Standard DTI FAQs
Do you have some evidence of stripped threads?
The only "overtensioned" or "overtightened" bolt is a stripped or fractured one. The Research Council on Structural Connections (RCSC) states that "... all bolt tightening methods MAY produce tensions in excess of the minimum, but this shall not be cause for rejection ..." Peter Gill's classic paper "The Yielding of Fastenings During Tightening" (see downloads on this website) explains why this is true, literally. ASTM A325 states that during some product tests some necking in the first few threads is to be expected. Heavy Hex structural nuts are extra deep in order to engage sufficient threads to develop the full strength of the bolt before anything in the thread fails.
But,
very occasionally, some thread stripping does occur. The attached photo
is of such an occurrence. This type of bolt thread stripping is very serious,
and must never be allowed to occur. It's occurrence a certain sign that
there is something seriously wrong with the bolts. If it is observed,
the bolting procedures must be re-examined and re-evaluated immediately.
If it is allowed to persist, serious consequences may result because the
bolts will not only not sustain a preload, but external loadings may be
transmitted directly to the bolts and cause joint failure.
Bolt stripping is invariably caused by a material failure in the bolt itself. It is not a result of faulty installation procedures, although over-zealous installers may in fact uncover some stripping failures. In every case, no heavy hex structural bolt should strip prior to fracture. Bolt fracture, probably in the first few threads preceeded by some necking down of the threads, must always preceed stripping, or the bolts are faulty.
For a more thorough discussion of bolt stripping, see the paper entitled "Bolts Should Stretch and Occasionally Fracture, Never Strip" available as a download from this website.
