Summerhill College by mikesanye


									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        Above you can see a crane         Above we can see a
pillar under              lifting a load.                   plank of wood with 3
Compression.              The crane is pulling up and the   forces acting on it.
                          load is pulling down, therefore
If the column is not      the cable is in Tension.         The forces are not in
constructed out of the                                     line with each other and
right materials then it   For a crane, the cable is made so if the material is not
would collapse.           of a number of steel cables that strong enough it will
                          are wound around each other.     Shear.
Concrete columns are      This increases the strength of
often reinforced by       the cable.                       A scissors cutting paper
steel cable or rods.      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

•   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

• 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    After 5 to 7 days the Lead strip
length at the start of the   is longer than it was at the start
experiment.                  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
This is only a method of testing the Hardness of a
material relative to another material, and it is not very
• 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
• 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     The softer material showing
where the hardened steel ball   where the hardened steel ball
penetrated less.                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
• Generally all metals become more Malleable as
  their temperature is increased and this allows
  them to be pressed or rolled into quite intricate
  • 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    A panel from a truck which was
to increase the steels Malleability.   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

• 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
• 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
• 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

• 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.
• 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
  are put to, such as         good Conductivity of
  electrical wiring, pots     heat and electricity,
  and pans, and               which makes it good for
  electronics.                cooking with, but very
                              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 is the ability of a material to be
  stretched or formed into another shape and
  then hold that shape, without breaking or

• 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 back 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.

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