The Low Cycle Fatigue Behavior Of Polycrystalline Copper

Copper is the second most demanded non-ferrous metal material in China. It is widely applied in the aerospace, the transportation, and the national defense industry. The fatigue behavior of copper is one of the key criterion to determine its range of application.Axial low-cycle single-step loading and two-step loading tension-tension fatigue tests were carried out under strain control at room temperature. The results show that all specimens experienced the same process: cyclic harding?circulatory stability?cyclic harding. The fatigue life under axial single-step loading increased as the strain amplitude decreased, while the correlation between fatigue life and the two-step loading is relatively complex due to the influence of loading history and the interaction between loads. The fatigue life under high-to-low cyclic loading is higher than under axial single-step loading with the high amplitude, but lower than under axial single-step loading with the low amplitude. However, the fatigue life under low-to-high cyclic loading is not only higher than that under axial single-step loading with high amplitude, occasionally even higher than the fatigue life under axial single-step loading with low amplitude.The basic linear accumulation damage rule introduced by Palmgren and Miner, and the frequently-used first and second modified exponential nonlinear accumulation damage rule proposed by Z.Hashin and A.Rotem were verified. None of the fatigue life predictions by the three theories is satisfactory. However, the dispersity of the fatigue life prediction under high-to-low two-step loading is significantly lower than under low-to-high two-step loading. While the predicted low-to-high two-step loading fatigue life is an over-estimate, the predicted high-to-low two-step loading fatigue life is an under-estimate.The fractography was also carried out on the fatigue tested specimens to observe their fracture characteristics and traits. It was revealed that the observed fracture process of polycrystalline copper had two stages, including the crack nucleation in the surface of the fatigue specimen and crack propagation until the final fracture. Strain amplitude is the major factor responsible for different fracture morphologis.