Study of a modified non-unified model for time-dependent behavior of metal materials

Based on the concept of the non-unified model (Kobayashi et al., 2003), a constitutive model was proposed to simulate the viscoplasticity and creep/relaxation behavior of metal materials. An additional steady part was introduced into the traditional unified viscoplasticity model to act as a supplement for the creep strain. The model was analyzed in both steady and viscoplasticity parts, which made it analogous to the non-unified viscoplasticity model. The total strain consisted of elastic, viscoplastic, and steady strains. The viscoplastic strain was calculated based on the viscosity function, and the steady part was a function of the equivalent deviatoric stress. Experiments involving a nickel-based super alloy (Zhan, 2004) and 40Sn-60Pb solder (Neu et al., 2000) were conducted to verify the utility of the proposed model. The model well described not only the creep/relaxation behavior for different stress/strain levels, but also the cyclic plasticity for materials at both room and high temperatures. The steady part may improve the results, even when the viscoplasticity part already has a static recovery term. Contrary to Akamatsu et al.(2008), all of the parameters for the viscoplasticity part were determined from experiments with different loading rates instead of the creep/relaxation results in the transient stage. The parameters for the steady part were obtained from creep experiments.