Initiation and Early-Stage Growth of Internal Fatigue Cracking Under Very-High-Cycle Fatigue Regime at High Temperature
Initiation and Early-Stage Growth of Internal Fatigue Cracking Under Very-High-Cycle Fatigue Regime at High Temperature
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高温甚高周疲劳状态下内部疲劳裂纹的萌生和早期扩展
DOI：
10.1007/s11661-020-05633-3
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发表时间：
2020-04-01
影响因子：
2.8
通讯作者：
Chen, B.
中科院分区：
文献类型：
--
作者：
Zhao, Z.;Zhang, F.;Chen, B.
研究了ni基定向凝固高温合金在室温、750℃和850℃条件下的高周疲劳萌生和早期裂纹扩展。与100 Hz相比，20 kHz频率对疲劳寿命的影响较小，变形和断裂机制也没有影响。位错缠结在γ / γ′界面处重新排列形成明确的网络，这是VHCF在850℃时疲劳强度比在750℃时增强的原因。在大多数情况下，铸造内部孔隙是裂纹起裂部位。裂纹萌生和早期扩展发生在{111}面的其中一个面或它们的相交面上，这是第一阶段裂纹的特征。利用优化后的间歇加载条件，基于I阶段裂纹区域内细小但可见的滩痕，成功地跟踪了裂纹的萌生和早期裂纹扩展过程。第一次登记的疲劳海滩标记距离裂纹起裂点仅为86 μ m，并且在整个早期裂纹扩展阶段，裂纹长度稳步增加。850℃时疲劳强度的提高可以用较高的早期裂纹扩展阈值来解释。早期裂纹扩展所消耗的疲劳寿命比例随着应力的减小而减小，最终导致初始控制的VHCF疲劳失效。讨论了这些结果对疲劳寿命和疲劳强度模型预测的意义。
The initiation and early-stage crack growth under very-high-cycle fatigue (VHCF) at room temperature, 750 degrees C, and 850 degrees C on directionally solidified Ni-base superalloy have been investigated. There was little frequency effect of 20 kHz on fatigue lives when compared with 100 Hz, nor did the deformation and fracture mechanisms. Dislocation tangles re-arranged themselves to form well-defined networks at interface of gamma/gamma ', accounting for the enhanced fatigue strength at 850 degrees C in VHCF regime when compared to that at 750 degrees C. In most cases, internal casting pore was the crack initiation site. Crack initiation and early-stage growth occurred on one of the {111} planes or their intersecting planes, a characteristic of Stage I cracking. With the use of optimized intermittent loading conditions, both the initiation and early-stage crack growth processes were successfully tracked on the basis of fine but visible beach marks within the Stage I cracking region. The first registered fatigue beach mark can be as close as only 86 mu m to the crack initiation site and the crack length increased steadily over the whole early-stage crack growth stage. The enhanced fatigue strength at 850 degrees C can be rationalized with the higher threshold for propagating the early-stage crack. The fraction of fatigue life consumed for early-stage crack growth reduces with the decreasing stress, eventually leading to the initiation-controlling VHCF fatigue failure. The implications of these results are discussed with respect to the model prediction of fatigue life and fatigue strength.