A three-dimensional analysis of fatigue crack paths in thin metallic sheets

Fatigue crack growth in thin sheets of 7075 T651 aluminium alloy and S355 steel were characterised in 3D, using crack front markings and topographic reconstructions of fracture surfaces. Tests performed in air or in salt water produced different crack paths for similar mechanical conditions, shear lips being reduced by corrosive environment, in the aluminium alloy as well as in the steel. Before the onset of shear lips development, tunnelling crack fronts were observed, due to the difference in closure effects at mid-thickness and near free surfaces. Tunnelling was progressively reduced and cancelled as slanted crack growth developed, even though ΔKI was reduced locally by crack twisting. This indicates a significant contribution of shear modes to the crack driving force, even though mode I striations are present in slanted zones. Elastic three-dimensional X-FEM computations were performed to analyse the observed crack growth kinetics, based on ΔKI, ΔKII and ΔKIII. The crack growth rates correlated much better to ΔKeq=ΔKI2+ΔKII2+ΔKIII2(1-ν) than to ΔKI. Elastic-plastic finite element simulations and the local application of a fatigue criterion with an amplitude-dependent critical plane were found to capture qualitatively the transition in fracture mode and its inhibition by side grooves.