13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Analysis of the transition from flat to slanted fatigue crack growth in thin metallic sheets Jean-Baptiste Esnault 1,2, Véronique Doquet 1,*, Patrick Massin2 1 : Laboratoire de Mécanique des Solides, CNRS, Ecole Polytechnique, Palaiseau France 2 LaMSid, EDF – CNRS - CEA, Clamart, France *corresponding author: doquet@lms.polytechnique.fr Abstract 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 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 the 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 (1 ) 2 2 2 ν− Δ Δ = Δ +Δ + III II I eq K K K K 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. Keywords fatigue crack, shear lips, thin sheet, mixed-mode, X-FEM, slanted crack 1. Introduction Fatigue crack growth normal to the tensile axis becomes unstable in thin metallic sheets, above a material, environment and frequency-dependent amplitude, even though, in many cases, small scale yielding conditions still prevail. Shear lips development has been investigated mainly in aluminium alloys by Schijve and coworkers [1-2] Zuidema et al. [3-4] or Shanyavsky and Koronov [5], while a few studies were devoted to steel [6] or titanium alloys [7]. Walker et al [7] first reported an influence of environment on shear lips development in titanium alloys. The corrosive environment appeared to postpone crack twisting in Ti-8Al-1Mo-1V, while no systematic effect was found in Ti-6Al-AV. Vogelesang and Schijve [2] observed complete crack twisting in 7075 T651 aluminium alloy for a lower apparent ΔKI in vacuum than in air and for a higher apparent ΔKI in 3,5%NaCl solution. Horibe et al [6] reported a similar effect of salt water in low strength steel, but a less pronounced effect of environment for high strength steel. Shanyavsky and Koronov [5] measured the shear lips width in thin aluminium alloy cruciform specimens cyclically loaded in two orthogonal directions and reported that a positive biaxiality ratio reduced the shear lips, while a negative ratio increased it. They also observed a reduction in shear lips width when the R ratio increased. This observation conflicts with the conclusion of Zuidema et al. [3-4] that the
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