High Tension Bolt

The second time you try to use it, the bolt will have stretched from the first time it was tensioned. Instead of gradually approaching the correct tension, it will hit the tension relatively suddenly, and have little room for fine adjustment. (That's layman's talk; if BoltIntegrity joins the thread, we'll get a better explanation.)

The bolts if properly tensioned reaches into the in-elastic regions (especially in the lower diameter regions eg the thread portion between the shank and nut).

Usually the high tensile bolts are manufactured to be ductile enough to undergo the first inelastic preload but may not have enough ductility left out to undergo the second or subsequent.

UD15

Banu...

When your bolted joint was originally designed, one of the trade-offs considered was bolt replacement versus size or weight.

Replacement high tension bolts/studs and nuts are part of the cost of business.

The only reason (aside from cracking) that any fastener cannot be re-used is permanent deformation....which you probably have.

A: Unfortunately, most assemblers do not control the bolting process. Without control, there is a risk that that the "required torque" might take the bolt into yield .

If the bolt has yielded, subsequent tightening (even if it's "controlled tightening") may not produce the same bolt load (clamp force) and thus the joint may fail

For the reason explained above, such a spec is always the case on fasteners that are to be tightened beyond yield (such as those on internal combustion engine heads). However, if the design does not call for yield tightening and if the bolting process is controlled, this practice is totally unnecessary - even on fasteners that are required to be tightened to upwards of 95% of yield .

Q: What is "control", you ask?

A: It's tightening a fastener while knowing how "tight" (or, in this case, how close to yield) the fastener is. This has absolutely nothing to do with the applied torque value! "Why not?", you ask incredulously. Well, it's because "torque" is just a value which expresses how much resistance is felt when trying to turn a nut. If you encounter less resistance than the Engineer assumed () when he converted the necessary bolt stress value to a nebulous torque figure, your "proper torque" will result in the fastener being too tight and possibly yielding!