Hypervelocity impact of small masses on large surfaces of piezoelectric ceramics

The hypervelocity impact of small masses on large surface piezoceramics was investigated to study the impact behavior of hypervelocity projectiles. From a linear elastic model obtained at lower velocities, solutions were found for the hypervelocity case which determine both the size and the momentum of impacting projectiles from the rising slope of the charge signal generated by the impact. The results lead to the development of a new generation of impact detectors for small masses at hypervelocities which consists only of a plate of piezoceramic material.     

Characterization of the mechanical nonlinear behavior of piezoelectric ceramics

A characterization of the nonlinear behavior with high signal excitation in piezoceramic resonators was carried out. The behavior of power devices working at resonance, in which high strains are involved, is explained. A theoretical model previously described is used to explain the motional impedance variation proportional to the square of the motional current. This impedance increase DeltaZ is independent of the frequency and explains: the nonlinear elasticity that produces the A-F effect, the nonlinear mechanical losses that increase greatly close to the resonance, and the hysteresis phenomenon produced with frequency sweeps. Different methods for measuring the mechanical nonlinear coefficients of piezoceramics with high signal excitation are presented. An experimental method is proposed to measure the mechanical loss tangent and the compliance variations as a function of the mean square strain in the piezoceramic. This consists in measuring the maximum admittance and the series resonance frequency for downward frequencies. At this jumping point, the phase angle remains zero whatever the amplitude of the excitation. Two main coefficients characterizing the material mechanical nonlinearity are deduced. Experimental measurements were carried out to compare the nonlinearity of different ceramic materials in longitudinal and transverse mode.     

Multi‐actuated functionally graded piezoelectric micro‐tools design: A multiphysics topology optimization approach

Micro‐tools offer significant promise in a wide range of applications such as cell manipulation, micro‐surgery, and micro/nanotechnology processes. Such special micro‐tools consist of multi‐flexible structures actuated by two or more piezoceramic devices that must generate output displacements and forces at different specified points of the domain and at different directions. The micro‐tool structure acts as a mechanical transformer by amplifying and changing the direction of the piezoceramics output displacements. The design of these micro‐tools involves minimization of the coupling among movements generated by various piezoceramics. To obtain enhanced micro‐tool performance, the concept of multifunctional and functionally graded materials is extended by tailoring elastic and piezoelectric properties of the piezoceramics while simultaneously optimizing the multi‐flexible structural configuration using multiphysics topology optimization. The design process considers the influence of piezoceramic property gradation and also its polarization sign. The method is implemented considering continuum material distribution with special interpolation of fictitious densities in the design domain. As examples, designs of a single piezoactuator, an XY nano‐positioner actuated by two graded piezoceramics, and a micro‐gripper actuated by three graded piezoceramics are considered. The results show that material gradation plays an important role to improve actuator performance, which may also lead to optimal displacements and coupling ratios with reduced amount of piezoelectric material. The present examples are limited to two‐dimensional models because many of the applications for such micro‐tools are planar devices. Copyright © 2008 John Wiley & Sons, Ltd.     

Characterization of piezoelectric polymer composites for MEMS devices

Composite piezoelectric ceramics are important materials for transducer applications in medical diagnostic devices and MEMS devices. In micrometer scale the material properties of piezopolymers or piezoceramics do not coincide with that of bulk materials. The present work is aimed at simulating the material properties of piezoceramics and piezo-polymer composite thin films in the micrometer scale and then to determine the piezo-composite material properties. Piezoceramics have very high electromechanical coupling coefficient (k). But they have very high acoustic impedance and they are very brittle especially when thin films are fabricated. Piezopolymer like PVDF has low acoustic impedance and can be fabricated into thin films but it has very low k value and high dielectric losses. The combination of piezoceramics and piezopolymers form the piezocomposites, which have suitable material properties for transducer applications. The composites can have different connectivities. For 2?C2 composite, we can select two layers or a stack of PZT and PVDF layers. It is intended to determine the material properties both analytically and by simulation using computer simulation ANSYS software which implements finite element method (FEM). Although the simulation process presents approximate results, it can be verified from the large available experimental data from the literature with the simulated data.     

