Greg P. Carman

Influence of temperature on the electromechanical and fatigue behavior of piezoelectric ceramics

This paper presents research results on the electro–thermomechanical behavior of piezoelectric ceramics for use in actuator applications with an emphasis on ferroelectric fatigue. The material being investigated is a lead zirconate titanate piezoelectric ceramic with the composition PbZr0.53Ti0.47O3 (PZT-5H). Results presented in this paper include an augmented constitutive model that accounts for the temperature-dependent piezoelectric properties. Using this model, nonlinear effects measured at one temperature can be extrapolated to other temperatures with good accuracy. Experimental studies into 180° and 90° polarization switching of PZT-5H indicate that the dielectric flux to dipole the material appears to be an adequate criterion for predicting this nonlinear switching behavior. Fatigue studies show that material degradation is strongly influenced by temperature and by the magnitude of the applied electric field. Above a critical temperature, PZT-5H no longer fatigues in the presence of large electric...

Large magnetostrictive response of Terfenol-D resin composites (invited)

In this article we present results for magnetostrictive composites containing between 10% and 50% Terfenol-D (Tb0.3Dy0.7Fe2) in a nonmetallic binder. The composite consists of aligned Terfenol-D particles representative of a [1-3] fiber composite. For this study we measure the modulus of elasticity and other pertinent magnetostrictive properties. The measured stiffness values fall between the upper and lower bounds predicted by a rule of mixtures approximation. The magnetostriction for the composites rival that of the monolithic at comparable magnetic field levels. A mechanically unloaded 20% volume fraction composite produces 900 ppm and a 30% volume fraction composite preloaded with 8 MPa produces 1000 ppm. Contrary to the belief that the greater the magnetostrictive particulate volume, the greater magnetostrictive properties—this study shows that there is an optimal balance of epoxy and magnetostrictive material that will maximize magnetostriction for a mechanically unloaded specimen. For the matrix ch...

Nonlinear Constitutive Relations for Magnetostrictive Materials with Applications to 1-D Problems

In this paper, we present a nonlinear constitutive relation for magnetostrictive materials that includes nonlinear coupling effects arising between temperature/preload and magnetic field strengths. The relations are derived from thermodynamic principles using Gibbs free energy expanded in a Taylor series with only the pertinent constants included as determined from experimental evidence present in existing literature. By assuming that the magnetostrictive material is operated in a biased magnetic field and perturbing this field with a small value, relations between the nonlinear material constants and linear coefficients present in existing literature are derived. The accuracy of the nonlinear constitutive relation is evaluated by comparing experimental results obtained on a Terfenol-D rod operating under both magnetic field and stress biases with theoretical values. Results indicate that the model adequately predicts the nonlinear strain/field relations in specific regimes. The nonlinear constitutive relations are also integrated into a 1-dimensional nonlinear finite element model for studying structural components. The coupled magneto elasticity problem is derived using a weighted residual method along with a Newton-Raphson iteration technique. Results of the analytical model indicate that linear approaches are inappropriate for modeling the response of this material in a structure.

Manufacturing issues of thin film NiTi microwrapper

Manufacturing issues related to a thin film NiTi shape memory alloy (SMA) microactuator (i.e. microwrapper) have been investigated using both wet and dry etching techniques. Results show that wet etching the amorphous film produces a cleaner pattern than the crystallized film. Transformation temperatures are not affected by the pre-exposure of the NiTi film to air before crystallization. However, this process produces breakage in the NiTi film at sacrificial layer steps. This is believed to be due to residual stresses developed between the film and substrate during sputtering. The film breakage is overcome by dry etching the film with an ion-milling technique. Curvature in the microwrapper arms is induced using either a bi-layer material (i.e. polyimide and NiTi) or a functionally gradated NiTi film. Results show that when heated the microwrapper arms flatten due to shape memory effect and curl up to form a cage-like structure when cooled.

Mesoscale actuator device: micro interlocking mechanism to transfer macro load

Abstract A novel proof-of-concept prototype Mesoscale Actuator Device (MAD) containing microscale components has been developed. The MAD is similar to piezoelectrically driven inchworm motors with the exception that mechanically interlocking microridges replace the traditional frictional clamping mechanisms. The interlocked microridges, fabricated from single crystal silicon, are designed to increase the load carrying capability of the device substantially. Tests conducted on the current design demonstrate that interlocked microridges fabricated with 30% KOH solution support a 9.6 MPa shear stress or that a pair of 5×5 mm locked chips supports a 500 N load. For high frequency operation, an open loop control signal is implemented to synchronize the locking and unlocking of the microridges with the elongating and contracting of the actuator. The system was successfully operated from 0.2 Hz to 500 Hz (or speeds from 2 μm/s to 5 mm/s). The upper limit (500 Hz) is imposed by software and hardware limitations and not related to physical limitations of the prototype device.

