Q. Sun, Y. He, Hong Kong University of Science and Technology, Hong Kong, China
In this paper we present our recent modelling and experiment on the tensile stress-induced phase transition process in NiTi shape memory alloy strips, focusing on the role of loading time scale and the thermo-mechanical two-field coupling in the localized deformation domain patterns. We first report the effects of loading time or stretching rate on the domain patterns observed in NiTi strips in the strain rate range of 10-4/s ~ 10-1/s. It was found that the nominal stress-strain curve of the specimen changed from near-isothermal plateau-type with distinct stress-drops in the low strain rate region to the near-adiabatic smooth hardening-type in the high strain rate region. The corresponding deformation mode changed from the localized-propagation mode with a few parallelepiped martensite domain patterns to the near-homogeneous multiple-nucleation mode with fine alternating austenite-martensite stripes. The number of the macroscopic domains (domain spacing) of the specimen increased (decreased) with the applied stretching rate in a power-law form, i.e., the higher the loading rate, the finer the deformation pattern. The analysis showed that such observed power-law dependency is originated from the coupling between the material’s nonlinear property (formation and growth of domains) and the transfer of latent heat. The important roles of heat transfer dynamics and loading time in imposing the observed emerging length scale in the deformation pattern of the material is demonstrated.
Summary: We present our recent modelling and experiment on the role of loading time scale in the deformation domain patterns in tensile loading of superelastic NiTi SMA strip. The strong effects of loading time or strain rate on the domain pattern spacing will be reported and analyzed. It is shown that the coupling between the material’s mechanical phase transition process and transfer of heat is the key underlying mechanism for the observed rate-dependent domain spacing. The important roles of heat transfer time and loading time in governing the observed emerging length scale of the domain structure of the deformation pattern is demonstrated.
