Uniaxial ratchetting and low-cycle fatigue failure of the steel with cyclic stabilizing or softening feature

The ratchetting behaviour and low-cycle fatigue failure, as well as the interaction between them were investigated by uniaxial cyclic stressing tests for 42CrMo steel with annealing or tempering heat-treatment. The ratchetting strain and fatigue life of the material were measured in the uniaxial cyclic stressing with different loading levels. The effects of mean nominal stress, nominal stress amplitude and stress ratio on the ratchetting strain and final failure life were discussed. Simultaneously, the variations of responded strain amplitude with the number of cycles were illustrated to discuss the interaction between ratchetting and low-cycle fatigue failure behaviour. The experimental results show that the ratchetting and fatigue failure behaviours of the annealed 42CrMo steel are different from those of the tempered steel, since different cyclic softening/hardening features are caused by different heat treatments, i.e., the annealed 42CrMo steel presents cyclic stabilizing feature, but the tempered steel is cyclic softening. Two kinds of failure modes (i.e., ratchetting failure with obvious necking due to large ratchetting strain and fatigue failure due to low-cycle fatigue with nearly constant responded strain amplitude) take place, depending on mean nominal stress, nominal stress amplitude and stress ratio of uniaxial cyclic stressing, as well as the heat-treatment.