高温疲劳短裂纹萌生与扩展的实验研究

SHORT FATIGUE CRACK INITIATION AND PROPAGATION AT ELEVATED TEMPERATURE

对 2 2 5Cr 1Mo合金钢进行高温疲劳表面短裂纹的观测试验 ,采用中断试验和复膜方法观测短裂纹萌生扩展的演化过程 ,研究短裂纹数密度和平均长度随循环次数及载荷条件的变化特征。结果表明 ,高温疲劳损伤是由晶界孔洞开始的 ,晶界孔洞相互联结合体形成细观组织短裂纹 ;短裂纹在接近应力轴垂直方向的晶界上随机萌生和扩展 ;在寿命前期和中期疲劳损伤以裂纹萌生为主要形式 ,寿命后期由于裂纹数目增多 ,裂纹间合体效果增强 ,促使主裂纹迅速形成和长大 ,从而导致最终的疲劳失效 ;加载应变范围越大或温度越高 ,裂纹萌生和扩展越快 。

High temperature fatigue tests were performed using cylindrical specimens of 2.25Cr-1Mo. Short cracks' behavior was investigated with the method of interrupt test and replication. It was found that surface inter-granular cracking is the damage mechanism. High temperature fatigue damage begins with the initiation and coalescence of cavities along grain boundaries. These cavities then grow into short cracks. Short cracks initiate and propagate at random. At the initial and medium stage of fatigue crack initiation and propagation are the leading mechanism of fatigue damage. During the final stage because of large number of cracks, coalescence effect increases, crack propagation and coalescence dominate the damage process. It was also found that the higher the total strain rate (or the higher the temperature), the more rapid short cracks initiate and propagate and so the lower the fatigue life.