The Boeing Dreamliner, the commercial plane closest to being all plastic, has a critical test to pass today – the “Ultimate Load Test” – in which the wings will have a stress of 150% of the expected maximum applied to for 3 seconds. The test will obviously be run on the ground and not in the air.
“In programs past, with traditional aluminum planes, Boeing has generally passed the ultimate-load test and then taken the pressure higher until the wing does break, to see where that point is, Gunter said.
That’s not planned today, however, because the reinforced composite wings are expected to be stronger and would need an amount of force that’s too risky to the equipment and personnel, she said.”
Those are some seriously large forces to be working with (maybe the Mythbusters can run the test some day in their typically schlocky fashion). I do find it somewhat strange that the test is defined in terms of the total deformation time (3 seconds) and that there is not a deformation rate associated with it. Composite materials certainly would show a change in deformation values with the stress rate, not just the total stress. Mid-air turbulence and other stresses have a higher frequency than 1/3 s-1. It is quite apparent that this test was originally designed for aluminum planes and is being grandfathered in. Worse yet, I found this line about the Airbus 380 disturbing:
”The FAA mandates that the aircraft survive three seconds of 150 percent stress in the “ultimate load test,” creating enough pressure to potentially snap the wings in two. That’s what happened with the Airbus SAS A380 in 2006. While the breakage was a “black eye” for the Toulouse, France-based planemaker, Airbus was able to convince authorities it knew exactly what the problem was and could fix it, without having to repeat the test, Weber said.”
While some such test is needed, the specifics should get a review as composite planes are not going to disappear anytime soon.
If the video is ever released, it would be fascinating to watch. A few years ago, Rolls Royce released video of engine testing that was reassuring but also personally troublesome. While the engine past the test so well, it was disturbing as it opened my eyes to a potential failure mode that I had never thought of before.