Numerical Simulation of Fatigue Growth of Cracks in the Inner Surface of a Pipe Elbow
| Autor: | LIU, CHENG-CHIEN, 劉政傑 |
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| Rok vydání: | 2018 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 106 When a pipeline is subjected to a periodic loading for a long period of time, cracks initiated on the inner wall of the pipeline may cause safety concern, which is important when cracks occur in the region near pipe elbow where stress distribution is rather complex. This paper studies the impact of the bent angle of a pipe elbow and the location of a crack in crack growth process. The finite element method is used to analyze different elliptical cracks in a curved pipe wall, and to solve stress intensity factors and energy release rates of elliptical cracks. The cracks are assumed to keep in elliptic shape when they propagate. Two different crack growth models are studied in the present work: the constant crack growth rate mode and the non-constant crack growth rate mode. The length of the long axis and the short axis of the elliptic crack increases with the same amount every computational step in the former mode. In the latter mode, the crack of the short axis advances at a preset propagation rate, therefore, fixed crack length increase every step, and the crack length increase of the long axis is determined by the crack propagation rate, which depends on the local stress intensity factors. The behaviors of stress intensity factors, energy release rates and fatigue loading cycles of cracks were investigated under the constant crack growth rate mode and the non-constant crack growth rate mode. The results show that the stress intensity factor is larger when the position angle of crack is smaller, i.e. outer side of the pipe elbow with smaller curvature. When the pipe elbow bends, the stress intensity factor will increase until the bent angle reaches about 15 degree, after which the stress intensity factors reduces a little and the level up. Therefore, it can be concluded that the fracture position angle is minimal and the elbow bending angle is easily affected by the crack at 15 degrees. In both crack growth modes, stress intensity factors and energy release rates of the initial long axis are smaller than that of the short axis, however, the variation of the stress intensity factors and energy release rates of the long axis is larger than those of the short axis, and the stress intensity factors and the energy release rates of the long axis increase at faster rates than that of the short axis. In the constant crack growth rate model, the long axis and the short axis of crack are fixed at length 0.1mm. Under the two crack growth modes, the initial aspect ratio of two axes has little effect on the fatigue cycle life of the long axis. The short axis will have a shorter fatigue cycle life as the initial short axis ratio is smaller. In the non-constant crack growth rate mode, the crack length of the long and short axes of the elliptic crack has a direct effect on the local stress intensity factors, which cause the crack growth rates of the crack at different location are different along the crack front. However, as the crack advances, the elliptic aspect ratio of the crack will approaches to 0.88 under the present study. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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