plain carbon steel (0.15 C) can have both of these structures of either a body centred
cubic or a face centred cubic, as shown in figure1.
The structure of Plain Carbon steel depends on where about it places on this
graph (Fig2) which illustrates temperature in which if it is below 910C it is a
body centered cubic and if it is above 910C then it is a face centered cubic
HDPE (High density polyethylene)
HDPE has an atomic structure consisting of a simple long
chain of hydrocarbon molecules with no side branches. The carbon atoms form the
spine of the molecule, whereas the hydrogen is the outer structure of the
molecule. Because there is no side branches this means that each chain is able
to be closely packed together which ensures that it has a high density. The
more rigid and orderly the structure results in a improved tensile strength and
a high resilience to high temperatures which usually means that once moulded to
a shape it cannot be remoulded.
This is a man-made ceramic produced from silica sand and
carbon. Silicon carbide exists in many different crystal forms but alpha
silicon carbide is the most common form. It is formed at temperatures exceeding
2000°C and has a hexagonal crystal structure.
Carbon Fibre Reinforced Polymer:
Fibre reinforced Plastics is a where strong stiff strands of
fibre are chemically bonded with a polymer or resin.
The fibres have high tensile strength and thus this augments
the low stiffness and the strength of the resin or polymer. The resin/polymers
purpose is to transfer loads onto the fibres which has a high tensile strength
and it also serves to protect the fibre from damage.
Above is an image of a Carbon fibre lattice structure
A piezoelectric is a type of Dielectric. Dielectrics are an
insulating material which is able to store an electrostatic charge. When this
is passed through an electric field it causes polarisation of its molecules.
If a mechanical stress is applied to this, it causes strain
which displaces molecules and therefore creating an electric field.
Austenitic SS: Austenitic Stainless Steel is mainly used for
cutlery and cooking equipment. It is classified as a metal. its main properties
corrosion resistant, non-magnetic, high
ductility, high tensile strength, tough when working at very low temperatures,
high melting point.
For these reasons it is a metal, with corrosion
resistance and high strength.
PVC: Polyvinyl chloride in its unplasticised form is used
for guttering and fascia boards on houses.
Its main properties are:
• easily UV stabilised, easy to mould, rigid, low density,
non-conductor of electricity, does not degrade, does not need surface
finishing/coating after being moulded (e.g. by extrusion).
• For these reasons it is a non-metal, with low density and
not requiring surface finishing after moulding.
Melamine: Its main properties are:
• good abrasion resistance, good resistance to attack by chemicals,
can be moulded with a decorative surface finish, non-conductor of electricity,
remains rigid at high temperatures, low thermal conductivity, low density.
• For these reasons it is a non-metal with low density and
Its main properties are:
• highly elastic, impervious to gases, not affected by UV
radiation, non-conductor of electricity, highly resistant to outdoor
• For these reasons it is a non-metal, with high elasticity
Its main properties are excellent thermal conductivity,
excellent electrical conductivity, high melting point, hardness increases when
cold worked, ductile, good tensile strength. For these reasons it is a metal,
with electrical conductivity and ductility
Its main properties are: extreme hardness, high abrasion
resistance, low coefficient of friction, low coefficient of thermal expansion,
low toughness, high light dispersion. For these reasons it is a non-metal, with
extreme hardness and low coefficient of thermal expansion
Its main properties are: very high strength to weight ratio,
mouldable into complex shapes, low impact resistance, can work at high service
temperatures, non-conductor of electricity. For these reasons it is a
non-metal, with strength to weight ratio and non-conductivity
Mechanical: These are properties which affect the way that a
material reacts when subjected to the application of force.
Tensile strength: this is the stress (measured in MPa) that
a material can withstand before it breaks. It is of interest to an engineer
because the values for all materials are well documented and can be used when
working out the cross-sectional area of load-bearing components
example, Vickers Pyramid Number (VPN). The hardness value indicates how well
the surface of a material will resist indentation and abrasion. It is important
to know this when designing components that slide against each other.
Physical: These are properties determined by the atomic
structure of the material.
Density: this is the mass per unit volume of a material. It
is of interest to an engineer when they are designing components to be used in
dynamic situations because low denisty objects need smaller amounts of energy
to move them around.
Glass transition temperature: this is the temperature at
which a polymer changes from being rigid and brittle to being flexible and
rubbery. It is of interest to an engineer because if a material such as
polythene is used below -120°C it will start to crack.
Permeability: the amount a material will magnetise when
placed in a magnetic field. Polarisation: the orientation of N and S poles.
when a material is magnetised. Application: permeability – calculating the
number of coils and armature dimensions for a solenoid that is being designed
to operate a valve.
Plain Carbon Steel.
In the temperature range 3 to 25°C mild steel has mechanical
properties such as It is tough, can withstand impact loads and shows ductile
fracture when it fails due to overloading. Sea water temperatures around
freezing occur when air temperatures are even colder, which means that the
ship’s hull could be well below freezing. The hull plates will have been
welded. Mild steel has a BCC structure
and at normal temperatures is a ductile material. As the temperature decreases,
the metal’s ability to absorb the energy of impact decreases and there is a
ductile to brittle transition.
is an alloy of aluminium and copper and its tensile strength can be greatly
enhanced by cold working and age hardening. This gives a very high strength to
weight ratio because of its low density compared to other metals. The problem
with this material is that it suffers from fatigue cracking, initiated by Concentrated
stress points and blemishes on the surface which is then extenuated by rhythmic
or random variations in stress levels. Every time an aircraft takes off and
lands, its airframe goes through a stress cycle because of the changing air
pressure on the outside of the cabin, which effectively expands and then
contracts. Additionally, air turbulence causes small continuous vibrations on
the wings and control surfaces. Over a period of time tiny cracks will form in
the duralumin but this is not a problem provided they are monitored and repair
panels fitted at the correct time intervals. If a designer knows the operating
conditions of the plane, they can calculate what this time interval should be similar
to the mileage recommendation for changing the brakes on a car.
Polybutylene can operate at temperatures up to 100°C without
softening or distorting and it does not degrade over time – unlike copper and
brass which react with steel components, such as radiators, and will corrode.
Its hoop stress is better than copper which means that pipes are stronger when
pressurised and it is chemically inert and therefore does not contaminate
drinking water. Polybutylene pipe has good dimensional stability, which means
that it can be joined using mechanical fittings containing O-ring seals and
stainless steel lock washers.