Composite-to-Metallic Single Shear, Multi-Row Joint Question 1

[Ryan_aero]

Is it common to assume load distributes evenly amongst all fasteners for a multi row, single shear joint where one plate is carbon fiber and the other plate is a ductile metal? Is this a valid assumption if the metal plate thickness is sized such that bearing yield in the metal plate has the lowest allowable amongst all joint failure modes?

Thanks

 

[SWComposites]

Ryan - prior to any yielding the fastener distribution in a two row joint will not be the same unless the straps are the same Et stiffness. In joints with more than two rows, the end fasteners will have more load than the interior fasteners. Once bearing yield starts in at least one fastener hole, then the load distribution will start to even out. With three or more rows, the end fasteners may still see more load than the middle fastener, and the critical margin may then be net section in one of the straps. You basically have to set up a load share analysis with the strap and fastener flexibilities, and then incrementally apply load once yielding starts.

 

[Ryan_aero]

Thanks Steve - Let's say we have a 3 row joint. Is it possible to forgo the load share analysis and and to use a fitting factor to account for load peaking that may be present if the end fasteners still see more load than the middle? or is the fastener load share too uncertain to set an appropriate fitting factor?

Is this strategy only applicable for ultimate loading? It was unclear to me if the bearing yield needed to redistribute load was enough to constitute a bearing yield failure as defined by MMPDS and if you will still need do a limit/yield check. I'm designing a very highly loaded joint and wrestling with different options to achieve positive margins.

 

[SWComposites]

Ryan -

For a three row joint, assuming the two straps have the same stiffness, then a 40/20/40 fastener distribution will be conservative, particularly at ultimate load. If the straps have different stiffnesses, then the fastener load will be higher at the inner row of the stiffer strap.

Its not too hard to set up a 1D linear load share analysis in a spreadsheet.

If you can show the metal strap clearly yields in all fasteners before a bearing or bearing-bypass tension failure in the composite strap, then you may be able to justify a uniform distribution at ultimate load.

Note that with a composite strap, and more than two fastener rows, it is hard to avoid a bearing-bypass interaction failure at the first (inner to the composite) fastener. Often it requires thickness tapering the straps.

If you have a metal strap, then you probably need to do a limit (yield) check (depending on the part and certification agency); check for bearing yield and net section yield.

 

[SWComposites]

oh, and just remembered, Brian Esp has a load distribution code here:

https://www.espcomposites.com/software/advanced.html

His book is also quite good.

 

[Ryan_aero]

Copy - that all makes sense. Brian's book is great - I have it open on my desk

 

[SWComposites]

Advances have provided great benefits.

 

[WKTaylor]

Be aware... fastened composite-composite or composite-metallic joints have MANY more aspects that simple mechanics... Dissimilar materials/finishes issues, thermal coefficients differences, load strain differences, electrical conductivity [lightning/static corona grounding bonding, fuel resistance, coatings, etc... The list of practical issues issues is long and very important... but not insurmountable.

Regards, Wil Taylor
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