Evolution kinetics in Shape Memory Alloys under arbitrary loading: Experiments and modeling

An important aspect of modeling Shape Memory Alloy (SMA) behavior is the description of evolution kinetics as a function of thermomechanical driving forces. Due to path dependency in SMA behavior, it is essential to have appropriate memory in the model. This becomes critical during incomplete transformations due to load fluctuations frequently encountered in real world applications. In this work, experiments are conducted on SMA wires to investigate the material behavior under fluctuating thermal and mechanical loads. The nature of memory under such transformation is discussed to motivate the modeling effort. Based on the concept of thermodynamic dissipation function, a suitable memory parameter is introduced to capture the transformation process under fluctuating loads. In the context of phase diagram, the dissipation potential can be mapped to the width of the transformation region in the phase diagram. Hence, inside a transformation zone, the distance of a point on the load path from the finish boundary is used as a measure of dissipation, and hence as a memory parameter. This is used as an additional criterion for evolution. Using the existing cosine based function for the phase fraction, this additional criterion is shown to result in the behavior consistent with the experimental observations. The proposed criterion is used to describe both stabilized and non-stabilized hysteretic behavior and hence is quite general. Numerical simulations are carried out to predict the behavior of SMA under arbitrary thermomechanical loads. The salient differences in the proposed approach vis a vis existing models are highlighted. Predictions from the proposed model are compared with experimentally observed results for typical NiTi based SMAs for arbitrary thermomechanical loading. The proposed models are shown to adequately capture the observed behavior.

► Both experiments and modeling of 1-D Shape Memory Alloy response under arbitrary thermomechanical loading are attempted.
► Shift in critical transformation temperatures and stresses and shakedown effects are discussed.
► Following the phase diagram based approach, evolution kinetics with additional distance based memory parameter is proposed.
► Non-stable response captured by an empirical relation involving a new material parameter and the memory parameter.
► Model predictions compare well with experiments for different types of arbitrary loading.