Mechanics of mixed mode small fatigue crack growth

An elastic-plastic analysis of a mixed mode crack was conducted under general yielding conditions using a finite element method. Based on the finite element analysis, a formula of the J-integral was developed, and the behavior of the crack opening or sliding displacement was investigated. Crack growth mechanics was discussed to explain mixed mode growth. A model expressed by the J-integral was proposed assuming that the crack growth is determined by the linear summation of relations of pure Mode I and II crack-tip deformation. The crack growth rate obtained using Inconel 718 thin-wall tubular specimens was correlated with the range of ΔJ evaluated from the proposed formula. The crack growth equations in terms of ΔJ for three different biaxial strain ratios were compared with the relations expected from the proposed crack growth model. The difference between the experimental results and the estimation was discussed from the viewpoint of crack closure and the geometry of the surface crack.