Describe of its surface when external stresses are

Describe
with the aid of sketches/images and graphs, the relevant principles and
appearance of:

 

Ductile fracture

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A ductile fracture occurs when a material is pulled apart. Upon
constant stress being applied to the material, necking will begin. Necking
tends to occur prior to the fracture itself.

 

Brittle fracture

 

A brittle
fracture is the failure of
a material with minimal amounts of necking deformation. If the broken pieces of
a brittle fracture are fitted back
together, the original shape and dimensions of the specimen would be restored. Brittle fractures are defined as fractures which occurs at or below the elastic limit of a
material.

 

Fatigue failure

 

Fatigue failure occurs when the surface of a material progressively
cracks due to the brittleness of its surface when external stresses are
applied. The degree of impact of the fatigue depends on the intensity and
frequency of the stresses applied to the material.

 

Creep failure

 

A creep failure refers to the progressive deformation of a
material when put under constant stress, this could be high temperature and
heavy applied loads on the material. Creep failures tend to happen slowly,
however the result is permanent. The graph below shows the strain on the
material increases quickly when the load is first applied, then the creep rate
increases at a steady rate before increasing rapidly up until the fracture
occurs.

 

Task 2 (P8)

 

Explain
using sketches/images where appropriate, the different processes of degradation
for each of the following material types:

 

Metals Metals
are susceptible to corrosion and degradation which leads to the component
weakening. Aqueous corrosion is an electrochemical reaction of materials in a
wet environment, this results in a deterioration of the material properties.
Galvanic corrosion is a process when a metal corrodes when it is in electrical
contact with another with the presence of an electrolyte. A type of corrosion
is rusting, this occurs when the metal reacts with the oxygen in the air. Some
metals like iron are more susceptible to corrosion than others, Aluminium is an
example of corrosion resistant material as it has a natural layer of aluminium
oxide.Polymers Polymer degradation changes the properties of the material.
Polymers normally degrade by disintegration, oxidation, hydrolysis and
radiation. Polymeric molecules are very large and any loss in chain length
lowers the tensile strength of the material and is a primary cause of premature
cracking. Polymers tend to discolour, the tensile strength will lessen and the
shape of the polymer may also change slightly when they come into contact with
light, heat, acids, alkalis and some salts. The degradation of polymers is useful
when it comes to recycling, however is more likely to have undesirable effects. Ceramics Ceramics
react humidity and frost. When ceramics are exposed to humid atmospheres, mould
begins to form on the material. This causes a discolouration, making the
ceramic look unsightly. High temperatures cause ceramics to warp and the
physical properties of the material would be affected. As ceramics have large
pores frost can also have an effect on the material. In wet conditions, water
can seep into the pores of the ceramic and when it freezes, the water expands
and makes the ceramic prone to cracks and breaks. Ceramics are very delicate,
so their surface can easily be chipped or scratched. A damaged surface would
leave the underneath of the material vulnerable to water and chemical damage. Task
3 (M3)  

Explain, using sketches/images,
how and why a harsh marine environment might affect the behaviour of steels
used for the manufacturer of ships (i.e. the hulls). 

Salt water accelerates the rusting process. This is because the
electrons in salt water move more freely than those in fresh water. This means
electrolysis reactions happen more frequently so corrosion occurs at a faster
rate. Galvanic reactions happen when there are two metals in contact with salt
water (electrolyte), in effect this creates a battery and speeds up the
corrosion rate of the metals. Harsh marine environments with lots of moisture
in the air and with the presence of water vapour have much faster rust rates
than dry areas. In order for rust to occur there must be the presence of
oxygen. As the hull of a ship is exposed to water constantly, the steel and
oxygen is always in contact which means the oxygen in the water reacts with the
steel much faster than if it were to be on dry land. When steel rusts, the
material weakens and the properties of the metal are detrimentally affected. To
slow the rate at which corrosion occurs, waxes and oils can be applied to the
surface of the hull. These act as a barrier and do not allow the water vapour
to come into contact with the hulls surface. As a result, the elements cannot
exchange electrons as freely and easily, meaning minimal amounts of rust occur.
Another way to prevent rusting, a zinc coating can be applied to the surface of
the ship. Zinc reacts very slowly with water, therefore any corrosion that does
occur is affecting the zinc coating and protects the hulls material.

 

Explain, using sketches/images
where appropriate, how exposure of thermoplastics to certain chemicals affects
their behaviour. Thermoplastic
materials are types of plastic that become soft when they are heated and hard
when they the cool down. Upon cooling, the properties of the material change
considerably. When thermoplastics are exposed to ozone 3, thermoplastics will
disintegrate and the properties of the material and the molecular weight will
change vastly. As a result, the material would begin to fall apart, this is
known as ozonolysis. Thermoplastics are also susceptible to chlorine gas. Upon
exposure, the material will crack. Chlorine attacks the weakest part of the
chain molecules and causes chain cleavage, this leaves a brittle crack on the
material.