In this video excerpt from NOVA’s "Making Stuff: Stronger", host and New York Times technology columnist David Pogue visits DuPont™, where Kevlar® was invented. After learning how this bulletproof material is made, David puts its strength to the test with an ice pick. A related demonstration tests and compares the tensile strength and elasticity of Kevlar®, nylon, steel, and cotton thread by using them to lift weighted buckets. Students learn that materials can be strong in different ways—some have high tensile strength, others are more elastic—and that materials scientists test the strength of materials by stressing them to their breaking point.
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“Strong as steel” is a familiar saying, and it’s an apt one. Steel, an iron alloy, is one of the strongest and most versatile materials around. But, as host David Pogue discovers in NOVA’s "Making Stuff: Stronger", scientists are creating new materials that push the idea of strong to extraordinary new limits.
The word strong actually refers to a range of properties, each defined by the ability to stand up to a different type of force. Strength is a measure of how well a material can resist a force (or load) before failing. The load is distributed over an area and is more accurately defined as stress (force per unit area). There are different kinds of stresses, including tension (pulling), compression (squeezing), impact (a sharp blow), torsion (twisting), and shearing (surfaces sliding past one another). We apply these stresses in our daily lives when we pull open a door, push a cart, or twist the cap off a bottle.
Materials scientists test the strength of materials by stressing them to the breaking point, called failure, at which point the material ruptures and cannot rebound to its original condition or shape.
Tensile strength is how much stress a material can withstand while being pulled in opposite directions. This stress causes the material to temporarily lengthen. If the stress is low enough, when you release the force, the material will return to its original length – this ability for the thread to stretch and rebound is a property called elasticity. The amount the material lengthens is called the elongation. (If you divide the elongation by the original length you get what is called strain.)
If you continue to pull, the bonds between atoms in the material will start to break, eventually reaching the point at which the material will not rebound when the stress is released; the deformation is permanent, or plastic. When enough bonds break, the material snaps apart. The amount of stress the material can endure at the time of failure is the strength of a material.
DAVID POGUE Kevlar is clearly some tough stuff.
Whoa!
But how does it work?
TUCKER NORTON If you were to zoom down at about 100-million times...
DAVID POGUE Well, we have a really nice camera, can you zoom down 100-million times?
Kevlar is a polymer, a long repeating chain of atoms, in this case, carbon, hydrogen, oxygen and nitrogen. Each chain is like a stiff piece of spaghetti. Gathered into bundles, the stiff chains create a thread-like fiber that has a hard surface, extremely high tensile strength and enough toughness to absorb the impact of a bullet.
Stopping a round is impressive, but there are other threats out there. How does Kevlar stand up to a knife or an ice pick?
Don't try this at home.
I'm not going to pay a lot for this muffler!
I said, "Caesar, on the side."
Twenty cents a text message? Are you nuts?
That's amazing. How can it do that?
TUCKER NORTON Well, it does it because of the tight weave and because of the strength and durability of Kevlar. The tight weave prevents the spike from getting all the way through, from working its way through. And the strength of that Kevlar actually helps blunt that tip and maybe even bend that tip if you are strong enough.
DAVID POGUE Since it was invented, Kevlar has been used in all kinds of products, from tires to parachutes and even cables. And that gives me an idea.
Tucker, I've got this friend. Um he's got a fleet of nuclear aircraft carriers and, they have these steel cables, a lot like this, that are designed to stop aircraft that are landing, 130 or 150 miles an hour. How does the strength of one of these cables compare with a steel cable?
TUCKER NORTON Well, we'd expect the strength of this, the tensile strength, to be actually equal or better than something like the steel cable.
DAVID POGUE And, it's lighter and it's less dangerous if it snaps?
TUCKER NORTON That's right.
DAVID POGUE Tucker doesn't think that anyone has ever tested Kevlar arresting cables, but I think I may be on to something.
This, this could be a sales opportunity for you.
TUCKER NORTON We appreciate that, thank you very much, David.
DAVID POGUE I'll look for my commission.
With its tensile strength greater than steel, extreme flexibility and heat resistance, in a package that weighs one-fifth as much as steel, Kevlar could, one day, be a replacement for steel cables.
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