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

采用正火0.37%碳钢和时效硬化铝合金7075-T6，在室内空气中进行了光滑试样的旋转弯曲疲劳试验。为了明确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。试验中使用的空气条件为：20ºC的湿空气（相对湿度h = 70-80%）、35ºC的湿空气和35ºC的干空气（h = 35-45%）。各气氛下疲劳行为的主要结果可总结如下：S-N曲线受大气影响。钢在20ºC时的疲劳寿命比35ºC时长，湿度的影响可以忽略不计。然而，铝合金在干燥空气中的疲劳寿命比在潮湿空气中的要长。20℃时钢的裂纹萌生寿命比35℃时钢的裂纹萌生寿命长，但湿度的差异不影响裂纹萌生寿命。另一方面，湿度对铝合金裂纹萌生寿命的影响比对温度的影响更大，在干燥空气中比潮湿空气中裂纹萌生寿命更长。对于这两种金属，表面裂纹的扩展速率在潮湿空气中比在干燥空气中更低，因为扩展速率接近阈值水平。在潮湿空气中生长迟缓可能是由于微动氧化物的形成。对于dl/dN大于10^6 mm/循环的裂纹，湿度对其生长行为没有影响。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用威布尔分布表示。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。由于实验室空气的不同，没有观察到CV，变异系数，与1关系的显著差异。

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.