Effect of Loading Angle on Crack Propagation in 6005A-T6 Aluminum Alloy Under Different Degradation Levels

To investigate the effects of loading angle on the crack propagation life and damage mechanisms of 6005A-T6 aluminum alloy under different fatigue degradation levels, a systematic fatigue crack propagation test was designed and conducted. The compact tension shear specimens were employed, and the comparative tests were performed under four degradation levels and four loading angles. The crack propagation rate was calculated and evaluated by using the seven-point incremental polynomial method. First, crack propagation curves were obtained under non-degraded and different degrees of pre-fatigue conditions. Second, the variation law of residual life at different loading angles was compared. Finally, the local strain distribution at the crack tip was analyzed by using digital image correlation (DIC) technology, and fracture morphology was observed to reveal the crack propagation mechanisms. The results show that the loading angle significantly affects the crack propagation behavior. As the loading angle increases, the crack propagation rate decreases significantly, and the residual life is greatly extended. Under the non-degraded condition, the degree of life extension can reach up to 186%. Under the 1.5 × 10⁵ cycles of degradation, the degree of life extension drops to approximately 140%, indicating that the effect of loading angle on life extension diminishes as degradation deepens. DIC results show that increasing the loading angle reduces the local strain at the crack tip, decreases the sensitivity of the crack to the main loading direction, and exhibits a stronger propagation hysteresis effect. In terms of fracture morphology, as the loading angle increases, the fracture roughness increases, and the cleavage steps and tear ridges gradually become coarser, with the crack propagation path becoming more complex, revealing the dominant role of loading angle in the crack failure mode.