huangyhg

re-use of pretentioned bolts
hi,
in my business it is common practice not to re-use bolts that are pretentioned/torqued.
now i have designed a clamp. the clamp uses 16 bolts of m36/1.5in size bolts. they are pretentioned to 40% of yield. yes, it is a big thing.
the clamp needs to be re-used a dozen times. following the general common practice, i would need 16 new bolts for every operation. this is not cost effective.
i have looked in the codes and standards in our bookshelf, i can't find anything about re-using pretentioned bolts. one could consider an inspection before re-use, but what kind would be suitable? mpi, dp, ut - not sure if they work on a bolt and if you can clean the bolt afterward.
any advice? experience? codes i can refer to to justify new bolts? (or re-use)?
regards

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imho it sounds like you have an important structure. i would just purchase 200 bolts in bulk and reuse them when appropriate. it sounds like an insignificant cost compared to the structure involved and would not risk using bolts that nave been stressed so heavily no matter what the literature says.
what grade are the bolts? if they are only stressed to 40% of yield, i would have no fear about reusing them. but how do you know how much they are stressed?
in my case bolts are 10.9 m36, loaded with 250kn
as for buying extra bolts - as an engineer i have to convince purchasing department, financial department, management etc. we are a project contractor, so naturally pennywise-poundfoolish, even if i don't like it.
so any handles to do one or the other are welcome.

"for want of a nail, a battle was lost"
"don't be penny-wise, and pound-foolish"
(buy a fortune cookie if you want more ...)
there ain't no free lunch ... if you want to re-use the bolts, have someone either ...
1) crack-check them after removal (how much will that cost?)
or 2) anayze them to this usage. if you preload them to 40% fty, it would be reasonable to assume that the maximum service load is maybe 40% fty (no gapping), possibly 60% fty (gapping in-service). the fatigue spectrum would be (for the latter) a cycle 0-60% and multiple cycles 60%-40% ... doesn't sound too bad. a healthy safe-life factor would help.
and 3) analyze for a failed bolt
fatigue load a bolt at a lab to 40% for 5x the cycles. if they survive, ok. if not...
you have to decide for yourself, but i have no problem re-using any structural item that shows no signs of ware (no worn threads for instance) and stayed in the elastic range. if you don't want to re-use them whats the great loss? new bolts are probably cheaper to install then re-using old bolts.

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