Strain rate effects on the localization of the stress-induced martensitic transformation in pseudoelastic NiTi under uniaxial tension, compression and compression-shear

Localization of the stress-induced martensitic transformation by nucleation and propagation of martensite bands is well-known in pseudoelastic NiTi shape memory alloys subjected to tensile loading; compressive loading, however, is associated with a more homogeneous deformation. In this study, we investigate the effect of different strain rates in the range from 10(-4) s(-1) to 10(-3) s(-1) on the thermo-mechanical behavior of pseudoelastic NiTi bar specimens and on the deformation mode with respect to localized transformation under uniaxial tension, uniaxial simple compression, and a special type of combined compression-shear loading. Dynamic compressive loading is performed using a specially designed drop tower setup (epsilon similar to 10(2) s(-1)) and a split Hopkinson pressure bar (epsilon similar to 10(3) s(-1)) with a pulse shaper. Surface strain fields are documented systematically with a digital image correlation in situ technique during both quasi-static and dynamic experiments in order to characterize the formation and growth of martensite bands. The transformation stress and the slopes of the stress-strain curves in the transformation region increase with increasing strain rate for all load cases, which can be attributed to self-heating effects. Multiple bands are formed during the stress-induced martensitic transformation at higher strain rates under uniaxial tension. Under simple compression, the deformation proceeds homogeneously without distinct strain localization and the deformation mode is hardly affected by an increase of the strain rate. Under combined compression-shear loading, the superimposed shear-stresses due to the special specimen geometry result in localized transformation even under high strain rates. Our results provide new insights into the interaction of low/high strain rates, self-heating of the material and transformation stresses, and their effect on localization phenomena and thermo-mechanical behavior of pseudoelastic NiTi in a wide variety of load cases. (C) 2015 Elsevier B.V. All rights reserved.