The capability of using recoverable martensitic transformation to modify the residual stress-state of hybrid Shape Memory Alloy (SMA) composites is explored. It is shown that through careful selection of a thermomechanical loading path the composite can be “processed” such that the constituent phases have a beneficial residual stress-state. Specifically, for materials which have preferred loading conditions (i.e., compression versus tension) resulting in improved material properties, such processing places the considered phase into a preferred stress state. This processing is explored here by considering composites with an SMA phase whose constititutive behavior is described by a recent phenomenological model and an elasto-plastic second phase. To consider realistic microstructural effects, a 3D numerical representation of the composite is generated using microtomography. It is shown that through an actuation (isobaric) loading path, the martensitic transformation of the SMA phase generates irrecoverable strains in the elasto-plastic phase which, upon unloading, results in a favorable residual stress-state. To consider the applicability of this methodology for a variety of composites, the effect of thermal residual stresses due to thermal expansion mismatch is identified and matrix phases with different elastic moduli and plastic hardenings are considered. Specifically, it is shown that martensitic transformation is the driving force behind the generation of the new composite residual stress-state. Through computational simulation, it is shown that increased elastic moduli or plastic hardening coefficients of the elasto-plastic phase yield small increases in residual stresses.
Stress state dependency of unloading behavior in high strength steels