Fatigue crack growth versus plastic CTOD in the 304L stainless steel

The plastic CTOD range, delta(p), which quantifies crack tip plastic deformation, is used to study fatigue crack propagation instead of the classical Delta K elastic parameter. A da/dN-delta(p) model was defined for the 304L stainless steel. da/dN was obtained experimentally in standard CT specimens while delta(p) was predicted numerically using the finite element method. The numerical models replicated the experimental tests in terms of geometry, material properties and loading. The da/dN-delta(p) model was used to predict the effects of stress state, load ratio and variable amplitude loading. All the results obtained respect the trends observed in literature, attesting that the da/dN-delta(p) model is able to produce good qualitative predictions. Additionally it was found that crack closure is a main phenomenon affecting delta p, and therefore da/dN, but there is also an effect of mean stress.