Working properties of materials: strength, hardness, toughness and more

Diamond is the hardest natural material known, and only another diamond can scratch it. Can anything break a diamond, then? Most people would say no, because something that hard sounds unbreakable. But diamond cutters split rough diamonds on purpose, with one sharp tap along the right line. And a hard knock from the wrong direction can chip the stone in a ring. So how can a material that almost nothing scratches be split by a single tap? We will crack the diamond puzzle once the working properties are in hand.

A working property describes how a material copes when a force acts on it: when you press it, pull it, hit it or bend it. Each one says what the material resists, or what it allows, in a product. Strength is the ability to withstand a force without breaking or bending for good. It always travels with its force, in one word. Strong in tension means it resists being pulled. Strong in compression means it resists being squashed, like a table leg under a heavy top. A climbing rope holds a climber's weight. Strong in tension, or in compression? Tension. The climber's weight hangs from it, so it must not stretch for good or snap. Now hardness. Hardness is the ability to resist scratches, dents and wear, and to resist being cut. Drag a nail across a hard material and it is barely marked. Toughness is a different property. Toughness is the ability to absorb a sudden knock without cracking or shattering. A car tyre hits kerbs and potholes for years and stays whole. A material that cracks or shatters when it is knocked is brittle, the opposite of tough. And a brittle material can still be very hard. That is enough to settle the diamond puzzle from the opening. Hard or tough: which does a diamond have, and which does it lack? A diamond is hard, but it is not tough. Being hard, it resists marks from almost everything. Being brittle, it cannot absorb a sudden blow, so the blow cracks it instead. Struck along its weakest line, it splits cleanly, which is what the cutters use. That makes hard and tough two different properties. Hard means resisting scratches and dents. Tough means absorbing a sudden knock without cracking. A phone screen shows the same pair. Your keys cannot scratch it, because the glass is hard. One drop onto a pavement can crack it, because the glass is brittle. Here's a sort to try. A ceramic mug, a pair of polycarbonate safety glasses, and a rubber mallet head. Which of the three are tough, and which one is hard but brittle? The mug is hard but brittle. A spoon cannot mark it, yet one drop on a tiled floor shatters it. The safety glasses are tough, so a flying chip does not shatter them, and the mallet head is tough too, so it survives every blow.

Malleability is the ability to be pressed, hammered or rolled into a new shape without cracking. Aluminium is rolled into kitchen foil, and steel sheet is pressed into the shape of a car door. Ductility is the ability to be pulled, or drawn, into a long thin wire without breaking. Copper is drawn through smaller and smaller holes to make the wire inside a cable. Both mean shaping without breaking. Malleable is squashed or rolled flat, and ductile is pulled long, so the direction is what tells them apart. Elasticity is the ability to stretch or bend under a load, then return to the original shape when the load is removed. A material that does both is elastic. That is two halves, every time. It stretches under the load, and it returns when the load goes. A steel spring does both, and so does elastane, the stretchy fibre in the cuff of a sports sock. Now a multiple choice, with four descriptions. A material that goes back to its shape after stretching. A material that can be pressed or rolled into a new shape without cracking. A material that resists scratches and dents. And a material that can be pulled into a long thin wire. Which one describes malleable, and what do the other three describe? Pressed or rolled into a new shape without cracking. Springing back after stretching is elasticity, resisting marks is hardness, and being drawn into a fine strand is ductility, the near neighbour of malleable. Here's a different kind of question, about that sock cuff. Pulling the sock over your heel stretches the cuff. If the cuff stretched but never returned, what would happen to the sock? It would stay baggy after the first wear, and the sock would slide down into your shoe. The cuff only grips your ankle because the elastane returns to its original size. One examiner's report records where that second half went missing. In some instances, students provided an example of a product that stretched which did not access the second mark for indicating how an elastic material returns to its original shape when no longer under load. So the fix is to write both halves every time: it stretches under a load, and it returns to its original shape when the load is removed. Stretchy on its own is only half the property.

