Fatigue cracking and fracture behaviors of coarse-grained copper under cyclic tension–compression and torsion loadings

In this study, fatigue damage and fracture behaviors of coarse-grained copper are investigated under cyclic tension–compression and torsion loadings. The cyclic stress response and dislocation patterns of coarse-grained copper under the two fatigue tests are compared. Fatigue crack initiation, propagation and fracture surfaces of the specimens are found to rely on the loading mode. In general, the initiation and early propagation of fatigue cracks are mostly controlled by the direction of the maximum shear stress. The effect of normal stress can be used to explain the common tendency for fatigue cracks to propagate from stage I to stage II. Combining the formation mechanism of fatigue striations with the analysis of stress state, it is found that fatigue cracks are much easier to transform from stage I to stage II under tension–compression fatigue than under torsion fatigue. But under torsion fatigue with higher strain amplitude, because the driving force is much higher, stage II fatigue crack propagation obviously exists. Additionally, by analyzing the growth condition of longitudinal and circumferential cracks, the fracture process of torsion fatigue with different strain amplitudes is discussed.