解理断裂微观准则的研究

An Investigation Into the Micro-criterion of Cleavage Fracture

对三点弯曲预裂纹试样断口形貌、裂尖附近微孔及残留裂纹进行了观察,并用有限元方法计算了裂尖应力应变场。结果表明,解理点距裂尖存在一个最小距离,在此距离内三轴应力度过低,仅能形成微孔而不能形成解理微裂纹,RKR模型中表述的特征距离并不存在。最小韧性取决于在最小距离上满足解理所需的最小载荷。解理的临界事件是由启裂的第二相尺寸的微裂纹向周围基体的扩展。韧性值的分散和脆性粒子在裂央前的随机分布及微观解理断裂应力的波动有关。

The distributions of stress and strain ahead of a precrack tip in three point bending specimen are calculated by finite element method and the fracture surfaces, the micro-cavities and micro-cracks in the vicinities of the precrack tip are also observed. The results show that there is a minimum distance from the cleavage to the precrack tip within which there are only micro-cavities instead of micro-cracks due to lower triaxiality of stress, and that the characteristic distance in RKR model does not exist. The lowest value of K1c depends on the critical load needed to propagate a crack within the minimum distance. The critical event of cleavage is the propagation of a second phase particle-sized microcrack into the neighboring matrix. The scatter of toughness is related to the random distribution of brittle particles ahead of precrack tip and the fluctuation of cleavage fracture stress σf.