Very high cycle fatigue behavior of riblet structured Alclad 2024 thin sheets

•VHCF tests were performed on flat and riblet structured Alclad 2024 thin sheets.•Specimens show a sharp transition from early failure to very long life.•This transition is attributed to the threshold for crack growth in the substrate.•The riblet structure does not significantly harm the fatigue performance. Fatigue properties in the very high cycle (VHCF) regime have been studied for Alclad 2024-T351 thin sheets. Furthermore, the influence of a riblet-like surface structure has been evaluated. For this we used a rolling technique to form a riblet structure into the Al clad layer. Fatigue testing was performed at ultrasonic frequencies of 20kHz, in order to ensure a reasonable runtime of the experiments, with fully reversed axial loading (R=−1). The clad material shows a sharp transition from early failure at some 106 cycles to run-outs at ⩾2×109 cycles. The corresponding threshold is higher for the structured samples, due to a modified stress distribution near the interface between Al cladding and Al 2024 substrate as well as due to a shorter crack length of the fatigue cracks formed in the cladding. Fatigue cracks are initiated at the surface of the clad layer and grow easily to the interface (cladding/substrate). The fatigue threshold for this compound material can be attributed to the threshold for further crack growth into the substrate.