STRESS INTENSITY FACTORS FOR UNUSUAL SEMI-ELLIPTICAL SURFACE CRACKS WITH RATIO A/L GREATER THAN 0.5

Most of the literature about fracture mechanics treats cracks with a flaw aspect ratio ale lower or equal to 0.5 where a is the crack depth and e the total length of the crack. The limitation to 0.5 corresponds to a semi-circular shape for surface cracks and to circular cracks for subsurface cracks. This limitation does not seem to be inspired by a theoretical limitation nor by a computational limit. Moreover, limiting the aspect ratio a to 0.5 may generate some unnecessary conservatism in flaw analysis. The present article deals with surface cracks in plates with more unusual aspect ratios a/l >0.5 (narrow cracks). A series of Finite-Elements calculations is made to compute the stress intensity factor K(I) for a large range of crack depths having an aspect ratio greater than 0.5. The K(I) values can be used with the same formalism as the ASME XI Appendix A, such that this approach can provide an extension above the inherent limitation to 0.5. Some of the results obtained are checked by using two different Finite-Elements softwares (Systus and Ansys), each one with a different cracked mesh. In addition, a comparison is made for some cases with results obtained by a XFEM approach (eXtended Finite-Element Method), where the crack does not need to be meshed in the same way as in classical Finite-Elements. The results show a reduction of stress intensity factor, sometimes significant, when considering a flaw aspect ratio above 0.5 instead of the conventional semi-circular flaw. They also show that it is not always possible to reduce the analysis of K(I) to only 2 points, namely the crack surface point and the crack deepest point. The growth by fatigue or by corrosion of a crack with such unusual shape should still be investigated.