Transient yielding in strain rate change tests

Abstract

Strain rate change (SRC) tests, during a tensile test, are carried out frequently to determine values of activation area and proper interpretation of these results may provide considerable information on dislocation activities during deformation. However, transient load drop is generally observed during low to high SRC and this drop increases with increasing deformation. This effect may thus be exacerbated, at least in principle, by conducting SRC tests on pre-crept samples. In the present work the authors report these results and also discuss the nature of transients during low to high SRC with reference to overall deformation processes. The material used for this work was a modified 9Cr-1Mo steel containing niobium and vanadium. All SRC tests were carried out at room temperature on an Instron by instantaneously changing the cross-head speed by a factor of 10. Strain gauges were used to not only continuously monitor the overall strain but also to obtain the magnitude of strain accompanying the change in cross head speed.