History Effect in Fatigue under Variable Amplitude Loading Conditions in a 316L stainless Steel

摘要

The context of this study is the prediction of fatigue crack growth in a rotor of a nuclear power plant. In these components, cracks are subjected to varying loads depending on the needs of delivering power. According to the operation phase, the cracks are subjected to mode I or mixed-mode dominated fatigue cycles. The studied material is a 316L stainless steel. This material displays a very significant cyclic hardening effect, which is expected to contribute to history effects in fatigue crack growth. As a matter of fact, cyclic plastic deformation within the crack tip region introduces internal stresses that modify the subsequent behaviour of the crack and are at the origin of spectacular history effects in fatigue. Consequently a model was developed for mode I fatigue crack growth under variable amplitude loading conditions that accounts for the kinematics hardening of the material. This model needs to be enriched so as to account for the isotropic hardening displayed by the 316L stainless steel. The model is set up using FE computations, but once identified it allows avoiding systematic and expensive FE computations so as to be easily used in an engineering context. First of all, the model is based on a multiscale approach. The idea is to partition the velocity field in the crack tip region into elastic,and plastic parts. Each part is the product of a reference spatial field and of an intensity factor which evolution allows monitoring the behaviour of the crack tip region at the global scale. The respective intensity factors are K_I, the Mode I pseudo-elastic intensity factor, and p1, the Mode I plastic intensity factor. FE computations allows generating evolutions of K1 versus p1, which are used to identify a global cyclic-elastic-plastic constitutive model for the crack tip region, i.e. a set of equations that allows predicting dp_I/dt. Its incremental formulation avoids using a cycle counting method. The first part of this paper is dedicated to present the experiments performed to characterize the fatigue behaviour of the stainless steel under variable amplitude loading. The second part is dedicated to the identification of the model for this material and to its evolutions to account for the specific features of the cyclic behaviour of the 361L.