Mixed-Mode fatigue crack propagation tests were carried out in stainless-steel cruciform specimens in which a center initial crack oriented at 45 deg was loaded biaxially. When the loadings were in-phase, the crack was deflected with the initial crack growth angles less than 50 deg and when they were out-of-phase, it was branched with the initial crack growth angles larger than 50 deg. In all the cases, the deflected or branched cracks propagated in Mode I, namely ΔKII was almost always zero along the crack paths. The relationship between deflected or branched cracks during propagation was also analyzed.

Symmetric branching was found in all fatigue experiments carried out in a cruciform specimen in which the initial crack was in the middle of the specimen and at 45 deg with respect to the loading axes. The loading was biaxial, sinusoidal along each axis and out-of-phase from each other. Stress intensity factor calculations using finite element methods showed ΔKII was almost zero along each branch for all four branches so that all branches were Mode I cracks. The propagation of branched Mode I cracks in the initial Mode II damaged zone was faster than in their own Mode I zone created afterwards. The two dominantly branched cracks shielded the other small cracks appeared during the initiation of branching.

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