Formation mechanism of inclined fatigue-cracks in ultrafine-grained Cu processed by equal channel angular pressing

For envisaged structural applications of ultrafine-grain (UFG) materials processed by equal channel angular pressing (ECAP), extensive attention has been paid to the fatigue performance. In the low-cycle fatigue tests of UFG metals, shear bands (SBs) and crack paths that were inclined 45° to the loading direction have been commonly observed. Up to now, SB formation and crack growth behavior of UFG materials have been mainly discussed from the viewpoints of microstructure and morphological features of surface damage. On the other hand, the discussion from the mechanical viewpoints should be done for a better understanding of the fatigue damage of UFG materials. However, such studies are few and certain questions remain unanswered.

In the fatigue process of smooth specimens, innumerable cracks are initiated over the whole surface, followed by propagation with coalescence and interaction with multiple cracks. However, a few cracks among such innumerable cracks play a major part in the fracture of the specimen. For an understanding of fatigue mechanism in equal channel angular pressing Cu, the effect of equal channel angular pressing microstructure on the growth behavior of a fatal crack, which led to the final fracture of the specimen, should be discussed. To achieve this, the fatal crack must be introduced into the surface with a specific positional relationship toward the orientated microstructure in the equal channel angular pressing samples. However, it is quite difficult to design the initiation of a fatal natural crack at a specific site on the whole surface of a smooth specimen. To overcome this difficulty, a shallow partial notch was introduced on the smooth specimen surface. Using this type of specimen, specification of the initiation site of the fatal natural crack could be achieved despite the microstructural inhomogeneity resulting from equal channel angular pressing.

The stress-controlled fatigue tests of smooth specimens were conducted at high and low stress amplitudes corresponding to low- and high-cycle fatigue regime, respectively. The crack face profile and the crack growth direction on the surface and inside the specimen were monitored. The values of mixed-mode stress intensity factors of an inclined semi-elliptical surface crack were estimated. The objective of this study is to clarify the physical background of the formation mechanism of inclined cracks in terms of the mixed-mode deformation at the crack tip and the microstructural evolution caused by cyclic stressing.