Endurance limit of die-cast magnesium alloys AM50hp and AZ91hp depending on type and size of internal cavities

The aim of this work was to investigate the influence of internal cavities on the fatigue properties of two of the technical most common die-cast magnesium alloys, AM50hp and AZ91hp. For this purpose the endurance limits of altogether three batches of S–N specimens, two conventional cast and one vacural cast, with varying internal defects have been measured. After fatigue failure the fracture surface of each sample has been analysed with respect to the site of crack initiation and, where appropriate, the size of the crack initiating cavity or pore. Moreover, on both alloys crack growth tests have been carried out and the thresholds ΔKth of the stress intensity factor have been measured. Finally, the experimental data from both, the S–N tests and the crack propagation measurements, were depicted in a modified Kitagawa–Takahashi diagram. Using El Haddad’s and Topper’s approach the distribution function of the endurance limit has been proposed, whose parameters could be determined by fitting them to the experimental results. The knowledge of these parameters allows the calculation of the fracture probability as a function of an equivalent crack length and the stress amplitude.