Analysis of the transition from flat to slanted fatigue crack growth in thin metallic sheets

Fatigue crack growth in thin sheets of 7075 T651 aluminium alloy and S355 steel were characterized 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 the corrosive environment, in the aluminium alloy as well as in 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 the free surfaces. Tunnelling was progressively reduced and cancelled as slanted crack growth developed, even though ΔK₁ 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 the slanted zones. Elastic three-dimensional X-FEM computations were performed to analyse the observed crack growth kinetics, based on ΔK_i, ΔK _ii and ΔK_iii. The crack growth rates correlated much better to ΔK_eq = √ ΔK_I² + ΔK_I I² + -ΔK²_iii than to ΔK_I. 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.