They both matter. In the past, separate full scale tests have been done with cabin pressure cycling and with flexing on some new airframes. I'm not sure if this is the case any more given the advances in the science, finite element modelling, and laboratory scale tests. I suspect that fairly large scale tests are still done on composites.
But most of that becomes irrelevant once the plane is airborne, which is when most of the vibration and flexing occurs.
I suspect we hear little of what happens with third world aviation unless it involves the deaths of presidents or large numbers of passengers.
It would appear that McDonnell Douglas and the Commonwealth of Australia Civil Aviation Safety Authority among others aren't so complacent about that.
Probably because most of the problems are inspected out before they become catastrophic. I believe that at least one crash was caused by metal fatigue, but that was in an engine part and not structural.
DC3/C47 are not pressurised ergo very little cabin flexure. And other source of fatigue- main spars etc - are much further from the failure point anyway - they are built to take 4g or so loads. you vcan bend a DC3 before it breaks, and many have been...
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