[PSUBS-MAILIST] O-rings and cold temperature

[Alec Smyth via Personal_Submersibles]

Here's one for the PSUBS brain trust.

I was reading today about the Challenger accident 40 years ago. The O-ring
that failed was Viton, and the failure was caused by cold weather in the
days prior to the mission. The low was 18F. Per the engineers interviewed
for the article, when an O-ring contracts due to extreme cold it sets at
its newly reduced size, without regaining its prior shape and elasticity as
temperatures go back up.

Here's my question. The variant of Viton used on Challenger has a service
temperature range with a lower end of -20F, way colder than +18F. Why would
18F have damaged it?

The O-rings on Shackleton are Buna-N, which is rated to -40F, but I'd still
like to understand the failure mechanism, since the weather forecast calls
for 3F (-16C) in a few days' time, and I'll be diving Shackleton in Florida
two weeks from now.

Best,
Alec
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