A new failure criterion for materials exhibiting ratcheting during very low cycle fatigue

Ratcheting or cyclic creep is the phenomenon of progressive accumulation of permanent deformation when a component is subjected to cyclic loads in the plastic strain range under stress controlled fatigue with non-zero mean stresses. This accumulation of plastic strain will finally lead to a shakedown, or a constant rate of ratcheting or very large ratcheting strains leading to failure of the material. This latter situation is encountered during the very low cycle fatigue (VLCF) of components when the stress levels are close to the ultimate strength and the component fails in less than 100 cycles. Large strains are accumulated leading to plastic instability/necking and final fracture. A failure criterion based on the initiation of plastic instability has been proposed in this work, for the life prediction of materials exhibiting ratcheting in the VLCF regime. The proposed model is supported by experimental observations made from the stress controlled VLCF tests with tensile mean stresses on smooth cylindrical mild steel specimens.