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Since fatigue life of a plain specimen of ductile metals is controlled mainly by the propagation life of a small surface crack, to clarify the growth behavior of a small crack is crucial to the safe design of smooth members. However, little has been reported on the growth behavior of small surface cracks in ultrafine grained (UFG) metals. In the present study, stress-controlled fatigue tests for coarse grained (CG) and UFG copper were conducted. The surface damage evolution during cyclic stressing was observed by optical microscopy, and the growth behavior of a small surface crack was monitored by a plastic replication technique. The physical background of fatigue damage for CG and UFG copper was discussed from the viewpoints of the initiation and growth behavior of small surface cracks.

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Two Cu samples: oxygen-free copper (99.99 wt% Cu) and deoxidized low-phosphorous copper (99.9 wt% Cu) were processed by equal channel angular pressing (ECAP). After the ECAP processing using 4 passes, equiaxed grains (~300 nm grain size) and elongated grains were formed for both samples. Fatigue strength of ultrafine grained Cu was enhanced by the addition of trace impurities. The formation behavior of surface damage and the change in surface hardness during stressing were monitored. A close relationship was observed between the change in hardness and the formation behavior of damage. The effect of trace impurities on the fatigue damage was discussed from the viewpoints of the grain coarsening and the crack initiation/growth behaviors.

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