A ductile fracture can occur when a material is subjected to
a tensile load over a period of time, leading to a progressive crack. This form
of fracture occurs in stages and a ductile rupture is first to appear. A
ductile rupture occurring essentially means that the material has reached the
point at which it will inevitably malfunction. This will appear as a small
disfigurement in the materials surface. This is whats known as necking/
extensive plastic deformation. Once progressed, the material is beyond a state
b) A brittle fracture occurs differently to a ductile fracture; brittle
fractures are categorised by the fact that there is little or no plastic
deformation prior to the extensive damage. This means that the materials in
question for brittle fracture are generally ceramics, glasses and some
polymers. Factors that can contribute to brittle fracture are temperature,
stresses, strains or a tensile load that the material is under.
c) Fatigue failure is caused by repetitive shear stresses and generally occurs
on metals. The first stage of a fatigue fracture can be referred to as
‘initiation’ and is the stage in which an irreversible change happens to the
metal. At the site of initiation, providing the fatigue fracture hasn’t
occurred to its full extent, there should be a small collection of
‘micro-changes’. This damage appears as a result of a large number of load
applications: load applications would be the material performing its function.
For example, a welding plate in a bodyshop is checked regularly for abrasion
and cleaned at intervals. This stages rate of progression is mainly dependant
on the geometry of the part; the smaller the angle of abrasion the faster the
damage will progress. The following stage of a fatigue fracture is propagation.
This is the stage when the microcrack will change direction and progressively
grow: this growth is perpendicular to the point of tensile stress.
Additionally, this is the stage that is most noticeable and therefore can be
addressed. Finally, the last stage of fatigue failure is a rupture occurring.
This is simply a result of the crack in the propagation stage progressing to a
point at which only one additional load application can be exerted on it before
it is no longer functional at all.
d) Creep failure is sometimes referred to as cold flow. It refers to the
characteristic of a solid material to progressively deform beyond a state of
repair. This reason behind this fault are the mechanical stresses exerted on
the material over time; despite mechanical stresses being below the yield
strength of the material, long-term exposure to these stresses ultimately
results in permanent damage. This fits with the title itself, as overall it
refers to a material reaching break point after a long period of gradual
Task 2 (P8)
A. Degradation of metals can occur in a few different ways. Some of which are
the progressive fractures mentioned above. Generally, in aluminium’s case, a
ductile fracture will progress, leading ultimately to a complete breakage in
the material’s structure. Creep failure is also common in metals, particularly
in the body in white stages of production; clamps are used in body shop 2 at
JLR to secure the body panels to a fixture and the material (small metal
plates) have cracks appearing on them. This is a result of small but repetitive
impacts on the material. Furthermore, oxidation can occur on some metals, which
leads to rusting. Iron commonly undergoes this process of degradation, which
may lead to brittleness and loss of functionality, depending on the component
B. Polymers, such as plastics, can degrade by brittle facture: due to their
dense structure and ease to bend, plastics can be snapped without previous
damage being inflicted on the material. Polymers can also undergo a process
involving degradation into an aqueous environment. This is called hydrolytic
bond cleavage, in which water soluble polymers dissolve into water, which leads
to erosion of the polymer itself. As well as water making contact with
polymers, heat can also lead to its degradation. When a polymer is placed next
to a heat source, it eventually leads to the material expanding. In certain
systems, this would lead to a loss of function of the polymer. For example, an
electrical system would require for wires insulation to remain in a serviceable
condition. However, if produced with the wrong type of plastic, the insulation
may expand, making the system unsafe.
C. Ceramics are also vulnerable to brittle fracture. The most common example
may be glass, as it can break under tensile stress without any previous signs
of damage. Additionally, ceramics can degrade by moulding. This occurs in warm
and moist environments and eventually leads to ceramic materials crumble.
Moreover, ceramics in cold moist conditions can develop a layer of frost. This
causes expansion of the material and causes it to crack.
Task 3 (M3)
a) In a marine environment, the probability that the material on a ship will
deteriorate is extremely high. Particularly the hull of the ship, as this is
the section which is submerged in sea. The reason for the high likelihood of
deterioration is because the marine environment is ideal for metal degradation;
tiny organisms in the sea effectively cling to the ships bodywork and develop
overtime. This results in the moisture collecting on the materials surface,
acting as an electrolyte to allow for the process of galvanic corrosion to take
place. Galvanic corrosion is the process in which
two different metals can bind when submerged in sea water. The galvanic scale
demonstrates that, depending on where the metal is on the scale, one causes the
other to deteriorate. The main material used in ships is stainless steel. Its
corrosion resistance depends on the formation of what is referred to as ‘a
passive chromium oxide film’ on the stainless steel’s surface. This layer of
chromium oxide is highly resistant to corrosion and therefore prevents the
process of galvanic corrosion.
Other failures associated with stainless steel include cracking and fatigue
fractures; general forms of degradation in the material can be addressed by
replacing them with a material such as duplex stainless steel, which is
essentially a reinforced product of stainless steel.
b) Explain, using sketches/images where
appropriate, how exposure of thermoplastics to certain chemicals affects their
Polymers can be affected by
chemicals in various ways. The way in which they are affected is dependent on a
few different factors. One of which is the polymer’s composition: the chemical
makeup of a polymer influences the way it reacts when exposed to different
chemicals. Other factors can accelerate the process of change, which include
the time of exposure, temperature and internal stress of the polymer (caused
through excessive processing). Interaction with a chemical can cause either a
physical or molecular reaction. Physical reactions can result in the polymer
softening. It may also cause swelling of the material. The reason being that
the chemical penetrates the surface and changes the structure in the ‘amorphous
regions of the polymer. Moreover, some polymers are referred to as semi
crystalline polymers. These polymers have lower free volume in their
crystalline regions. This means that they are less likely to react with
chemicals, as the substances have less chance to penetrate at the surface.
These types of polymers are beneficial in industry, as a process involving
removing the chemical and drying the polymer can potentially return them to the
original state they were in.