Mechanical Prestress Research Articles

Ultrahigh electrostrictive strain and its response to mechanical loading in Nd-doped PMN-PT ceramics

Electrostrictive materials play an important role in the development of high precision actuators, due to the advantages of high resolution, low power dissipation and fast response time. However, the practical application of electrostrictive materials has been hindered by their limited displacement output. To address this, we conducted a study where we doped Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) ceramics with Nd2O3, aiming to significantly improve the electrostrictive effect by enhancing local structural heterogeneities that disrupt the formation of long-range ordered ferroelectric domains. We obtained an optimized electrostrictive strain of 0.231 % with negligible hysteresis at an electric field of 50 kV/cm in 4 %Nd-doped PMN-PT ceramic, surpassing the performance of state-of-the-art lead-based electrostrictive ceramics. Of particular significance is that a high electrostrictive coefficients M33 = 12.2 × 10−16 m2/V2, together with a high mechanical work density of 0.035 J/cm3 and a low power dissipation of 14 %, was achieved under a mechanical prestress of 50 MPa and an electric field of 10 kV/cm. Additionally, this material exhibits excellent fatigue resistance, with less than 3 % variation over 106 operation cycles. All these findings position 4 %Nd-doped PMN-PT ceramics as a promising candidate for high-performance electromechanical actuator applications.

A Computational Model for All-Solid-State Batteries Coupling Electrochemistry and Solid Mechanics on Resolved Microstructures Enabling Optimization of Battery Electrode Design

All-Solid-State Batteries (ASSB) are a promising technology to replace conventional Lithium-Ion Batteries (LIB) since the energy density and fast charging capability of LIBs are projected to reach a physical limit soon. In this regard, the ASSB technology is considered superior, especially if the incorporation of the lithium metal anode, or other high-capacity anode materials like silicon is achieved. However, these systems still require intensive research as many physical effects, especially those concerning the interaction of electrochemistry and solid mechanics, are to date not well understood [1]. Most experimental investigations on ASSB cells are performed on laboratory scale and are usually operated at elevated mechanical pre-stress to obtain satisfactory system performance. The delamination of active materials and solid electrolyte during cycling is expected to be one of the key effects for the required mechanical pre-stress. Furthermore, a lot of experimental research focuses on the development of new material classes and their stability in the cell compound, while a systematic optimization of the cell setup is missing.Comprehensive computational models can help to generate insights in these regards as they allow for detailed investigations of local effects, which is often not possible in experimental setups. Another advantage of computational models compared to experiments is the ability to change input parameters in a fast and systematic way to quantify their influence on the battery cell performance and thereby help to speed up the optimization of battery designs. Since the multitude of physical effects that are relevant in ASSB are computationally challenging, a lot of modeling approaches rely on numerous simplifications like spatial homogenization or neglect of mechanical effects to name just two. However, such assumptions can significantly limit the validity of the model.We propose a novel computational model for ASSB [2] to thoroughly investigate the interdependence of electrochemistry and solid mechanics on resolved microstructures that vastly influence the behavior and the performance of an ASSB cell. It is based on nonlinear continuum mechanics and accounts for large deformations due to lithiation dependent volume changes of the active materials. The mass and charge conservations are consistently formulated to also ensure the conservation property in large deformation scenarios. Furthermore, the model is extended by interaction effects of electrochemistry and solid mechanics like a nonlinear contact formulation at the electrode-electrolyte interfaces to account for delamination phenomena and the subsequent absence of charge transfer reaction (see Fig. 1). We thereby show how delamination phenomena drastically change available percolation paths in the microstructure. The efficient parallel implementation of the computational model allows to investigate realistic three-dimensionally resolved microstructures of ASSB cells while accounting for the coupling effects of electrochemistry and solid mechanics. A combination of elaborate physical models and probabilistic methods enables to gain further understanding and to quantify main influencing factors on the cell performance. To showcase these capabilities and how they can be exploited to increase system understanding, we analyze how statistical values like the porosity, or the composition of the electrode microstructure influence the capacity of an ASSB cell and thereby contribute to find optimal electrode designs.

