I should have been more specific: the o-ring did not fail in an unexpected way.
There was no defect in the o-ring. The design of the entire joint it was sealing was suspect, and was known to perform in a way that was not satisfactory. It did perform just as it was expected to (they expected a failure under the conditions) by the people that had the technical details.
True indeed, but it's important to not lose the uncertainly they had at the time, and the degree of o-ring failure being not so binary.
According to what I recall of Allan McDonald's version of things, they had a good amount of data that colder temperatures meant worse sealing performance from the o-rings (soot making its way past the first o-ring and in some cases damaging the second, basically). Like you said, it was a well-known issue in some circles. They also knew the Challenger launch the next morning would be very cold indeed, something wild like at or just above freezing, I think.
The engineers at Thiokol raised their concerns and were asked what a safe temperature to launch is and said something like 53F, basing this on the fact that a previous launch at that temp was successful. NASA (and Thiokol) management balked at this because the booster's certified minimum launch temp was something lower like 30 or 40F. Then they basically asked them to prove it would catastrophically fail at the temperatures expected the next morning, which they couldn't conclusively do since they didn't have the data to back it up. Management reversed the no-go recommendation based on this.
So yes, the o-rings performed as expected insofar as colder = worse, but it was a matter of how much worse at temperatures lower than any successful previous launch.
> Then they basically asked them to prove it would catastrophically fail at the temperatures expected the next morning, which they couldn't conclusively do since they didn't have the data to back it up.
Turns out they _did_ conclusively prove that, just not within the time or budget (or casualty) constraints demanded by management at the time.
There was no defect in the o-ring. The design of the entire joint it was sealing was suspect, and was known to perform in a way that was not satisfactory. It did perform just as it was expected to (they expected a failure under the conditions) by the people that had the technical details.