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
