Abstract: Focusing on the fatigue failure issues of the servicing aircrafts in coastal areas, non-corrosion and 3.5% NaCl salt spray in-situ corrosion multiaxial fatigue tests with 200 MPa, 250 MPa and 300 MPa of equivalent stresses were conducted on 7075 aluminum alloys with self-developed corrosion devices. Comparative analyses of multiaxial fatigue failure mechanism were implemented by cyclic loop and fracture morphology and modified Manson-Coffin-Basquin(MCB) criterion was brought up for life prediction. Results show that with the increase of equivalent stresseses, multiaxial fatigue life of both non-corrosion and in-situ corrosion 7075 aluminum alloy decreases. Under the same equivalent stresses, multiaxial fatigue life of in-situ corrosion specimens significantly deduces and area of both axial and torsional hysteresis loops augment in different extents compared with the non-corrosion specimens. With the equivalent stresses of 200 MPa, multiaxial fatigue life of specimen is 99 675 cycles under non-corrosion condition while decreases to 55 284 cycles under in-situ corrosion conditions. Cracks initiate at the outer surfaces of specimens in non-corrosion and in-situ corrosion environments. Sized salt occurs at the crack propagation regions for in-situ corrosion specimens. Inducing the influence of loading conditions and environmental factors, the modified MCB criterion gives fine prediction results that all predicted life values are within 2x scatter bands.

Key words: 7075 aluminum alloy, multiaxial fatigue, in-situ corrosion, failure mechanism, life prediction

摘要： 针对沿海地区飞机服役过程中的疲劳失效问题，利用自行开发的腐蚀装置对7075铝合金进行未腐蚀和3.5%NaCl盐雾原位腐蚀条件下，等效应力分别为200 MPa、250 MPa和300 MPa时的多轴疲劳试验。结合循环曲线和断口形貌对未腐蚀和原位腐蚀条件下的多轴疲劳失效机理进行对比分析，并提出改进的Manson-Coffin-Basquin（MCB）准则进行寿命预测。结果表明，随着等效应力的增大，7075铝合金未腐蚀和原位腐蚀条件下多轴疲劳寿命均下降；等效应力相同时，相对于未腐蚀试样，原位腐蚀试样的多轴疲劳寿命显著缩短，轴向和扭向滞回线的面积均不同程度增大；200 MPa等效应力条件下，试样多轴疲劳寿命在未腐蚀条件下为99 675周次，原位腐蚀条件下下降至55 284周次；未腐蚀和原位腐蚀条件下，裂纹源均出现在外表面，原位腐蚀试样扩展区有盐粒出现；修正的MCB准则引入加载条件和环境因素，得到了良好的预测效果，寿命预测值均位于两倍分散带内。

关键词: 7075铝合金, 多轴疲劳, 原位腐蚀, 失效机理, 寿命预测