Accurate Position Tracking Control of SMAAs Based on Low-Complexity Self-Sensing Model and Compound Control Strategy

Shape memory alloy actuators (SMAAs) exhibit high strain, high power-to-weight ratio, and self-sensing ability, which are promising characteristics for application in micro-actuator systems. However, the complex self-sensing and nonlinear hysteresis characteristics of shape memory alloy (SMA) seriously affect the position tracking accuracy of SMAAs. In order to realize the accurate position tracking of the SMAAs without sensors, this article proposes a low-complexity self-sensing of modeling methods, which does not require complex mechanical structure, specific prestress, and complex mathematical models. Furthermore, a compound control strategy of generalized Prandtl–Ishlinskii (GPI) inverse model and fuzzy-proportional–integral derivative (PID) was presented to compensate the nonlinear hysteresis and time-varying characteristics of SMAAs. First, macroscopic and microcosmic characteristics of SMA were analyzed to explain the negative effect of the rhombohedral ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${R}$ </tex-math></inline-formula> ) phase on the self-sensing characteristics of SMA. Second, a NiTi-based SMA wire without <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${R}$ </tex-math></inline-formula> phase was selected as a self-sensing modeling object by comparative evaluation experiments, which has similar linear self-sensing properties under various pretension force conditions, a self-sensing model containing only quadratic polynomials is constructed by an offline identification. Third, a fuzzy-PID controller is designed based on the transfer function obtained from the SMAA’s system identification, further, a nonlinear hysteresis feedforward compensation controller is designed based on the GPI model. Finally, comparative experiments with the compound controller and fuzzy-PID controller for multistep and sinusoidal position tracking responses were performed, and the experimental results show that the proposed compound controller has a faster response and higher control accuracy in position tracking control.

Theoretical and experimental study on optimum operational conditions of a magnetostrictive force sensor

Terfenol-D, as a giant magnetostrictive material, is used to fabricate different force sensors. In these sensors, bias magnetic field and mechanical pre-stress are applied to improve the sensor’s characteristics. The present paper proposes the optimum values of the above operational conditions to reach maximum sensitivity and linear measuring of the sensor. Initially, a sensor’s model based on coupled piezomagnetic equations is developed and affective parameters on the output voltage are recognized. An experimental setup is fabricated and used to obtain magnetization and magnetostriction curves of Terfenol-D at different mechanical pre-stresses to study the parameters. Analyzing the experimental curves, the sensor's effective parameters, including magnetostriction constant, magnetic permeability, and their variation with axial stress, are calculated and imported to the model. Hence, the sensor's sensitivity and linear measuring range are obtained at different values of bias magnetic field and mechanical pre-stress and optimum values considering the external force’s amplitude are introduced. The maximum sensitivity factor is 0.004515 mV/Ns−1 which obtained at mechanical pre-stress of 6.89 MPa and bias magnetic field of 25kA/m. In these conditions the linear measuring range of the sensor is 260–500 N. To validate the model, the results are compared with previously reported experimental results, and the minor errors show the precision of the model and experimental investigation.

Functionally Graded Soft Dielectric Elastomer Phononic Crystals: Finite Deformation, Electro-Elastic Longitudinal Waves, and Band Gaps Tunability via Electro-Mechanical Loading

Active tuning of elastic wave band gaps received significant attention in the recent past. To this end, soft dielectric elastomers (DEs) have indicated great promise owing to their capability to undergo large reversible deformations and deformation-dependent constitutive properties. In this paper, we investigate the longitudinal wave band gaps an infinite periodic soft functionally graded dielectric elastomer subjected to electro-mechanical biasing fields. Material properties of the functionally graded DE are described by the power law. The nonlinear hyperelastic compressible neo–Hookean model is employed for characterizing the constitutive behavior of the soft functionally graded dielectric elastomer. The finite element method with Bloch–Floquet theory is employed for finding the band-structure of the infinitely periodic soft functionally graded DE structure. A parametric study brings out the effect of biasing fields and power law exponent on the band gap characteristics. The biasing mechanical prestress, the electric displacement field show significant influence on position and the width of band gaps. The inferences reported here can find their potential use in the design of soft DE wave devices with tunable band structures.

