Properties of Materials
1. Strength
• Strength is the ability of a material to
withstand the forces of compression,
tension and shear.
• The Strength property of a material is very
important in areas such as building
construction, bridges and aircraft.
• In the diagrams below the forces are
represented by the arrows.
Above we can see a
pillar under
Compression.
If the column is not
constructed out of the
right materials then it
would collapse.
Concrete columns are
often reinforced by
steel cable or rods.
Above you can see a crane
lifting a load.
The crane is pulling up and the
load is pulling down, therefore
the cable is in Tension.
Above we can see a
plank of wood with 3
forces acting on it.
The forces are not in
line with each other and
For a crane, the cable is made so if the material is not
of a number of steel cables that strong enough it will
are wound around each other.
Shear.
This increases the strength of
the cable.
A scissors cutting paper
Materials that do not have
is Shearing the paper.
enough strength would stretch
and break in this situation.
2. Ductility
•
Ductility is the ability of a material to
be stretched out by a force into thin
wire.
•
Materials such as copper, aluminium and
gold have excellent Ductility.
•
Steel also has excellent Ductility when it
is heated.
•
By applying enough heat you can change materials
from one state to another. So if you apply a small
amount of heat to a Solid you are slightly changing
the material towards a Liquid state.
•
It is important to remember that a Ductile material
can be stretched a long way before it breaks or
fractures and it retains the shape you stretch it to.
•
Take a piece of chewing gum, chew it for a while, and
then stretch it. The chewing gum stretches for a good
distance before it will break.
If it didn't break and let one end go, the chewing gum
doesn't return to its original shape. This is Ductility.
•
In the diagram below you can see a simple
diagram of how a metal is drawn into a thin wire.
3. Creep
• Creep is the stretching of a material
due to a constant force over a period of
time.
• This is an unusual property because it is
difficult to see, however you can conduct
an experiment in the metalwork room to
demonstrate Creep
Creep Experiment
The Lead strip is a fixed
length at the start of the
experiment.
After 5 to 7 days the Lead strip
is longer than it was at the start
of the experiment.
4. Hardness
• Hardness is the ability of a material to
withstand scratching or penetration.
• Glass is an example of a hard material.
• If you try to scratch it with a Scriber you
will find that it takes more effort on your
part than scratching plastic.
A simple way of testing for the hardness of materials is
to use a scriber to scratch the surface.
The more force you have to use the harder the material
is.
This is only a method of testing the Hardness of a
material relative to another material, and it is not very
accurate.
• A more accurate way of comparing the
Hardness of two materials is to use a Bench
Vice and a hardened steel ball.
• As the Bench Vice is tightened the hardened
steel ball will be pushed further into the softer
material.
• This simple method is the way Engineers use to
find the Hardness of different materials.
• Engineers use machines like the
Rockwell or Brinell Indenter
for this purpose.
• These machines press a hardened ball or point
into a material using a limited force, and then
measure the depth of the indentation
The harder material showing
where the hardened steel ball
penetrated less.
The softer material showing
where the hardened steel ball
penetrated more.
5. Malleability
• Malleability is the ability of a material to be
stretched or shaped in all directions without
breaking or fracturing.
• Copper, gold and aluminium have good
Malleability.
• Generally all metals become more Malleable as
their temperature is increased and this allows
them to be pressed or rolled into quite intricate
shapes.
• Some products that are manufactured as a
result of the material being Malleable are
girders, sheet metal and car panels.
A girder made from hot rolled steel
to increase the steels Malleability.
A panel from a truck which was
bent into shape from sheet steel.
6. Elacticity
• Elasticity is the ability of a material to return
to its original shape after it has been
stretched.
• A regular elastic band is a good example of the
Elastic property of a material.
• An Elastic band is made from rubber which has
good Elasticity.
• You can stretch it a long way and when you it go,
the elastic band returns to its original shape.
• All materials do, however, have an Elastic limit.
• This is reached when the material is stretched so far
that it will not return to its original shape, and
sometimes the material will break or fracture.
• Metals have a reasonably good level of Elasticity,
which you might find surprising. Special tests can be
conducted on materials to see how far they can be
stretched before the Elastic limit is reached,
• When constructing buildings and structures, it is
important to know the Elasticity of the materials
being used especially if the forces or loads on
the structure are changing.
• If the wrong material is being used then
eventually the structure will fail, often with
catastrophic consequences, due to the Elastic
limit being exceeded.
7. Toughness
• Toughness is the ability of a material to
withstand impact.
• The larger the impact needed to fracture or
break a piece of a material determines how
tough the material is.
Differences with Hardness & Toughness
• Example 1: Glass which is Hard but definitely not Tough.
• Example 2: Steel is a very tough material and it can be
alloyed with other materials in order to make it Tougher
still.
• However by making a material Tougher you also reduce
its Hardness.
• A good example of this is High Speed Steel. This
material is Harder than mild steel but it is not as Tough.
• We could demonstrate this with a Hammer, but safety
precautions must be observed as this is a dangerous
experiment.
• Toughness in a material is important.
• Think of a car, the body needs to be Tough in
order to protect the driver and passengers. So
anywhere there is likely to be impact we need a
material that can withstand the impact.
• One of the best materials for withstanding
sudden impacts is actually the polymer known
as ‘Kevlar®’.
8. Brittleness
• A Brittle material can be easily broken or
fractured by an impact.
• Brittleness is the opposite of Toughness, so you
should be able to tell, at this stage, that glass is
Brittle.
• It is very important to know if a material is Brittle
in case you use it in a situation where an impact
may occur.
• Materials become more Brittle as
temperatures decrease, which is one of
the reasons that the Titanic sank,
• The Steel hull of the Titanic lost its
Toughness and became more Brittle in the
cold waters of the North Atlantic and when
the ship hit the iceberg the hull was no
longer able to withstand the impact.
9.Conductivity
• Conductivity is the ability of a material
to allow heat or electricity to flow
through it.
• Metals in general make very good
Conductors and this is because of their
Chemical Bonding.
• Copper, Aluminium and
Gold are very good
Conductors of heat and
electricity, and you can
see this in many of the
uses that these materials • Water also has very
good Conductivity of
are put to, such as
heat and electricity,
electrical wiring, pots
which makes it good for
and pans, and
cooking with, but very
electronics.
dangerous near
electrical equipment. You
should never put
electrical equipment in
water, and never swim
during a thunder storm.
• Materials which have good Conductivity, like metals, are
called Conductors ,
• Materials which have very poor Conductivity, like plastics,
are called Insulators.
• You will often see Conductors and Insulators working
together. Take the example of a normal plug.
• The pins are made from Brass to
conduct the electricity, but the
casing is made from plastic, to
save you from an electric shock.
• The wires from the plug to the
electric appliance are also
covered in plastic as protection.
Plasticity
• Plasticity is the ability of a material to be
stretched or formed into another shape and
then hold that shape, without breaking or
fracturing.
• This property is very different from Elasticity, as
the material does not return to its original shape.
By heating metals you can increase their
Plasticity, and this goes pack to breaking some
of the Chemical Bonds that hold the Molecules
of a Solid together.
•You will have seen a horse-shoe at some stage in
your life. The horse-shoe was manufactured by a
blacksmith using the Plastic property of the metal.
•The blacksmith heats the metal to increase its
Plasticity and then deforms the metal by
hammering it into the horse-shoe shape.
•When the metal cools the horse-shoe retains its
distinctive shape. If the metal had broken or
fractured the Plasticity of the metal would have
been exceeded.
•This could have been prevented by heating the
metal more.