Increasing Bearing Strength Locally at a Few Fasteners for CFRP Double Shear Joint

[Ryan_aero]

Hey All,

Is there a good way to increase the bearing capability local to a few fasteners for a single row (double shear)joint with many bolts? The applied line load is concentrated at a few fastener locations, causing them to transfer additional load than the rest.

Current ideas:
1) Use bigger bolts in the area near the concentrated line load
2) Build up the laminate thickness locally
3) Add a 2nd row of fasteners locally
4) Abandon the idea and increase bearing capability uniformly

Seems like options 1&2 may have minimal gains due to attracting additional load to the area?

 

[SWComposites]

It largely depends on the type of loads causing the peaking at the joint ends: thermal, enforced displacement or applied mechanical loads. All 3 options can work.

Do you have positive margins at limit load using bearing yield allowables? If so, you might be able to use nonlinear bearing load-displacement data at ultimate to redistribute bearing loads.

Post a sketch of the joint and fastener loads.

 

[SWComposites]

Oh, double shear joints are uncommon is aerospace structures; what is the application?

 

[Ryan_aero]

The load concentration is mechanical in nature due to a geometric discontinuity near the joint. It's a joint between two tubes for a launch vehicle application - I attached a sketch. I have positive bearing yield margins at limit load at the fasteners away from the load concentration, but not currently at the highest loaded fasteners. I was planning to run FEA to assess how fruitful the different options are if there isn't a clearly best/standard method. I'll also look into the considering a nonlinear fastener load share, I hadn't thought of that.

 

[SWComposites]

Presumably the center strap is the composite part and the outer straps are metal, correct?

For a launch vehicle, if this is a one time use vehicle, and the critical load case is a launch load case, you may not need to consider a yield margin if all of the ultimate margins are positive. But check the certification requirements.

If you do use a nonlinear load share analysis, then you need to check each fastener displacement at ultimate load (since the fastener load is limited to the ultimate bearing load) and be sure the displacement is not excessive, and is covered by bearing test data for the specific layup, thickness, temperature, configuration, etc.

 

[Ryan_aero]

The outer straps are composite and the center strap is metal.

Roger that - will do. Hardware isn't built yet so all options are still on the table.