复合型裂纹起裂角厚度效应的实验研究

AN EXPERIMENTAL STUDY OF THE THICKNESS EFFECT ON THE INITIAL ANGLE OF MIXED MODE CRACK

完成了飞机结构铝合金LC4CS的2、4、8和14mm四种不同厚度试样在I+II复合加载条件下的复合型断裂实验,系统分析了厚度和复合载荷对裂纹起裂角的影响,揭示了常用复合型断裂准则的厚度适用范围,用三维断裂理论对结果进行了讨论。结果表明:复合型裂纹起裂角具有明显的厚度效应;最大周向应力准则能够准确预测薄试样和厚试样(厚度为2 mm 和14 mm)在各种复合加载条件下的起裂方向,但是不适用于中间厚度的试样,尤其是8 mm厚度的情况。最大三轴应力准则试图考虑裂纹尖端三维约束对裂纹起裂的影响,但是结果并不理想。最小应变能密度因子理论的预测结果与最大周向应力准则的预测结果非常接近,但同样不能预测8mm厚度试样的起裂方向。非常有必要建立一个普遍适用的三维复合型断裂准则。

Mixed-mode I/II fracture experiments were performed on aircraft structure aluminum alloy LC4CS. Compact tension (CT) specimens of four kinds of thickness, 2, 4, 8 and 14 mm were used. The coupled effects of thickness and mixed mode I/II loading on the initial angle of crack are first analyzed systematically, and the applicable scope of the conventional mixed mode fracture criteria is revealed. The experimental and theoretical results are discussed by three-dimensional fracture theory. It is shown that the mixed mode crack has an obvious thickness effect. The maximum tangential stress (MTS-) criterion established on the basis of plane hypothesis is adequate to predict the crack initiation direction of the thin (2mm) and thick (14mm) plates under mixed-mode load conditions. However, it is not suitable to middle-thick plate, especially the 8 mm-thick plates. Although the maximum triaxial stress (M-) criterion considers the influence of three-dimensional constraint on crack initiation, it is not applicable to the aluminum alloy in this paper. Crack initiation angle predicted by the minimum strain energy density (S-) criterion is very close to that by MTS-criterion. It is necessary to develop a universal three-dimensional mixed mode fracture criterion that is valid for various thickness conditions.