INSITU SEM EXPERIMENTS CONCERNING THE MECHANISM OF DUCTILE CRACK GROWTH

Experiments are described in which growing fatigue cracks with simple and well reproducible crack geometries can be observed during cyclic loading inside a SEM. Copper and Fe-2.4%Si single crystals of appropriate shape and orientation were examined. For the investigation of the slip processes at the apex of the crack tip a commercially available tensile stage was modified to allow for cyclic deformation. It was found that the same mechanism for ductile crack propagation is operative in copper and in Fe-2.4%Si. In these crystals various predictions of the alternate slip model were verified. In both materials the alternate activation of the slip systems can be observed directly. In the Fe-2.4%Si single crystals with their larger variety of slip systems an orientation can be chosen, which further simplifies the stress-strain distribution near the specimen surface. In these crystals even the motion of the crack tip due to the alternating slip-the very mechanism of crack advance-can be observed directly. © 1979.