Freeze Plug and O/S Fastener

[737NG]

Suppose you have a damaged fastener joint. You repair by opening up the hole, putting a freeze plug (dia. D) then installing nominal fastener (dia. d) through the freeze plug.
My question is how to treat this repair when analyzing bearing capability.
Which should I use to compute edge margin, D or d?

 

[Sparweb]

Taking a hint from Bruhn, check both!

Depending on the material of the plug and parent material, the fastener may be critical in bearing against the plug, or the plug against the parent material. If the OD of the plug gives you a problem with the edge distance, treat it like a lug, and analyze whatever failure methods could apply.

If this fastener is part of a long row, then the effect of the plug may not be too different from oversizing it, but I suppose you would have oversized it in the first place, if it was a continuous row.


Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout

 

[Aconnell]

Every thing SparWeb wrote is correct. However to add a little, in my experience for freeze plugs if the wall thickness is above 0.030 for steel plugs and 0.050 for aluminum plugs there is usually no need to check the bushing for bearing. You should however check for bearing, shear tear out and possibly a net tension failure of the part based on D(the bushing outer diameter). I think the section that Spar Web is refering to in Bruhn is on page D1.5. The formula that applies to a shear out analysis in case it is not obvious from Bruhn is as shown below or you can use the bearing-shear efficiency charts in Bruhn. I don't know if you require it but if you require a fatigue analysis this can some times by applying the severity factor analysis discussed in Michael Niu, however you have to make some assumptions.

Shear Tear-Out
Allowable Load=2(EM-S)(Fsu)(t)
S=DCos40/2
EM=Edge Margin, Fsu=shear allowable for part,
t=part thickness at hole loaction
This formula is from the course notes for the Boeing Repair Design course.

I hope this helps

Alex Connell

 

[Jimneeson]

In answer to your question, which is, how to compute Edge Margin, it is the same for the fastener and plug, assuming they are both have incidental centrelines, which is a fair assumption.

If it is to get a joint shear capability you are actually asking for,then all said before is applicable. Minimum joint is dependant on all components of the repair.

What is it you are repairing anyway?