A7085铝合金Ⅰ-Ⅱ复合型疲劳裂纹扩展及其数值模拟

Ⅰ-Ⅱ mixed-mode fatigue crack propagation of A7085 aluminum alloy and its numerical simulation

为探究不同加载角度下A7085铝合金Ⅰ-Ⅱ复合型疲劳裂纹扩展机理,在MTS疲劳试验机上采用紧凑拉伸剪切试件(CTS)对A7085铝合金进行不同加载角度的疲劳实验;用有限元分析计算不同裂纹扩展长度的裂纹尖端应力强度因子,通过七点递增多项式法对数据进行处理,计算出A7085铝合金Paris公式中的参数C和m.结果表明不同加载角度的裂纹基本沿着与外载荷垂直的方向扩展,裂纹扩展路径近似为一条直线,裂纹扩展角测量结果基本符合最大环向拉应力理论;Ⅰ-Ⅱ复合型裂纹一旦发生扩展,Ⅱ型应力强度因子KⅡ所占比例急剧减小,Ⅰ型应力强度因子KⅠ不断增大,此后KⅡ远远小于KⅠ,有效应力强度因子(KⅠ和KⅡ的组合)基本等于KⅠ,相当于裂纹扩展主要受Ⅰ型应力强度因子控制,研究结果有助于对Ⅰ-Ⅱ复合型疲劳裂纹扩展机理的理解.

Compared with other types of aluminum alloys,A7085 aluminum alloy has a series of excellent properties such as high strength,high toughness,and high fatigue resistance. These advantages meet the requirements of aircraft performance; thus,A7085 aluminum alloy is widely used for fabricating aircraft components. The shell cracks in aeronautical structures are often mixed-mode cracks,i. e.,comprising open type and sliding type,and they are also known as the Ⅰ-Ⅱ compound crack. It has been found that fatigue fracture is the main reason for the failure of most specimens. At present,most studies on fatigue crack are focused on mode Ⅰcrack,but the load on the specimen is usually not a single pure type Ⅰ,Ⅱ,or Ⅲ mode. It is usually a combination of these three kinds of loads. When the crack is subjected to Ⅰ-Ⅱ mixed-mode loads,its crack growth rate and crack growth path are affected by the loading conditions. To investigate the mechanism of Ⅰ-Ⅱ mixed-mode fatigue crack growth of A7085 under different loading angles,mixed-mode( Ⅰ-Ⅱ) fatigue crack growth tests were performed on compact tension shear( CTS) specimens using a servo-hydraulic fatigue testing machine. The stress intensity factor of the crack tip was calculated by finite element analysis. Furthermore,C and m in the Paris law were calculated using seven-point incremental polynomial methods. The results show that when under different loading angles,cracks will extend along the vertical direction of the external load. Moreover,the path seems to be a straight line. The results of experiments agree with the maximum tensile stress theory. Once the crack expands,type Ⅱ stress intensity factor KⅡbasical-ly remains at 0,while type Ⅰ stress intensity factor KⅠincreases gradually. The stress intensity factor amplitude is almost equal to KⅠ,and crack propagation is mainly controlled by KⅠ. The result is helpful to understand the mechanism of the Ⅰ-Ⅱ fatigue crack propagation.