The Use of Plastic Strain Control in Thermomechanical Fatigue Testing

Abstract

Among the variables that govern the mechanical behaviour of materials, the stress σ and the plastic (inelastic) strain rate e pl play a distinguished role as so-called state variables in mechanical equations of state, cf. Hart [1]. The strain rate plays also an important role in cyclic deformation [2,3], in particular at high temperatures. As is well known from the literature, the effect of (plastic) strain rate on the cyclic stress-strain behaviour as well as on cyclic lifetime gains importance with increasing temperature. Furthermore, many alloys which are used for intermediate and high temperature applications exhibit dynamic strain ageing effects. In this case, as a consequence of the interaction of dislocations with point defects, which are considered to be responsible for the occurrence of dynamic strain ageing, the sign of the strain rate sensitivity changes with temperature. Whereas at low as well as at high temperatures an increase of the strain rate leads to an enhanced stress amplitude in isothermal fatigue tests at a given constant strain amplitude, in an intermediate temperature interval the opposite effect can be observed. Open image in new window