Four more working properties come next: stiffness, strength-to-weight ratio, plasticity and impact resistance. One takeaway uses all four. Stiffness, also called rigidity, is the ability to resist bending or flexing under a load, so a part keeps its shape. A corrugated card pizza box lid is stiff. Its wavy middle layer holds the two flat faces apart, so the lid does not sag onto the pizza. Strength-to-weight ratio is how much strength a material gives for its weight, and a high ratio means strong but light. Corrugated card is mostly air, yet one box carries a hot pizza and takes the weight of more boxes stacked on top. That lets a driver carry a whole pile at once, and very little card goes into each box. An aluminium bike frame works the same way: strong, but easy to lift. Stiff and strong are another pair that gets muddled. Stiff means it does not bend. Strong means it does not break. Acrylic is stiff, yet it is brittle, so a stiff material can still snap. Now try that pair on a fishing rod. It bends deeply under a big fish, but it does not snap. Is the fishing rod stiff, strong, or both? Strong, but not stiff. It flexes a long way, so it is not stiff, yet it carries the catch without breaking. Plasticity is the ability to be reshaped for good by a force, often with heat, and keep the new shape without cracking. Here, plastic is a property, not the name of a material. The takeaway's sushi tray shows it. A HIPS sheet, short for high impact polystyrene, is heated until soft, then sucked down over a mould. Once it cools, it keeps the tray shape for good. If the HIPS sheet were elastic instead, what would happen once the suction stopped? It would spring back flat. An elastic material springs back, and a plastic one stays in its new shape, so plasticity is the opposite of elasticity. High impact is right there in the name of HIPS. Impact resistance is the ability to absorb a sudden knock or blow without cracking, which is toughness by another name. That is why a dropped tray survives the counter. One more word is fine to use if you say what it resists. Durable means it resists wear and weather over time, so a garden bench stays sound for years.

Now every property has to become a linked answer: the property, what it resists or allows, then what that means for this product. Here are two answers about an oak table. One says: oak is hard. The other says: oak is hard, it resists dents and scratches, so the table top stays smooth. Which one scores, and why does the other one not? The second. The first is a bare word with no reason. The second says what hardness resists and what that does for the table. Thick and cheap fail for a different reason. Thickness is a size and cheap is a price, so neither one is a property at all. Now a bigger one. A zip wire at an adventure park hangs from a steel cable. Explain two working properties the cable needs, saying what each one resists or allows. Try it on paper before I show you the answer. First, strength in tension. The rider's weight pulls along the cable, so it must hold that load without stretching for good or snapping. Second, toughness. As a rider launches and brakes, the cable takes sudden jolts. A tough material absorbs each jolt without cracking, so a sudden jerk never snaps it. A good third is ductility. Steel can be drawn into thin wires, and many thin wires twisted together make a cable that bends round the pulley wheels. One examiner's report gives the wording that earned the mark for that first property. What was required to gain the mark was strong in tension. So the fix is to put the force beside the word. Strong in tension for something pulled, and strong in compression for something squashed. Another examiner's report records the words that worked. Popular correct answers were scratch and dent for hardness and break and bend for strength. Those are plain words, and they do the job once each one is attached to a product. Each material group leans on its working properties. Metals are malleable and ductile, so they can be pressed into panels and drawn into wire. Hardwoods like oak are hard and tough, so a chair survives years of knocks. Many polymers are tough, so a dropped lunch box bounces instead of breaking. And elastane gives fabrics stretch and return, so a pair of leggings keeps its shape.

Time to check the property words. Four questions, and the last one goes back to the diamond.

A wooden chopping board survives years of knife cuts. Which working property is that? Hardness. It resists knife cuts and scratches, so the surface stays usable. Now a hair tie stretches round a ponytail, then goes back to size. What property is that? Elasticity. It stretches under the load, and it returns when the load is removed. Here's gold, beaten into leaf thinner than paper without cracking. Which property allows it? Malleability. Hammered flat into a new shape, and it never splits. A different angle on the diamond to end: why would it make a poor hammer head? A hammer head must be tough, to absorb blow after blow. A diamond is hard but brittle, so one swing could crack it.

If all four came straight back, you've got it, so give this video a thumbs up and it will show as done when you scroll back through. If the pairs still blur, write hard and tough out once, side by side with an example each, because that one written line tends to stay.

Next in the chapter: Properties at work: why products are made from the materials they are, where every property word gets a named material and a real product. Hopefully that's next in your feed, but if not, search for ScholaFly D T oh three, oh three.

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