EFFECT OF ATMOSPHERE ON THE FATIGUE BEHAVIOR AND ITS STATISTICAL CHARACTERISTICS OF Al ALLOY SPECIMENS

在室内空气中使用标准化的0.37%碳钢和时效硬化的铝合金7075-T6对光滑试样进行旋转弯曲疲劳试验。为了阐明空气条件对疲劳损伤的影响，每次试验都改变实验室空气的温度和湿度。测试中使用的空气条件为20 ° C下的潮湿空气（相对湿度h = 70-80%）、35 ° C下的潮湿空气和35 ° C下的干燥空气（h = 35-45%）。S-N曲线受大气影响。钢在20 ℃下的疲劳寿命比在35 ℃下的长，而湿度的影响可以忽略不计。而铝合金在干燥空气中的疲劳寿命要比潮湿空气中的长. 20 ℃时的裂纹萌生寿命比35 ℃时长，但湿度的差异不影响裂纹萌生寿命。湿度对铝合金裂纹萌生寿命的影响比对温度的影响更大，且在干燥空气中的裂纹萌生寿命比在潮湿空气中的裂纹萌生寿命长;两种金属在潮湿空气中的表面裂纹扩展速率都比在干燥空气中的低，当裂纹扩展速率接近临界值时，裂纹的扩展速率比在干燥空气中的低。在潮湿空气中的生长速率的延迟可能是由于微动氧化物的形成。对于dl/dN超过10^6 mm/循环的裂纹，湿度不会影响其生长行为。温度对裂纹扩展速率也有影响，但影响不大.裂纹萌生寿命、扩展寿命和疲劳寿命分布均服从威布尔分布。裂纹扩展速率的统计分布由对数正态分布和威布尔分布函数表示。由于实验室空气的差异，未观察到CV（变异系数）与1的关系存在显著差异。

Rotating bending fatigue tests of the smooth specimens were carried out in room air using a normalized 0.37 % carbon steel and an age hardened aluminum alloy 7075-T6. In order to clarify the effect of air conditions on the fatigue damage, both the temperature and humidity of laboratory air were changed for each test. Air conditions used in the tests were moist air (relative humidity h = 70-80%) at 20ﾟC, moist air at 35ﾟC and dry air (h = 35-45%) at 35ﾟC.The main results of fatigue behavior in each atmosphere may be summarized as follows :1. The S-N curve is influenced by the atmosphere. The fatigue life of steel at 20ﾟC is longer than at 35ﾟC, while the effect of humidity is negligible. However, the fatigue life of Al alloy is longer in dry air than moist air.2. The crack initiation life of steel is longer for 20ﾟC than 35ﾟC, however the difference in humidity does not affect the crack initiation life. On the other hand, crack initiation life of Al alloy is strongly affected by the humidity than temperature and it is longer in dry than moist air.For both the metals, the growth rate of a surface crack is lower in moist air compared with dry air, as the growth rate approaches the threshold level. The retarded growth rates in moist air may result from the formation of fretting oxides. For cracks with dl/dN in excess of 10^6 mm/cycle, humidity does not influence the growth behavior. The temperature also affects the crack growth rate, but this effect is not large.3. The crack initiation life, growth life and fatigue life distribution is represented by Weibull distribution. The statistical distribution of crack growth rate is expressed by both a log-normal and a Weibull distribution function. No significant difference in the CV, the coefficient of variation, versus 1 relationship is observed due to a difference in laboratory airs.