Micromechanisms of a macrocrack propagation behavior affected by short to long fatigue microcracks

The propagation behavior of the macrocrack and microcracks from short to long is investigated based on the Muskhelishvili's complex potential method and the equivalent crack method. The effect of microcracks on the macrocrack propagation is analyzed. The improved stress intensity factor at the crack tip and the theoretical solution of plastic zone are obtained. The equivalent crack method is applied to compute trajectories of the kinked microcracks. The direction of crack propagation is calculated based on the maximum circumferential tensile stress criterion. Three configurations of microcracks in front of the macrocrack tip are considered: arbitrarily oriented and located microcracks, parallel microcracks, and fan-shaped microcracks. Parallel distributed microcracks allow the macrocrack to propagate at the fatigue threshold Delta K (th) rapidly, then fan-shaped microcracks, followed by arbitrarily oriented and located microcracks. The results show that the physically short crack can propagate well below the Delta K (th) while the crack surface is closure-free. Moreover, the closure effect may delay the propagation process of long crack, taking longer for it to coalesce with the microcrack. The results are consistent with the experimental observations and can provide some useful information to predict the fracture or damage behaviors at the macrocrack tip for the material containing microcracks.