Enhanced thermal buckling resistance of folded graphene reinforced nanocomposites with negative thermal expansion: From atomistic study to continuum mechanics modelling

Materials with tunable negative thermal expansion (NTE) are of crucial importance in engineering applications. Currently, the route to achieving large tunability for the coefficient of thermal expansion (CTE) in metal matrix composites (MMCs) is very limited. This paper develops folded graphene (FGr) structures to engineering the CTE of MMCs by embedding the FGr into a copper (Cu) matrix through the molecular dynamics (MD) method. Moreover, the present work investigates the thermal buckling resistance capability of the NTE material composite beam using the classical Euler-Bernoulli beam theory and Ritz method. Atomistic studies based on the MD simulations show that the CTE of FGr/Cu nanocomposite can be effectively tuned by mechanical prestress and temperature, especially within the range from 200 K to 400 K. By using the origami-patterned FGr designed herein, the maximum negative and positive CTEs can reach approximately −95.42 × 10−6 K−1 and 56.32 × 10−6 K−1 under the prestress of 1000 MPa at 300 K and 200 K, respectively. More importantly, the quantitative relationship of the temperature- and prestress-dependent material properties of the FGr/Cu composites are built. Based on the built material model, the FGr/Cu composite beam exhibits considerably improved thermal buckling performance than the pristine graphene/Cu beam.

Experimental Modeling of Magnetic Hysteresis Power Loss of Terfenol-D at Different Values of Frequency and Mechanical Pre-Stress

یکی از محدودیت‌های مهم استفاده از ترفنل-دی در محرکه‌ها و حسگر‌ها، اتلاف ناشی از پسماند مغناطیسی آن است که منجر به کاهش بازده مکانیکی در محرکه‌ها و کاهش دقت و کوچکتر شدن محدوده اندازه گیری خطی در حسگر‌ها می‌شود. در مقاله حاضر، افت توان ناشی از پسماند مغناطیسی در ترفنل-دی و عوامل موثر بر آن مورد مطالعه و مدلسازی تجربی قرار گرفته است. با استفاده از یک مجموعه آزمایشگاهی ابتکاری، رفتار مغناطیسی ترفنل-دی شامل منحنی‌های پسماند آن در چرخه‌های بزرگ و کوچک و در مقادیر مختلف فرکانس، شدت میدان مغناطیسی و پیش‌تنش مکانیکی به دست آورده شده است. از منحنی‌های پسماند به دست آمده به عنوان اطلاعات ورودی در رابطه تحلیلی افت توان ناشی از پسماند استفاده شده است. نتایج نشان دهنده آن است که افت توان ناشی از پسماند با افزایش فرکانس میدان مغناطیسی، افزایش مقدار ماکزیمم شدت میدان مغناطیسی در هر چرخه پسماند و کاهش پیش‌تنش مکانیکی، افزایش می‌یابد. سپس یک مدل تجربی مبتنی بر پارامترهای توانی با در نظر گرفتن سه عامل فرکانس، شدت میدان مغناطیسی و پیش‌تنش مکانیکی به منظور پیش بینی افت توان ناشی از پسماند مغناطیسی ارایه شده است، که اعتبار سنجی این مدل در شرایط متفاوت و جدید، نشان دهنده دقت قابل قبول آن می‌باشد. مزیت اصلی مدل تجربی ارایه شده نسبت به مدل تحلیلی این است که تنها با تعیین پارامترهای مربوط به شرایط کاری ترفنل-دی، امکان پیش بینی افت توان فراهم می‌گردد و به منحنی‌های پسماند مغناطیسی ترفنل-دی به عنوان ورودی مدل نیاز ندارد.

