Investigation of High Temperature Shape Memory Alloys thermo-mechanical behavior under complex loading paths

This work focuses on the understanding of complex thermo-mechanical loadings involving simultaneous stress and temperature cycling in high temperature shape memory alloys (HTSMA). Two new characterization loading paths are investigated: out-of-phase (OP) involving a simultaneous increase in stress and decrease in temperature and in-phase (IP) involving a simultaneous increase in stress and temperature. This work aims to propose methods of characterization for HTSMA that allow to capture complexities sometimes missed by the isothermal and isobaric characterization loads.

First, an experimental investigation of the mechanical responses associated to different loading paths is performed. The actuation and residual strains, transformation temperatures and work output are compared between complex paths and the traditional isobaric and isothermal paths. Full-field investigation using Digital Image Correlation (DIC) of IP and OP paths is performed to identify mechanisms of deformation and propagation of martensitic transformation.

A crystal plasticity model is then used to interpret experimental observations for IP and OP paths. A self-consistent mean field approach is implemented for smooth behavior prediction. The model is calibrated using isobaric responses to evaluate if complex responses can be accurately described. Good prediction capability was achieved but some limitations were observed concerning the hysteresis width and total strain at low stress levels for complex paths. This aims to open discussions on how to predict responses of HTSMA to complex loading paths.