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RE: Thermal props of high tensile bolts

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Could it be that the observed loss of prestress, assuming it really
happened, resulted not from differential expansion between high tensile bolt
and mild steel components, but from a little extra localized compressive
yielding of the mild steel in addition to what may have already existed at
bolt contact spots, due to the effects of the temperature on the mild steel? 

I am imagining a slight loss in elastic "zip" of the mild steel where the
pinch stress was highest, and a little bit of plastic flow that of course
did not return when the assembly cooled. After cool-down, the gripped parts
would not be quite as thick between bolt head and nut as previously, and
loss of prestress would be detected as a result.

Charles O. Greenlaw, SE   Sacramento CA

        Original message, in part:
>I have a problem where an argument has been raised regarding the loss of
>pre-tension in a torqued screw, where the screw is high tensile and the
>components are mild steel.  The claim is that after heating the assembly to
>130°C (266°F) for a couple of hours, the thermal differences in the two
>steels causes significant loss of prestress.
>Any comments?
>Dave Meney
>Structural Engineer
>Yenem Engineering Services
>54 John Street
>Phone  (08)9257 2695
>Fax    (08)9257 2264
>Mobile  0417 949 374