On characteristic identification for prestressed human skin

In the paper, we present a model of skin in the form of viscoelastic layer-like structure that is inhomogeneous in thickness and consists in turn of three layers: subcutaneous fat, dermis and epidermis. We also assume the presence of an inhomogeneous uniaxial prestress in the skin layer arisen as a result of relaxation after the skin lifting (tightening) surgery procedure. We formulate the problem on the basis of the general problem statement on steady-state vibrations of an inhomogeneous body, taking into account the initial stress-strain state. Using the correspondence principle, the elastic moduli in the problem statement are replaced by complex analogs corresponding to the model of a standard viscoelastic body. The inverse problem is to determine the mechanical characteristics (complex analogs of the Lame parameters) and prestress using the data on the acoustic response for a periodic probing non-invasive effect on the layer surface. By using the Fourier transform in the longitudinal coordinate, we reduce the original problem to solving a number of simpler problems in transforms. Based on the combination of different loading modes, we present a two-stage scheme for identifying mechanical properties and prestress. Within this scheme, at the first stage, complex analogs of the Lame parameters are successively determined using the iterative approach and the Tikhonov regularization method. At the second stage, by using the functions found at the first stage, prestresses are determined using the proposed projection technique. We illustrate the approach developed to the considered skin integument problem by performing computational experiments showing the efficiency of the techniques proposed in the ranges corresponding to the real values of the sought-for parameters. Additionally, we provide some recommendations on the choice of frequency ranges for the best identification at each stage.

New insights into the mechanical pre-stressing of piezotransducers

Although pre-stressing is a requirement for the operation of a high-power piezo transducers, its impact on the model, the performance and the design of such devices is still not yet fully covered in the literature. This work attempts to bridge this gap by analyzing how the components of the Butterworth-Van Dyke model, the resonances, electrical impedances, electromechanical coupling factor, mechanical losses, and the tuning inductance are determined as a function of mechanical pre-stress. We have employed the Butterworth Van-Dyke model to study piezoelectric transducers tightened with a range of pre-stressing levels (12.2–60.1 MPa). As expected, the experimental results indicate that the components of the Butterworth-Van Dyke model are dependent upon the mechanical pre-stressing. The conclusions point out that, under mechanical pre-stressing levels between a threshold value and 60.1 MPa, the transducers used in this work tend to stabilize and enhance their performances. Also, upon stabilization, the transducers can be tuned with a constant inductance, which may also aid the design of ultrasonic systems. Overall, this work contributes to the definition of pre-stressing levels and its relationship with the length of the transducer, the mechanical losses, and the tuning.

Leveraging Monostable and Bistable Pre‐Curved Bilayer Actuators for High‐Performance Multitask Soft Robots

AbstractSoft actuators are typically designed to be inherently stress‐free and stable. Relaxing such a design constraint allows exploration of harnessing mechanical prestress and elastic instability to achieve potential high‐performance soft robots. Here, the strategy of prestrain relaxation is leveraged to design pre‐curved soft actuators in 2D and 3D with tunable monostability and bistability that can be implemented for multifunctional soft robotics. By bonding stress‐free active layer with embedded pneumatic channels to a uniaxially or biaxially pre‐stretched elastomeric strip or disk, pre‐curved 2D beam‐like bending actuators and 3D doming actuators are generated after prestrain release, respectively. Such pre‐curved soft actuators exhibit tunable monostable and bistable behavior under actuation by simply manipulating the prestrain and the biased bilayer thickness ratio. Their implications in multifunctional soft robotics are demonstrated in achieving high performance in manipulation and locomotion, including energy‐efficient soft gripper to holding objects through prestress, fast‐speed larva‐like jumping soft crawler with average locomotion speed of 0.65 body‐length s−1 (51.4 mm s−1), and fast swimming bistable jellyfish‐like soft robot with an average speed of 53.3 mm s−1.

