Torque to Yield Head Bolts...

[dang]

This subject has always had me curious so I thought I'd ask you guys. When you use torque-to-yield head bolts, or the type that stretch when tightened, I understand the concept of the bolt reaching the yield point and relaxing a bit, but does the bolt "reharden" itself after stretching? In otherwords, what's going on at the molecular level? I don't know very much about how metal/steel can be heat treated, tempered, annealed, etc., but I am familiar with some welding and understand how steel can be heated and distorted and after normalizing still have the same strength or properties as before.

Any insight?

Dan

 

[x_atlas0]

A torque-to-yeild bolt takes advantage of a phenomenon known as "Strain Hardening", which is when the Young's Mod and yeild strength rise in responce to loading. If you look at a stress/strain curve, you can see it very easily, as a little bit of strain increases the strength of the material. You actually have to deform the metal though, and cause some plastic deformation.

http://en.wikipedia.org/wiki/Cold_work