Shape Memory Alloy Material Behavior Research Articles

A finite element framework for a shape memory alloy actuated finger

This article presents on finite element modeling of an artificial finger driven by shape memory alloy wires. These alloys appear as a promising transduction technology, due to their inherently high energy density which makes them a good choice for compact, lightweight, and silent actuator systems with many applications in the robotic field, ranging from industrial to biomedical ones. However, the complex nonlinear and hysteretic behavior of the material makes it difficult to accurately model and design shape memory alloy–actuated systems. The problem is even more challenging when shape memory alloys are used as actuators in articulated structures, adding complex kinematics and contact situations to the picture. In this article, a finite element model is developed to describe the behavior of a finger prototype, in which a bundle of shape memory alloy wires works against an extension spring. The commercially available software COMSOL is used for implementing the coupling and contact issues between the finger structure and the shape memory alloy wires. To describe the shape memory alloy material behavior, a COMSOL implementation of the Müller–Achenbach–Seelecke model is presented. By means of different experiments, it is demonstrated how the model predicts the prototype behavior in relation to different power stimuli and actuator geometries.

Thermal Vibration Characteristics Analysis of a Composite Thin-Cylindrical Shell Embedded with Shape Memory Alloy Wires

Thermal vibration characteristics of the composite thin-cylindrical shell embedded with shape memory alloy (SMA) fibers are investigated. The shape memory alloy material behavior is programmed in the form of an ABAQUS user subroutine code (UMAT), and this program is successfully verified by using the response of pseudoelasticity and the shape memory effect (SME) at various temperatures and stress levels. Based on the ABAQUS UMAT code, the natural frequency of the composite thin-cylindrical shell embedded with SMA wires under thermal load and mechanical load is calculated respectively. Then the effects of the SMA wire temperature and the content on the thermal vibration characteristics of the structure are discussed. Numerical simulation results show that the temperature and the volume content of the SMA wires increase the stiffness and buckling critical load of the structure.

A homogenized free energy model for hysteresis in thin-film shape memory alloys

Thin-film shape memory alloys (SMAs) have become excellent candidates for microactuator fabrication in microelectromechanical systems due to their capability to achieve very high work densities, produce large deformations, and generate high stresses. In general, the material behavior of SMAs is nonlinear and hysteretic. To achieve the full potential of SMA actuators, it is necessary to develop models that characterize the nonlinearities and hysteresis inherent to the constituent materials. We develop a model that quantifies the nonlinearities and hysteresis inherent to SMAs. The fully thermomechanical model is based on free energy principles combined with stochastic homogenization techniques. It predicts rate-dependent, polycrystalline SMA behavior, and it accommodates heat transfer issues pertinent to thin-film SMAs. The main advantages of this model are that it admits a simple, low-order formulation suitable for implementation and subsequent control design, and that most of the model parameters are identifiable directly from standard measurements. We illustrate aspects of the model through comparison with thin-film SMA superelastic and shape memory effect hysteresis data.

Finite element analysis of the behavior of shape memory alloys and their applications

A nonlinear finite element procedure is developed which incorporates a thermodynamically derived constitutive law for shape memory alloy material behavior. The constitutive equations include the necessary internal variables to account for the material transformations and are utilized in a one-dimensional finite element procedure that captures the unique shape memory alloy responses of pseudoelasticity and of the shape memory effect at all temperatures, stress levels and loading conditions. Detailed material properties for the alloy used are necessary for the analysis. The solution of the geometrically and physically nonlinear problem is achieved by application of a Newton's method in which a sequence of linear problems is numerically solved. Due to consistent linearization, a quadratic rate of convergence is obtained.Several test cases are presented to illustrate the potential of the finite element procedure. Cases simulating the stress-strain behavior of a bar of shape memory alloy under simple uniaxial loading as well as restrained recovery responses at different temperatures compare well with experimental and analytical results. Two further generalized applications are examined : the use of a shape memory alloy ring as a pipe connector and eigenfrequency tuning of a composite beam with embedded shape memory wires. The results of these analyses correlate well with analytical results and the methodology for use of the finite element procedure in general cases is demonstrated. The finite element procedure is thus shown to be a powerful tool for studying various applications of shape memory alloys.