UL bearing allowable in mixed material joints 1

[Aerodesign]

I am analysing joints that consist of metallic plates with thermoplastic (TP) skins. The load path uses a metallic plate 1 plus TP skin with load passing to a thicker matallic plate 2. The assumption for bearing is that the ult bearing joint allowable is the metallic plate 1 allowable plus TP allowable. The strain when the ult allowable is produced is different for each material and hence there is a counter-argument that says the joint allowable should be lower tahn the sum of their respective allowables.

In theory this is correct, the metallic part will strain less and hence achieve its bearing allowable first, the total allowable being the metallic plate 1 plus the portion of bearing load achieved at the same strain for the TP skin.

Does anyone have a suitable method for me to reference. Any alternative theories are welcome.

 

[RPstress]

Are the allowables to MIL-HDBK-5-type methods? If so, then the ultimate allowable is just the max load seen in the test irrespective of deformation. Arguably you might have to mess about with load deflection curves from test if you needed to increase the combined allowable over the metallic only. On the other hand, if it's an unreinforced thermoplastic does it really add that much? (And if it's reinforced it should have a pretty low strain.)

However, if it's b10-type allowable then that's the load at 1.0% deformation, and so in theory you could just add them.

 

[Aerodesign]

The allowables are proprietary for certification. The approach we have adds them together at ultimate. Any additional loading will result in a failure at teh highest loaded fastener. This is now agreed with the client. What they now want is to look at the limit load conditon (a proof load analysis) to see if any fastener gets over loaded before limit load is reached. At that point the local deformation must be determined to be non-detrimental.

The thermoplastic in practice takes a lower load then the joint analysis suggests (a view reinforced using a FEM with flexibility of each fastener modelled). The ultimate load in the joint is less then the sum total of bearing allowable in the metallic and thermoplastic combined, so it appears that we are OK.

Thanks for your response.