Effect of cyclic softening and mean stress relaxation on fatigue crack initiation in a hemispherical notch

Corroded surfaces of offshore mooring chains typically show a very irregular shape with different pit geometries. Fatigue tests were performed in three‐point bending on specimens with hemispherical notches, representing an idealized geometry of a corrosion pit. Digital image correlation was used to detect crack initiation lives and locations. The aim of the study was to quantify mean stress relaxation (MSR) and its influence on fatigue initiation life. MSR data were obtained from strain‐controlled fatigue tests on R4‐grade offshore mooring chain steel. Interestingly, MSR was observed even at a stress amplitude below the cyclic yield stress. For initiation lives below 100,000 cycles, significant improvement in predictions was obtained if cyclic softening and MSR were accounted for. Using only the monotonic stress‐strain curve led to less accurate predictions. For longer lives, scatter in the base material S‐N data and residual stresses from machining of the notch made predictions less accurate.