FATIGUE LIFE PREDICTIONS OF NOTCHED SPECIMENS UNDER NON-PROPORTIONAL LOADINGS

A series of low cycle fatigue tests were conducted for notched specimens of Mod.9Cr-1Mo ferrite steel under multi-axial loadings. The crack initiation life was measured by the direct current potential difference method (DC-PDM). The effects of the notched radius and the strain path on crack initiation life were discussed. It is shown that the ratio of the crack initiation life to the entire fatigue life is related to material type, strain path, and especially the notched radius. Under the same strain path, the ratio increases with the notched radius. Local stress and strain were calculated based on the Neuber rule, and the results were employed to predict the fatigue life using the Smith-Watson-Topper (SWT) approach and the Kandil-Brown-Miller (KBM) approach. On the whole, the prediction from the SWT approach was optimistic, except for the uni-axial and proportional strain paths. On the other hand, the KBM approach offered a more reliable prediction, apart from being conservative under circumstances of uni-axial strain paths. The overall prediction results fell within a two-time scatter zone.