I irrationally dislike composites (Rant)

A composite material is just two or more different materials put together, with the materials inside remaining the same. This is what I dislike about composites, they aren’t actually any stronger on a microscopic scale than normal materials. As an example, take pre stressed concrete beams. This is a concrete structure with a steel beam running through. The steel provides resistance against stretching, and the concrete provides resistance against being squashed, combining properties just like a composite. But if we took a powerful force that rips atomic bonds apart (with a larger range than electromagnetic force) and used it, the concrete would start unraveling while the steel held up better (or would it be the other way round?). This would leave just the steel (or concrete?) on its own. Since the properties of the materials making up the composite are unchanged, this means the composite still has exactly the same strength and properties on a microscopic scale, and increased strength just happens on a macroscopic scale where forces don’t just pry apart specific atoms, but push/pull on all the atoms together. Similiarly, if a carbon fibre composite was heated to a temperature of 2000 degrees celsius, then one material would just melt, causing the composite to unravel and leaving just the carbon fibre. So the composite still has the exact same properties as the materials that make it up on a microscopic scale and it’s not any stronger (maybe it is, but the fact remains that the materials making up the composite remain unchanged, and if a force was specifically prying apart the atoms the performance would be the same; if I’m wrong correct me), but regular materials are actually stronger on the microscopic scale, that is, their atomic bonds require more force to break (at least for materials that are only made of one element, for element combinations, I’m not sure).To me that means regular materials are stronger. You can say we don’t care about microscopic scales, but this is just my opinion.

Welcome to PF.

An epoxy filled carbon fibre composite, is stronger under tension than epoxy, and stronger under compression than carbon fibre. The composite is not as strong under tension as carbon fibre, nor as strong as epoxy under compression. The physical and chemical specifications tend to follow the lowest common denominator.

By adjusting the ratio of carbon fibre to epoxy, you can juggle the ratio of the tensile to compressive strength limits of the composite. By sensibly aligning the fibres, you can benefit more.

What is there to not like about that?

Baluncore said:

Welcome to PF.

An epoxy filled carbon fibre composite, is stronger under tension than epoxy, and stronger under compression than carbon fibre. The composite is not as strong under tension as carbon fibre, nor as strong as epoxy under compression. The physical and chemical specifications tend to follow the lowest common denominator.

By adjusting the ratio of carbon fibre to epoxy, you can juggle the ratio of the tensile to compressive strength limits of the composite. By sensibly aligning the fibres, you can benefit more.

What is there to not like about that?

Under microscopic scales too? How is that even possible?!

Baluncore said:

Welcome to PF.

An epoxy filled carbon fibre composite, is stronger under tension than epoxy, and stronger under compression than carbon fibre. The composite is not as strong under tension as carbon fibre, nor as strong as epoxy under compression. The physical and chemical specifications tend to follow the lowest common denominator.

By adjusting the ratio of carbon fibre to epoxy, you can juggle the ratio of the tensile to compressive strength limits of the composite. By sensibly aligning the fibres, you can benefit more.

What is there to not like about that?

And I should add: can’t composites break under less stress than their normal strength if you just tear apart the two materials? There is delamination, which I think is exactly that, and it’s what doomed the Titan submersible.

TitaniumVCarbon said:

Under microscopic scales too?

If you mix the fibre and the filler at the microscopic scale, it will work for objects at the microscopic scale. The structural resolution does not require mixing at the microscopic level, when the object is macroscopic. It is sufficient that the materials be chemically bonded.

TitaniumVCarbon said:

There is delamination, which I think is exactly that, and it’s what doomed the Titan submersible.

The Titan was placed under hydrostatic pressure. The vessel was compressed by a few percent. That shortened the carbon fibres and so took them out of tensile play. The entire hoop crush on the carbon fibre hull was then carried by the polymer filler, which was progressively crushed on each dive. That further removed the carbon fibres from tensile play. There came a time when there were insufficient fibres remaining under tension to prevent the side buckling inwards.

