Polymers travels between the grains rather than the grains

Polymers and metals can experience ductile fracture. “A fracture is referred to as one body
going through separation due to imposed stresses.”1
Extremely ductile materials experience rupture rather than fracture and the
materials simply pulls apart rather than slowly fracture.

As a ductile material undergoes tensile
stresses, plastic deformation occurs known as necking, of which there are 4
stages: Void nucleation where small pockets of tears form and create gaps in
the material. As further stresses occur, these gaps meet or ‘coalesce’ and void
coalescence occurs. Further stresses cause a crack to propagate and slowly spread
throughout the material. If subjected to more stress, the material will fail
and separate and this is often identified by a rough cup and cone shaped
surface.

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Ductile fracture is preferred to
brittle fracture as there as visual indications of plastic deformation and
stretching of the material caused by ductile properties before ultimate failure
occurs.

 

 

 

 

 

 

 

 

 

 

 

 

b)

As brittle materials undergo tensile stresses, there is
almost no indication failure is going to occur. Crack propagation occurs
suddenly and rapidly, often shattering materials into many pieces without any
visual indication of necking or ductility resulting in a brittle fracture.
Brittle fractures often occur at low temperatures due to metals and many other
materials having a ‘brittle to ductile’ temperature and low temperatures change
materials from ductile to brittle.

There are two types of brittle fracture than can occur in
metals: Intergranular and transgranular. In transgranular fractures the crack
spreads through the lattice of the metal following the path of least
resistance. Intergranular fracture travels between the grains rather than the
grains themselves.

 

 

 

 

 

 

 

 

 

 

 

c)

Fatigue failure occurs in materials over time as they are
subjected to the repeated removal and reapplication of loads. As a material is
subjected to forces which are below that which cause fractures, microscopic
fractures known as cumulative damage occur in the material in the process of
initiation. As the load is repeatedly added, the cumulative damage increases,
and the microcrack changes direction to expand perpendicular to the stresses in
the propagation stage. As further loads are applied, the crack propagates more
and reduces the cross-sectional area, and thus the strength of the material.
Eventually, the crack expands throughout the material and one further loads
causes the material to fail in the final rupture process. The end fracture can
either be ductile or brittle.

d)

Creep failure occurs when a material is subjected to a
constant load over a period of time which is below its yield strength. Creep
failure causes a material to elongate slowly over time which introduces
deformation to the material. There a three stages of creep failure: Primary,
secondary and tertiary. During the first two stages of creep no material
strength is lost. Creep can be useful to utilise in certain applications as the
strain relieves stress in material.

In the primary stage, strains are rapidly introduced into
the material which slow over time. In the secondary stages, the strains gradually
increase over time at a uniform rate and finally in the tertiary stage strains
occur at a rapid rate once again and necking deformation and material fracture
can occur if the strains build up too much causing failure of the component or
material.

b

Task 2 (P8)

a)

Metals are used for a variety of purposes and subjected to
different environments which can affect the properties of the metal and cause
it to degrade over time.

Metals can corrode when they react with other chemicals or
compounds to create a more stable compound. Often metals react with oxygen in
the atmosphere to form an oxide, the most commonly known one being rust which
is iron oxide. They can react with sulphides and hydroxides. As corrosion
changes the composition of components, the property of the component is also
affected. Often the new compound after a reaction loses strength and other
properties. Corrosion can easily be avoided by coating metals with paint or
other chemicals which provide a barrier between the metal and oxygen or other
reagents. Passivation is another technique to create a barrier between metals
and a reagent by creating a film of oxidised metal which does not permeate
through the metal. The layer of oxidised metal prevents oxidation of the metal
below.

Metals can also suffer from fatigue. Fatigue is inherent to
any metals which undergo repeatedly applied loads, however the lifecycle of the
metal can be improved by reducing loads they are subjected to, or engineering
components to spread loads better across the material. Residual stresses in a
material can contribute to fatigue failure and cold rolling helps to ensure any
stresses are kept at the surface of the material and does not permeate
throughout it.

b)

Polymers are generally non-reactive; however, they can suffer
from UV degradation. When subjected to ultraviolet light such as those from the
sun, the UV can break the bonds between the atoms creating free radicals which
then react with oxygen in the atmosphere. UV degradation can cause discoloured
and cracks or even complete disintegration in some polymers. To overcome this
hazard, polymer components can be coated in a UV absorbing coating which
absorbs the UV radiation and protects the polymer underneath.

