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European researchers claim to have developed a composite material that is basically uncuttable, so with this material, your bike might never be stolen.

Hosted by: Hank Green

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Sources:
https://doi.org/10.1038/s41598-020-65976-0
https://doi.org/10.1016/B978-0-12-813489-4.00008-8
https://link.springer.com/book/10.1007/978-0-387-74365-3
https://doi.org/10.1016/0010-4361(91)90549-V
https://doi.org/10.1016/j.ijimpeng.2020.103609

Image Sources:
https://upload.wikimedia.org/wikipedia/commons/1/1a/Proteus-Alciato.gif
https://www.nature.com/articles/s41598-020-65976-0/figures/4
https://www.istockphoto.com/photo/close-up-of-bench-grinder-machine-gm1190888255-337759839
https://www.nature.com/articles/s41598-020-65976-0/figures/10
https://www.istockphoto.com/vector/vector-superhero-team-silhouette-in-the-city-stock-illustration-gm1168482172-322652942
https://www.istockphoto.com/vector/abstract-geometric-gradient-shape-background-gm1158846602-316696714
https://www.nature.com/articles/s41598-020-65976-0#rightslink
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[upbeat electronic music]

In comic books, heroes are often equipped with practically indestructible materials like adamantium or vibranium. Unfortunately, as anyone who's ever had a bike lock snipped with bolt cutters can tell you, we have not figured out how to make something that tough. But we're getting pretty close, because European researchers claim to have developed a composite material that is basically uncuttable.

Composites are made up of two or more distinct materials that usually have different properties and therefore end up working better together. It's like the Captain Planet of material science. Concrete, for instance, is made up of cement, rocks, and sand. When their powers combine, they form a strong, durable building material. The new composite the researchers developed is a unique mixture of aluminum, steel alloy, titanium, and ceramic, and even though it's six times less dense than steel, it's incredibly difficult to cut through. 

Here is how they made it. First, the researchers mixed powdered aluminum with titanium dihydride, which was used as a foaming agent. The foaming agent's job is to create tiny pockets within the aluminum by releasing gas when heated - in this case, hydrogen gas. Them, they compressed this aluminum mixture into rods. The rods were stacked in alternating rows with ceramic spheres that were about 13 millimeters in diameter, and then those rows were laid one on top of another, creating a checkerboard pattern if you looked at it sideways.

And all of this was then heated to 760 degrees Celsius. This caused the aluminum to melt and the titanium dihydride to release all of that hydrogen gas. Those bubbles made the aluminum expand out around the ceramic balls. Once everything cooled, the researchers were left with their new composite: ceramic balls encased in what is essentially an aluminum foam matrix.

The researchers decided to name the composite. They called it Proteus from the shape-shifting Greek god from mythology. And now all they had to do was test it.

First, they took four-centimeter-thick plates of Proteus and tried to cut through them using an angle grinder. And, spoiler alert, the blades didn't get far. The angle grinder disc became unusable in just over a minute after it had made its way through a mere one-fifth of the material. Yeah, Proteus is that tough.

And, according to the researchers that designed it, there's two main reasons for that. Right off the bat, the rotating disc started breaking off tiny, sand-sized particles of the hardened aluminum foam and ceramic spheres. This is exactly what the researchers wanted to happen, because small particles have high amounts of fracture toughness, which is just a fancy way of saying that small particles are harder to break into pieces, and that meant the particles ended up behaving like a rough sandpaper, helping wear down the cutting disc.

Small particles are also harder to penetrate, as long as the penetrating load or force is applied quickly. That's because, when there's a large load applied to small particles over a short amount of time, the particles squeeze together and become more compact. That increases the friction between them, and that friction helps them grip onto one another, increasing their resistance to being pierced. This, for example, is why bags of sand can stop bullets.

So Proteus's tiny particles resisted the grinder disc while wearing it down. Then, as soon as the blade really hit Proteus's ceramic spheres, it was game over, thanks to vibrations. Vibrations are the back and forth movement of particles, and, over time, one vibrating particle can cause others to vibrate, creating a ripple effect throughout the material.

When an angle grinder disc starts spinning, it generates vibrational waves that radiate out from the center of the disc. In this case, as the angle grinder made contact with Proteus, those waves started passing into the material, which caused the ceramic spheres to begin vibrating, too. And so, once the vibrating disc came into contact with one of those vibrating spheres, that sphere applied a point load to the disc, a force that is concentrated in one specific place.

In this case, that point load was applied right on the edge of the grinder's cutting disc, and that sent vibrations back into the disc itself. That's just Newton's third law of motion and action. And the researchers believe these forward and backward vibrational waves really dulled the blade. See, if the vibrations going back into the cutting disc matched the frequency of the vibrations coming from it, then the size of the blade's vibrations could have gotten larger due to resonance. And, if that happened, then all of that wiggling of the blade could have further ground down the edge of the disc.

More research is needed to conclusively show that this is what's happening, though. One way or the other, the blades could not get more than a centimeter in. The researchers also tried a power drill and a water jet cutter with similar results. As long as the cutting implement hit a ceramic sphere, it was stopped in its tracks. 

But to make Proteus even tougher, the team tried tossing in some short nichrome wires. These helped to improve the tensile strength of the aluminum, its ability to withstand being stretched or pulled. And, once they did that, nothing made it more than several millimeters.

A potentially uncuttable material like Proteus is especially exciting for material scientists, because there are a lot of ways you can customize it. By adjusting the foaminess of the aluminum or the size or number of ceramic spheres, the researchers believe they could make Proteus adaptable to all kinds of uses. Bike locks, yes, but also maybe armored doors or maybe impenetrable show soles like Black Panther. So, while Proteus isn't exactly adamantium or vibranium, it could make for a pretty impressive shield.

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[upbeat electronic music]