Catherine Gardin

3D Numerical Study on how the Local Effective Stress Intensity Factor Range Can Explain the Fatigue Crack Front Shape

The plasticity-induced crack closure of through-thickness cracks, artificially obtained from short cracks grown in CT specimens of 304L austenitic stainless steel, is numerically simulated using finite elements. Crack advance is incremented step by step, by applying constant ΔK amplitude so as to limit the loading history influence to that of crack length and crack wake. The calculation of the effective stress intensity factor range, ΔKeff, along curved shaped crack fronts simulating real crack fronts, are compared to calculation previously performed for through-thickness straight cracks. The results for the curved crack fronts support that the front curvature is associated to constant ΔKeff amplitude, thus assumed to be the propagation driving force of the crack all along its front.

Specificity of Crack Initiation under Equibiaxial Fatigue: Development of a New Experimental Device

Fatigue lifetime assessment is essential in the design of structures. Under-estimated predictions may result in unnecessary in-service inspections. Conversely, over-estimated predictions may have serious consequences on the integrity of structures. In some nuclear power plant components, the fatigue loading may be equibiaxial because of thermal fatigue. So the potential impact of multiaxial loading on the fatigue life of components is a major concern. Meanwhile, few experimental data are available on austenitic stainless steels. It is essential to improve the fatigue assessment methodologies to take into account the potential equibiaxial fatigue damage. Hence this requires obtaining experimental data on the considered material and with a strain tensor in equibiaxial tension. This paper describes an experimental program on austenitic stainless steel carried out on the new experimental fatigue device FABIME2 developed in the LISN in collaboration with EDF and AREVA. This new device allows accurate quantification of the effects of both equibiaxial strain state as well as structural parameters (such as mean stress) on the fatigue life. It also allows studying the complexity of combinations between potential detrimental effects like surface roughness, mean stress and equibiaxial loading. Different load ratios can be tested by adjusting the loading conditions. A Finite Element Modeling is performed in order to obtain a precise description of the strain state in the specimen. The results of the on-going test campaign will be presented.

Influence of crack front shape on 3D numerical modelling of plasticity- induced closure of short and long fatigue cracks

The simultaneous effect of crack length and crack front shape on plasticity-induced crack closure (PICC) for a 304L austenitic stainless steel is simulated through 3D numerical modelling using finite element software Abaqus for through-thickness cracks with different curved crack fronts in CT specimens in comparison with bidimensional through crack with a straight front. The influence of possible loading history effect is avoided by applying constant K amplitude. The local stress intensity factor range for crack opening Kop is evaluated from the simulation of the loss of the last local contact between the crack lips near the crack tip. The pertinence of the different crack front shapes is discussed in term of the effective stress intensity factor range Keff and in comparison with the experimental crack front observations.