Averaged stress intensity factors for semi-elliptical surface cracks

The measurement of material resistance against crack propagation is mostly performed with (in principle) one-dimensional cracks as usual for standard test specimens as for instance DCB (Double Cantilever Beam) specimens. Such a crack exhibits a constant stress intensity factor along the crack front that allows simple computations of crack extension. In the case of 2-dimensional cracks, e.g. semi-elliptical surface cracks, the calculation of strength and service life under subcritical crack growth is considerably more complicated. The main reason for this is the fact that the stress intensity factor changes along the crack front. In this report, averaged K-factors for virtual crack propagation in both axial directions of the semi-ellipse are determined. For this purpose, virtual crack area increments according to Cruse and Besuner are used. The stress intensity factors for the deepest point of the crack and the surface terminating points are given by polynomials obtained via curve fitting of the numerical results. In addition, the averaged K-factors under concentrated forces were estimated.