The delamination arises because the cylindrical composite was wound as a tape onto the cylinder, then impregnated with resin and cured. When that structure failed, patches of woven tape would remain intact, partly covered on both sides by sharp grains of crushed polymer. The collapse failure would travel at the speed of sound in filler and slack fibre. Those are very different speeds, so the energy would follow the contact of the resin with the fibre, shearing the two and causing the delamination.

TitaniumVCarbon said:

But if we took a powerful force that rips atomic bonds apart (with a larger range than electromagnetic force) and used it, the concrete would start unraveling while the steel held up better (or would it be the other way round?). This would leave just the steel (or concrete?) on its own.

Sure


... but we do not always design for nukes.

And for many other cases, composites are (very) good, quite often better than the constituting materials on their own.

That's all. The right material for the right job. Your choice.

Rive said:

Sure
View attachment 329511

... but we do not always design for nukes.

And for many other cases, composites are (very) good, quite often better than the constituting materials on their own.

That's all. The right material for the right job. Your choice.

I never mentioned nukes. Just a force that is prying apart the atoms and just doing that specifically. Not equally pushing on the atoms, but prying apart specific ones in the material

TitaniumVCarbon said:

Just a force that is prying apart the atoms and just doing that specifically. Not equally pushing on the atoms, but prying apart specific ones in the material

And why should we care about this terrifying (albeit hypothetical) force? There are plenty of real problems without worrying about hypotheticals.
You do call your worry irrational in the title...

Baluncore said:

If you mix the fibre and the filler at the microscopic scale, it will work for objects at the microscopic scale. The structural resolution does not require mixing at the microscopic level, when the object is macroscopic. It is sufficient that the materials be chemically bonded.

Baluncore said:

If you mix the fibre and the filler at the microscopic scale, it will work for objects at the microscopic scale. The structural resolution does not require mixing at the microscopic level, when the object is macroscopic. It is sufficient that the materials be chemically bonded.The Titan was placed under hydrostatic pressure. The vessel was compressed by a few percent. That shortened the carbon fibres and so took them out of tensile play. The entire hoop crush on the carbon fibre hull was then carried by the polymer filler, which was progressively crushed on each dive. That further removed the carbon fibres from tensile play. There came a time when there were insufficient fibres remaining under tension to prevent the side buckling inwards.

The delamination arises because the cylindrical composite was wound as a tape onto the cylinder, then impregnated with resin and cured. When that structure failed, patches of woven tape would remain intact, partly covered on both sides by sharp grains of crushed polymer. The collapse failure would travel at the speed of sound in filler and slack fibre. Those are very different speeds, so the energy would follow the contact of the resin with the fibre, shearing the two and causing the delamination.

Umm… the material I only think is stronger against external force (e.g. impact from high speed atom). Internal force resistance (a force from inside the object) remains the same, as the bonds holding the two materials together as only as strong as the adhesive, (if it was stronger than the materials making up the composite then why not use it as a standalone material) and the atomic bonds in both materials are exactly the same strength as before, having remained unchanged. An analogy is the reinforced concrete, if we made a sphere of influence that pries apart atomic bonds and that was centred inside the concrete, then the steel bar wouldn’t help as the concrete would still break apart due to being exposed to tension forces. So the components in the reinforced concrete are still the same, no stronger. Why wouldn’t this apply to composites?

P.S. the fact the I have your message twice, it’s down to some stuff that happened, ignore that

hutchphd said:

And why should we care about this terrifying (albeit hypothetical) force? There are plenty of real problems without worrying about hypotheticals.
You do call your worry irrational in the title...

If you don’t understand what I mean when I talk of the force, think of electromagnetic force but it only pries apart atoms and doesn’t affect the stuff within atoms, and it works over much larger distance. The materials in the composites would have the exact same performance as if they were seperate. What I knew when writing this was composites worked well enough, what were are currently discussing is whether the materials inside the composites or the composite actually has stronger atomic bonds.

You messed up the BBcode, click on [] to toggle and edit BBcode.

