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 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, but tunnelling was progressively reduced and cancelled as slanted crack growth developed, even though KI is reduced by crack twisting. This indicates a significant contribution of shear modes to the crack driving force, even though mode I striations are still present on slanted zones. KI, KII and KIII were computed by X-FEM taking into account the real crack path. The crack growth rates correlated much better to than to KI. Elastic-plastic finite element simulations and the local application of a fatigue criterion with an amplitude-dependent critical plane qualitatively captured the transition in fracture mode in 7075 T651.