Finite Element Solutions for Magnetic Field Problems in Terfenol-D Transducers

An appropriate magnetic design helps ensure that the Terfenol-D (Terbium- Dysprosium-Iron alloy) rods in giant magnetostrictive transducers have the perfect magnetostriction ability. To determine the optimum Terfenol-D rod state, a segmented stack configuration comprised by the Terfenol-D rods and NdFeB (neodymium-iron-boron) permanent magnets is presented. The bias magnetic field distributions simulated through the finite element method indicate that the segmented stack configuration is one effective way to produce the desired bias magnetic field. Particularly for long stacks, establishing a majority of domain to satisfy the desired bias magnetic field range is feasible. On the other hand, the eddy current losses of Terfenol-D rods are also the crucial to their magnetostriction ability. To reduce eddy current losses, the configuration with digital slots in the Terfenol-D rods is presented. The induced eddy currents and the losses are estimated. The simulations reveal that the digital slots configuration decreases the eddy current losses by 78.5% compared to the same size Terfenol-D rod with only a hole. A Terfenol-D transducer prototype has been manufactured using a Terfenol-D rod with a mechanical prestress of about 10 MPa and a bias magnetic field of about 42 kA/m. Its maximum transmitting current response of 185.4 dB at 3.75 kHz indicates its practicability for application as an underwater projector.

Stability analysis of charge-controlled soft dielectric plates

We examine the stability of a soft dielectric plate deformed by the coupled effects of a mechanical pre-stress applied on its lateral faces and an electric field applied through its thickness under charge control. The electric field is created by spraying charges on the major faces of the plate: although in practice this mode of actuation is harder to achieve than a voltage-driven deformation, here we find that it turns out to be much more stable in theory and in simulations.First we show that the electromechanical instability based on the Hessian criterion associated with the free energy of the system does not occur at all for charge-driven dielectrics for which the electric displacement is linear in the electric field. Then we show that the geometric instability associated with the formation of small-amplitude wrinkles on the faces of the plate that arises under voltage control does not occur either under charge control. This is in complete contrast to voltage-control actuation, where Hessian and wrinkling instabilities can occur once certain critical voltages are reached.For the mechanical pre-stresses, two modes that can be implemented in practice are used: equi-biaxial and uni-axial. We confirm the analytical and numerical stability results of homogeneous deformation modes with Finite Element simulations of real actuations, where inhomogeneous fields may develop. We find complete agreement in the equi-biaxial case, and very close agreement in the uni-axial case, when the pre-stress is due to a dead-load weight. In the latter case, the simulations show that small inhomogeneous effects develop near the clamps, and eventually a compressive lateral stress emerges, leading to a breakdown of the numerics.

Electromechanical instabilities in periodic dielectric elastomer composites

Several types of electromechanical instability emerge in periodic dielectric elastomer (DE) composites – either limiting the range of electromechanical actuation or providing an opportunity to harness instabilities for enhanced functionality. The electromechanical pull-in instability – extensively studied in monolithic DEs – is also encountered in periodic DE composites, and additional instability types – microscopic, macroscopic, and interfacial instabilities – arise in DE composites. For general DE composites subjected to arbitrary electromechanical loading, numerical simulation is required to detect all types of instability. In this paper, using a finite-element-based numerical simulation capability, we consider electromechanical instabilities in DE composites consisting of periodic lattices of aligned circular-cross-section fibers embedded in a matrix. We report on several types of electromechanical instabilities that are known to occur in this type of DE composite: (1) pull-in, (2) microscopic, (3) macroscopic, and (4) interfacial instabilities, while surveying a comprehensive parameter space. Namely, the roles of shear modulus contrast, electric permittivity contrast, fiber volume fraction, chain-locking behavior, mechanical prestress, prestretch, and confinement are considered. The result is an overarching view of the instability landscape in periodic, fiber-based DE composites. We conclude by showing that microscopic instabilities may be harnessed to invoke electric-field-triggered pattern transformations in certain cases.

