On the thermomechanical coupling of shape memory alloys and shape memory alloys composites

Smart materials have received much attention in recent years, especially due to their various applications in smart structures, medical devices, actuators, space and aeronautics. Among these
materials, shape memory alloys exhibit extremely large, inelastic, recoverable strains (of the order of 10%), resulting from transformation between austenitic and martensitic phases. This
transformation may be induced by a change, either in the applied stress, the temperature, or both.

From a macroscopic point of view, one may separate the observable behavior of shape memory alloys into two major phenomena. The first one is known as pseudo-elasticity, in which
nonlinear elastic behavior is observed. Here, very large strains upon loading occur, but full recovery is achieved in a hysteresis loop upon unloading. When shape memory alloys experiences the
shape memory effect, it exhibits a large residual strain after loading and unloading. This strain may be fully recovered simply by raising the temperature of the body.

In addition, these materials exhibit full coupling between their mechanical and thermal response. In other words, the temperature of the alloy changes upon applied force, and the mechanical response changes upon temperature deviation. In this study, this phenomenon is considered using a micromechanical model. This model, referred to as "high-fidelity generalized method of cells" (HFGMC), is capable to predict the behavior of multiphase inelastic composites with periodic microstructure by employing the homogenization technique. In this research, it is generalized to incorporate the thermomechanical coupling in shape memory alloys.

To this end, this model is applied to predict the response of composites that consist of shape memory alloy fibers embedded in metallic and polymeric matrices. In particular, the induced
average temperatures that result from the thermomechanical coupling are computed and presented under various circumstances. Results are given for shape memory alloy continuous fibers
composites with a metallic (aluminum) matrix and polymeric (epoxy) matrix. It is shown that the thermomechanical coupling has a negligible effect on the average stress-strain response of the
composite, but a significant effect on the induced temperature that is generated due to the application of mechanical loadings. This effect is mainly due to the inelastic strain rate in metallic
materials, and to a less extent, to the transformation strain rate in the shape memory alloys fibers. These results are given at two reference temperatures at which shape memory and pseudoelasticity effects in the shape memory alloy fibers take place.

It may be emphasized that the thermomechanical coupling is so significant in case of SMA/aluminum composite, that the composite's temperature varies by approximately 40o upon only five loading/unloading cycles. Consequently, a structure that was originally designed to incorporate with the shape memory effect, may exhibit a pseudoelasticity behavior instead.

A detailed research paper may be found in the following reference: 

Aboudi J. and Freed Y., Two-way themomechanically coupled micromechanical analysis of shape memory alloy composites, Journal of Mechanics of Materials and Structures, 1:937-955 (2006).