MAXIMAL COCK TIP SHIELDING BY MICROCRACKING IN ISOTROPIC ELASTIC SOLIDS
各向同性材料中微裂纹的最大屏蔽效应

In Hutchinson's (1987) opinion, there are two sources of microcrack shielding, one is the reduction in the effective elastic moduli resulting from microcracking and the other is the strain arising from the release of residual stresses when micro-cracks are formed. This residual stresses in question develop in the fabrication of polycrystalline or multi-phase materials due to thermal mismatches between phases or thermal anisotropies of the single crystals. In this paper, besides the two sources of the redistribution of stresses in the near-tip stress field of the macroscopic crack,the third source of microcrat shielding was proposed, i.e., the redistribution of stresses in the near tip induced by far field stresses while microcracking. That is to say, the microcrack could relieve the far field stresses and produce strains, this trains are different with the strains produced by the residual stresses motioned by Hutchinson, it could be noted as release of induced stresses. The influence of these three sources on shielding and anti-shielding effect of the main macrocrack due to near-tip microcracking is discussed.When considering the reduction in the effective elastic moduli resulting from microcracking and the strains produced due to release of induced stresses, the shielding and anti-shielding effect of the main macrocrack due to near-tip microcracking is then evaluated. It is concluded that: the oriedations of microcrack don't normalized to the direction of maximal tension stress when the maximum shielding effect occurs in the point under investigation. This result is different from the conclusion derived by Ortiz (1989) in isotropic brittle solids.