Fatigue strength and failure mechanisms of wrought aluminium alloys in the VHCF-region considering material and component relevant influencing factors

Abstract

Investigations at room temperature were carried out with smooth specimens made of the aluminium alloys EN AW 6056 and EN AW 6082. Different failure mechanisms were investigated for instance according to material and number of cycles. The influence of mean stress on fatigue strength of smooth specimens made of EN AW 6056 and transition of failure mechanism from the high cycle fatigue (HCF) to the very high cycle fatigue regime (VHCF) are discussed. Fracture surface and microstructure of the crack initiation sites were observed by means of scanning electron microscope (SEM), electron back scatter diffraction analysis (EBSD analysis) and metallographic methods. Transmission electron microscopy (TEM) was used to find differences between unstressed and tested material of EN AW 6082. The results reveal for both aluminium alloys that the fatigue strength decreases with increasing number of cycles, which is known for materials with face-centred cubic lattice. In the very high cycle regime the crack initiation sites of the smooth specimens were internally. The crack initiation sites of the notched specimens were always at the root of the notch independent of the number of cycles. The microstructures of fracture surfaces were different for both aluminium alloys. Slightly notched and sharp notched (screws M10, thread rolled before heat treatment) specimens made of EN AW 6056 were investigated. Fatigue tests were done with a servohydraulic testing machine and a resonant frequency machine. Tests run up to a maximum number of N = 2 × 10 8 cycles (EN AW 6056) or N = 10 9 cycles (EN AW 6082). A comparison of the experimental results with estimated fatigue strength of a design code (FKM-Guideline, 2003 [1]) is presented.