Potentials and limitations in finite element based crack propagation analysis using FRANC3D

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Examensarbete för masterexamen
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2020
Författare
Haeg, Jonas
Elfving, Filip
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Sammanfattning
About 80% to 95% of all structural failures occur due to a fatigue mechanism1. Accurate fatigue assessment is therefore of great importance. This thesis focuses on a finite element analysis based approach, implemented in the program FRANC3D, to simulate crack propagation under fatigue loading. This approach is not commonly used at GKN, where the thesis was carried out. FRANC3D offers the users many options, the effect of which has not been previously investigated at GKN. In this thesis, a sensitivity study of various settings in FRANC3D was performed for propagation analyses of a semi-elliptical surface crack in a rectangular test specimen. A group of settings were identified to be recommended and were later used for fatigue assessment of Kb test specimens for two different load types: tensile and four-point bending. The results were validated against handbook-type solutions, provided by NASGRO, and towards test results. Fatigue crack growth assessment was also performed for an embedded crack in a circular specimen—a load case not supported within NASGRO. A statistical evaluation of the finite element based approach gave similar results as the handbook-type solutions and matched the test results with respect to mean fatigue life. Thus, this thesis can be considered to have validated FRANC3D for GKN’s internal use. This opens up the opportunity for more accurate fatigue life calculations for components with complex geometry and complex thermal and mechanical loading histories.
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Fatigue, crack propagation, FRANC3D, NASGRO, acture mechanics, SIF, M-integral, surface crack, embedded crack
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