Mechanics of 1–3 piezocomposites subjected to creep–fatigue loads

1–3 piezocomposites are extensively used for sensor, actuator, naval and aerospace applications owing to their enhanced electromechanical coupling, wider bandwidth and reliability. However, their performance gets adversely affected when they are subjected to combined static and dynamic loads that initiate both creep and fatigue damage, simultaneously. 1–3 piezocomposites undergo damage due to creep–fatigue interactions because of the passive and viscoelastic polymer matrix and inherent domain switching mechanisms. In this work, experiments are conducted to obtain the electromechanical response of 1–3 piezocomposites subjected to electrical fatigue and mechanical compressive creep loads. It is observed that, the electromechanical response degrades with increase in number of electrical fatigue cycles, matrix volume fraction and mechanical compressive pre-stress. A nonlinear phenomenological combined creep–fatigue damage model is developed within a thermodynamic framework to account for the creep–fatigue interactions in 1–3 piezocomposites. The model predictions are found to be in agreement with the experimental observations.

Thermal and prestress effects on nonlinear magnetoelectric effect in unsymmetric composites

Prospects shown by Magneto-Electric (ME) composites for use in sensing and energy harvesting applications have encouraged experimental and theoretical characterization. This work examines the effect of the bending-extension coupling in unsymmetric composites on the ME effect. A theoretical framework has been established to determine the frequency dependent response of the structure under an applied magnetic field, giving due consideration to the nonlinearity of the ferromagnetic phase. A finite element formulation has been used to predict the resulting deformation and hence the developed electrical response. The model also analyzes the influence of mechanical pre-stress and thermal environments on the nonlinearity and hence the ME coefficient of the composite. Experiments are performed in order to validate the model and the results are in agreement. Further, the ME coefficient is optimized with respect to parameters like frequency, temperature, prestress, etc. The results indicate enhanced ME responses under tensile prestress and low operating temperatures.

Experimental characterization and viscoelastic modeling for thermo-mechanical creep of 1-3 piezocomposites

1-3 piezocomposites are often subjected to mechanical pre-stress under elevated temperatures in naval and offshore applications, actuators, and so on, where the presence of the ductile epoxy matrix causes considerable creep in the system. This deteriorates the system efficiency and its overall performance. To address this issue, experiments are performed to capture the electromechanical response for thermo-mechanical creep of 1-3 piezocomposites for different fiber volume fractions under various thermal loads, wherein mechanical depolarization is observed. The piezo-coupling coefficient is observed to degrade. Higher thermal loads resulted in an increase in creep strain coupled with a decrease in creep polarization. A Kelvin–Voigt fractal derivative viscoelastic model is used to capture the creep strain. The creep strain is decomposed into ferroelastic and anelastic parts. The evolution of creep polarization is obtained using the ferroelastic component of the creep strain. The model predictions are found to be in agreement with the experimental observations.

Shape memory alloy–actuated prestressed composites with application to morphing automotive fender skirts

This work presents smart laminated composites that enable morphing vehicle structures. Morphing panels can be effective for drag reduction, for example, adaptive fender skirts. Mechanical prestress provides tailored curvature in composites without the drawbacks of thermally induced residual stress. When driven by smart materials such as shape memory alloys, mechanically-prestressed composites can serve as building blocks for morphing structures. An analytical energy-based model is presented to calculate the curved shape of a composite as a function of force applied by an embedded actuator. Shape transition is modeled by providing the actuation force as an input to a one-dimensional thermomechanical constitutive model of a shape memory alloy wire. A design procedure, based on the analytical model, is presented for morphing fender skirts comprising radially configured smart composite elements. A half-scale fender skirt for a compact passenger car is designed, fabricated, and tested. The demonstrator has a domed unactuated shape and morphs to a flat shape when actuated using shape memory alloys. Rapid actuation is demonstrated by coupling shape memory alloys with integrated quick-release latches; the latches reduce actuation time by 95%. The demonstrator is 62% lighter than an equivalent dome-shaped steel fender skirt.

