Mode I Crack Propagation under High Cyclic Loading in 316L Stainless Steel

Abstract Mode I fatigue crack propagation under high mechanical-thermal stress in large-scale yielding condition is observed in an experimental setup (named PACIFIC) developed by EDF. However, the crack growth within the ductile zones cannot be quantified numerically: the traditional methods based on the fracture mechanics such as the Paris propagation law are not appropriated, and no other ductile fatigue propagation law is yet approved. The objective of this study is to come up with a robust fatigue propagation law in large-scale plastic condition. An incremental model was developed for mode I fatigue crack growth under complex load spectra in consideration of non- linearity of material in the crack tip region. The displacement field in the crack tip region is partitioned into an elastic field and a plastic field which presents the plastic deformation within the crack tip region. Each field is approached by the product of a spatial reference field and its intensity factor. This approach allows dealing with the crack growth at large-scale. Our study aims at extending this model to account for the large-scale plasticity. For this purpose, besides the elastic and plastic field, the velocity field of the crack tip region is approached by a third field which is assumed to introduce the large-scale plasticity and is in the form of a product of a spatial reference field and its intensity factor rate ġ. Non-linear FE analyses were used to show that such an approximation is reasonably precise. The analyses conducted in this study indicate that the large yielding plasticity has no direct contribution to the crack propagation, thus the crack growth rate ȧ is merely proportional to the small-scale plasticity rate as in small-scale yielding condition. In addition, the 316L displays a very significant cyclic hardening effect and a memory effect. A constitutive law for this material in large strain range was identified that accounts for this memory effect, and the simulations are in good agreement with the experiments.