Under prolonged operational conditions, the servo motor housing is prone to developing numerous fatigue cracks, which compromise its structural integrity and service lifespan. To address this issue, we apply a fatigue life prediction method rooted in small crack theory to the servo motor housing, while also exploring innovative approaches for damage tolerance design and life prediction. This study conducts extensive tests on the propagation of both long and small cracks in the L-T and T-L directions at room temperature, utilizing 7050 aluminum alloy single-sided notch tensile specimens. Based on the propagation data of small and long cracks, the da/dN-ΔK data of crack propagation are obtained by fitting. Scanning electron microscopy（SEM） is used to conduct microscopic analysis of the specimen’s fracture surface and to quantitatively assess the material’s initial defects. Based on the initial crack size and crack propagation data, the three-dimensional crack propagation theory analysis method based on fracture mechanics and the finite element simulation method of ABAQUS-Franc3D software are used to predict the fatigue life of the servo housing. The predicted results are evaluated using the actual fatigue life, and the predicted and experimental results are in good agreement.

Flaw tolerance performance of TC4 alloy was investigated for helicopter transmission systems design. The impact pit flaws and scratch flaws were investigated by fatigue specimens with same flaw depth for flaw tolerance design, and high-cycle fatigue S-N curve tests were conducted. The results show that both scratch flaws and impact pit flaws significantly reduce the fatigue limit of the TC4 titanium alloy. The scratch flaw has a higher flaw influence factor（K_flaw） of 2.29 compared to the impact pit flaw’s K_flaw value of 1.75 under the same flaw depth conditions. Fracture surface analysis provided laws of the fatigue crack initiation and propagation associated with these flaws. Scratch flaws mainly exhibit multiple sources, with crack initiated at the root of the scratch, while the characteristics of fatigue crack initiation for impact pit flaws vary with the applied stress. Multiple sources are observed under high stress lever, with crack initiated on the surface of the pit, while under low stress, the crack initiates at a subsurface of the pit.