Anisotropy of Crack Propagation and Fracture Surface in Fatigue of Rolled Steel with Discoid Inclusions

Abstract

Rotating bending fatigue tests and pulsating tensile fatigue tests were carried out using the notched and CT specimens cut out in various directions from a rolled steel. The material used had discoid inclusions with a remarkable directionality, in spite of its slightly laminated structures. The crack growth rate in each directional specimen was measured by an optical microscope and the fracture surfaces were observed by a scanning electron microscope. The main results obtained are as follows. (1) As for the anisotropy in the crack initiation process and in the process of initial crack propagation, the effect of discoid inclusions was large, as in the case of the anisotropy of the fatigue limit. This is because in the direction where the inclusions give effect, the cracks propagate immediately from the discoid inclusions, while in the direction where the inclusions give no effect, many repetitions are needed until the cracks initiate. (2) As for the anisotropy of the propagating process of large cracks, the crack growth rate was largest in the case of specimens whose cracks propagate along the inclusions, but the anisotropy of the crack propagation was small in comparison with that of the fatigue limit. This is because the fatigue limit is determined by the limiting condition for the propagation of cracks initiated from inclusions, while the crack growth rate is determined by the average properties of the crack front.