On the Transcrystal Fatigue Crack Propagation Mechanism of bcc Metals

Abstract

On the basis of the observed cristallographic features of the fatigue crack propagated planes in Fe-3%Si single crystals, the transcrystal fatigue crack propagation mechanism of bcc metals was studied. To make sure the one cycle-one striation relation which forms a basis of the fatigue crack propagation, program load fatigue tests were carried out. The relation was confirmed all over the fatigue cracked surfaces. The dependence of crystallographic orientation on the crack propagating rate and the direction of the propagation was investigated using the coarse-grain thin plate specimens. The two dimensional slip-off process model was modified under the three dimensional condition, taking the shear stresses on slip planes into account. This model shows that the crack plane and the fatigue striation have a tendency to coincide with {110} and ⟨111⟩ respectively, where the rate reaches a maximum value. The model also shows that this tendency appears slowly in polycrystals, and that the effect of the stress ietensity factor of the whole specimen and that of the local one are in equilibrium with each other and the fatigue crack plane and the striation can lie on an unspecified plane and direction respectively in single and coarse grain crystals. These were in good agreement with the observations. Considering the strain hardening in the cracking process, the variety of the contrast and the profile of the striation could be explained.