Frequency effects on cyclic plastic strain of polycrystalline copper under variable loading

Studies of material frequency effects in cumulative damage are rare. Since new work has shown that the frequency of cycling affects persistent slip band (PSB) formation, with large effects on cyclic plastic strain in load control tests, step-descending tests have been performed at various frequencies. The results show that in regimes of stress-strain where loop patches and PSBs are the prevailing structures, cyclic deformation following a load-descending step is significantly affected by frequency. For example, a frequency of 0.5 Hz at the low level can increase the current plastic strain in the specimen by as much as nearly two times over the same step test performed at 8 Hz. If the prevailing structure from which the step test is initiated consists of cell structure, the effect of frequency differences is found to be small. These results, interpreted in terms of structural changes and secondary dislocation behavior, show that frequency effects could be significant in the accumulation of fatigue damage, especially in variable loading.