A Discussion on the Performance of Continuum Plasticity Models for Fatigue Lifetime Assessment Based on the Local Strain Appraoch

This paper presents a discussion on the performance of continuum plasticity models for fatigue lifetime assessment according to the local strain approach. Several cyclic plasticity phenomena such as the cyclic hardening/softening, ratchetting, cyclic mean stress relaxation and non-proportional cyclic hardening require, in general, specialized continuum plasticity models. Continuum plasticity models, available in commercial finite element codes (e.g. ANSYS^®), with linear, multilinear and nonlinear kinematic hardening are identified using the experimental information available for a pressure vessel steel — the P355NL 1 steel. The potentialities of these plasticity models to describe the material cyclic behaviour are discussed, limiting the discussion to proportional loading. The plasticity models are applied to evaluate the strain ranges and mean stresses of a nozzle-to-plate connection. Two analysis strategies are applied to extract the strain ranges, namely the Twice Yield (TY) and the Cycle-by-Cycle (CBC) methods. The mean stress is only evaluated using the CBC method since the TY method has been proposed only for evaluation of the strain ranges. It is demonstrated that the TY and CBC methods gives similar results for the linear and multilinear kinematic hardening plasticity models. The plasticity model can have an important effect on the evaluation of the mean stresses and thus on predicted strain-life results, if mean stress effects are taken into account in the local strain approach. Finally, the calculated strain ranges and mean stresses are used in the evaluation of the fatigue life of the nozzle-to-plate connection using a local strain approach, and predictions are compared with available experimental results. The effect of the mean stress is important for long lives and is very dependent on the continuum plasticity model and on the number of cycles modelled in the CBC extraction method. Although differences are observed in the estimation of the strain ranges, using the several plasticity models, relatively small differences in fatigue life estimations were resulted.