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I started with a moderate element size of 20 mm (based off of Tommy’s mesh independence study) and used default mesh settings for the gussets. This generates a single layer of rectangular prism elements for the gussets, with some elements having a visually high aspect ratio. In CFD, high aspect ratios are usually bad unless you know for sure that the gradient of the parameter you are trying to calculate is low. (Maybe this isn’t relevant for FEA?)
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After running a simulation, the maximum deformation was only 47 nm. This seems unrealistic in a 5G crash scenario. The maximum combined stress in the beam elements was 361.4 MPa.
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It should be noted that these tests don’t prove which element types are more accurate, only that there is a difference between them. I would say that I intuitively believe that the chassis will deform by more than 47 nm in such a collision. Although I cannot say if 4.2 mm is accurate, I think that number is more probable and hence tetrahedral elements are probably better for modelling the gussets than the default elements. As for why there is such a large difference, I found this article from SimScale that may be relevant. They discuss a concept called “locking” which is where improper modelling of certain features limits their mobility. In our case, modelling thin features like gussets with a single layer of linear mesh elements can lock them and prevent them from bending. This artificially increases the stiffness of the gussets, which I’m thinking reduces the overall deformation of our chassis. Tetrahedral elements help with this issue by creating a greater density of smaller elements but another method that could help is to switch to higher order mesh elements. ANSYS has the capability to use quadratic elements, so I ran a simulation where the gussets were represented by quadratic tetrahedral elements and the bulkheads were represented by quadratic quadrilateral elements. The max deformation increased slightly to 4.33 mm (only 3% difference from linear elements) and the max combined stress increased to 368 MPa.
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