预蠕变损伤对高温疲劳宏观裂纹扩展的影响

针对ＳＵＳ３０４不锈钢光滑试棒预先导入１０７３ Ｋ·ｃｐ－ｔｙｐｅ条件下的预蠕变疲劳损伤，然后开小切口进行时间依存性 ９２３ Ｋ·ｃｐ－ｔｙｐｅ，１０７３ Ｋ·ｃｐ－ｔｙｐｅ及循环数依存性９２３ Ｋ·ｐｐ－ｔｙｐｅ的宏观裂纹扩展试验，考察了试棒内部因预蠕变疲劳而产生的大量的粒界微小裂纹对高温疲劳宏观裂纹扩展的影响．结果如下： １．预损伤加速了９２３ Ｋ·ｃｐ－ｔｙｐｅ下的蠕变裂纹扩展，对于同一蠕变Ｊ积分范围△Ｊｃ，损伤值越大，裂纹扩展速度ｄｌ／ｄＮ也越大．这种加速起因于主裂纹与微小裂纹的合体． ２．１０７３ Ｋ·ｃｐ－ｔｙｐｅ下的预损伤材料和处女材料的ｄｌ／ｄＮ在同一△Ｊｃ。下相等．即，损伤材料的裂纹扩展速度的上限值由１０７３ Ｋ·ｃｐ－ｔｙｐｅ下的处女材料的ｄｌ／ｄＮ－△Ｊｃ关系给出． ３．在９２３ Ｋ·ｐｐ－ｔｙｐｅ条件下，对于同一疲劳Ｊ积分范围△Ｊｆ，预损伤材料的ｄｌ／ｄＮ要比处女材料快１０倍左右．一般ｐｐ－ｔｙｐｅ的破坏形式为粒内破坏．预损伤材料的场合，因为试棒内部分布有大量的微小粒界裂纹，主裂纹便沿这些破坏阻抗最小的微小裂纹边合体边扩展，主要在粒界上扩展．即微小粒界裂纹是裂纹扩展阻抗减小的主因．

Effect of pre-creep damage on fatigue crack propagation at high temperatures

The effect of pre-creep damage on successive macrocrack propagation was examined on the basis of a nonlinear fracture mechanics concept. Fatigue loading with tension hold was subjected to smooth specimens of a Type 304 stainless steel at 1073 K for giving a pre-creep damage. After a notch was provided at the center of the pre-damaged specimen , the crack propagation tests were conducted in time dependent fatigue at 923 K as well as 1073 K and cycle dependent fatigue at 923 K. As the results, the pre-damage significantly accelerated the successive crack propagation rate. In the time dependent fatigue , the propagation rate of pre-damaged specimen presented a few times faster than that of virgin one for an equal value of creep J-integral (C parameter). It was clarified that the acceleration was caused by the reduction of crack propagation resistace due to multiple small cracks distributed in the vicinity of the macrocrack, which was the substance of pre-damage introduced by the creep fatigue. In the c ycle dependent fatigue ,the propagation was drastically influenced by the pre-damage. The rate was over ten times faster than that of virgin material for an equal value of fatigue J-integral range (cyclic Jintegral) , and the crack , which usually propagated through grains in the fatigue of virgin material ,exhibited the intergranular path. This pointed out that the coalescence with the small cracks, which were the substance of the predamage ,accelerated the macrocrack propagation.