I contribute
not as an aviation engineer, but as a test engineer who was responsible for investigating failures of experimental vehicles - a sort of accident investigation engineer. The failures were not previously documented (as the vehicles were the first examples of their type) so it was important to quarantine the vehicle, interview the operator(s) then sift through the information and evidence, remaining open-minded (including 'meteor strikes' and the like) until a sufficient amount of information had been collected to create a plausible scenario. Of course I then had to reproduce the failure in order to prove that the analysis was accurate, rather than merely 'adding a patch' (which probably came later, then I had to prove that the fix solved the problem before it could be released for production). Of course we had rigs on which to test (as well as accelerated vehicle operating schedules) in order to speed up the overall development duration.
Think Comet fatigue test tank:-
The Ministry of Civil Aviation decided upon a unique test to find out. They built a tank large enough to hold one of the grounded Comets. The wings protruded from water-tight slots in the sides of the tank. Then the tank and cabin were flooded with water. The water pressure inside the cabin would be raised to eight and a quarter pounds per square inch to simulate the pressure encountered by a Comet at 35,000 feet. It would be held there for three minutes and then lowered while the wings were moved up and down by hydraulic jacks. The hydraulic jacks would simulate the flexing that naturally occurs in aircraft wings during flight. This process continued non-stop, 24 hours a day. This torture test continued until the cabin in the tank had been subjected to the stresses equivalent to 9,000 hours of actual flying. Suddenly, the pressure dropped. The water was drained and the fuselage examined. The investigators were horrified to find a split in the fuselage. It began with a small fracture in the corner of an escape hatch window and extended for eight feet. Metal fatigue! Had the Comet not been under water, the cabin would have exploded like a bomb. Several months later the results of this test were corroborated when an Italian trawler recovered a large section of cabin roof from the sea. A crack had started in the corner of a navigation window on top of the fuselage. Like the escape hatch window of the test Comet, it had square corners. The square design of the windows was the major flaw that doomed the Comet.