Fatigue Crack Growth Estimation in 3D Stress Fields

Abstract

Different approaches for the estimation of fatigue crack growth in 3D stress fields are compared as part of this study. An approximate estimation of fatigue crack growth in a crack along the lateral side of a beam in bending is presented as a practical example, utilizing three- and four-point bending test specimens. There is no solution available in the literature to compute the Stress Intensity Factors (SIFs) for these configurations, as it results in a three-dimensional problem. Different solutions are analyzed, including scenarios in which the beam is sliced initially in a horizontal direction and afterwards in vertical slices. For these slices there are analytical expressions to compute SIFs (that vary along the crack front locations) and correction factors that account for the effects of free boundaries. Other techniques that are used and compared include: XFEM computation; carrying out the appropriate fatigue experiments (4-point bending with a very fine-grain steel test-piece). The limitations of these techniques are shown; also the approximation and decisions, which seem more reasonable to make, are discussed.Once the SIFs are estimated, it is possible to estimate the crack propagation along the crack front. Thereafter an assessment about the remaining fatigue life, until reaching the material critical SIF, is obtained. The time consumption and accuracy in terms of component life, accuracy on crack size and shape are discussed for the different approaches and in comparison with the experimental evidence.