Numerical Investigation of Mixed-Mode Crack Growth

Sammanfattning

Fatigue crack growth poses a critical challenge to service life of components, especiallyin safety-sensitive industries such as aviation and railway transportation,where structural components are subjected to millions of load cycles during service.Reliable prediction of crack growth direction and rate is essential for damage-tolerantdesign and maintenance planning.Stress intensity factor-based criteria are widely used when the crack is loaded inone single mode. However, their applicability for mixed-mode crack growth undernon-proportional loading conditions remains limited. The Vector Crack Tip Displacement(VCTD) criterion has been proposed in the literature as an alternative,offering advantages in generality by relying on displacement measures rather thanassuming linear elastic material with small-scale yielding. However, its performanceunder general mixed-mode fatigue loading conditions requires further investigation.In this study, several mixed-mode fatigue crack growth experiments from the literatureare modeled within a two-dimensional Finite Element (FE) framework. Theexperiments include Four Point Bending, Compact Tension Shear, two variations ofbiaxial stress experiments, and a Twin-disc experiment. The performance of threeformulations of the VCTD criterion are investigated in terms of their ability to predictcrack growth direction. In addition, crack growth rate using Paris law and theresulting fatigue life are assessed independently along the predicted crack paths, andsequentially compared against experimental data from the literature.The VCTD criterion successfully captures crack growth direction in cases wherethe crack propagates along a straight path, with good agreement between predictedfatigue life and experimental results from the literature. However, all three formulationsconsistently fail to predict abrupt changes in crack direction, such as kinkingand branching behavior. The origin of this discrepancy is discussed, consideringthe following possible contributing factors: microstructural features that may drivekinking in the physical experiments in ways not captured by the FE model, simplificationsinherent to the FE framework that may affect the displacement fields atthe crack tip, and a potential fundamental limitation of the VCTD criterion itself,which may lack a mechanism to detect abrupt directional changes.