Derivation of the localization and homogenization conditions for electro-mechanically coupled problems

The challenges in high-performance mechatronics and micromechatronics are the modeling, analysis and design of electro-mechanical systems. Piezoceramics are the prototype of materials which exhibit characteristic electro-mechanical coupling effects used in a wide range of industrial applications. In complex designs of piezoceramic devices concentrations of stresses and electric field occur, which may lead to nonlinear electro-mechanical responses, typical for the large signal range. The challenge in the field of ferroelectric ceramics is the modeling of the complicated interactions between electrical and mechanical quantities on the macroscale, which are caused by switching processes on the microscale. In this paper we derive the basic equations for a possible direct two-scale homogenization procedure of electro-mechanically coupled boundary value problems.     

Numerical aspects of non-local modeling of the damage evolution in elastic–plastic materials

The design of mechanical systems in modern industrial plants requires reliable and efficient methods to predict the behavior of structural materials. For complex loading conditions, the behavior of the structural materials is determined by damage evolution, strain rate and temperature. The subject of the article is the modeling of the damage evolution in elastic–plastic materials of structural components, which are utilized at various temperatures. To achieve this goal, a hybrid model of steel cracking is applied. The hybrid model uses a finite element simulation combined with an experimental test realized in the macroscale. By using the hybrid model, the modeling of the damage evolution affords possibilities of determining macroscopic effects of the steel micro-defects. An essence of solving the predicting behavior of structural materials with micro-defects consists in time integration procedures for constitutive equations. In the article a semi-implicit time integration procedure is presented. The semi-implicit time integration procedure is suitable for the inelastic materials (compressible or incompressible) with the combined kinematic–isotropic hardening behavior. Its numerical solutions are stable, namely without the oscillatory behavior. By spatial averaging over a representative volume (RV), the homogenization technique (HT) is used for the defining of non-local variables in the constitutive equations. Evolutionary algorithms (EAs) based on local selections are applied to perform the homogenization technique. Within the framework of the large strain theory, the non-local continuum satisfies the objectivity requirements. Limitations on applicability of the -integral approach to construct crack growth resistance curves are also presented.     

Identification of elastic, dielectric, and piezoelectric constants in piezoceramic disks

Three-dimensional modeling of piezoelectric devices requires a precise knowledge of piezoelectric material parameters. The commonly used piezoelectric materials belong to the 6mm symmetry class, which have ten independent constants. In this work, a methodology to obtain precise material constants over a wide frequency band through finite element analysis of a piezoceramic disk is presented. Given an experimental electrical impedance curve and a first estimate for the piezoelectric material properties, the objective is to find the material properties that minimize the difference between the electrical impedance calculated by the finite element method and that obtained experimentally by an electrical impedance analyzer. The methodology consists of four basic steps: experimental measurement, identification of vibration modes and their sensitivity to material constants, a preliminary identification algorithm, and final refinement of the material constants using an optimization algorithm. The application of the methodology is exemplified using a hard lead zirconate titanate piezoceramic. The same methodology is applied to a soft piezoceramic. The errors in the identification of each parameter are statistically estimated in both cases, and are less than 0.6% for elastic constants, and less than 6.3% for dielectric and piezoelectric constants.     

A domain evolution model for hysteresis in piezoceramic materials

In this paper, we use a circular distribution to quantify certain domain properties and model the hysteresis behavior of piezoceramic materials. The model is constructed by bridging the characteristics of microscopic domain distribution into the macroscopic (or bulk) behavior. Contributions, other than those associated with the polarization of domains, to bulk quantities are also counted. A domain orientation distribution function is first selected and the corresponding distribution function parameters are chosen as the internal state variables. For the two-dimensional model, a von Mises-Fisher circular distribution is used. Instead of micromechanical analysis of domain motions that would involve large computation efforts, the delineation of domain evolution is simplified by considering the evolution of the domain orientation distribution, which is determined by the dynamic variations of the internal state variables. We also develop a procedure to identify the material constants introduced in the constitutive equations. The models are used to quantitatively characterize various hysteresis loops observed in piezoceramic materials.     

