Experimental and numerical investigations on homogeneous vs. localized deformation modes during shear-compression loading of pseudoelastic NiTi

The shape memory community’s conventional wisdom holds that localized deformation (transformation via the formation and growth of martensite bands) only occurs under tension, whereas compressive loading is supposed to result in homogeneous deformation. In this contribution, we present experimental results on shear-compression testing using a special type of specimen that allows to combine shear stresses with predominantly compressive loading. We analyze strain fields measured by digital image correlation in tension, compression, and shear-compression experiments on binary NiTi at quasi-static as well as at dynamic stain rates. We demonstrate that the localized nucleation of martensite bands is not limited to tensile loading, but that it can also occur when shear stresses occur in combination with compression – a novel finding with implications for the design of shape memory devices, functional and structural fatigue and damage modeling. Tensile loading, nevertheless, is required for martensite bands to grow by propagation of austenite-martensite interfaces. This asymmetric mechanical behavior, which particularly differs in terms of critical stresses for the onset and finish of the stress-induced transformation in tension and compression, is simulated in a Finite Element model utilizing a Drucker-Prager based approach that allows for strain softening in tension and for strain hardening in compression. By comparing our numerical results with the new experimental observations, we explain how different loading conditions favor distinct types of martensite band formation and growth.