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This article was first published in 2007.
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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
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.
Elasticity
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.
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Before
we go any further, think about just the two characteristics of tensile strength
and elasticity. A suspension arm may be made from material with a high tensile
strength and high elasticity. That means it won’t break, but it may deflect a
long way under loads, so changing the suspension geometry. A spring steel, for
example, has high elasticity and high tensile strength. It’s not suitable.
But
a metal with high tensile strength may be too inelastic – rather than
deflecting, it breaks off. Again, it’s not suitable for a suspension arm.
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Ductility
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
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.
Interrelationships
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.
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