research.

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.

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 this paper, surface conductive heating was utilized to actively control the stiffness of lattice metamaterials manufactured employing multi-material 3D printing. To create an electrical surface conduction, additively manufactured samples in single and dual material configurations were dip coated in a solution of carbon black in water. Electro-thermo-mechanical tests conducted successfully demonstrated that the low-cost conductive coating can be used to actively alter the stiffness of the structure through surface joule heating. The process was found to result in repeatable and reproduceable stiffness tuning. Stiffness reductions of 56% and 94% were demonstrated for single and dual material configurations under the same electrical loading. The proposed methodology can be implemented to actively control the properties of polymeric lattice materials/structures where the change in the composition of polymers (introduce bulk electrical conductivity) is difficult and can have a wide range of applications in soft robotics, shape-changing, and deployable structures.

A self-powered and durable pressure sensor for large-scale pressure detection on the knee implant would be highly advantageous for designing long-lasting and reliable knee implants as well as obtaining information about knee function after the operation. The purpose of this study is to develop a robust energy harvester that can convert wide ranges of pressure to electricity to power a load sensor inside the knee implant. To efficiently convert loads to electricity, we design a cuboid-array-structured tribo-pizoelectric nanogenerator (TPENG) in vertical contact mode inside a knee implant package. The proposed TPENG is fabricated with aluminum and cuboid-patterned silicone rubber layers. Using the cuboid-patterned silicone rubber as a dielectric and aluminum as electrodes improves performance compared with previously reported self-powered sensors. The combination of 10$\,wt\%$ dopamine-modified BaTiO₃ piezoelectric nanoparticles in the silicone rubber enhanced electrical stability and mechanical durability of the silicone rubber. To examine the output, the package-harvester assemblies are loaded into an MTS machine under different periodic loading. Under different cyclic loading, frequencies, and resistance loads, the harvester's output performance is also theoretically studied and experimentally verified. The proposed cuboid-array-structured TPENG integrated into the knee implant package can generate approximately 15$\,\mu$W of apparent power under dynamic compressive loading of 2200 N magnitude. In addition, as a result of the TPENG's materials being effectively optimized, it possesses remarkable mechanical durability and signal stability, functioning after more than 30 000 cycles under 2200 N load and producing about 300 V peak to peak. We have also presented a mathematical model and numerical results that closely capture experimental results. We have reported how the TPENG charge density varies with force. This study represents a significant advancement in a better understanding of harvesting mechanical energy for instrumented knee implants to detect a load imbalance or abnormal gait patterns.

One of the biggest challenges in structural health monitoring for rotor blades in wind turbines is to provide enough energy to power wireless sensor nodes. Batteries are not an adequate solution due to their limited lifetime and conventional cabling fails due to the rotation of the rotor blade. Therefore, we present an electromagnetic energy harvester that is specifically designed to be operated inside rotor blades and can generate a sufficient amount of energy. It uses the changing gravitational force vector to move a permanent magnet in a tube and converts this mechanical into electrical energy by coils arranged around the tube. Finite element methods simulations were performed to estimate the generated energy and an extensive parameter sweep of several key design parameters provided guidance for an optimized performance of a prototype. This device was characterized in the lab followed by a field test in a wind turbine where it was operated for several days and provided a continuous and rectified power of 6 mW, enough to power conventional wireless accelerometers, typically used within a predictive maintenance concept for the vibrational monitoring of rotor blades.

To deal with the limited availability of long-lasting power sources for sensor nodes in industrial environments, a novel piezoelectric energy harvester with high efficiency and a wide working bandwidth was designed to harvest broadband and random vibrations from the ambient environment. The developed energy harvester adopts a doubly clamped piezoelectric beam with a peanut-shaped auxetic structure to improve the power output. It also incorporates a sliding proof mass for frequency self-tuning, enabling a wider working bandwidth. As the doubly clamped beam exhibits geometry nonlinearity under large vibration amplitudes, the power output of the energy harvester can be further enhanced in the frequency self-tuning process. Finite element simulations are conducted to evaluate the impact of the auxetic structure and the position of the proof mass on the performance of the energy harvester. Experiments are performed to examine the energy harvesting performance of the proposed energy harvester. Under an excitation acceleration of 0.3 g, the use of the sliding proof mass widens the working bandwidth of the auxetic energy harvester (AEH) by 9 Hz, with the maximum root mean square output power of AEH reaching 18.78 μW, which is much higher than that of the plain energy harvester (PEH) or the AEH with a fixed proof mass. The developed energy harvester can successfully power a wireless temperature and humidity sensor node based on the vibration produced by a centrifuge, which demonstrates the practical feasibility of the proposed energy harvester for industrial applications.

Inflatable structures, promising for future deep space exploration missions, are vulnerable to damage from micrometeoroid and orbital debris impacts. Polyvinylidene fluoride-trifluoroethylene (PVDF-trFE) is a flexible, biocompatible, and chemical-resistant material capable of detecting impact forces due to its piezoelectric properties. This study used a state-of-the-art material extrusion system that has been validated for in-space manufacturing, to facilitate fast-prototyping of consistent and uniform PVDF-trFE films. By systematically investigating ink synthesis, printer settings, and post-processing conditions, this research established a comprehensive understanding of the process-structure-property relationship of printed PVDF-trFE. Consequently, this study consistently achieved the printing of PVDF-trFE films with a thickness of around 40 µm, accompanied by an impressive piezoelectric coefficient of up to 25 pC/N. Additionally, an all-printed dynamic force sensor, featuring a sensitivity of 1.18 V/N, was produced by mix printing commercial electrically-conductive silver inks with the customized PVDF-trFE inks. This pioneering on-demand fabrication technique for PVDF-trFE films empowers future astronauts to design and manufacture piezoelectric sensors while in space, thereby significantly enhancing the affordability and sustainability of deep space exploration missions.

