Abstract: The driving force for the propagation of the macrocrack is reduced by the microcrack zone around the crack tip region of the macrocrack,meanwhile, damage is occurred in the macrocrack tip region, in which the materials is weakened by microcrack damage. Then the fracture toughness of the materials in the macrocrack tip region is decreased. The evaluation of the effect of the microcrack damage on the mechanical behavior must take into account both the stress-shield effect and the decreasing effect of the frac...