FINITE ELEMENT SIMULATION OF LOCALIZED PHASE TRANSFORMATIONS IN PSEUDOELASTIC NITI SHAPE MEMORY ALLOYS SUBJECTED TO MULTI-AXIAL STRESS STATES

摘要

The formation and propagation of distinct transformation bands in pseudoelastic NiTi shape memory alloys can be observed under various loading conditions. As such localization phenomena affect the macroscopic mechanical response of NiTi specimens and components, there is a need for dedicated simulation approaches that consider nucleation and growth of transformation bands on several length scales. In the present study, we use the Finite Element Method to simulate localized deformation of NiTi on the macro and meso scales. A simple, but physically motivated, material model incorporating strain softening is used to study the mechanical behavior of a flat tensile specimen (uniaxial load case) and of a plate with a circular hole (uniaxial loading, but multi-axial stress-state near the edge of the hole). Our results agree well with recent experimental observations; we find that certain stress states favor the formation of fully developed transformation bands, while more complex strain states lead to a more homogeneous deformation. But even in these cases, the transformation occurs in well-defined regions near geometric imperfections, and strain states clearly differ from predictions by simulations that exclude strain softening.