Very high cycle fatigue behavior of riblet structured high strength aluminum alloy thin sheets

摘要

Fatigue testing was performed on two age hardened high strength aluminum alloys (AA 2024 T351 and AA 7075 T6) at ultrasonic frequencies of around 20 kHz in fully reversed axial loading (R = -1). Tests were carried out on flat and riblet structured thin sheets in order to evaluate their usability for a novel technique for aerodynamicdrag reduction as well as for gaining further insight into the relevant degradation and failure mechanisms. The studied riblets were of semi-circular geometry and produced by a flat rolling process which was developed at the Institute of Metal Forming (RWTH Aachen University). Important aspects of the present work are the influence of commercially pure CP Al claddings – which are frequently used for the prevention of corrosion – as well as of different riblet dimensions on the fatigue performance.Whereas the bare material shows a continuous transition from high cycle fatigue (HCF) to very high cycle fatigue (VHCF), for clad sheets a sharp transition from HCF failure (up to some 1e6 cycles) to run-outs (at ≥ some 1e9 cycles) is observed. Particularly in the megacycle regime, the fatigue life of the structured bare material is – compared to the non-structured case – significantly reduced by stress concentrations induced by the surface structure. However, the fatigue performance of clad material is not negatively affected by the riblets. In this case, the threshold value at which the transition from HCF failure to run-outs occurs was even higher than in the flat case. The transition stress differs with cladding thickness as well as with riblet geometry. 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.Specimens only fail, if the threshold for further crack growth into the substrate is exceeded. The fatigue limit of both, the flat and riblet structured clad material can thus be described by a fracture mechanics approach using a Kitagawa-Takahashi diagram.In the case of structured clad material, the threshold for fatigue failure is not only directly affected by the remaining thickness of the cladding below the riblet structure. Finite element (FEM) simulations demonstrate that due to plastic deformation a stress redistribution in the CP Al layer occurs which modifies the effective stress at the interface (cladding / substrate). The effective interface stress is thus as well a function of cladding thickness, which therefore, besides the direct effect, also indirectly influences the stress intensity of through-cladding cracks. Further FEM simulations demonstrate that riblets can be optimized with respect to VHCF performance, if the thickness of the clad layer below the riblet valleys is around 25% of the riblet diameter.The failure mechanisms of both tested alloys are similar to each other. Further aspects covered in this work are a detailed analysis of material changes induced by the structuring process and the development of a bending testing setup in which the loading conditions resemble the exposure during use in active drag reduction systems.