Umm… the material I only think is stronger against external force (e.g. impact from high speed atom). Internal force resistance (a force from inside the object) remains the same, as the bonds holding the two materials together as only as strong as the adhesive, (if it was stronger than the materials making up the composite then why not use it as a standalone material) and the atomic bonds in both materials are exactly the same strength as before, having remained unchanged.

Why wouldn’t this apply to composites?

Because the inner/outer differentiation does not seem to make sense.

TitaniumVCarbon said:

I never mentioned nukes.

The term 'nuke it' does not necessarily means usage of actual nuclear devices.
It ~ means usage of excessive/powerful force (that rips atomic bonds apart ) far outside of expected tolerance.

So, yeah. This rant, in short: composites won't work when nuked.
But ... should they?

TitaniumVCarbon said:

P.S. the fact the I have your message twice, it’s down to some stuff that happened, ignore that

Because you replied inside a quote, you cannot be quoted by others. You can edit your post to fix it. Just edit then toggle BBcode with [].

Baluncore said:

You messed up the BBcode, click on [] to toggle and edit BBcode.

Because the inner/outer differentiation does not seem to make sense.

I mean that the force comes from inside the composite. What do you not get?

Baluncore said:

You messed up the BBcode, click on [] to toggle and edit BBcode.

Because the inner/outer differentiation does not seem to make sens

If the force is external, then the force is absorbed by both components of the object. If it’s internal, then that doesn’t happen. The reinforced concrete analogy might help you to understand what I mean.

I think you must consider two cases; 1. external hydrostatic pressure, or 2. internal fluid pressure.
The wall thickness changes are not a direct problem with pressure. It is the hoop compression or tension that leads to failure.

TitaniumVCarbon said:

If the force is external, then the force is absorbed by both components of the object.

A submersible is subjected to external pressure that removes hoop tension from the composite. There is little to no tension in the fibres. The entire external pressure is carried by the compressed filler between the fibres. There is no surface tension in the fibres to prevent an unstable buckling and implosion of the external pressure vessel.

TitaniumVCarbon said:

If it’s internal, then that doesn’t happen.

Internal pressure pushes the walls out, stretching the fibre hoops and keeping them tight. The hollow structure is stable in the same way that a party balloon, or a soap bubble is stable, it has surface tension that prevents surface buckling.

TitaniumVCarbon said:

... As an example, take pre stressed concrete beams. This is a concrete structure with a steel beam running through. The steel provides resistance against stretching, and the concrete provides resistance against being squashed, combining properties just like a composite. But if we took a powerful force that rips atomic bonds apart (with a larger range than electromagnetic force) and used it, the concrete would start unraveling while the steel held up better (or would it be the other way round?). This would leave just the steel (or concrete?) on its own

The strength of each material is not everything resisting loads.
The geometric location of each is very important to be considered as well.
Once the concrete fails under tremendous compression, the remaining steel will not be loaded at all as it was before that rupture.

In order to resist buckling and bending loads, the material should be located as far as possible from the neutral line of internal forces.
Composites for airplanes, for example, allow that, while keeping the weight very low (think of the shape of a honeycomb created inside a wooden beehive, but at microscopic level).
To achieve the same with aluminum would be impossible.

A truck's tire is another example of rubber, nylon and steel working together as a semi-flexible structure.
Rubber keeps the air inside and the nylon cords in the correct place.
The steel ring keeps the rubber from jumping over the wheel.

Lnewqban said:

The strength of each material is not everything resisting loads.
The geometric location of each is very important to be considered as well.
Once the concrete fails under tremendous compression, the remaining steel will not be loaded at all as it was before that rupture.

In order to resist buckling and bending loads, the material should be located as far as possible from the neutral line of internal forces.
Composites for airplanes, for example, allow that, while keeping the weight very low (think of the shape of a honeycomb created inside a wooden beehive, but at microscopic level).
To achieve the same with aluminum would be impossible.

A truck's tire is another example of rubber, nylon and steel working together as a semi-flexible structure.
Rubber keeps the air inside and the nylon cords in the correct place.
The steel ring keeps the rubber from jumping over the wheel.

