Abstract
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.
Highlights
Fatigue crack growth normal to the tensile axis becomes unstable in thin metallic sheets, above a material, environment and frequency-dependent amplitude
These authors observed that shear lips started to develop when a threshold crack growth rate was reached (5 to 7.5μm/cycle in titanium alloys, according to Walker et al [6], about 0.1μm/cycle in aluminium alloys, according to Zuidema et al [4], while Hudson and Scardina report 0.2 to 0.7μm/cycle for 7075-T6 alloy [7])
Crack growth was substantially accelerated by the NaCl solution in the aluminium alloy, but the acceleration was rather limited in steel
Summary
Fatigue crack growth normal to the tensile axis becomes unstable in thin metallic sheets, above a material, environment and frequency-dependent amplitude. Based on “constant ∆KI tests”, empirical relations between the steady-state shear lips width, the loading frequency and the “effective ∆KI” were derived for aluminium alloys [4,5]. This parameter will be denoted below by “apparent ∆KI”, since it is computed in 2D for a normal crack of same length as that observed on the free surface, with an empirical correction for closure effects deduced from the R ratio. The meaning of “constant ∆KI tests” is questionable in view of the large gradient in KI, KII and KIII along the front of a partially or completely slanted crack and of the reduction in ∆KI associated with crack twisting
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