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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.

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This paper presents results on the electro-thermo-mechanical behavior of piezoelectric materials for use in actuator applications with an emphasis on durability performance. The objective of this study was to compare the performance of different commercially available actuator systems and to determine the properties necessary for the design of such actuator systems. Basic piezoelectric properties of five stack actuators were determined as a function of mechanical preload and temperature. Changes in these properties during ferroelectric fatigue up to 107cycles were determined from strain-field relations after a specified number of fatigue cycles. Experimental results indicate a strong dependence of piezoelectric properties and power requirements on mechanical loading conditions. Results indicate that the optimum operating conditions (i.e., mechanical preload) that will improve actuation capabilities of piezoelectric stack actuators can be determined. That is, strain output was found to increase by 60% for some actuators upon the application of certain compressive prestress. Results of fatigue tests indicate negligible degradation in strain output for some stack actuators even when operated under mechanical preload that causes large displacements through domain wall motion. Similar trends in strain output and current degradation curves (as a function of fatigue cycles) suggest that material degradation can be indirectly inferred from simply measuring the current being dissipated by the material and the fatigue predicted by measuring the strain output (quantity related to domain motion). Finally, temperature rise of lead zirconate titanate stacks due to self-heating should be taken into account when designing actuator systems, since temperature changes were found to significantly influence both strain output and power required to drive piezoelectric stack actuators. Physical mechanisms of ferroelectric fatigue are explored.

The results of investigations of {100}, the nongrowing faces of sodium chlorate crystals in supersaturated solutions, are presented. It occurred that some of the faces did not grow, whereas the neighbor faces of the same crystals grew. The dissolution and refaceting of the crystals did not affect the probability of the nongrowing face appearance and the reduction of the range of face growth rates. Nongrowing faces are very stable; they start to grow at a relatively high supersaturation, face by face or several faces simultaneously. The order of the start of the growth is random, subsequently grown neighbor or opposite faces. Possible reasons for coexistence of growing and nongrowing faces, Ostwald ripening, microscopic conditions in the crystallization cell, microstructure of crystal face, and the impurity effects are discussed.