Effect Of Strain Rate On The Localization Behavior Of Pseudoelastic NiTi During Simple Compression and Compression-Shear Testing

Numerous experimental studies of the thermo-mechanical properties of NiTi shape memory alloys have been performed under quasi-static uniaxial tensile loading. It is well known that pseudoelastic NiTi exhibits a distinct mode of localization of deformation (formation of martensite bands) under uniaxial tension, whereas under compression, from a simple macroscopic point of view; the deformation appears to be homogeneous. We have recently demonstrated that under quasi-static compression-shear loading, the superimposed shear stresses can trigger the localization of deformation even under predominantly compressive loading. In this study, we consider the interaction of strain rate, heat effects and transformation stresses, and their effect on the thermo-mechanical behavior of NiTi under these stress states. We characterize the thermo-mechanical response of pseudoelastic NiTi to quasi-static (10^-4 s^-1) and dynamic (10² s^-1) loading conditions. Mechanical testing at low strain rates is performed in a conventional tensile/compressive testing machine; dynamic experiments are performed in a specially designed drop tower setup. An in situ optical technique (digital image correlation) is used to measure the surface strain fields and to characterize the formation and growth of individual martensite bands. We discuss how self-heating of the specimen near martensite bands affects localization events under compression and shear-compression, and we compare these new results to well-established thermo-mechanical models for the tensile load case.