[thsksbd]

Composite failure is very hard to predict and hard to evaluate. By comparison metal fatigue is very well understood.

Compression loads on submarines are extreme and cyclical. This causes the fibers to delaminate loosing strength. Furthermore, De-lamination is difficult to measure non-destructively (I think only xrays work?)

Manned conposite subs should always have their chief designer or financier aboard

[avs733]

ultrasound works quite well as well - but no NRE technique is perfect

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7503374/

[eternityforest]

Didn't they specifically refuse to do any testing because they trusted that proprietary acoustic system?

[thsksbd]

The paper is from 2020. My understanding is that composite testing is still in its infancy.

Also note that the hull was 5 inches thick (safety factor > 2.5). Depending on the dispersion properties of the medium ultrasound might not penetrate and get a good result. In Ti (some alloys at least), for example, you cant get much signal past a couple of inches due to the scattering (from the grains?)

[namibj]

That was supposed to hear the hill cracking/creaking.

Ultrasound for delamination is like medical ultrasound and looks for a discontinuity in the sheet from the delamination.

[eternityforest]

I think someone wanted to do the delamination hull scan and they skipped it presumably because they trusted the creak listener

[fecal_henge]

Just to add, isnt the reason why metal fatigue is well understood because it has killed a lot of people?

[adrian_b]

No, it is well understood because the failures of metals are much more reproducible than of other materials.

Both for ceramics and for composites the failures are caused by microscopic defects that are never the same for different samples, so the conditions for failure are very variable from sample to sample. Therefore the results of experiments with such materials have lower predictive ability for the behavior in a real application.

[thsksbd]

To some extent. "Mistakes were made" as they say. But its a lot more nuanced than that.

Metal fatigue is understood because we have a mechanistic understanding of what it is, how fatigue accumulates, and how different metal crystal strictures respond to fatigue.

We know, for example, that metals like aluminum will always fail from fatigue given enough loading cycles, no matter how small the applied stress.

We also know that other metals, like iron and titanium, have a "fatigue limit" below which fatigue doesn't accumulate and these metals can endure infinite loading cycles.

We have, to some extent, the ability to repair metal fatigue.

We build airplanes from aluminum knowing their aluminum hulls and wings will fail (whereas if it were built with Ti fatigue failure could be eliminated) because metal fatigue is very predictable and we can withdraw a hull from service after a regulatory determined number of landings.

So yes, until we developed our current understanding of fatigue, people died. But, often, this was from a callous disregard to traditionally accepted safety factors by cowboy "innovators".

(Im a materials eng. PhD in polymers w/ background in Eng. Phys. Im not a metallurgist for what its worth)