How does adding material affect stress in FEA results?

[Shumon Hossain]

Hello,

I have a fairly simple question but I guess I am having a hard time trying to understand it. I have a plastic connection of two different geometries (see attachment). When I hold end of the front portion (of the uniform diameter) and apply torque at the bigger cylinder geometry, there is a stress that is lower than if I run the FEA holding the front portion (not uniform) diameter? I increased the diameter in the non uniform connection and would have thought it would have lowered the stress instead of increasing the stress. I understand that in order to really lower the stress the wall thickness would need to increased, but I really would like to know why if I add material, the stress is going up. I have seen this in quite a few simulations. Does it have to do something with how the energy has to go somewhere when applying the load? Does adding the additional step always increase stress since it's not necessarily the load but the transition of geometry? Please see the attachment for better understanding of my question.

 

[Mech_Engineer]

It looks to me like a stress discontinuity due to proximity to the fixed boundary condition. I'd recommend taking a close look at the mesh geometry in that region before making decisions about the stress, the maximum stress might be driven up by a stress concentration due to a poorly-formed mesh element(s).