Abstract. The rate of crack propagation is determined by the rate of dislocation evolution. However, dislocation evolution is dominated by the loading condition in fatigue. To study the dislocation structure evolution affected by reversal slip systems, the rates of crack propagation variation were measured by changing the R ratio (fixed maximum loading and increased minimum loading) under pure tension condition in fatigue. In this study, compact tension specimens of polycrystalline copper were completed as the following ASTM. In order to measure the rates of changing in the experiment, a steady state rate of crack propagation was reached by Instron fatigue machine, and then it was subjected to an increasing of the minimum loading. The microstructure at crack tip was observed by means of the back electron image (BEI) of scanning electron microscope (SEM). In order to ensure that the dislocation development is affected by reversal slip system, the low cycle fatigue was performed for different R ratios at the same fatigue cycles (constant maximum loading) under pure tension loading. At the same time, after a complete low cycle, the specimens' micro-hardness was measured. The results reveal that: (1) the crack propagation rate is decreased and the range of dislocation evolution is widespread when the R ratio increases; (2) the micro-hardness of low cycle fatigue specimens is increased with an increased R ratio; (3) the dislocation structures in low cycle fatigue specimens show ladder-like wall structure embedded in loop patches at low R ratio and the cells' structure is created by multiple slip systems induced by hardness at a high ratio; (4) no matter what the crack propagation and low cycle fatigue, the results were determined by the reversal slip systems operation.

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