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Simulating Fatigue Cracks Growth in Spiral Bevel Gears Part 8

Tham khảo tài liệu 'simulating fatigue cracks growth in spiral bevel gears part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 5.4.3 Approximations of Method The accuracy of the proposed method is limited by the accuracy of the BEM results. In addition several approximations and simplifications in the loading conditions and crack growth rate models could affect the accuracy. The shape of the elliptical contact patches is estimated by straight lines. When the applied traction is far enough away from the crack front such that the deformation due to local contact stresses does not contribute to the crack tip field St. Venant s principle holds. In this case the shape of the traction area is inconsequential and the straight line approximation is legitimate. However if the crack trajectory is significantly close to a contact patch then this assumption is no longer valid. As a result the accuracy of the SIFs could be comprised. Furthermore the size and location of each of the fifteen contact areas are kept constant throughout the crack propagation. In reality the change in flexibility of the tooth could change the contact areas and in turn affect the SIF distribution in the later stages of propagation. This aspect will be investigated in Chapter 7. However if the same contact areas are used during the entire crack propagation simulation the fatigue life predictions will most likely be conservative. This is because the SIFs continuously increase as the crack advances when the loading scenario is kept constant and the crack length is increasing. It is assumed that the crack front conditions are characterized by plane strain along the entứe front. This is consistent with the maximum principal stress theory equations implemented to calculate the kink angle. However shallow cracks and portions of edge cracks near the free surface are usually characterized by plane stress not plane strain. Nevertheless crack growth rates will be larger in plane strain conditions than plane stress. This assumption errs on the conservative side and will predict shorter fatigue lives. The size of the plastic zone in the .