The Uniaxial Unloading Behavior of Two Engineering Alloys at Room Temperature

Abstract

A computer-controlled servohydraulic testing machine was used to study the unloading behavior from the fully plastic region of AISI Type 304 Stainless Steel and of the Ti-7Al-2Cb-1Ta Titanium alloy at room temperature. Load and displacement control were used and in some tests the magnitude of the loading rate was changed by two orders of magnitude at the point of unloading. Unloading behavior from the fully plastic region depends strongly on the control mode (the boundary condition) and is not linearly elastic, but curved. In displacement control, positive slopes larger than the elastic one are obtained leaving the unloading point. Small negative slopes and creeping out may be observed in load control. At some region below the maximum stress and above zero stress the unloading curve attains an elastic slope. At zero stress the slope is less than the elastic one. The observed behavior is influenced by the presence of significant rate dependence of the deformation behavior (viscosity effects).