Modeling of Cyclic Ratchetting Plasticity, Part II: Comparison of Model Simulations With Experiments

Abstract

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.