I mean, that was part of NASA's investigation conclusions. The design error wasn't using an industrially rated part, it was using it without proper screening, testing, and failure to follow manufacturer recommended safety margin.

One of the purposes of using the "right rated" parts is that you get to off-load some of the testing and screening that you might otherwise wish to do to a component vendor (who is better equipped to absorb the price of testing, and because they have tighter integration with manufacturing, a better ability to achieve specifications).

Like I totally get the idea of using lower cost components. But sometimes they are actually the solution. And sometimes finding the problem is incredibly expensive, if not in $$$, then at least in PR.

Americans love to shit on gold plating, but they also love to shit on greedy corporations minimizing the bottom line.

No, that's "a bad batch."

The issue here (as a sibling post points out) is QA.

And "industrial grade" includes parts where the manufacturing is that unreliable and nobody has tested them either?

The QA on aerospace grade material is much more severe as are the manufacturing standards. As a result the spread in critical characteristics between batches is much smaller, thus reducing the risk of a batch bad enough to fail in the normal use case. But yes, there are no guarantees, even aerospace grade stuff will fail. Testing isn't perfect, it simply reduces the risk.

As evidenced by that explosion, yes.

I don't see how the explosion helps distinguish between "industrial grade" and "below industrial grade"?

The distinction is between aerospace grade and industrial grade. Aerospace grade is definitely not "below industrial grade". Aerospace grade parts will be subjected to far more rigorous testing.

The problem SpaceX had was using industrial grade parts when they should have used aerospace grade parts.

The post I was replying to was making a distinction between aerospace grade and industrial grade.

I was looking at the details of the problem, and suggesting that those parts were neither aerospace grade nor industrial grade, but something lower still.

Non-rigorous testing is a reasonable thing for "industrial grade". I am not convinced that being far under half of the rated strength is a reasonable thing for "industrial grade".

The manufacturer suggested a load factor of 4x. Most industries probably don't subject their hunks of stainless steel to temperatures of -300°F. Cast parts also have a tendency to be more brittle when exposed to certain types of forces.

Suggested load factor shouldn't mean anything. There is a spec, and the parts are supposed to meet the spec. And most of the struts did. It wasn't a case of blatant misuse. Something went wrong with a subset of them.

The load factor is part of the spec, usually (I didn't check the specs). Maybe blatant misuse is a little bit strong, but one could get that SpaceX took slightly higher risks than traditional companies.