That’s actually tension, concrete is very bad with tension which is why the steel breaks later than the concrete, the force mentioned is like electromagnetic force but with a longer range and that pries apart atomic bonds and not atoms. I don’t know what you mean by the neutral line of internal forces, but the atomic bond strength and thus energy needed to break the material is still the same. So prying apart the material, atom by atom, would require the same amount of energy as if they were seperate.

Baluncore said:

I think you must consider two cases; 1. external hydrostatic pressure, or 2. internal fluid pressure.
The wall thickness changes are not a direct problem with pressure. It is the hoop compression or tension that leads to failure.

A submersible is subjected to external pressure that removes hoop tension from the composite. There is little to no tension in the fibres. The entire external pressure is carried by the compressed filler between the fibres. There is no surface tension in the fibres to prevent an unstable buckling and implosion of the external pressure vessel.

Internal pressure pushes the walls out, stretching the fibre hoops and keeping them tight. The hollow structure is stable in the same way that a party balloon, or a soap bubble is stable, it has surface tension that prevents surface buckling.

I didn’t assume it was coming from the centre, it could have come from inside the hoops, but the tension is only as strong as the atomic bonds. Also we are not talking about the Titan right now, we are talking about the actual strength of the atomic bonds (how much force would be needed to completely seperate two atoms in the composite). If we were to pry apart the material, atom by atom, the same amount of force would be needed as if the materials were all seperate.

Baluncore said:

If you mix the fibre and the filler at the microscopic scale, it will work for objects at the microscopic scale. The structural resolution does not require mixing at the microscopic level, when the object is macroscopic. It is sufficient that the materials be chemically bonded.The Titan was placed under hydrostatic pressure. The vessel was compressed by a few percent. That shortened the carbon fibres and so took them out of tensile play. The entire hoop crush on the carbon fibre hull was then carried by the polymer filler, which was progressively crushed on each dive. That further removed the carbon fibres from tensile play. There came a time when there were insufficient fibres remaining under tension to prevent the side buckling inwards.

The delamination arises because the cylindrical composite was wound as a tape onto the cylinder, then impregnated with resin and cured. When that structure failed, patches of woven tape would remain intact, partly covered on both sides by sharp grains of crushed polymer. The collapse failure would travel at the speed of sound in filler and slack fibre. Those are very different speeds, so the energy would follow the contact of the resin with the fibre, shearing the two and causing the delamination.

That still exposes the bad things about composites, their ‘strength’ is not inherent in the material and if we were to pry apart the material atom by atom, the same force would be needed as if the materials were seperate.

TitaniumVCarbon said:

That still exposes the bad things about composites, their ‘strength’ is not inherent in the material and if we were to pry apart the material atom by atom, the same force would be needed as if the materials were seperate.

That’s the great thing about composites, we can taylor the strength for the application.

TitaniumVCarbon said:

That still exposes the bad things about composites, their ‘strength’ is not inherent in the material and if we were to pry apart the material atom by atom, the same force would be needed as if the materials were seperate.

Irrelevant. This is like complaining that doped semiconductors don't exhibit useful properties when you look at them on the scale of an atom or two. They're not supposed to. That's not their scale of operation. I don't dislike my kitchen scale because it can't weigh an elephant. It's not supposed to and that's not a bad thing.

TitaniumVCarbon said:

If we were to pry apart the material, atom by atom, the same amount of force would be needed as if the materials were all seperate.

Any macroscopic force is spread out over large sections of the material involving huge numbers of atoms. This means that the material best able to resist compression, tension, shear, etc will have to be overcome for the material to fail. A tension force large enough to overcome the epoxy or plastic in carbon fiber reinforced polymers will not break apart these materials because the carbon fiber is holding things together. A compression force larger than the carbon fiber can withstand will not break the material until it overcomes the epoxy/plastic filler.

This 'rant' makes no sense. You are ignoring the exact reasons composites are useful materials and inventing things to dislike. You might as well get upset that light bulbs can't be used on the Sun.

Thread locked.