Proper way of extracting component and bolt forces in an assembly

[humbleninja]

I need to do a shock analysis on an assembly that can be simplified into a fixed plate with tapped holes, and a cantilevered beam with a flange bolted to the plate. My objective is to find stresses in the bolts and the beam. I am using ANSYS Mechanical to solve this problem. The system is setup as Static Structural (preload bolts) -> Modal -> Response Spectrum.

My question mainly revolves around contacts. In the past, we have setup two models - one with bolts, and one without. The philosophy behind this is that the model without bolts would have a bonded contact between the entirety of the plate and cantilevered beam's flange and be used to find stresses in the beam. The other model has bolts, and has frictionless contact between the plate and beam flange, which would result in the highest stresses in the bolts.

My questions are as follows:

1. Is it appropriate to use two different models to solve for stresses in the beam and bolts? Or can we use one model?
2. Is it appropriate to first solve the frictionless model with bolts to confirm that there is no gapping between the plate and beam flange? If there is gapping, then the bonded model is no longer valid, correct?

 

[SWComposites]

If you are gapping the joint you do not have a good joint design. Either increase fastener torque, or add bolts, or something.

You can use either one or two models, just have to consider the limitations of each. Both are an approximation to reality.

 

[humbleninja]

If you are gapping the joint you do not have a good joint design. Either increase fastener torque, or add bolts, or something.

You can use either one or two models, just have to consider the limitations of each. Both are an approximation to reality.

Thanks for the reply. I agree that a gapped joint is a bad design, but I should confirm that right?

I would ideally use only one model to save time and work, but I think the only option to do that is to use a frictional contact. I think this is the most accurate way of representing the contact, but my thought was frictionless contact is more conservative when solving for bolt stresses. Thoughts there?

 

[SWComposites]

are you going to model the bolts with solid elements?
or are you modelling the bolts with bars/springs/etc?
are you going to attempt to model the fastener preload? which is quite difficult.
there have been many thousands of aerospace fastened joints analyzed with FE models with simple fastener connections to shell element nodes without any contact elements, where fastener loads are extracted and then fastener shear stress, sheet bearing stress, etc are calculated outside the FEM.
what exactly are you modelling and why do you need the complexity of contact elements?