RELATIONSHIP BETWEEN SHEAR PLANE OF THE FINAL PRESSING AND FATIGUE CRACK GROWTH BEHAVIOUR OF ROUND-BAR SPECIMENS OF CU PROCESSED BY ECAP

The formation mechanism of inclined fatigue cracks in ultrafine-grained Cu processed by equal channel angular pressing was studied by using a smooth specimen with a small blind hole. The crack growth direc- tion depended on the location of drilling hole along the circumferential direction of the round bar specimen and on the applied stress amplitudes. Although the low-cycle fatigue crack growth paths inclined 45° and 90° to the loading-axis were observed in the different locations on the surface, crack faces in these cracks were extended along one set of maximum shear stress planes, corresponding to the shear plane of the final processing. To study the crack growth behaviour, surface damage around the crack paths formed by the two-step fatigue stress tests was observed. Profile of crack face was examined, showing the aspect ratios ( b / a ) of b / a = 0.38 and 1.10 for the cracks with 45° and 90° inclined path directions with respect to the loading axis, respectively. The role of the microstructure and deformation mode at the crack-tip areas on the formation of crack paths parallel to the shear plane of the final pressing was discussed in terms of the microstructural evolution caused by cyclic stressing and the mixed-mode stress intensity factor.