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

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 Ⅰ crack, 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 K_Ⅱ basical-ly remains at 0, while type Ⅰ stress intensity factor K_Ⅰ increases gradually. The stress intensity factor amplitude is almost equal to K Ⅰ, and crack propagation is mainly controlled by K_Ⅰ. The result is helpful to understand the mechanism of the Ⅰ-Ⅱ fatigue crack propagation.