I’m familiar with a structure that is actually stronger after it fails. In that case strength is not the primary requirement because the application requires stiffness to combat a magnetic load that increases with displacement.
For an aircraft structure, I think the chance of designing two parallel load paths with stiffness, elongation and strength identical to the point that they fail simultaneously under a static load test is zero. I also think it’s safer that one of the load paths fails first under static loading, as well as providing redundancy if one fails due to long term fatigue and crack growth. A great big bang followed by flying home is better than the wing falling off. An aircraft structure that is not fault tolerant for any one of its loading conditions is not ideal IMO.
In fail-safe design, the failure of the primary load path is due to crack growth (from fatigue initiation of material/manufacturing defect). And the remaining structure is only good to withstand limit load.
So in an ultimate load test, both the primary and secondary load path might fail together. Or even if it is not a waiting failsafe configuration the secondary load path might fail first.
aart wrote:
A perfectly designed wing would fracture at various different points or ideally disintegrate all over. If not, all portions that did not break were simply too strong and thus too heavy. I guess we’l get there one day.
Perhaps for Experimental Category structures. Otherwise see Page 23 of AC 23-13A.
Many manufacturers claim to support amazing overload situations but I would say it makes the plane too heavy. All parts should be build to support a defined load and then break. This way the plane ends up as light as possible and in flight strength from some parts does not help much if not all structural parts support the load.
Exactly. Plenty of impressive videos on the web of static tests to destruction where you can see wings bend upwards to crazy angles where you start wondering what margins we are using. But of course fatigue is a different issue. But then these static tests often show a failure at just one point of the wing. A perfectly designed wing would fracture at various different points or ideally disintegrate all over. If not, all portions that did not break were simply too strong and thus too heavy. I guess we’l get there one day.
aart wrote:
Nice static test. A dynamic test with these folks dancing in a coordinated way may provide a less pretty picture after a while.
The “dance test” has to do more with aero-elastic frequencies (mass & elasticity & true TAS speeds) than aerodynamic forces (geometry & strength & indicated IAS speeds), something that goes into the certification requirements for high speed designs VNE via flutter requirements
So the aircraft may go ugly with barely 2 guys depending if they do their rock party in “2 temps” or “4 temps” while it is rock solid with 100 sitting on the wings 
I recall studying this for bridges: there is combination of structure shape, mass and excitation frequency where any structure no matter how solid it is will wiggle until it breaks, aircraft will have same airframe-airflow resonance at specific frequencies (more to do with TAS, mass and shape than the amount of aerodynamic force which relate to geometry & indicate airspeed), for bridges, most are designed to take lot of force but they do fail at the wrong harmonics, Angers bridge case: could handle the weight of 10 million soldiers but failed with just 200 marching on it in sync
https://en.wikipedia.org/wiki/Angers_Bridge
https://en.wikipedia.org/wiki/Tacoma_Narrows_Bridge_(1940)
Many manufacturers claim to support amazing overload situations but I would say it makes the plane too heavy. All parts should be build to support a defined load and then break. This way the plane ends up as light as possible and in flight strength from some parts does not help much if not all structural parts support the load.
Nice static test. A dynamic test with these folks dancing in a coordinated way may provide a less pretty picture after a while.
Regarding the mooney remember it is negative load so it brings it up to max give or take.
