We all know that different metals have different properties. Bend a piece of aluminium, and then bend a piece of steel that’s the same size, and differences become immediately apparent. But try to use this knowledge to select, say, the best type of steel for an anti-roll bar, and it all starts to become very difficult indeed. In fact, before you can even start selecting the best material for the job, you need to know what characteristics there are, and how they vary.
So here’s a brief, easy to understand coverage of the main properties that you need to know about.
Tensile strength is the amount of smoothly applied pull that will stretch a piece of metal so far that it breaks apart.
The way it is most commonly expressed is in psi, that is, pounds per square inch. If the piece of metal being stretched by the horses is one square inch in area, and the horses can apply a pull of 3000 pounds before it breaks, the tensile strength of the material is 3000 psi.
Most metals are not brittle – they bend before they break. For example, a piece of steel subjected to the tensile test described above will stretch before it breaks. Elasticity refers to the deformation that the material can experience under load and yet still return to normal when the load is removed.
To put this another way: an elastic specimen returns to its original shape after the load has been removed. If the material does not return to its original shape after the load has been removed, it has exceeded its elastic limit and has started to yield. (See also Making Things, Part 6 .) Normally, a metal that in design use has started to yield is said to have failed.
The ductility of a material refers to the degree to which it can be permanently deformed without failure. That doesn’t sound like a property that’s desirable - but in many applications it is. If you need to bend the material, or shape it in a press, you want the material to have sufficient ductility that it doesn’t crack during this forming process.
Some high strength steels being used in today’s car bodies have high tensile strength and low elasticity. However, they also have low ductility, meaning that, if involved in an accident, they cannot be panel-beaten back into shape. (See Advanced High Strength Steels, Part 1 and Ultra High Strength Steels, Part 2.)
Another way of thinking about ductility is to take the example of a material that has very low ductility - we call it brittle. A brittle substance is one that fails without appreciable deformation. For example, carbon fibre is far more brittle than nearly all steels. Brittle substances do not give much warning before failure.
Hardness refers to the resistance that the material presents to being penetrated by another material.
It’s easiest to understand if some of the tests for harness are described. In one, test, a hardened steel ball is pushed into the material by a known force. How far the ball penetrates, as indicated by the diameter of the indent, is measurement of the hardness. Another type of hardness test uses a diamond-tipped pyramid that is forced into the material.
If the hardness a piece of steel is increased (eg by a post-production hardening process), the tensile strength will also increase. (But that doesn’t mean that all hard materials have high tensile strength!) The hardness/strength relationship shows how if one characteristic is altered, other characteristics will also change. This means that, for a given application, you need to consider all characteristics, not just focus on one.
No material has all the most desirable characteristics for an application – there will always need to be trade-offs. For example, if the material has to be deformed to make the finished product, it will need to be sufficiently ductile. If a wear surface is present, it will need to be hard. It might also need to have high tensile strength, or be high (or low) in elasticity.
A very high tensile bolt may be too brittle for the application, or a metal might be too hard for the tools that need to cut it.
There are no easy answers in metals selection, but use the ideas of tensile strength, ductility, elasticity and hardness when talking to experts about the best material for the application.