Accounting for internal forces when comparing FEA reactions to FBD

[mfritze]

Hello,

Recently I submitted a structural analysis report to a major aerospace prime for an electronics enclosure (read 'box') we are under contract to design. The box is supported by four bolts at each corner. As part of my stress report I did a FBD showing the free-body reaction at each of the bolts due to the applied load factors (some acceleration in all 3 axis simultaneously).

In the FE model I am modeling the bolts as springs (think CBUSH) of stiffness equivalent to the bolt stiffness axially and arbitrarily 100x bolt stiffness in the bolt shear direction.

The problem arises in that the FBD reactions do not match the FE reactions because the high shear restraint makes the structure indeterminate. The reviewer did not like that the FE reactions differed from the FBD reactions and that the FE reaction loads were pretty uneven among the 4 bolts of the basically symmetric structure.

The reason the FE reactions are uneven is because of the Poisson effect internal forces are generated between the rigid supports. When I relax the shear stiffness of the supports the FE solution approaches the FBD solution.

I am a new analyst and I believe this issue is probably very common.

How do you account for comparing a FBD to FEA results when a structure is indeterminately supported and develops internal (Poisson related) forces?

Thanks,

 

[SWComposites]

If you use "rigid" bolts, either via rigid elements or very stiff cbush spring stiffness, you will get unrealistic fastener loads from the FEM.

Search eng-tips and/or google for "huth fastener flexibility" formulas for calculating in-plane (shear) stiffness values.

 

[rb1957]

however you've supported your model ('cause it's not really clear to me), if the FEA results don't match simple hand calc FBD (given a simple structure, like a box) then there's something to be explained.

but i have to ask, why do FEA on an electronics box enclosure ? i'd've thought that you should be able to react large inertial accelerations with any sort of fasteners ? based on a simple hand calc.

another day in paradise, or is paradise one day closer ?

 

[mfritze]

Thanks for your response.

I will look at the Huth formula and see if I can come up with a good argument to reduce shear stiffness of my CBUSH supports.

As for why we are doing a full FEA study that's a good question --a detailed stress analysis is a contractual requirement here.

 

[SWComposites]

A detailed stress analysis does NOT require a FEM. Cripe.