The mesh and engineering judgement have a large impact on the quality of the FEA results. After all, FEA is a numerical methods approach towards the real solution. Therefore, hand calculations should always be performed to verify the results. Remember that a static FEA does not consider instability, inertial effects, fatigue, etc.
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Meshing the entire structures is not always feasible or particle. A good engineering practice is to break up the structures into simpler elements for inspection and analysis [1].
At least three mesh layers are required to capture the change in bending stress through the thickness [2]. Five is a desirable lower limit.
Pay special attention to the transitioning between mesh densities, since abrupt transitioning introduces errors of a numerical nature [3]. The results may be overly conservative and non-realistic. Mesh transitions should be located away from the areas of interest in a region.
For simple elastic structures, use a coarse mesh for strength analysis and a fine mesh for fatigue analysis [4]. More complex shapes or steep stress gradients may require finer meshes. Brittle materials are defined as materials with a final strain before material rupture of less than 5% (εf < 0.05). They require a much finer mesh as the lack of ductility does not permit localized yielding.
For simple elastic structures, the suggested maximum element dimension is 10 times the smallest element dimension in a strength analysis [4]. Even with more complex shapes, this serves as a good starting point with further iterations of refinement from there.
Remember to defeature you model for the mesh. The solid part may contain unnecessary details that should be omitted when you set up your FEA model. Common details to defeature include small holes, faces, engravings, faces, features, etc.
Use localized mesh controls to fine-tune your mesh to adequately model the transition zones. The standard automatic mesh provides a good starting point, but should be fine-tune through iterations and should be refined at hole locations, curves, corners, etc.
Remember to split the face to accurately model load effects, corners, areas of interest, etc. For example, in this bolt-bearing region, the entire interior face of the hole does not bear against the bolt. Rather the bearing zone from the bolt was modeled at a 25° zone and the face was split [2]. The corners and areas of concern where also split to refine the mesh density.
Remember to fillet interior corners as sharp re-entrant corners result in stress singularities. The more you refine, the more the stress never converges to the true value. Adding a fillet removes the infinite stress, but interior fillets usually need a reasonable amount of mesh refinement [2]. Exterior corners are less important at getting accurate results.
If the stress does converge but are still extremely high and unexpected, you may be modeling a stress peak. Stress peaks or concentrations do not significantly affect ductile failure. The effects of stress concentration are ignored since the material will yield locally at the stress-riser while the material further away from the riser remains below yield strength [5]. This is further verified by the allowance of highly localized stress that are blunted by confined yielding [6].
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Eric Kuusisto is a registered Professional Engineer (Civil-Structural). He has worked in a wide range of structural engineering projects, from skyscrapers to transmission towers to oil & gas. Currently works for HALFEN USA as an Engineering Technical Representative. Please like and comment!
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