A complex crack trajectory has been obtained by subsequently changing the orientation of a straight crack at the center of a thin circular disk of aluminum 2024-0 with respect to uniaxially applied constant amplitude fatigue loading. The propagation of the main crack has been performed by producing a series of kinks and pseudo-asymmetric forks predominantly in the plane stress state. The experiments are performed for two mixed mode load ratios. The complex crack trajectory has been analyzed with Sih's [Strain energy density factor applied to mixed mode crack problems. Int. J. Fracture 10, 305-321 (1974)] strain energy density theory. Appropriate stress intensity factors at any point on the cracked trajectory have been calculated on the basis of existing solutions for stress intensity factors on curved and kinked cracks presented by Cotterell and Rice [Slightly curved and kinked cracks. Int. J. Fracture 16, 155-169 (1980)[. In calculating stress intensity factors on the successive kinks, the procedure adopted by Suresh [Crack deflection: implication for the growth of long and short fatigue cracks. Metall. Trans. 14A, 2375-2393 (1983)] for a doubly kinked crack has been followed.
In predicting the number of load cycles required to reach the locations observed in the experiments, the fatigue crack growth criterion based on the strain energy density concept proposed by Sih and BarthelamY [Mixed mode fatigue crack growth prediction. Engng Fracture Mech. 13, 439-451 (1980)] has been used. The predicted values are in close agreement with the experiments, but in predicting the successive kink initiation angles on the basis of the strain energy density theory, significant deviation from the experiment is observed. Some merits and demerits of the criterion are discussed. A few interesting observations made in the experiment are consistent with the analysis.