One big fastener surrounded by several small 1

[A380istoobig]

I am looking for the explanation why it is not a good idea to have one big fastener (5/16") in a repair that has otherwise 1/8" dia fasteners. I know it has to do with load distribution and the fasteners will not have an equal load. I have had a quick look through Bruhn and Niu and I can't find it.

Any help where I can find a published answer will help me.

Many thanks!

 

[Sparweb]

If you pulled on a continuous joint of 1/8" rivets, then each rivet would displace an equal amount. The load, displacement, and shear transferred through the rivets would be constant along the length of the joint.

Now, by replacing a 1/8" rivet with, say, a 5/16" screw, things change. Assuming a constant load along the length of the joint again, under that load, the 5/16" screw will displace much less than the 1/8" rivets. About 19 times less, in fact. This "hard point" prevents the adjacent rivets from taking their fair share of the load. The sheet transfers a large load locally through the screw.

Associated with installing a screw is the hole that isn't filled by an expanded fastener. The stress concentration is high at an overloaded, loose fastener, cutting down the fatigue life.

That's my off-the-cuff explanation. Flip through an article by Swift, or Eastin, and they can explain it in much more detail, plus some analytical rigour. Their articles can be found with a Google search.


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