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The fatigue strength and the transition of failure mechanism from the high cycle fatigue (HCF) to the very high cycle fatigue regime (VHCF) for EN AW 6056 were investigated at mechanically polished, smooth specimens. Fatigue tests were conducted with a servohydraulic testing machine and a resonant frequency machine at room temperature. Tests run up to a maximum number of cycles N=2⋅108.The results reveal that the fatigue strength decreases with increasing number of cycles, which is known for materials with facecentred cubic lattice. In the very high cycle regime the crack initiation sites of the smooth specimens were internal. Fracture surface and microstructure of the crack initiation sites were investigated by means of scanning electron microscope (SEM), energy dispersive X-ray analysis (EDX), electron back scatter diffraction analysis (EBSD) and metallographic methods. The influence of notches was investigated at slightly notched (Kt=1.75) and sharp notched (screws M10, Kt=4.7) specimens. The influence of compressive residual stresses was investigated by a comparison of precision-turned and shot-peened, slightly notched specimens as well as screws of with thread rolling before heat treatment and thread rolling after heat treatment. The fatigue strength of the specimens without compressive residual stresses decreases with increasing number of cycles. The fatigue strength of the specimens with compressive residual stresses shows a horizontal SN-curve after a point of deflection. The crack initiation sites of the notched specimens were always at the root of the notch independent of the compressive residual stresses and the number of cycles

procedia engineering

Fatigue strength and failure mechanism for the aluminium wrought alloy EN AW 6056 in the VHCF-region and influence of notches and compressive residual stresses