Modeling of Short Crack Growth Under Biaxial Fatigue: Comparison Between Simulation and Experiment

Grain boundaries, microstructural barriers and differences in stress/strain state play a dominant role in the early stages of the fatigue crack growth of metals. Many studies on the growth of short cracks have revealed anomalies in the behavior predicted by LEFM analysis. A simulation of fatigue crack growth is presented. The polycrystalline metal was modeled as an aggregate of hexagonal grains with a different crystallographic orientation of each grain. The effect of grain boundaries on Stage I crack growth is considered in the model. The mode of shear crack growth is used to compute the crack growth. This mode is analyzed on the basis of microstructural crack growth within the first few grains, where the crack growth decelerates as the crack tip gets closer to the grain boundary. Normal crack growth has been considered for those cracks which are longer than microstructural cracking (physically short cracks). The transition from Stage I to Stage II growth is considered. The model is applied for thin-walled tubular specimens of the ferritic steel AISI 1015 and the aluminum alloy AlMgSi1 subjected to tension and torsion loading as well as in-phase and out-of-phase combined tension-torsion loading, sequential tension and torsion loading. The microstructural crack pattern and crack distribution can be successfully simulated with the model, and the simulated microstructural crack growth rate is presented.