The distinct influences of pre-strain on low cycle fatigue behavior of CP-Ti along rolling direction at different strain amplitudes

In this paper, the distinct influences of pre-strain on low cycle fatigue (LCF) behavior of CP-Ti at different strain amplitudes are clarified from macroscopic and microscopic aspects. At low strain amplitude, the sample of as-received material (AR) shows pronounced secondary cyclic hardening, however, this phenomenon gradually disappears in pre-strained (PS) sample with the increase of pre-strain. As cyclic stress amplitude and mean stress of PS sample increase with the decrease of strain amplitude, the detrimental effect of pre-strain on LCF life becomes more significant at lower strain amplitude, indicating the pre-strain dependent LCF behavior. At high strain amplitude, due to the more significant cyclic softening and mean stress reduction behavior, cyclic stress amplitude and mean stress rapidly decay to the value of AR sample in the initial few cycles, indicating the pre-strain independent LCF behavior. As the reduction of LCF life is related to the increase of mean stress and stress amplitude, the asymmetry coefficient related model can be applied to predict LCF life of CP-Ti. Further TEM observation shows that at low strain amplitude, dislocation structures of AR sample are mainly composed of planar slip structures and the secondary cyclic hardening is caused by corduroy structure. After pre-strain, both planar slip and wavy slip structures are observed in PS sample, indicating that the irreversibility of the dislocation structures inherited from pre-strain causes the decrease of LCF life. At high strain amplitude, the dislocation structures introduced by pre-strain can be erased completely during subsequent cyclic deformation, consequently, dislocation configurations of AR and PS samples are composed of the same wavy slip structures.