Concept and model of a piezoelectric structural fiber for multifunctional composites

The use of piezoceramic materials for structural sensing and actuation is a fairly well developed practice that has found use in a wide variety of applications. However, just as advanced composites offer numerous benefits over traditional engineering materials for structural design, actuators that utilize the active properties of piezoelectric fibers can improve upon many of the limitations encountered when using monolithic piezoceramic devices. Several new piezoelectric fiber composites have been developed, however almost all studies have implemented these devices such that they are surface-bonded patches used for sensing or actuation. This paper will introduce a novel active piezoelectric structural fiber that can be laid up in a composite material to perform sensing and actuation, in addition to providing load bearing functionality. The sensing and actuation aspects of this multifunctional material will allow composites to be designed with numerous embedded functions including, structural health monitoring, power generation, vibration sensing and control, damping, and shape control through anisotropic actuation. A one-dimensional micromechanics model of the piezoelectric fiber will be developed to characterize the feasibility of constructing structural composite lamina with high piezoelectric coupling. The theoretical model will be validated through finite element (FE) modeling in ABAQUS. The results will show that the electromechanical coupling of a fiber-reinforced polymer composite incorporating the active structural fiber (ASF) could be more than 70% of the active constituent.     

Electromechanical characterization of a active structural fiber lamina for multifunctional composites

Piezoelectric fiber composites (PFCs) are a new group of materials recently developed in order to overcome the fragile nature of monolithic piezoceramics. To resolve the inadequacies of current PFCs, a novel active structural fiber (ASF) was developed that can be embedded in a composite material to perform sensing and actuation, in addition to providing load bearing functionality. The ASF combines the advantages of the high tensile modulus and strength of the traditional composite reinforcements as well as the sensing and actuation properties of piezoceramic materials. A micromechanics model and a finite element model have been developed to study the effective piezoelectric coupling coefficient of the ASF as well as the ASF lamina. In order to evaluate the performance of the ASF when embedded in a polymer matrix and validate the model’s accuracy, single fiber lamina have been fabricated and characterized through testing with an atomic force microscope. The results of the testing demonstrate the accuracy of the model and show that ASF composites could lead to load bearing composites with electromechanical coupling greater than most pure piezoelectric materials.     

Optimal design of laminated piezocomposite energy harvesting devices considering stress constraints

Energy harvesting devices are smart structures capable of converting the mechanical energy (generally, in the form of vibrations) that would be wasted otherwise in the environment into usable electrical energy. Laminated piezoelectric plate and shell structures have been largely used in the design of these devices because of their large generation areas. The design of energy harvesting devices is complex, and they can be efficiently designed by using topology optimization methods (TOM). In this work, the design of laminated piezocomposite energy harvesting devices has been studied using TOM. The energy harvesting performance is improved by maximizing the effective electric power generated by the piezoelectric material, measured at a coupled electric resistor, when subjected to a harmonic excitation. However, harmonic vibrations generate mechanical stress distribution that, depending on the frequency and the amplitude of vibration, may lead to piezoceramic failure. This study advocates using a global stress constraint, which accounts for different failure criteria for different types of materials (isotropic, piezoelectric, and orthotropic). Thus, the electric power is maximized by optimally distributing piezoelectric material, by choosing its polarization sign, and by properly choosing the fiber angles of composite materials to satisfy the global stress constraint. In the TOM formulation, the Piezoelectric Material with Penalization and Polarization material model is applied to distribute piezoelectric material and to choose its polarization sign, and the Discrete Material Optimization method is applied to optimize the composite fiber orientation. The finite element method is adopted to model the structure with a piezoelectric multilayered shell element. Numerical examples are presented to illustrate the proposed methodology. Copyright © 2015 John Wiley & Sons, Ltd.     

