There's always some concern that the math models correctly reflect reality. That's why there's a 50% safety factor, and it's backed up by the engineers' experience that things "look right", bench testing, and flight testing. (In early flight test, they'll tape strain gauges all over the place to see if the math predictions of the stress matches reality.)

Did you ever experience any times when a part passed the math model, but didn't "look right"?

BTW, this happens all the time when using structural optimization programs, where you tell the computer what material you're using, how it's constrained and loaded, and then it figures out what it thinks is the optimal shape for the part.

It means somebody left out a load or a constraint, or messed up the material properties, which is all another way of saying the math model is wrong.

So when a part doesn't look right but seems to pass the math, be very suspicious of the math! :-)

Sort of. I spent much time arguing with Stress over how to calculate the torsional rigidity of the elevator structure.

I don't mean to be too hard on Stress. They're good guys, we both had the best interests of the design at heart. Their manager paid me a great compliment by wanting me to join their team. But I liked design work too much.

I believe that Boeing pushes for a little margin so they can revise the design without having to redesign a whole bunch of parts.