Comparison of multiaxial low cycle fatigue behavior of CP-Ti under strain-controlled mode at different multiaxial strain ratios

Abstract Multiaxial low cycle fatigue (MLCF) tests were performed on commercial pure titanium (CP-Ti) at different multiaxial strain ratios (λ = 0.865, 1.73, 3.46, 5.19). A higher λ accelerated the initial cyclic hardening for the axial stress. For torsional stress responses, all specimens showed initial cyclic hardening characteristics. Optical microscopy (OM) observations of the fracture surfaces showed that the critical plane of CP-Ti is better aligned with the maximum principal strain plane rather than the maximum shear strain plane. The fatigue-life curves obtained from the tests were constructed using the Zamrik, ESN, SWT, FS, and KBM models. According to the modified critical plane, a KBM-P model based on the maximum principal strain plane is proposed. A unified life prediction KBM-U model incorporating the effect of λ is further proposed. Scanning electron microscopy (SEM) observations of fatigue fractures showed that fatigue cracks initiated from the outer surface of the specimen at all λ. Many small cleavage surfaces of different sizes and heights were observed in the crack propagation zone. At higher λ, these cleavage characteristics were accompanied by shear dimples.