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 hemispherical notches, representing an idealized geometry corrosion pit. Digital image correlation was used to detect crack initiation lives and locations. The aim the study quantify mean stress relaxation (MSR) its influence fatigue life. MSR data obtained from strain-controlled R4-grade chain steel. Interestingly, observed even at amplitude below cyclic yield stress. For 100,000 cycles, significant improvement predictions if softening accounted for. Using only monotonic stress-strain curve led less accurate predictions. longer lives, scatter base material S - N residual stresses machining notch made accurate.