The behaviors of model-I fatigue crack propagation behaviors under different strain cycles in single crystal aluminum have been systematically investigated by molecular dynamic and quasicontinuum method with embedded atom potential. Four different crack orientations: (010)[001], (111)[11-2], (110)[001] and (101)[10-1] are investigated by using the edge-crack model. Different fatigue crack growth mechanisms such as cleavage crack propagation, twinning and dislocation emission are observed. Premature crack surface contact during the unloading path is also observed for the (010)[001] crack, which is consistent with the crack closure hypothesis in the classical fatigue theory. The relationship between local deformation and crack growth kinetics are identified by using crack tip increments and crack tip opening displacement (CTOD) profiles at the selected stress cycle. The results show that crack only grows during part of the loading path and no crack growth during the unloading path, which are well in agreement with our previous in-situ SEM observations.

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