06/24/2016

Composites and Planned Obsolescence

Being an engineer, and a particularly old school engineer, I spent the early part of my career trying to avoid composites. The thing was that I didn’t trust what I was being told and learning about the material. Technically, any material that lattices two different materials together is a composite: concrete and metal, glass and plastic, clay and stainless steel, wood and gypsum for a few. There are good reasons for compositing materials, but there can be huge pitfalls. I must stress that it’s not all a rosy picture, but there are some good things.

 

In basic chemistry we learn about covalent and ionic bonds. It isn’t until organic chemistry (usually in college) that we learn about van der waals bonding. This is the type of bonding that keeps plastics together. This isn’t the same as the previous two. There are a ton of extra caveats to this type of bond, it is much weaker than ionic, covalent, or even the crystalline structure of metals. It breaks down in light, is more prone to corrosion, has certain toxicity malfunctions, and even degrades over time. Overall plastic is an inferior material when considering durability, robustness, cyclical use, safety, and product lifetime.

 

Planned obsolescence is a term coined to describe what economists have decided to call durable goods. One of the most glaringly obvious and hideously polluting faults of durable goods is the usual failing of a small plastic part. This is why industrial equipment rarely incorporates the material, and when said industrial equipment makes use of plastics it is different. They amp up the thickness, choose particularly durable plastics, and hedge all bets.

 

Not to be redundant (he he) but typically redundant systems are robust and rarely fail. The chemistry of plastics dictates that they will fail, fail often, fail under corrosion, break down in light (UV) and fail, spew tiny amounts of toxins that are imperceptible, and generally make our lives hell. Yes, there are benefits, and we all know those benefits, but why the railing against plastics?

 

Aircraft composites are plastics. Albeit plastic with something else inside, but still contain plastics. This material is extremely strong for its weight (referred to as strength-to-weight by engineers). Another caveat is that this composite is extremely strong in one direction, and weak in the others. This is called anisotropy. Metals are not perfectly isotropic (uniformly strong in all directions) but they are close enough to generally consider them so. Composites are not isotropic at all and can be very strong in one direction, but very weak in another.

 

For decades, there were three main aircraft structural materials namely, aluminum,corrosion resistant steel, and titanium. Hopefully, the materials were used in this order because the strength to weight relied highly on the use of aluminum with titanium and cres filling in only the gaps. This new ‘composite’ material throws a whole lot of junk into the mix that confuses everyone. This confusion is particularly true of executives, reporters, and generally anyone who doesn’t understand math and physics well.

 

— warning note: be aware of the term ‘factor of safety’ here —

 

If I was a billionaire, and I wanted to fly around in a jet, I would completely ban plastics and composites from my aircraft on the mere grounds that it was a bad investment. At the point that a salespeople interjected some random composite material benefits, I would ask for a deep discount for the aluminum equivalent. I consider aluminum to be the superior material. At this point we are going to have to go over the differences and start splitting hairs.

 

Aluminum has several widely used variants. Its main advantage is that for its weight it is very strong and relatively cheaply bought strength. It is highly corrosion resistant, has a long lifetime, and is the choice metal for durability in aircraft over time. However, it has a lifetime. This lifetime is very predictable. So predictable, that you can calculate very precisely when it will fail, but it will eventually fail. This failure mode is called fatigue.

 

Steel has many variants, but only cres can be used in conjunction with aluminum. Two metals tend to create a battery, and one corrodes away. Cres is the way to avoid this battery-corrosion problem. Steel (and corrosion resistant steel) have a particular behavior that causes it to have potentially/possibly infinite fatigue strength, whereas aluminum is not capable of this infinite behavior. However, steel is much heavier per strength than aluminum and a fully steel aircraft would suffer extreme performance/weight problems.

 

Titanium is pretty much relegated to two jobs. One, where there is a high temperature application, as it is hard to melt. Two, where there is a particular shortcoming that steel will not suffice for. Titanium might as well be called ‘incorruptible unmeltable brittle steel’. It is expensive, hard for communists to obtain, and serves pretty much as a replacement for steel as their strength to weights are pretty close. Although titanium is a bit stronger. Also, titanium corrodes far differently (less) than steel does.

 

Keep in mind that balsa wood and burlap/cloth ply are the ‘really old school’ materials, but are no longer used widespread. Also, ultralights are aluminum tubes with nylon cloth.

 

Composites are the new kid on the block. This material has ‘huge’ strength to weight benefits. The material has annoyingly hard to calculate stresses that only apply in one or two directions, take more manpower to produce, and confuse the crap out of everyone who isn’t a scientist/engineer. The benefit is that the strength-to-weight can be off the chart if put together right. However, the achilles heel is not the lack of understanding, or high cost of production. The achilles heel of this material is the almost fraudulent lack of lifecycle, effectivity, or longevity calculations.

 

— Please read the last sentence of the previous paragraph a few times. The last sentence of the previous paragraph is the point of this article/post —

 

Let that sink in.

 

To be clear, composites have a clear advantage over the other material choices. For example, in non-reusable rockets it should be the prime material due to its peculiar properties. Let’s say that the thrust reversers on an engine are ‘replaceable’ then this would definitely be the place to use them. Imagine a case where something needs to be extremely strong, but need not last past 5 years of repeated use, and this is the place for this material.

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