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March 1999

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Fri, 26 Mar 1999 15:22:07 EST
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This may be a duplicate , but I got a server generated message saying I
confirmed the wrong code and to resend the message corresponding to that
code,or something...

Subj:[log in to unmask], [log in to unmask]
Date:   19/03/99
To:     [log in to unmask], [log in to unmask]

In a message dated 19/03/99  05:34:16PM, you write:

> ........
>  I have some concern with the mechanical stress that it may see at
>  different use temperatures.  But tell me if my thinking is correct here:
>  The epoxy is cured at an elevated temperature (150 to 160 C).  So, given
>  that it is stress free at the cure temperature, at room temperature, the
>  epoxy should be in tension.  Right?


The answer is definitely maybe.
It depends on the configuration obviously, but don't forget that the adhesive
will most likely be transitting through its Glass transition (Tg) temperature
on cool down.
Tg is the temperature at which the hard glass like resin transits to/from a
softer state.  The change is from hard to soft on warm up and the reverse on
cool down. It is marked by a change in thermal coefficient of expansion  which
can be quite large.
Above the Tg the relaxed bonds in the epoxy will most likely abosrb stress and
stay relaxed till the Tg is passed, thus any stress will only be from the Temp
Tg down, - from say 70C to ambient at 25C for example.
You need to check your data sheets for the actual numbers.

[And if any proper polymer chemists are out there I recognise that I have
simplified this explanation somewhat.]

HAving said all that the use of adhesives to stabilise components in harsh
mechanical environments, or where overly large or heavy components are
attached to relatively small pad sizes is arecognised technique going back
some years. (eg I first saw it 14 years ago on a then old military assembly).


Mike Fenner

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