A hybrid phenomenological model for thermo-mechano-electrical creep of 1–3 piezocomposites

ABSTRACTPiezocomposites find extensive use in sensors and actuators, naval applications, etc. owing to their characteristic coupling between electro-mechanical domains. These materials being relatively soft and ductile show a viscoelastic response to applied thermo-mechano-electrical loads. This causes a drift in the system response and creep is observed, which affects system efficiency. During prolonged service periods, self-heating occurs. Often, piezocomposites are subjected to mechanical pre-stresses prior to electrical input. To understand the response of 1–3 piezocomposites under these intricate loading conditions, experiments are performed to study the electro-mechanical response of 1–3 piezocomposites under thermo-mechano-electrical creep loads. Mechanical depolarisation is observed during the creep process. Creep strain is observed to increase with matrix volume fraction, mechanical pre-stress, electric field and temperature. A decrement in creep polarisation is seen with the increase in matrix volume fraction, mechanical pre-stress and temperature. The system performance is observed to be strongly dependent on the fibre volume fraction. A hybrid model is proposed within a thermodynamic framework to predict the creep response of 1–3 piezocomposites under thermo-mechano-electrical loads. The model is simultaneously representative of the macroscopic phenomenological details and mesoscopic electro-mechanical insights. While the free energy potential and domain switching concepts are characteristic of macroscopic modelling, the representative volume element (RVE) and its description with regard to domain wall resistance can be related to the mesoscopic length scales. The model predictions are found to be in agreement with the experimental observations.

Electro-mechanical creep of [formula omitted] piezocomposites: Theoretical modeling and experimental approach

Experimental and theoretical characterization on creep behavior of 1−3 piezocomposites subjected to a constant electric field and mechanical pre-stress is carried out. For curved surfaces or systems undergoing large deformations, piezo-ceramics are unsuitable since these are brittle in nature. Hence, piezo-composites are used as replacements. Creep is an important aspect with these systems due to a softer viscoelastic matrix involved. Electrical and electro-mechanical creep are studied for these systems. Electrical creep is observed to be maximum at the coercive field. Also, it is inferred that the volume fraction of fiber in 1−3 piezocomposites affects the system performance. The electro-mechanical creep is observed to reduce exponentially with increasing compressive pre-stress. Mechanical depolarization effect is observed during compressive loading. A thermodynamically-consistent macro-mechanical model is proposed to capture the creep behavior of 1−3 piezo-composites. It is found that the model predictions are in agreement with the experimental observations.

Thermoelastic vibration and stability of temperature-dependent carbon nanotube-reinforced composite plates

The present article investigates the thermoelastic vibration and stability characteristics of carbon nanotube-reinforced composite (CNTRC) plates in thermal environment. The CNTRC plates are made up of four different types of uniaxially aligned reinforcements. The single-walled carbon nanotubes (SWCNTs) reinforcement is either uniformly distributed (UD) or functionally graded (FG) according to linear functions of the thickness direction. The material properties, of both matrix and CNTs, are temperature-dependent and the effective elastic coefficients are evaluated by using a micromechanical model. The governing equations (GEs) are derived in their weak-form by using Hamilton’s Principle in conjunction with the method of the power series expansion of the displacement components. The Ritz method, based on highly stable trigonometric trial functions, is used as solution technique. Convergence and stability of the proposed formulation have been thoroughly analyzed by assessing many higher-order plate models. Thermal and mechanical pre-stresses are taken into account. Moreover, the effect of significant parameters such as length-to-thickness ratio, volume fraction, aspect ratio, loading-type, CNTs distribution as well as boundary conditions is discussed.