[Arnt]

I'm curious:

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)?

[HeyLaughingBoy]

Because it's not just about testing. Like in high-quality software, testing is only the final step. The primary determinant of quality is the source material and how it's processed, and testing can't completely prove whether or not it was processed correctly.

[Brian_K_White]

What tests? I just told you there are no tests which can actually tell you what you need to know. Even destructive tests.

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.

[Arnt]

There's a bottle of water on the table next to me. The label says it's... basically official wording for "high quality" and that the source, result and production process was thoroughly tested on a named date, and with a less thorough using continuous testing regime.

I find it difficult to believe that aircraft metal production has less testing. And if not less, then surely that which the manufacturer does can be repeated on a sample by the aircraft company?

[Brian_K_White]

There are all kinds of tests you can do, and they do do all those tests of course, and they can tell you a lot, but they can not tell you what kolnowledge of the raw material source and production process tells you. Tests can prove that a part is bad, but can not prove that a part is not bad.

All the xrays and ultrasound and strain guages and spark chromatography in the world don't tell you how a part will perform and develop over time. But prior observation of a parts full lifetime and knowledge of it's production does.

Even destructive examination of random samples aren't sufficient for high stakes items when the total quantity is small.

Find it difficult to believe all you want. Or look into it and then not rely on uneducated lack of credulity to decide if something is bs or not.

When it comes to a chunk of alloy, the only way to trust the end product is to know that you created it according to a known protocol that previously has been shown to produce a certain performance result.

That protocol starts right with where the raw materials were sourced from, and every process they've been subject to along the way.

The only way to really know is if you did it all yourself.

Next best is to have documentation that you have reason to trust, ie, the supplier has a valuable reputation that they wouldn't dare risk all future jobs for the small short term gain from lying about any one job.

In this case, the supplier was a nameless supplier several subcontractor levels deep away from Boeing, and had no such reputation to worry about. The small immediate gain from a single sale was all they were ever after and they got it. Tomorrow they can do the same thing again just fine under a new random name to a new customer. And most customers won't even care because they are making bike parts and camping equipment and gimmik wallets and phone bodies not jet parts.

[Arnt]

I hear what you're saying, but I don't think you understand my question. No doubt my phrasing is bad.

The manufacturer had to produce the material in a certain way, right? Mix specific amounts of other things into the titanium, use specific heat, specific cooling. (I don't know anything about metal, really, just assuming that these things are like how high-performance concrete is made.) Now, the choice of additives, amounts, temperatures, pressure etc. is based on testing, right? Someone chose a particular pressure after doing many tests using a range of pressures. The manufacturer isn't allowed to just set up a production process that matches that spec and just assume that the result will match the results elsewhere. But the manufacturer can't take decades to check the product at the normal passage of time either. So the manufacturer has to do some sort of accelerated test to check that the production process works as intended.

That testing is naturally not perfect. I understand that. And whatever testing Airbus/Boeing can do after taking delivery is also not perfect.

My question was rather: Why can't Airbus/Boeing reach the same standard of testing as the manufacturer? If the manufacturer can do some tests and document them (or just fake the documentation) and assert that its production process matches the spec, then I don't see why Airbus/Boeing can't. I do realise that it isn't sure to match reality, the thing I don't see is why Airbus/Boeing can't get as close to testing the spec as a (proper) manufacturer can. I'd like to understand that.

If the answer is that some significant aspect is unobservable afterwards, then my next question is how that was chosen to begin with.

Does this make more sense?

[Brian_K_White]

The testing that produced the recipe is the full eventual observed lifetime performance in the past.