Ok, I've done a little more research, particularly on Torque To Yield
(TTY) bolts. In a TTY application, the bolt is torqued right up to it's
elastic limit (IOW, stretched as far as it will stretch without
deforming), as opposed to the non-TTY procedure of torqueing to 75% of
the elastic limit. One reason TTY bolts are discarded afer each use is
that it's so easy to exceed the limits and stretch the bolt, rendering
it unsuitable for further use. Another is that heating and cooling
during engine operation may cause the bolt to exceed its stretch limits.
In a non-TTY application there's a 25% safety factor left in so the bolt
won't exceed it's stretch limits. If a bolt's stretch limits aren't
exceeded, it will return to its previous length and not lose its tensile
strength.
Now, in the example you saw, was the "correct" torque value at the
edge of elasticity for the bolt, or below it? It makes a big
difference, according to my research. Also, what do you mean by
"preload"? The articles I've read talk about "clamping force", might be
the same thing. To measure clamping force you could take a couple of
blocks, put a strain guage between them, pass the bolt thru one and
thread it into the other, then tighten the bolt and measure the output
of the strain guage. Did they do something similar?
This is interesting stuff. One thing I know from professional
experience is that you can't evaluate the results of a test unless you
know how the test was run.
Cheers,
CR
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