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Unit 10: Properties and Applications of Engineering Materials


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Task 1-

Question- Using
diagrams to illustrate your answers describe the structure (including the
atomic structure) of the following materials:

• plain carbon steel (0.15 C), as used to make a body panel
for a car.

• high density polyethylene (HDPE), as used to make
non-carbonated drinks.


• silicon carbide, as used in ball type flow control valves.

• carbon fibre reinforced polymer (CFRP), as used in the
construction of aircraft.

• piezoelectric, as used in the sensor which fires the airbag
in a car.

Carbon Steel (0.15C)

Plain carbon steel is a type of ferrous metal (contains iron
so it is able to rust, rust is an iron oxide which means the oxide reacts with
the iron causing the metal to rust.). It also contains between 0.1 and 1.7%
carbon together with little amounts of manganese, phosphorus, silicon and

One of the properties that are associated with ferrous metals
are that it can be magnetic, but this does not apply to plain carbon steel and
the reason for this is because of its structure compare to other ferrous metals.

Plain carbon steel isn’t just made up of one element as it contains
little amounts of manganese, phosphorus, silicon and sulphur. This means that it
becomes an alloy as two or more different elements are joined together to make
a stronger material. However plain carbon steel is an alloy we don’t refer it
as an alloy compared to brass or bronze etc.

Atomic Structure for Plain Carbon
Steel (0.15C) (Figure 1)-











Lattice structure for plain carbon steel (Figure 2)-

The crystal structure is that of iron and below 910°C is body
centred cubic (BCC). This means that there is an extra atom in the centre body
of the atomic structure.

Above 910°C the structure changes to face centred cubic (FCC)
resulting in the material being more malleable, i.e. it can be easily hot
worked. This means that there is an extra atom in the face of the cube.

The unit cube is the basic building block for iron and these
build up to produce crystals (also called grains). The carbon atoms are much
smaller in size than iron and fit into the crystal lattice to produce an
interstitial solid solution. This interstitial carbon distorts the crystal structure
and makes the steel harder and stronger.

Density Polyethylene (HDPE)

High density
polyethylene (HDPE) has a specific atomic structure which is made up of long
hydrocarbon chain molecules with no side branches. In the atomic structure the
carbon atoms are in the middle (spine). There is no side branches which means
that the molecules fit together in a closely packed way. This means that HDPE
has a higher density than LDPE. Furthermore HDPE has a more orderly structure
which gives it an improved tensile strength, higher maximum service temperature
and greater impact resistance than LDPE. In some atomic structures there can be
pairs of atoms. Each pair of carbon atoms and associated hydrogens is called a
mer and poly means many.

Atomic structure of HDPE (Figure 3)




Structure of HDPE and LDPE comparison
(Figure 4)




Silicon Carbide

Silicon Carbide is a manmade ceramic which is produced from
silica sand and carbon. It exists in many different crystalline forms but alpha
silicon carbide (?-SiC) is the most common one. This is formed at temperatures
higher than 2000°C and has a hexagonal crystal structure.

Carbide Structure (Figure 5)





structure for silicon Carbide (Figure 6)




Fibre Reinforced Polymer (CFRP)


 Carbon Fibre
reinforced polymer (CFRP) materials consists from stiff, strong fibres bonded
together with a polymer or resin matrix. The fibres have high tensile strength
and enhance the low stiffness and strength of the resin. The resin’s main
purpose is to transmit loads into the stiff brittle fibres and to protect them
from damage. The carbon can be in the form of a woven mat or as strands when
wound onto circular formers for making tubes.

Fibre Reinforced Polymer layered construction (Figure 7)


Insulating materials are known as dielectrics and have the
capacity to store an electrostatic charge. When a dielectric is placed in an
electric field, polarisation of its molecules occurs. If a mechanical stress is
applied to it, this will induce a strain which displaces molecules in the
polarised material, creating an electric field.

Structure (Figure 8)








Task 2

The following table contains the names of engineering
materials. Classify each ones as either a metal or a non-metal according to
their properties.

Austenitic Stainless Steel


Melamine Formaldehyde (M-F)







Piezo Crystal



•          Austenitic Stainless Steel

•          PVC

•          Copper

•          Melamine Formaldehyde (M-F)

•          Duralumin

•          Butyl


•          Diamond


•          CRFP


•          Porcelain


•          Piezo Crystal


Task 3

The properties of materials can be classified as:

            • mechanical

            •  physical

            • thermal

            • electrical

            • magnetic.

For each category, pick two specific properties, describe
them and say why they are of interest to a design engineer.

From each category pick one specific property and state its
application in an engineering context, i.e. as it relates to the choice of
material for a particular product.




Tensile strength:

this is the maximum internal 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
(area=load/tensile strength).


for 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. Application: tensile strength –
calculating the diameter of tie rods was once used in the roof structure of a


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 lightweight 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 crack and fail. Application: density – producing
lightweight suspension components for F1 racing cars.


expansion coefficient: this is the amount a material
will change its shape when heated or cooled. It is of interest to an engineer
because when components are fitted together if some change their shape more
than others the result will be distortion due to thermal stresses.

conductivity: this property is about how well a material does or does not conduct
heat through itself. It is of interest to engineers when designing components
to be used in places where heat is generated and needs to be dispersed to
prevent overheating.

expansion coefficient – calculating the machined diameter of a ground shaft so
that when it runs in a plain bearing the clearance is correct at a given high operating


Electrical resistivity: this indicates how well a
material will or will not conduct electricity. It is of interest to engineers
when designing components that carry electricity because too much resistance
will cause unwanted heating effects.

this indicates how well a material can hold an electrical charge and is of
interest to an engineer designing electrical circuits that use capacitors.
Application: resistivity – calculating the voltage drop and power loss in a
long electrical cable


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.

permeability – calculating the number of coils and armature dimensions for a
solenoid that is being designed to operate a valve.

Task 4

a) Plain carbon steel (mild steel) is widely used in the construction
of ships hulls. It is easy to work with, low cost and has good structural
integrity when ships are operating in normal water temperatures (3°C to 25°C).
Explain what happens to the material if a ship is operated for long periods of
time in water temperatures that are at or near to freezing.

b) Aluminium alloy (code 2024 age hardened, also called
Duralumin) is widely used in the construction of passenger aircraft. Explain why
this is and the reasons for putting a finite life on certain parts of the

c) The Hep20® push fit plumbing system uses fittings and pipe
work manufactured from polybutylene (PB). Making reference to its properties
and structure explain why this material is an effective substitute for
traditional copper and brass.

carbon steel (a)

In the temperature range 3 to 25°C mild steel has
mechanical properties as given in reference tables.

 It is tough,
can withstand impact loads and exhibits ductile fracture when it fails due to

 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


Duralumin 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 stress concentrations and surface blemishes and
then propagated by cyclic 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 propagate in the duralumin but this is not a
problem provided they are monitor 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 polymer timing belt on a car engine).


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.

pipe has good dimensional stability, which means that it can be joined using
mechanical fittings containing O-ring seals and stainless steel lock washers.



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