Oversized Fasteners in Repairs 2

[SuperStress]

Many structural repair guidelines specify to use a 2nd oversize fastener when replacing an existing fastener in an assembly (especially when adding a doubler).

My understanding is that this is done for two primary reasons:

1. To restore the desired fastener fit.

2. To compensate for the added flexibility in the joint resulting from the increased eccentricity.

Are there any references around that give some details about the joint flexibility aspect?

Short of building a spring model for each repair, is there a simple way to determine when a 2nd oversize fastener is actually required?

This issue is coming up because of limited availability of 2nd oversized fasteners for field repairs.

Any help is appreciated.

SuperStress

 

[AeroE]

Installing a first or second oversize fastener is only useful for obtaining a good fit in a hole where the original fastener had to be drilled out or otherwise removed such that the hole was slightly damaged, without going up to the next size fastener which might incurr net section tension problems (static and fatigue).

If a doubler thicker than about 15 to 20 percent of the fastener diameter is installed, the inherent bolt bending cannot be accomodated by a first or second oversize fastener unless the loads are so small that large basic margins on the shear strength exist in the blueprint condition. This problem makes a case for steel or A286 repair doublers that give good stiffness (for picking up its share of the load) in thin gages, remain bearing critical, and minimize bolt bending.