3D constraint effect on 3D fatigue crack propagation

As typical examples, tensile round bars with elliptical surface cracks are used to illustrate the three-dimensional (3D) constraint effect and the characteristics of 3D fatigue crack propagation. Based on the latest plastic constraint theory and the concept of equivalent thickness Beq, a practical 3D crack closure model, which can be as well used to obtain material da/dN−ΔKeff curves, is developed to eliminate the 3D constraint effect on 3D fatigue crack propagation. Further, in order to verify the present Beq based 3D crack closure model, the fatigue crack propagation lives of 20CrMo steel round bars with elliptical surface cracks and 7475-T761 aluminum alloy sheets with corner cracks in edge region of their holes are predicted in this paper. The analytical results based on the present 3D model show that the 3D constraint effect on 3D fatigue crack growth can be eliminated, and fatigue crack propagation lives for both 20CrMo steel round bars and 7475-T761 aluminum alloy sheets with straight holes can be predicted accurately. The Beq based 3D crack closure model in the present paper can also be applied to analyze other complicated 3D fatigue cracks.