PATH DEPENDENCE OF SHAPE-MEMORY ALLOYS DURING CYCLIC LOADING

It has long been recognized that Ni-Ti shape memory alloys (SMAs) beha ve pseudoelastically above the austenite finish temperature (A(f)) wit h a nearly perfectly plastic character during the initial cycles of tr ansformation. Under conditions of cyclic loading with a maximum strain epsilon(max), the critical stress to initiate stress-induced martensi tic (SIM) transformation decreases, the strain-hardening rate increase s, residual strain accumulates and the hysteresis energy progressively decreases over many cycles of loading. Hence the hysteresis energy av ailable for dissipation gradually decreases during cycling. Recent wor k (Miyazaki et al., 1981, 1986; Contardo and Guenin, 1990; Filip and M azanec, 1994) has shown that dislocations are generated in the alloy d uring the phase transformation to accommodate formation of SIM, giving rise to the change of hysteresis behavior of the SMA. Upon loading th e SMA to a strain level higher than epsilon(max), the alloy behaves al most identical to the ''virgin material''. Likewise, it has been shown (Huo and Muller 1993) that the stress at which either the forward or reverse transformation occurs, even in the absence of significant matr ix dislocation effects, depends upon the strain level (transformation level) prior to the last unloading event. This behavior is attributed to the distribution and configuration of austenite-martensite interfac es which evolve during the transformation and requires additional inte rnal state variable(s) beyond the mass fraction of martensite for desc ription. These path dependent mechanisms are expected to invalidate cu rrent internal state variable models which assume that the free energy depends only on strain, temperature and martensite mass fraction. In this paper, some modelling features are discussed to address the effec ts of dislocation arrays generated in the parent phase as well as the distribution of transformation product/interfaces. Several uniaxial ex periments are reported on Ni-Ti to highlight the path dependence of th e cyclic deformation behavior and progressive decrease of dissipated e nergy. Implications for multiaxial loading behavior are also discussed .