Numerical and experimental investigations of the influence of corner singularities on 3D fatigue crack propagation

In the case of surface breaking cracks, the typical square-root stress singularity is generally not sustainable and a 3D corner singularity in the vicinity of the intersection of crack front and free surface has to be considered. Only the crack front intersection under a special angle γr ensures a valid square-root stress singularity and the applicability of the classical SIF-concept. In this paper, the theory of the numerical determination of the intersection angle γr is briefly described and the influence of the 3D corner singularity on fatigue crack growth is experimentally investigated. Therefore, 3D fatigue crack propagation experiments under pure mode-I are performed. Transparent specimens of PMMA are used, in order to be able to observe and to document accurate sequences of real 3D crack front evolution profiles via in situ photographic measurement.