The influence of prior strain rate on stress relaxation in solder alloys

The prior deformation history-dependence of the stress relaxation behavior of 60Sn–40Pb, 96Sn–4Ag, and 96.2Sn–2.5Ag–0.8Cu–0.5Sb (Castin™) is investigated. In new experiments, stress relaxation was measured immediately after three different applied strain rates: 10−3, 10−4, and 10−5 s−1. These tests revealed the unexpected result that the different prior deformation histories imposed affect the observed relaxation behavior such that higher strain rates during prior deformation result in greater total stress relaxation as well as lower relaxed stress. The McDowell model with a rate-dependent limiting value for short-range kinematic hardening predicts the stress relaxation behavior. Though the model predicts flow stress well during strain rate jump test simulations, it tends to over-predict the material response during the transient response just after each jump in strain rate. In another exercise, the inclusion of a rate-dependent isotropic hardening relation in the model does not improve the transient response of the model during strain rate jump simulations and simulates other material behavior that is not observed in the experimental data.