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

Abstract

Corroded surfaces of offshore mooring chains typically show a very irregular shape with different pit geometries. Fatigue tests were performed in threepoint 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.

Mean stress adjustment models for stress‐life fatigue of notched components

“…In both stress and strain‐controlled tests, neither the stress nor the strain in the notch is controlled; effects such as strain ratcheting and mean stress relaxation may occur together 16 . Elastoplastic modeling of notches to understand the details of their stress state remains an active area of research 17 . Modeling attempts along these lines are predicated on a sound understanding of the material's cyclic stress–strain response.…”

“…16 Elastoplastic modeling of notches to understand the details of their stress state remains an active area of research. 17 Modeling attempts along these lines are predicated on a sound understanding of the material's cyclic stressstrain response. In application, this information is often not available, forcing the employment of more approximate methods.…”