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The choice of mesh elements seems to have a large effect on the deformation in a simulation. This is very important for future simulations for 2 reasons. First, the regs state that the occupant cell must not deform by more than 25 mm so we need to be able to accurately predict how the chassis will deform. Second, the deformation of the chassis will likely impact the stresses in the chassis. It might seem like the results presented here contradict that, since the maximum stresses do not significantly change when different types of elements are used. However, the maximum stresses in this collision scenario occur right above the collision object. I would guess that the gussets have a very minor role in protecting this area on the chassis from this collision scenario and that it is likely that greater stress differences will likely be seen in other collision scenarios.
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Having 4.2 mm of deformation makes more sense to me than 47 nm. I would be interested to hear what others think of this if anyone else has encountered lower deformations than expected in their simulations but for now I will continue using tetrahedral mesh elements to model the gussets. Even though this study focused on the gussets, if this is indeed a thin feature locking effect, these results could also apply to bulkheads and any sheet metal reinforcements as those are also thin features which are only being modelled by single layers of mesh elements.
Update
I also tested different mesh element types on another collision scenario, the top 30 collision. I did not see any difference in deformation or evidence of locking behaviour, so this could have just been an error in my front bar collision setup. Another possibility is that the even locked gussets do not help protect the chassis in a top 30 collision scenario.