Analysis of the correlation between micro-mechanical fields and fatigue crack propagation path in nodular cast iron

The morphology of the fatigue crack in nodular cast iron has great influence on the material performance under cyclic loading. In contrast to previous phenomenological studies, the present work attempts to unravel a relation between the crack propagation path and the micro-mechanical fields developing at the microstructural level. To this end, a fatigue crack in a compact tension specimen is imaged with X-ray tomography and a new segmentation procedure based on digital volume correlation is used to distinguish the part of the crack growing in the matrix from the graphite nodules. The micromechanical fields surrounding the tip of the notch as well as the tip of the fully developed fatigue crack are estimated via 3D microstructure-resolved finite element models, whose size is maintained small compared to the specimen by using boundary conditions measured in-situ with digital image correlation. The numerical fields are compared to the location of the crack nucleation site as well as to the crack growth direction. A weak correlation is found between the nucleation site and the equivalent plastic strain, suggesting that nucleation is controlled by plasticity occurring at a lower scale. On the other hand, a clear correlation is revealed between the crack growth direction and the direction perpendicular to the maximum principal stress. Based on this result, a mechanism is proposed to explain the step-like features characterizing the shape of the fatigue crack in nodular cast iron.