Effect of mixed-mode loading on surface crack propagation in steels

Abstract

To understand the effect of mixed-mode loading on surface crack propagation in structural alloy steels, fatigue crack growth tests were performed on surface crack tension (SCT) specimens with initial semi-elliptical surface cracks. SCT specimens with cracks normal to the load axis were used for the mode I tests. Experiments with mixed-mode loading were conducted on SCT specimens with inclined surface cracks. A specific feature of the surface crack propagation, in the segmentation of the crack front, the occurrence of «factory roof» patterns, and deviations in the crack front from a planar shape were established under mixed-mode loading. A reverse engineering technique was applied in a numerical study to reproduce a complex fracture surface under mixed-mode loading. The equivalent stress intensity factor (SIF) formulation was used to interpret the fatigue-crack growth rate. A significant reduction in the fatigue crack growth rate was established for mixed-mode loading compared with pure mode I conditions. Several fracture mechanisms were identified along the surface crack front under mixed-mode loading. The areas corresponding to changes in the dominant fracture mechanism were identified using detailed fractography of the fracture surfaces.