Fatigue Behavior of Riblet Structured High Strength Aluminum Alloy Thin Sheets at Very High Cycle Numbers

Abstract

The VHCF behavior of age hardened 2024 and 7075 aluminum sheets was studied. The experiments were performed at frequencies of ≈ 20 kHz with fully reversed axial loading (R = -1). Special focus was put on the influence of AA 1050 claddings and riblet-like surface structures, which are used in aerospace applications to reduce aerodynamic drag. The fatigue life and fatigue limit of the AA 2024 bare material are – compared to the non-structured case – significantly reduced by the stress concentrations induced by the riblet structure. However, the fatigue behavior of the clad AA 2024 material is less sensitive to the surface structure. In this case, we obtained a sharp transition from HCF failure up to 5x106 cycles to run-outs at ≥ 2x109 cycles. This threshold value for failure differs with cladding thickness as well as with riblet geometry. We attribute this to the modified stress distribution near the interface (cladding/substrate) as well as to a locally reduced thickness of the cladding in the riblet valleys. Fatigue cracks are – even in the case of run-outs – always initiated at the surface of the clad layer and grow easily to the substrate. Samples only fail, if the threshold for further crack growth into the substrate is exceeded. Both Alclad 2024 and 7075 show the same failure mechanism.