This seems to be about this titan: «Boeing and Airbus both said their tests of affected materials so far had shown no signs of problems.» I read this as implying that Airbus has been buying other things from the same source and done its own tests on samples: «“Numerous tests have been performed on parts coming from the same source of supply,” an Airbus spokeswoman said…»
Is the documentation process expensive enough that it's worth faking it even when the tested material is OK? Weird if so.
You can't prove the material is good, you can only trust that the material is good, and 50 years later observe how it held up.
You can't find out the distribution of the alloy ingredients, or detect voids, or crystal structures, or traces of other elements, except by sawing the part in half and looking at the cut surface.
You can't find out the critical properties by looking at it. All you can do is be sure you know the full truth of the history of the material and the part. You only know that if a certain recipe is followed, then the material will be good. You have to trust that the supplier did do the recipe exactly as specified. You can't look at the part after the fact and tell that. Even stress testing to failure doesn't tell you that because the material may pass the test today but fail from fatigue over time.
The only empirical test is actual use in actual conditions for the full actual time.
You can accelerate some tests, and failing an accelerated test obviously proves the material was bad, but it doesn't go the other way. Passing an accelerated test does not prove that the material is good for actual use in actual conditions for the full normal time.
The end of the article has it right, if the parts seem ok from what testing is possible, then they are probably ok for this minute, and it's probably good enough to just replace them at the first opportunity during routine maintenance.
I assume that the documentation asserts something acceptable about the manufacturer testing (accelerated, destructive, what have you). In theory it could assert that the production process was such and such without any information about resulting quality assurance, but that seems improbable.
Why can't those tests be repeated (on samples, obviously)?
Even if there was a destructive test that actually predicted lifetime performance, the total sample sizes are probably too small for statistics to be valid unless you destroy something like 10% or more. If you only have say 100 of something, a random samling of 1% is too few. 2 or 3 is no better. Maybe 10, IF all 10 gave perfectly consistent results. But there is no such test anyway.
By interior composition and distribution I'm not talking about anything as comically stupid as plating like the inside is aluminum.
The surface of a finished part is routinely intentionally quite different from the interior, ie spin casting and case hardening etc. Frequently the performance of the part actually requires that the interior be different from the surface, ie hard shell resilient interior.
You can observe a lot about a finished part in various ways, like just tapping it and observing the sound can be more useful than an xray. But there's a lot you can not know after the fact through observation, except by observation of the eventual failure or not.
For one example, dissimilar materials, either within a casting or even just 2 parts in contact with each other, or a part and a brazing material, can migrate and diffuse into each other over time. Small differences in the initial conditions change how that develops over time, and can result in big changes in the performance of a part later.
You can't examine a finished part to determine that it was fabricated according to the recipe. You can only detect gross problems. You must trust that the supplier and their suppliers all followed the various recipes.
Here's another angle:
They first detected the forged paperwork because the guys on the factory floor observed that the material looked wrong.
So, it's the opposite of "you can't detect the difference". They detected a difference just plain visually.
The counterfeit parts might actually be perfectly sound. We don't know they will fail early, we only know that we can't trust the paperworks claims about how they were produced, where the materials were sourced from, how they were processed etc. Whatever the source and processes actually were, the end result might be inferior, or might be equivalent or even superior. (although detecting pitting they didn't expect does not lean towards the parts being superior)
They are able to observe that there is something different about these parts. They visually looked different enough to raise the question. Yet so far, they haven't been able to say that the parts are actually unsound through any testing or that initial visual observation.
It's not only that a part that looks perfect might not be, it's also true that even when you do detect a difference, it doesn't mean the part is bad.
You can observe a lot, but there is no amount of after-the-fact observation or testing that can replace knowledge of how a thing was produced.
