复合型韧性断裂实验和控制参数

通过对不同韧性材料在各种平面复合载荷形式下裂纹启裂阶段裂端变形、启裂位置和扩展方向的系统的宏微观实验验证及计算分析，考察了韧性断裂参数空穴扩张比的分布特征和裂纹启裂及扩展方向的关系．得到：对于不同韧性的材料，在裂端的钝化变形区域，空穴扩张比的极大值区对应干裂纹的启裂位置，裂纹启裂时钝化裂端前缘空穴扩张比的临界值不敏感于复合比的变化．而对于裂纹启裂后的扩展方向，则需根据具体材料在相应的特定区域中比较空穴扩张比参数极大值的分布特征，需经进一步的分析，从而确定裂纹的扩展方向．实验及计算结果表明，尽管复合型断裂时裂纹启裂及扩展的机理极其复杂，用于韧性材料复合型断裂的空穴扩张比参数仍能很好地预测裂端的启裂及扩展方向，可作为复合型韧性断裂过程的控制参数．

MIXED MODE DUCTILE FRACTURE EXPERIMENT AND ITS CONTROLING PARAMETER

In this paper, the experiment of a series of plain strain mixed mode ductile fractureis carried out in two kinds of ductile materials (quenChed aluminum alloy LY12-CZ and annealedaluminum alloy LY12-M). The microscopic characteristic behavior of deformation and the initiationof blunted crack tip is investigated by using SEM near the area ahead of the blunted crack tip inthe middle thickness profile at the critical range of crack initiation. The adaptability of ductilefracture parameter--void growth ratio is re-examined by the experimental data and numericalanalysis in large deformation process of the ductile metal materials under mixed mode loading.The comparison between the results from the experiment and those from FEM analysis is basedon the same deformation at the blunted crack tip with the aid of the same crack tip openingReceived 20 October 1997, revised 9 June 1998.displacement (CTOD) or crack tip sliding displacement (CTSD), then the critical value VG atwhich the crack initiation occurs is evaluated. The directions of trace line of maximal value of VGdistribution and the mixed mode fracture angles are discussed in different materials and mixedmode loading conditions.From the results under comparison and discussion it could be concluded that: (1) The deformation of blunted crack tip appeals as non-symmetry blunted deformation, the initiation positionof crack tip is always in the area of violent blunted deformation, specially in the condition of pureshear loading. (2) The distribution of VG near the blunted crack tip is different from which faraway from the crack tip. This property is different from that in Liner Elastic Fracture Theory inwhich the distribution of VG is unchanged with the distance from the crack tip. The transferredarea of VG distribution is affected by the loading condition, for example, this transferred areais near the crack tip in pure mode II and it is far away from the crack tip in pure mode I. (3)The position of maximal value of the void growth ratio near crack tip is consistent well with theposition of the initiation in the blunted crack tip. And the critical value of the void growth ratiois insensitive to the differences of mixed mode in a definite ductile material. (4) When a kind ofcharacteristic distance which depending on the material properties is considered, the direction ofthe crack growth is closed with the direction of the distribution of the maximal value of the voidgrowth ratio in a definite distance. So we can conclude that the ductile fracture parameter--voidgrowth ratio could be used to predict the initiation and growth direction of the ductile mixed modefracture when considering a material characteristic parameter.