MODE Ⅰ AND MODE Ⅱ CRACK TIP ASYMPTOTIC FIELDS WITH STRAIN GRADIENT EFFECTS

<正> The strain gradient effect becomes significant when the size of frac-ture process zone around a crack tip is comparable to the intrinsic material length l,typically of the order of microns. Using the new strain gradient deformation theorygiven by Chen and Wang, the asymptotic fields near a crack tip in an elastic-plasticmaterial with strain gradient effects are investigated. It is established that the dom-inant strain field is irrotational. For mode Ⅰ plane stress crack tip asymptotic field,the stress asymptotic field and the couple stress asymptotic field can not exist si-multaneously. In the stress dominated asymptotic field, the angular distributions ofstresses are consistent with the classical plane stress HRR field; In the couple stressdominated asymptotic field, the angular distributions of couple stresses are consistentwith that obtained by Huang et al. For mode Ⅱ plane stress and plane strain cracktip asymptotic fields, only the stress-dominated asymptotic fields exist. The couplestress asymptotic field is less singular than the stress asymptotic fields. The stressasymptotic fields are the same as mode Ⅱ plane stress and plane strain HRR fields,respectively. The increase in stresses is not observed in strain gradient plasticity formode Ⅰ and mode Ⅱ, because the present theory is based only on the rotational gradi-ent of deformation and the crack tip asymptotic fields are irrotational and dominatedby the stretching gradient.