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Comment by JKCalhoun | Hacker News Reader

I've been a long time carbon-fiber skeptic.

Only an armchair materials enthusiast but carbon fiber construction seems to lack the margin for fatigue that aluminum has. It seems to me that when carbon fails it fails catastrophically, while aluminum allows some "give".

Now it seems carbon fiber doesn't have the longevity either? I am aware the aluminum can "work harden" over time and become more brittle as well. No material appears to be perfect but carbon has always felt to me to not be going in the right direction.

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Keep in mind we have 30-year old A320s flying around using carbon-fiber, so we would seem to be out of the trial-period phase.

This issue appears to be a cosmetic one with regards to the paint layers, not structural (see my other comment).

Apparently A350 are built with Teijin Tenax thermoplastic carbon. Brochures say it cures in one minute, eliminating autoclave. Could be different from normal resin based carbons.

Yes, and quite possibly they did not factor in the long term differences between interaction of paint and materials.

Why would you think that? You think these aerospace engineers are a bunch of goof offs?

Because sometimes process optimizations have complex downstream effects.

Aerospace engineers are anything but a bunch of goof offs, which is why flying is as safe as it is. And that's why they pulled the aircraft from service even though it is 'just a surface issue' they want to make sure that it really is just that before releasing them - freshly painted - back into service again.

The history of aerospace quality control is written in blood, not because they are goof offs but because materials science is complex and sometimes the only warning you get that something is wrong is a crash. Avoiding those is what this sort of action is all about, in any other industry they would have slapped some new paint on it and kept going.

Is the paint for anything more than cosmetics/branding purposes? Does it provide a benefit like UV shielding etc? In otherwords, do we use the paint for anything other than ego of corps owning them?

Some paint is more slippery than the substrate it is painted on for lower air resistance:

https://www.skybrary.aero/index.php/Surface_Coatings_and_Dra...

Then there is corrosion protection, protection from ablation (especially on leading edges which usually have other protection against this as well), ice adherence and so on. So it is far from just cosmetic, and given the weight of a single coat of paint (500+ pounds on a 777 for instance) airlines make sure they optimize for fuel consumption because just hauling 500+ pounds along for the lifetime of the aircraft costs a pretty penny.

Aircraft are fairly regularly stripped and re-painted too (I lived near a special purpose facility in Canada that did this, pretty weird to see a widebody come down in a location that is not near any major airfield, the first time I saw that I thought it was about to crash, but it turned out it was there for a paint job).

You want your plane to be mostly white in color to reflect solar heat and reduce the load on the air conditioning packs.

White paint also weighs slightly less which, across a fleet, adds up to an economically relevant fuel cost.

Sounds plausible, if someone told me that this can of paint designed for that epoxy destroys this thermoplastic, I’d just believe it.

That could be, or it could be a process related issue, or a change in formulation of a paint that performed well, or a change in formulation of the polymer that the fiber is embedded in, there are a very large number of options to choose from.

there are also a number of enthusiastic road cyclists riding 20, 25 year old carbon fiber frames - if not crashed or mechanically damaged (in a way that would also total beyond repair a thin walled aluminum frame like a cannondale CAAD8), they last quite a while.

I had one of these until very recently, a first generation CADEX frame (1987, so now a good 35 years old), still as good as new, it went to Germany and is now plying the streets of Berlin.

Unless you smash them up just keep them clean and they seem to last quite long, at least as long as comparable steel or aluminum frames.

The weak points are where aluminum is bonded to the carbon, those spots you need to monitor and if there is any damage at all you should count the frame as lost.

When I raced mountain bikes I always found the feel of aluminium frames to deaden after a few years. They would be super responsive at first and then slowly lose that snappiness and with it trail feel as the years went by.

road cycling is probably less harsh than a plane cycling through 0 to 30k+ feet multiple times a day, though.

The loads and operating conditions are quite different for bicycles and airplanes.

As I wouldn't use the characteristics of my steel coffemaker to predict the behaviour of an LNG vessel I wouldn't do the same with bicycles and airliners.

If it's cosmetic only, why the grounding?

Carbon fiber and other composites are actually much better wrt fatigue than aluminium. In fact they are pretty much immune to it, if you mean the word in the sense it's used in metallurgy. They are however much worse for impact resistance.

Does the impact resistance degrade during the expected lifetime of those composites?

No, except inasmuch as the resin may degrade over time, but that's not just an issue for impact resistance.

To elaborate, carbon-fiber reinforced carbon, the expensive ceramic used for very heat-resistant lightweight structural members (like the leading edges of the space shuttle, and some jigs for use in metallurgy furnaces, where it's lack of deformation and low heat capacity is beneficial) can handle something around 20% of it's ultimate tensile strength for millions of full load cycles.

This is, going by fatigue-limited-strength to weight ratio, far better than steel or aluminium. The difference is mostly that it's a brittle ceramic and retains most of it's strength at temperatures where steel can be poured.

Which is why Aluminium drop-handlebars on bikes freak me out - when they fatigue they become brittle and can just snap one day if you put too much weight on them (e.g. out of the saddle sprinting).

The only time I've ever had aluminum drop bars go out on me was when the salt brine from sweat corroded the aluminum a lot.

I was on a hill in a mild rain riding next to a friend. I managed not to take her out and stay upright which was great. The cycling team I participated on in college said road bars need to have the tape off and inspected every year. They were right.

It's way to early to speculate about this, as we don't really have information from either side.

The fact that only Qatar Airways seem to have such problems (for now) is also interesting.

The reason could be anything from manufacturing errors with a specific batch of them which happened to have been bought by Qatar Airways to wrong maintenance by Qatar Airways or the planes having been to often exposed to to extreme weather conditions as they are not that unusual in Qatar. And even this "wide range" speculation is not really usable given the facts we (don't) have.

A likely reason for why only Qatar airways reports this is that they are notorious about regularly publicly complaining about “quality issues” in the press during commercial negotiations. It’s a trick to pressure the aircraft manufacturer.

That’s not to say that quality issues don’t happen, they certainly do. But QTR is known to be very quick to refuse to work with their supplier and publish the complaint in the press, instead of working it out like is the norm, especially if it’s near a time of contract negotiation.

Yeah I worked in NDT for a bit and it's very difficult to test carbon fibre for damage or delamination under the surface. There are techniques but not as good as for aluminium. And as you said they don't have a nice plastic region like metals.

> [carbon fibre] don't have a nice plastic region like metals.

An amusing turn of phrase, given we are presumably talking about carbon fibre reinforced plastics.

Metals don’t have a “nice plastic region “ either. If the parts of your aircraft have been permanently elongated by plastic deformation, you’ve destroyed it.

If anyone is curious about material science, some keywords are "stress strain curve", "plastic vs elastic deformation", and "fatigue limit" (that last one is really cool).

https://en.wikipedia.org/wiki/Stress–strain_curve

https://en.wikipedia.org/wiki/Deformation_(engineering)#Plas... vs https://en.wikipedia.org/wiki/Deformation_(engineering)#Elas...

https://en.wikipedia.org/wiki/Fatigue_limit

I am more worried about fire safety. Fire in CF is difficult to extinguish. How well and for how long does it protect occupants from fire, does it produce poisonous gases and lose structural integrity as it heats up?

Fatigue is not a concept applicable to carbon fiber reinforced materials. Carbon is anisotropic, unlike bulk metals which can be modeled as isotropic. Carbon may suddenly delaminate or whatever, but it doesn’t have a cycling limit.

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