Ultrasonic and conventional fatigue tests were carried out on the AISI-SAE AA7075-T6 aluminum alloy, in order to evaluate the effect of artificial and induced pre-corrosion. Artificial pre-corrosion was obtained by two hemispherical pitting holes of 500-μm diameter at the specimen neck section, machined following the longitudinal or transverse direction of the testing specimen. Induced pre-corrosion was achieved using the international standard ESA ECSS-Q-ST-70-37C of the European Space Agency. Specimens were tested under ultrasonic fatigue technique at frequency of 20 kHz and under conventional fatigue at frequency of 20 Hz. The two applied load ratios were: R = −1 in ultrasonic fatigue tests and R = 0.1 in conventional fatigue tests. The main results were the effects of artificial and induced pre-corrosion on the fatigue endurance, together with the surface roughness modification after the conventional fatigue tests. Crack initiation and propagation were analyzed and numeric models were constructed to investigate the stress concentration associated with pre-corrosion pits, together with the evaluation of the stress intensity factor in mode I from crack initiation to fracture. Finally, the stress intensity factor range threshold ΔK_TH was obtained for the base material and specimens with two hemispherical pits in transverse direction.

中文翻译：

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航空铝合金7075-T6的人工和诱导预腐蚀的超声和常规疲劳强度

为了评估人工腐蚀和诱导预腐蚀的影响，对AISI-SAE AA7075-T6铝合金进行了超声波和常规疲劳测试。人工预腐蚀是通过在试样颈部的两个直径为500μm的半球形点蚀孔进行的，并按照测试样品的纵向或横向方向进行加工。使用欧洲航天局的国际标准ESA ECSS-Q-ST-70-37C实现了预腐蚀。样品在超声疲劳技术下以20 kHz的频率进行测试，并在常规疲劳下以20 Hz的频率进行测试。施加的两个负载比率为：超声疲劳试验中R = -1，常规疲劳试验中R = 0.1。主要结果是人工腐蚀和人工腐蚀对疲劳强度的影响，以及常规疲劳测试后的表面粗糙度改进。分析了裂纹的萌生和扩展，并建立了数值模型来研究与预腐蚀坑相关的应力集中，并评估了从裂纹萌生到断裂的模式I中的应力强度因子。最后，应力强度因子范围阈值ΔK对于基材和在横向上具有两个半球形凹坑的样品，获得了_TH。