Three-dimensional thin-film shape memory alloy microactuator with two-way effect

A novel thin film (micrometer thickness) shape memory alloy (SMA) micro actuator is presented in this paper. The thin film SMA with composition of approximately 50:50 nickel titanium (NiTi) is sputter-deposited onto a silicon wafer in an ultra high vacuum system. Transformation temperatures of the NiTi film are determined by measuring the residual stress as a function of temperature. The transformation temperature is independent of the presence of chromium (Cr) used as an adhesion layer, or being exposed to air before annealing. A mixture of hydrofluoric acid (HF), nitric acid (HNO/sub 3/) and deionized (DI) water is used to etch the film. Different etch masks are evaluated to protect the NiTi film during the etching. Among the masks tested, a thick photoresist (AZ-4620) produces the best result. The NiTi membrane is hot-shaped into a three-dimensional (3-D) dome shape using a stainless-steel jig. Results indicate the membrane exhibits two-way effect. The performance of the SMA micro actuator is characterized with a laser measurement system for deflection versus input power and frequency response.

Response of piezoelectric stack actuators under combined electro-mechanical loading

The response of six piezoelectric stack actuators under electrical, mechanical, and combined electro-mechanical loading was investigated. The focus was to understand the behavior of piezoelectric materials under combined electro-mechanical loading scenario, and to determine fundamental properties necessary to model the constitutive response and optimize the actuator performance. Parameters that were evaluated include strain output, permittivity, mechanical stiffness, energy density, and coupling coefficients as a function of mechanical preload and electric field values. Results indicate that for certain actuators stiffness values change by more than 100% depending on the operating conditions. The magnitude of the applied compressive load significantly influences strain output, electrical and mechanical energy density, and coupling coefficients. Initially, the strain output and energy density is enhanced with an increase in mechanical preload by as much as 60% for some actuators, with maximum values occurring at preloads between 25 and 40 MPa. A similar trend is observed under combined out-of-phase electro-mechanical loading. However, linear superposition of strain outputs obtained under constant loading tests, does not lead to accurate predictions over the broad range of combined electro-mechanical loading conditions. This indicates a need for more accurate constitutive relations to predict the highly non-linear processes related to combined electro-mechanical loading.

Mesoscale actuator device with micro interlocking mechanism

A novel proof-of-concept prototype Mesoscale Actuator Device (MAD) has been developed. The MAD is similar to piezoelectric driven inchworm motors with the exception that mechanically interlocking microridges replace the traditional frictional clamping mechanisms. The interlocked microridges, fabricated from single crystal silicon, increase the load carrying capability of the device substantially. Tests conducted on the current design demonstrate that the interlocked microridges support 16 MPa in shear or that a 3/spl times/5 mm locked chip supports a 25 kgf load. To operate the MAD device at high frequencies an open loop control signal is implemented. Synchronizing the locking and unlocking of the microridges with the elongating and contracting actuator requires minor perturbations in the voltage signal supplied. The system was successfully operated from 0.2 Hz to 500 Hz (or speeds from 2 /spl mu/m/s to 5 mm/s). The upper limit (500 Hz) is imposed by software limitations and not related to physical limitations of the current device.

Stresses in piezoceramics undergoing polarization switchings

Piezoelectric materials exhibit nonlinear behavior when subjected to large electrical or mechanical loads. This strong nonlinear material behavior is induced by localized polarization switching (i.e., change of the polarization direction) at the subgrain level. In this article, a nonlinear finite element code is described to model polarization switchings in piezoceramics subjected to large electromechanical loads. The local polarization switching criterion is based on electric displacement combined with stability arguments. To evaluate the model, three cases are studied in this article: a partially electroded specimen simulating 180° polarization switching, a fully electroded specimen simulating 90° switching and a plate containing a void. The analytical results are compared with experimental data obtained from Moire interferometer with reasonable agreement. Stresses in the vicinity of the voids are computed and the results are used to explain fatigue behavior observed in piezoceramics. It has been determ...

Effect of loading parameters on the fatigue behavior of impact damaged composite laminates

The long-term mechanical fatigue of AS4/3501-6 graphite/epoxy quasi-isotropic laminates has been investigated to determine the influence of loading parameters on impact-induced delamination growth during constant amplitude, block, and spectrum fatigue loading. From the observed fatigue damage growth it is suggested that conservative damage tolerance criteria in the presence of barely visible impact damage should be based on static loading tests and a limited number of fatigue tests to determine the load levels that initiate delamination growth. For the type of impact damage and laminate configuration investigated in this study the fatigue limit is found to be 60% CSAI (compression strength after impact) in constant-amplitude compression-compression loading, and 65% CSAI under the spectrum loading conditions. Furthermore, the results of spectrum loading tests suggest that the durability testing can be accelerated by omitting from the standardized test sequence load levels in which the stress range is less than 20% CSAI.