Section 220.127.116.11. Demonstration of Initial Flaw Sizes Smaller Than Those Specified
The choice of smaller initial damage must be justified either
through an NDI demonstration or a proof test.
The NDI demonstration program is described in JSSG-2006 paragraph
4.12.1.a. The program must be
formulated by the contractor and approved by the Air Force and must verify
that, for the particular set of production and inspection conditions, flaws
will be detected to the 90% probability level with 95% confidence.
Where no other means of NDI is available or where it is cost
effective, the proof test can be an effective means of screening structure for
flaws. Proof testing generally has been
successful for the more brittle materials which exhibit plane strain fracture
behavior (e.g. high strength steels) and for small structural components. The application of proof testing to complete
airframe structure in the USAF has been somewhat limited. The notable exception has been the cold
proof tests (-40°
F) of the F-111 aircraft to clear the D6AC steel wing carry-through and
appendage components for flight usage.
In general, proof testing has only been used on major airframe
components as a last resort to allow operation (usually restricted) until
extensive modifications are made to the structure (e.g. wing reskin
modification of the B-52D). In deriving
estimates of the initial flaw size associated with the proof test conditions,
approved upper-bound fracture toughness values shall be used for the materials
under proof test conditions. Section 3
also presents more information on the proof test concept.