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中文摘要：
进行了不同应力水平下和不同应力比下2A12铝合金试验件的疲劳试验，并对试验件疲劳断口显微结构进行分析和对比，以揭示疲劳裂纹成核与扩展的微观特征。结果表明：2A12铝合金的疲劳裂纹在靠近试验件表面较为粗大的第二相粒子或试验件棱角缺陷处成核；成核位置距试验件表面的距离与应力水平和应力比有关，疲劳裂纹扩展区域也与应力水平和应力比有关。应力水平较小或应力较大时，疲劳裂纹沿晶扩展和穿晶扩展尤为明显。疲劳断口的裂纹成核粒子越小（大），试验件疲劳寿命越短（长），微裂纹成核寿命是疲劳全寿命计算中不可忽视的部分。
英文摘要：
The fatigue tests were performed for 2A12 aluminum alloys specimens at various stresses and stress ratios, and the microstructure of fatigue fractography was observed by optical microscope and scanning electron microscope. The microstructure features of crack nucleation and propagation were revealed. The results show that the fatigue cracks usually initiate in the larger second phase particles near the free surfaces or the defects in corner angle of the specimen. The distance between the crack initiation and the surface is related to stresses and stress ratios, and fatigue crack propagation zone is also related to stresses and stress ratios. The phenomenon of crack intergranular propagation and crack transgranular propagation under smaller stresses or higher stress ratios is particularly obvious. A small （large） crack-nucleating particle on fatigue fractography results in a short （long） life of specimen. Micro crack nucleation life play an important role in the calculation of holistic fatigue life.
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