Influence of riblet geometry on fatigue life of surface structured AA 2024 thin sheets

• Riblets in bare AA 2024 sheets significantly reduce VHCF performance.
• VHCF life of clad AA 2024 sheets depends strongly on riblet geometry.
• The optimum cladding thickness below the riblets depends on the riblet diameter.
• Optimized structured clad sheets can outperform even structured bare material.

Fatigue testing was performed on riblet structured AA 2024 T351 thin sheets. Main focus was put on the influence of different riblet geometries on fatigue life in the very high cycle fatigue (VHCF) regime. Experiments on bare as well as on clad sheets were conducted at an ultrasonic frequency of around 20 kHz with fully reversed axial loading (R = −1). While for bare material the riblet structure reduces lifetime by around one order of magnitude, the clad material is not negatively affected by the surface structure. The threshold for fatigue failure of clad material is governed by the remaining thickness of the cladding below the riblet structure. Finite element (FEM) simulations show that an optimal VHCF performance is reached, if the thickness of the clad layer below the riblet valleys is around 25% of the riblet diameter.