 

c)

Ceramics often have inherent instabilities in the material,
known as inherent vice, which occur during the manufacturing process. This can
result in efflorescence of the material. As water enters a ceramic and reacts
with the chemicals it is made of, ceramic dust mixes with the water. As the
water migrates and subsequently evaporates, this dust can be deposited
elsewhere on the ceramic causing extrusions to appear and alters the appear and
structural stability of an object. Inherent vice can be reduced by using higher
quality mixing and filling materials, using the correct firing techniques and
ensuring the shape and construction of a ceramic component is fit for purpose.

Ceramics are also especially affected by changes in thermal
conditions. Repeated expansion and contraction caused by rapid changes from a
hot to cold environment cause cracks to appear in the material which causes the
component to weaken and eventually fail.

Task 3 (M3)

a)

Marine environments are harsh environments for steels to be
present in. Salt water corrodes metals five times faster than fresh water and
marine air causes them to corrode ten times faster than normal air.2
This is due to the presence of salt. The sodium chloride and metal facilitate a
process known as electrochemical corrosion. The metal ions of steel dissolve in
water and salt water conducts electricity due to having an ionic bond.
Electrochemical corrosion occurs when electrons from other compounds are
attracted to the metallic ions. These electrons and ions bond with the metal
ions to form new compounds and structures from the parent metal weakening the
structure of the metal and potentially causing failure. Anaerobic corrosion is
also facilitated in marine environments. Sea water contains sulphates which
surround and can be deposited on the steel. Hydrogen sulphide is extremely
reactive with metals and will react causing change in the chemical composition
of steel. Marine environments are also host to a range of bacteria life whose
biological processes can be detrimental to steel. Certain bacteria use hydrogen
and iron present in steel and can excrete rust. Rust can propagate on the
surface of steel due to the presence of iron in it. A cumulation of these
circumstances can cause significant changes to the composition of steel and
affect the behaviour and properties of the metal. It will most likely result in
the weakening in the structure, cracks to appear and increased fatigue on the
metal. This is detrimental as the steel used for boats or other marine
equipment will often be structural and a weakening of structural components can
cause the object to collapse and be destroyed or otherwise rendered useless. Corrosion
eats away at metals and often leaves gaps as the remaining compounds are eroded
by the sea. This can cause cracks and eventual holes in the metal cause leaks
which can get to electrical equipment on board marine equipment and cause
severe damage. Stainless steels are often used in marine environments as the
reactions which occur are beneficial. Stainless steel has a composition
including no less than 10.5% chromium. This chromium reacts with oxygen instead
of the iron to form a chromium oxide passive layer protecting the steel
underneath by the process of passivation.

b)

The behaviour of thermoplastics can
be altered by the addition of certain solvents. Thermoplastics can be remoulded
once subjected to heat, however this may not always be feasible. Although many
thermoplastics are resistant to acids, alkalis and other inorganic compounds in
the presence of specific solvents, thermoplastics can be made to behave as a
liquid once more. The solvent weakens the bonds between the polymers and the
polymer chains are free to move around in the compound and can attach to other
free moving chains, however as it is now a liquid it loses rigidity and its
strength until it sets again. This is useful for a process known as solvent
welding which is often used to join to pieces of thermoplastic together.
Solvent is applied to weaken surface polymers; the pieces are attached together
and the polymer chains attach to each other. The material is held together
until they are set once more, as the solvent permeates through the surface and
out to the environment leaving a solid, bonded polymer chain with no mobility
and the strength and rigidity it had before treatment.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1 https://www.corrosionpedia.com/definition/421/ductile-fracture

2 https://sciencing.com/effects-saltwater-metals-8632636.html