The initiation and arrest mechanisms of small internal cracks in very high cycles fatigue regime in titanium alloys

近年来，当载荷循环次数超过107次（即确定材料的常规疲劳极限）时，发现金属材料仍会发生疲劳断裂，并且断裂模式由表面向内部转变。然而，由于内疲劳断裂的“隐形”特性，其研究面临着很大的挑战。在本研究中，采用多尺度SR-CT观察了自然引发的内疲劳裂纹的全寿命扩展行为。裂纹萌生和扩展对疲劳寿命的影响分别为57%和43%。试样断裂后，各循环的裂纹前缘叠加在断口上。裂纹扩展过程由多个面形成和随后在光滑区对应的基体中扩展组成。此外，在小于100 μ m的晶体水平上，裂纹扩展消耗了约95%的疲劳寿命。这一结果说明了极小尺寸内裂纹的重要性，对构件设计和工业维护具有重要意义。同时，观察到生长行为与断口表面特征有较强的相关性，可为事故调查等断裂分析领域提供有价值的证据。此外，β钛合金的内部裂纹扩展速度比表面裂纹慢，但比应用最广泛的（α + β）钛合金的内部裂纹扩展速度快20~100倍。

Recently, when the number of load cycles exceeds 107 where the conventional fatigue limit of the material is determined, it has been discovered that fatigue fractures still occur in metallic materials, and the fracture mode shifts from surface to interior originated. However, studies on internal fatigue fracture are facing great challenges due to its “invisible” feature.In the present study, a full-life growth behavior of a naturally initiated internal fatigue crack was observed by multiscale SR-CT. Crack initiation and propagation contributed to 57% and 43% of the fatigue life, respectively. After specimen fracture, the crack fronts at various cycles were superimposed on the fracture surface. The crack propagation process consisted of multiple facets formations and subsequent growth in the matrix corresponding to the smooth area.Moreover, around 95% of the fatigue life was consumed by the crack growth at a crystallographic level below a size of 100 um. This result indicates the significance of the very small size internal crack, which is meaningful for component design and industrial maintenance. Meanwhile, the growth behaviors were observed to have strong relations with the fracture surface feature, which could provide valuable evidence in the fracture analysis field such as accident investigation.In addition, the internal crack propagation rate in beta titanium alloy was found to be slower than its surface crack, but was 20~100 times faster than the internal crack in the most widely used (alpha+beta) titanium alloys.