Equation of state and mechanical response of NiTi during one-dimensional shock loading

Abstract

The equation of state and the mechanical response (dynamic tensile strength and dynamic shear strength) of the shape memory alloy NiTi have been investigated using plate impact. The Hugoniot has been extended with additional data and a nonlinear behavior of the Hugoniot (shock velocity-particle velocity) has been noted. A bilinear representation has been proposed for the trend. These two behaviors were attributed to the shock-induced phase transformation from B2 to monoclinic. However, this phase transformation seems to have no influence on the dynamic tensile strength. A minimum impact stress value was found necessary to create the spallation in NiTi but the pull back stress remains near constant above this value. A negative strain-rate dependency was also noted on the spallation. The shear strength (τ) of NiTi appears to increase continuously with the impact stress. The evolution of τ behind the shock front seems to be linked to the phase transformation observed in determination of the Hugoniot. Indeed, It was observed that there was no change of τ until an impact stress value corresponding to the inflection point on the Hugoniot, followed by increase behind the shock front above this value.