On propagating instabilities in nickel-titanium and steel alloys.

Abstract

A study of material systems that exhibit material level instabilities is presented. In recent years, there has been growing interest in using NiTi Shape Memory Alloys (SMAs) for novel applications. Unfortunately, the complex material behavior has made development of robust constitutive models difficult, prompting the need for more experimental data. A new experimental setup for uniaxial testing SMAs is developed that overcomes some long-standing difficulties including extreme rate and environmental sensitivities, propagating transformation fronts, and associated gripping artifacts. A special temperature control apparatus is used to reduce the effects of gripping and to better control the ambient environment, resulting in accurate measurement of nucleation and propagation stresses without machining the specimen. Additionally, the setup allows full-field optical and infrared imaging to monitor transformation fronts kinetics. Cyclic experiments are performed using this setup where the front kinetics are measured during cyclic softening for the first time. The surprisingly large change in mechanical response for only an 8°C change in temperature results from a change in the front kinetics, and the stabilized mechanical response is shown to still occur through inhomogeneous deformation. Additionally, nucleations are seen to occur at the locations of previous nucleation or front coalescence, suggesting additional damage at these locations, which could lead eventually to fatigue failure. A parameter study of the ambient media and rate sensitivities for monotonic loading is presented using a finite element analysis (FEA) with a special non-isothermal plasticity model. The parameters of interest are shown to be the non-dimensional thermal conductivity and the non-dimensional heat transfer coefficient, which correlate to the number of transformation fronts and to the average temperature rise, respectively. Non-dimensional design curves are developed to categorize the resulting behaviors as isothermal, transitional, and adiabatic. Unlike NiTi, steel's material instability does not exhibit a thermomechanical interaction, yet a similar modeling method can be used to investigate the interaction of a local material instability with a global instability (column buckling). The FEA shows that the symmetry or asymmetry of imperfections plays an important role in the evolution of deformation, and helps to explain the scatter seen in the experimental data.