In this paper, a piezoelectric energy harvester composed of a buckled beam and an extended beam with a tip mass is proposed. This study develops a mathematical model and a prototype of the energy harvester. The performance of the energy harvester is influenced by the axial load applied at the end of the buckled beam. Under an axial force below the critical load, the energy harvester exhibits a pre-buckling state with a hardening nonlinear characteristic. Conversely, when the axial force exceeds the critical load, a post-buckling state with a softening nonlinear characteristic is observed. Simulation results are validated through experiments, and the relationship between axial displacement and axial force is obtained through experimental data. Moreover, increasing the tip mass enhances the output voltage under the same acceleration. The energy harvester demonstrates superior performance in terms of output power and strain distribution compared to a cantilever counterpart.

Mechanical to electrical energy conversion is a well-established energy transduction approach. However, cases in which a mechanical energy source is not available call for new approaches to harvest electrical energy. In the present study, we demonstrate energy harvesting in soft dielectric elastomer (DE) tubes. Broadly, energy harvesting is obtained through inflation of the tube, electrical charging of the DE layer, and deflation, which results in a decrease in capacitance and an increase in voltage. We propose two methods to mechanically charge (or inflate) the system: (1) active, in which the tube is inflated through the application of mechanical pressure, and (2) passive, in which a passive cylindrical component placed inside the DE tube deforms radially in response to an environmental stimulus such as thermal excitation or water uptake and inflates the DE tube. To demonstrate passive charging, we consider gels as the passive component and employ well-known models with the properties of the commonly employed DE VHB 4910 to simulate the mechanical response of the system and estimate the harvested electrical energy. Our findings reveal that energy-densities in the order of ∼10–50 mJ cm^–3 can be harvested. The proposed approach and the inclusion of a passive component to mechanically charge the system opens new opportunities to generate energy in environments lacking traditional mechanical energy sources.

The twisted and coiled polymer actuator (TCPA) has a complex multi-scale structure consisting of crystalline micro-fibrils and an amorphous matrix at the micro-scale, which are organized into a macro-scale fiber. When the polymer fiber undergoes twisting and coiling, its mechanical and thermal properties become variable. In this study, we developed a multi-layer modeling framework capable of accurately predicting the effective mechanical and thermal properties, as well as the thermo-mechanical responses of the TCPA. Our numerical results demonstrate that the effective mechanical and thermal properties of the TCPA are influenced by the radius and twisting angle of the polymer fiber. By analyzing the precise mechanical and thermal properties, the numerical calculated driving responses exhibit good agreement with experimental data. We also examined the influence of initial helical radius, helical pitch and fiber radius on the driving responses of the TCPA. The proposed numerical model can be further utilized to optimize the driving responses of the TCPA by adjusting geometric parameters and the twisting angle of the polymer fiber.

In this paper, a Lamb-wave based structural health monitoring for multi-damage localizations in large composite plates is presented. The Lamb waves are generated and received by piezoelectric transducers, which are arranged in array on the composite plate. In the experiments, three composite plates with various laminate stacking sequences and taper designs were prepared. The damages were created on the specimens by impact testing. In each specimen, 24 piezoelectric transducers were utilized and mounted on the specimen surface. This study proposed an algorithm to identify the damage localizations. The transducer layout is classified by 10 subsets. In each subset, the wave propagation paths can be grouped into path groups pivoted by actuators and that by sensors. Based on the damage index, the mean angle line for each path group in a subset can be obtained. By assuming that the mean angle line passes through the actual damage, the damage localization can be achieved if there exist more than two mean angle lines in one subset. In this study, two exclusion rules are proposed to exclude a path group from the damage localization calculations. The damage localization results show that, for a composite plate with multiple damages, their locations can be identified by using multiple subsets. The damage localization results show that the damage location can be accurately predicted for the case that a damage exists in the interior of a subset. The experiment results also show that the Lamb wave characteristics and the localization results are not affected by the thickness variation of the plate, indicating that the proposed algorithm is available for tapered composite plate.

Magnetoactive soft continuum robots (MSCRs), capable of controllable steering and navigation, hold substantial promise for healthcare applications. However, advancements in MSCRs have been hindered by a limited understanding of MSCR dynamics and a lack of effective control methods. Addressing these gaps, this study presents a novel, time-dependent, and computationally efficient analytical model of MSCR, alongside a new optimal closed-loop control strategy for precise high-frequency trajectory tracking. A finite element (FE) model of the MSCR is initially developed, with its validity confirmed through rigorous laboratory measurements. Using the formulated FE model, a new and computationally efficient analytical model is subsequently developed to accurately predict the highly nonlinear response of MSCR. This model operates as a system of switched linear models, each of which is a reduced-order version of its corresponding high-order linear model extracted from the FE analysis. This innovative approach not only maintains the predictive accuracy of the FE model but also significantly reduces computational demands, operating in just a few seconds. The results highlight that the developed model can accurately predict the dynamic responses of the MSCR while significantly reducing the computational load by almost 80 orders of magnitude compared with the FE model on the same simulation platform. The proposed model has been effectively utilized to develop a novel optimal control strategy using the feedforward interval type-2 fractional-order fuzzy-PID method. A hardware-in-the-loop experimental test has been finally designed to demonstrate the superior performance of the MSCR under the proposed controller.