The role of mixed-mode deformation at the crack tip on shear banding and crack propagation of ultrafine-grained copper

•The crack paths was 45° inclined to the loading axis in the Y-plane.•The crack paths was 90° inclined to the loading axis in the Z-plane.•The aspect ratios were 0.38 for the Y-plane crack and 1.1 for the Z-plane crack.•The crack face shape depended on the in-plane shear-mode deformation at the crack tip areas. Stress-controlled fatigue tests of ultrafine-grained copper round-bar specimens were conducted. Although the crack paths inclined 45° and 90° to the loading-axis were observed in the different locations, crack faces were extended along one set of maximum shear-stress planes, corresponding to the final ECAP shear plane. Profile of crack faces was examined, showing the aspect ratios of 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 deformation mode at the crack tip on shear-banding and crack growth behavior were discussed in terms of the mixed-mode stress intensity factor.