Hello all, Sunfish especially.
OK, let us clarify some things related to certification:
Limit loads = the maximum loads to be expected in service - FAR 25.301 (a)
Ultimate loads = limit loads multiplied by prescribed factors of safety - FAR 25.301 (a)
Factor of safety = 1.5 (unless stated otherwise) - FAR 25.303
therefore:
Ultimate load = 1.5 x limit load
There are no Design Loads defined in certification documents.
In order to release the first FTA (Flight Test Article) to start flying, usually, CAA responsible for certification asks for completion of a batch of limit loads tests.
It is up to CAA to define, according to the relevant regulations and its judgment, which tests are required.
For the completion of certification process all the ultimate loads tests have to be satisfactorily completed. It is up to CAA, again, and the responsible party (OEM) to agree on what has to be done if there is some unacceptable test result (Margin of Safety < 0).
That can be a repeated test, a change in FEM, a weight penalty or whatever they agree on. It is FLEXIBLE.
As for the statement that structure is fail-safe this is wrong since this concept was left in early 70s with introduction of Damage tolerant structure.
Last fail safe model was 747.
Cheers