2022 Fiscal Year Annual Research Report
The initiation and arrest mechanisms of small internal cracks in very high cycles fatigue regime in titanium alloys
| Project/Area Number |
22J10639
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| Allocation Type | Single-year Grants |
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| Research Institution | Hokkaido University |
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Principal Investigator |
薛 高格 北海道大学, 大学院工学院, 特別研究員(DC2)
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| Project Period (FY) |
2022-04-22 – 2024-03-31
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| Keywords | Very high cycle fatigue / Internal crack / Titanium alloy / Crack growth |
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| Outline of Annual Research Achievements |
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.
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| Current Status of Research Progress |
Current Status of Research Progress
1: Research has progressed more than it was originally planned.
Reason
The present progress follows the designed experimental steps. Thanks to the unprecedented results, the publication has also been accepted smoothly.
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| Strategy for Future Research Activity |
The present progress has already observed the internal crack growth behaviors. The author's topic has been accepted for collaboration with the National Institute of Material Science (NIMS@Tsukuba). Their state-of-the-art will help us to investigate the mechanism of internal crack growth. In the future, the study will also proceed in comparing the internal crack behaviors with other high-strength materials to establish the overall comprehension of internal fatigue fracture behaviors.
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