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

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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.