You see, when it comes to materials, most people's idea of what "strength" means is inaccurate. Our expert on material strength, Mark Eberhart explains...
In casual conversation the word "strong" has multiple meanings, most of which are quite different from what a scientist means when using this word. For example, upon seeing bullets bounce off a sheet of steel, we might describe the steel as strong. A rock climber might be thankful for the strong rope that brought her to a stop after a thirty-foot fall. And, a builder might represent an I-beam as strong because it bends immeasurably when supporting an entire building. Actually, none of the materials in these examples is necessarily strong...
Strength is a measure of how much load something can carry before breaking. Imagine that from several different materials, say aluminum, steel, and glass we make rods 1 inch in diameter and five feet long. We suspend these rods from a crane by one end and to the other we begin to hang weights. As the weight increases the rods will stretch, for the same weight aluminum will stretch the most and glass the least. The amount of stretch for a given load is a measure of stiffness--glass is stiffer than aluminum. The I-beam the builder described as strong is actually stiff.
At some point as weights are added to the rods those made of metal will continue to elongate without the addition of more weight. Finally these rods will reach some critical stretch and break, this phenomenon is called yielding and the amount of weight needed to begin this process is a measure of the material's (yield) strength. The glass rod will not yield, when the weight exceeds some critical level the glass rod will simply break. The amount of weight needed to break the glass rod is a measure of its fracture strength. Of the three materials, the least strong is aluminum and the strongest is glass!
Now there is also a material's property called toughness. Toughness tells us how much energy a material can absorb before it breaks. That rock climber owes her life to a tough rope. We can find the toughness of our rods by multiplying its stretch at breakage by the weight needed to break the rod, i.e. its fracture strength. Obviously, a tough material is one that has good strength but also stretches a lot before breaking. Because the glass stretches very little before breaking, it is not very tough. When building things that need to absorb energy, as for example, cars that may collide, we build them out of tough materials. Of our three rods, the one of steel will be toughest while the glass rod is the least tough.
Oh, I almost forgot. The bullets bounced off that sheet steel because it was hard--another property we will have to deal with. Stay tuned.
Since there are different kinds of strength, during our research we often received different answers when we asked experts "what is the strongest material?" The answers typically corresponded to the focus of the expert's field. It quickly became obvious that it was important to address the variety of the answers in our program, and what better place to capture those varying responses on video than the annual meeting of a science society whose members consist of almost 15,000 materials scientists, engineers, physicists and more?!
So, David polled a variety of attendees at the MRS meeting, from college students who had just entered the world of materials science to top experts in the field. As expected, there was an array of responses. "The strongest material? At the moment, I'd say graphene." "Diamond is the strongest, of course." "I'd say carbon nanofibers or carbon nanotubes are the strongest material." Even "silkworm silk may actually be the strongest!" All were quick to follow up their answers with "well, it really depends on what kind of strength you're talking about."
So what is the strongest material in the world? You'll find out on "Making Stuff!"
Publicist's Note: MAKING STUFF: Stronger, Smaller, Cleaner, Smarter will premiere Wednesday, January 19, 2011 at 9pm ET/PT on PBS