A nonlocal lattice particle model for J2 plasticity

In virtue of their intrinsic integro-differential formulation of underlying physical behavior of materials, discontinuous computational methods are more beneficial over continuum-mechanics-based approaches for materials failure modeling and simulation. However, application of most discontinuous methods is limited to elastic/brittle materials, which is partially due to their formulations are based on force and displacement rather than stress and strain measures as are the cases for continuous approaches. In this article, we formulate a nonlocal maximum distortion energy criterion in the framework of a lattice particle model for modeling of elastoplastic materials. Similar to the maximum distortion energy criterion in continuum mechanics, the basic idea is to decompose the energy of a discrete material point into dilatational and distortional components, and plastic yielding of bonds associated with this material point is assumed to occur only when the distortional component reaches a critical value. However, the formulated yield criterion is nonlocal since the energy of a material point depends on the deformation of all the bonds associated with this material point. Formulation of equivalent strain hardening rules for the nonlocal yield model was also developed. Compared to theoretical and numerical solutions of several benchmark problems, the proposed formulation can accurately predict both the stress-strain curves and the deformation fields under monotonic loading and cyclic loading with different strain hardening cases.     

Simulation of Contaminates Around the Solid Immersion Lens in a Near-Field Optical Recording System

Accurate predictions of contamination in next-generation optical storage drives are paramount when active gap control is employed. In near-field recording devices, the read/write interface can be on the order of 20-30 nm, which means that the gap could be quite susceptible to contamination. Predictive modeling approaches for studying the behavior of contaminates in nanoscale hydrodynamic interfaces are needed. Here, we present such a model. The interface consists of a flat disk surface translating under a solid immersion lens (SIL) of hemispherical geometry. We present the computational modeling simulation results for nano-scale contaminates around the near-field SIL. The simulation shows that the discrete contaminates actually circumnavigate the SIL/disk interface during operation. We identify and discuss the external influences on the discrete contaminate particle behavior     

Modeling short- and long-term time-dependent nonlinear behavior of polyethylene

A new methodology for developing macromechanical constitutive formulations for time-dependent materials is presented in this article. In particular, two phenomenological constitutive models for polymer materials are illustrated, describing time-dependent and nonlinear mechanical behavior. In this new approach, short-term creep test data are used for modeling both short-term and long-term responses. The differential form of a model is used to simulate typical nonlinear viscoelastic polymeric behavior using a combination of springs and dashpots. Unified plasticity theory is then used to develop the second model, which is a nonlinear viscoplastic one. Least squares fitting is applied for the determination of material parameters for both models, based on experimental results. Due to practical constraints, experimental data are usually available for short-term time-frames. In the presented proposed formulation, the material parameters determined from short-term testing are used to obtain material parameter relationships for predicting the long-term material response. This is done by extending short-term information for longer time frames. Finally, theoretical and experimental results of tensile tests on polyethylene subjected to various load levels and test times are compared and discussed. Very good agreement of the modeling results with experimental data shows that the developed formulation provides a flexible and reliable framework for predicting load responses of polymers.     

基于Preisach理论的形状记忆合金温度-位移迟滞仿真研究

摘 要：智能材料如形状记忆合金(Shape Memory Alloy,SMA)已经广泛应用于驱动器和传感器的设计,实现定位和主动控制目的。然而,受迟滞影响,SMA驱动器的工作精度大大降低,限制了其应用。多数智能材料中,选择Preisach理论成为迟滞建模工具,近年来,也涉及到SMA材料系统。本文,讨论运用Preisach模型描述SMA驱动器系统的迟滞行为,尤其针对驱动器系统的模型建立过程,修正经典Preisach模型的几何解释和数值实现方法。最后,引入Gobert给出的Preisach平面的辨识函数执行仿真计算,数值结果表明该模型能够很好地描述SMA驱动器的迟滞行为。     

