在不同应力幅下（60%σ_s，70%σ_s，80%σ_s，90%σ_s，σ_s为试验合金屈服强度）对7075-T651铝合金进行了应力控制下的疲劳试验，研究了其循环应变响应，观察了疲劳断口形貌、表面损伤形貌以及显微组织，分析了疲劳裂纹的萌生及扩展机制。结果表明：试验合金中析出了微米级的Al₇Cu₂Fe颗粒、纳米级的η'（MgZn₂）相和尺寸较大的η（MgZn₂）相，此外，还存在尺寸为3~10 nm的细小球状GP区；在较高应力幅（80%σ_s，90%σ_s）控制下，试验合金表现出先软化后硬化直至断裂的疲劳行为，而在较低应力幅（60%σ_s，70%σ_s）下则先软化后明显硬化并趋于稳定；试验合金主要发生微孔聚集韧窝型断裂，在较高应力幅下，裂纹源位于粗大夹杂物Al₇Cu₂Fe和第二相MgZn₂处，位错大量缠结，而在低应力幅下，裂纹源位于基体轻微撕裂处，位错形态为分散的短或长直位错线。

Stress controlled fatigue tests at different stress amplitudes, 60%, 70%, 80% and 90% of yield strength (σ_s) of the tested alloy, respectively, were conducted on 7075-T651 aluminum alloy. The cyclic strain response of the alloy was studied, the fatigue fracture morphology, surface damage morphology and microstructure were observed and the initiation and propagation mechanisms of fatigue crack were analyzed. The results show that micro-scale Al₇Cu₂Fe particles, nano-scale η' (MgZn₂) phase and relatively large sized η (MgZn₂) phase were precipitated in the tested alloy. Moreover, fine and sphere-like GP zones with size of 3-10 nm were observed. Under the control of relatively high stress amplitude (80%σ_s,90%σ_s), the tested alloy showed a fatigue behavior of first softening then hardening until fracture, while first softening then hardening until becoming stable relatively at low stress amplitudes (60%σ_s, 70%σ_s). The tested alloy mainly fractured in a micropore gathered dimple manner. At high stress amplitude, fatigue cracks initiated at the coarse inclusion of Al₇Cu₂Fe and second phase of MgZn₂, with dislocation tangling a lot. Whereas at low stress amplitude, the crack source was at the slightly torn position of the substrate, and the produced dislocation showed shapes of dispersed short or long straight dislocation lines.