The material constants of the new plasticity model proposed in the first part of the paper can be divided into two independent groups. The first group, c(i) and r(i) (i = 1, 2, ..., M), describes balanced loading and the second group, χ(i) (i = 1,2, . . ., M), characterizes unbalanced loading. We define balanced loading as the case when a virgin material initially isotropic will undergo no ratchetting and/or mean stress relaxation, and unbalanced loading as the loading under which a virgin material initially isotropic will produce strain ratchetting and/or mean stress relaxation. The independence of the two groups of material constants and the interpretation of the model with a limiting surface concept facilitated the determination of material constants. We describe in detail a computational procedure to determine the material constants in the models from simple uniaxial experiments. The theoretical predictions obtained by using the new plasticity model are compared with a number of multiple step ratchetting experiments under both uniaxial and biaxial tension-torsion loading. In multiple step experiments, the mean stress and stress amplitude are varied in a stepwise fashion during the test. Very close agreements are achieved between the experimental results and the model simulations including cases of nonproportional loading. Specifically, the new model predicted long-term ratchetting rate decay more accurately than the previous models.
Approximation of mean stress relaxation by numerical simulation using the Jiang model and extrapolation of results