Shell theory for vibrations of piezoceramics under a bias

A consistent derivation of the shell theory in invariant form for the dynamic fields superimposed on a static bias of piezoceramics is discussed. The fundamental equations of piezoelectric media under a static bias are expressed by the Euler-Lagrange equations of a unified variational principle. The variational principle is deduced from the principle of virtual work by augmenting it through Friedrich's transformation. A set of two-dimensional (2-D), approximate equations of thin elastic piezoceramics is systematically derived by means of the variational principle together with a linear representation of field variables in the thickness coordinate. The 2-D electroelastic equations accounting for the influence of mechanical biasing stress accommodate all the types of incremental motions of a polarized ceramic shell coated with very thin electrodes. Emphasis is placed on the special motions, geometry, and material of the piezoceramic shell. The uniqueness of the solutions to the linearized electroelastic equations of the piezoceramic shell is established by the sufficient boundary and initial conditions.     

Topology optimization design of flextensional actuators

Flextensional actuators can be defined as a piezoceramic (or a stack of piezoceramics) connected to a flexible mechanical structure that converts and amplifies the output displacement of the piezoceramic. Essentially, the actuator performance depends on the distribution of stiffness and flexibility in the coupling structure and, therefore, on the coupling structure topology. In this work, we propose a general method for designing flextensional actuators with large output displacement (or generative force) by applying the topology optimization method. The goal is to design a flexible structure coupled to the piezoceramic that maximizes the output displacement (or force) in some specified direction. Static and low frequency applications are considered. To illustrate the implementation of the method, 2-D topologies of flextensional actuators are presented because of the lower computational cost; however, the method can be extended to 3-D topologies. By designing other types of coupling structures connected to the piezoceramic, new designs of flextensional actuators that produce output displacements or forces in different directions can be obtained, as shown. This method can be extended for designing flextensional hydrophones and sonars.     

Future challenges in CMOS process modeling

Development and optimization of electronic devices in industrial and academic environments would hardly be conceivable without the numerical simulation of their processing and electronic behavior. In the past, model development efforts aimed especially at predicting junction depths. With the paradigm shift towards ultra-thin body silicon-on-insulator devices and FinFET architectures, the main emphasis changed to activation and lateral diffusion. Based on simulations of the electrical behavior of such advanced devices, the requirements on simulation of doping profiles will be explained. To achieve the high dopant activation needed to reduce contact and channel access resistances, active concentrations if possible above solid solubility are required. The concepts pursued involve annealing with low thermal budgets as well as defect engineering. A further paradigm shift concerns the semiconductor material used for future devices. While silicon, especially in a strained state, is still in the lead, research is also looking for alternative materials like germanium, germanium-rich silicon-germanium alloys, and III-V compounds. In order to be helpful, models for such processes and materials have to be provided as soon as possible even if the complexity of models for alternative materials lags behind contemporary models for silicon. The personal view of the authors is guided also by the International Technology Roadmap for Semiconductors for which one of us coordinates the modeling and simulation chapter.     

Efficiency of excitation of piezoceramic transducers at antiresonance frequency

The efficiency of piezoceramic transducers excited at both the resonance and antiresonance frequency was investigated. Losses in piezoceramics are phenomenologically considered to have three coupled mechanisms: dielectric, mechanical, and piezoelectric losses. Expressions for the resonance and antiresonance quality factors, which ultimately determine transducer efficiency, have been received on the basis of complex material constants for both stiffened and unstiffened vibration modes. Comparison of electric and mechanical fields, thermal and electrical losses of power supply, and their distribution in the transducer volume have been made. For a given constant mechanical displacement of the transducer top, the required electric voltage applied to the transducer at the antiresonance frequency is proportional to the resonance quality factor, but the changes in the intrinsic electric and mechanical field characteristics in the common case are not too essential. The requirements on the piezoceramic parameters, types of transducer vibration, and especially on the factor of piezoelectric losses in a range of physically valid values were established to provide maximal quality factors at the antiresonance frequency.