Fractures Fracture – In ductile fractures extensive

Fractures are a term describing a body of
material going through a separation due to imposed stresses.

A) Ductile Fracture – In ductile fractures
extensive plastic deformation or ‘necking’ occurs before complete fracture.
Necking is permanent deformation that occurs once a material reaches it’s
ultimate stress point. Necking is shown below:

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When the neck forms it is permanent, and
indicates failure, but should the stresses placed on the material continue they
will fracture. Ductile materials can withstand between 50-100% strain before
fracturing under favourable loading and environmental conditions. The fracture
point is typically determined by the purity of the material, for instance, pure
iron can withstand 100% deformation under strain before fracturing.

Once necking occurs, voids form at the neck
point, and then coalesce resulting in a crack which reaches out from the void
to the edge of the material then failure causes separation. This fracture makes
the material pull apart, resulting in a rough surface. i

B) Brittle Fracture – A fracture which
occurs at or below the elastic limit of a material at high speeds of upto
7000ft/sec (in steel)ii.
Once a brittle fracture has occurred, unlike in a ductile fracture, the two
separated ends would fit back together. There is no discernible deformation and
appears to be a clean break due to no plastic deformation occuring before the
fracture.

Repetitive shear stresses placed on the
material cause initiate irreversible changes resulting in cumulative damage.
This damage is very small and only extends around 2 to 5 grains away from the
origin point.iii
The fatigue then enters into a propagation stage, where the microcrack will
change direction and grow perpendicular to the application of the tensile
stress. The crack then grows and then reduces the cross-sectional area of the
material, causing rupture. As shown in the diagram below, (c) shows a britte
fracture in comparison with a ductile fracture – displaying how one is
separated and deformed, whereas the brittle fracture looks like it would fit
back together and doesn’t show deformity.

C) Fatigue Failure – This is the weakening
of a material due to repeatedly applying loads and causes progressive and
localized structural damage. Fatigue causes the majority of engineering
failures and can occur with little warning if the cracking isn’t initially
spotted. An example of fatigue failure is bending a thin wire rod back and
forth in the hands – The spot where the rod has been bent back and forth will
suffer from localized damage and minor cracking, which eventually results in it
breaking at the point it has been bent. Where the rod is bent is the initiation
of the fatigue, which with repeated bending propagates the damage, and
eventually leads to rupturing. iv

D) Creep Failure –  This is also known as cold flow and describe
a material slowly moving or deforming permanently under high levels of stress.
Creep generally occurs at high temperatures over time, but can also take place
at room temperature in some materials, such as glass. The rate at which the
material deforms is called the ‘creep rate’ and is a slope over time as shown
below:

Primary creep starts at a rapid rate and
slows with time, with secondary creep having a relatively uniform rate and
tertiary creep having an accelerated rate until the material ruptures. The rate
at which creep occurs depends on the temperature and the amount of stress being
applied to the material over a certain amount of time. Hight temperature and/or
high stress being applied over a short amount of time causes a high amount of
strain and can result in rupturing.

When creep fatigue occurs, adjacent grains
or crystals start to move as a unit relative to each other, causing voids or
pores to develop, which can cause deformation.

Task 2 (P8)

Degradation of materials is the loss of
relevant properties which occurs gradually due to exposure to various
conditions.

A.) 
Metal degradation – Most metals are corroded due to oxidation – the
reaction to oxygen within the atmosphere, which affects ferrous metals such as
steel. When two different metals are in contact with one another and they are
exposed to rain water, the water acts as an electrolyte, causing one of the
metals to degrade and be eaten away. This is known as galvanic corrosion and
acts in accordance with the Galvanic series. Corrosion of metals results in a
reduction of the material thickness which reduces the strength of the metal and
can cause structural failure. Weld decay is a process by where stainless steel
loses its chromium content in the areas where welds are formed and can lead to
corrosion in the heat affected zones. Some metals can be corroded by anaerobic
sulfate-reducing bacteria which are active in the absence of oxygen and can
produce hydrogen sulfide which results in sulfide stress cracks in the material
– weakening it significantly. An example of microbial induced corrosion is
shown below:

 

B.) Polymers – Plastics can degrade when in
prolonged contact with elastomers and suffer from corrosion and melting. UV
light can discolour and affect the surface finish of various polymers, and
exposure to high heat can result in deformation, structural weakening and
melting, as well as low temperatures causing some plastics to become brittle
and crack under pressure. Coming into contact with acids or alkalis can also
weaken the bonds within the polymer chain can cause pitting, melting and
discolouration.

Some degradation of polymers is beneficial,
in biodegradable plastics such as polylactide acid (PLA) or Polyhydroxybutyrate
(PHB) microorganisms can develop when the polymer is exposed to light, heat and
moisture and they are slowly broken down into reusable natural materials.

C.) Ceramics – Ceramics are very resistant
to degradation, but they can warp over time if a high enough heat is applied to
them. Due to ceramics being heat-treated, rapid and extreme temperatures are
required to warp or deform the material. Mold can build up on ceramic materials
if the humidity is optimal, as some microbes will thrive in these conditions.
Extremely low temperatures can cause frost to build-up on the surface of the
ceramic and enter the pores of the material, this can then expand causing
cracks to appear and weaken the ceramic. v
If ceramics are buried in the ground for long periods of time, soluble salts in
the soil can enter the clay and are affected by the humidity, becoming soluble
in high humidity and then freezing in low humidity – again, expanding and
causing cracks in the ceramic. The effect of soluble salt and frost on
ceramics:

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

 

ii  Rolfe, John M. Barsom, Stanley T. (1999).
Fracture and fatigue control in structures: applications of fracture mechanics
(3. ed.). West Conshohocken, Pa.: ASTM. ISBN 0803120826.

iii https://www.tec-eurolab.com/eu-en/stages-of-fatigue-failure.aspx

 

iv https://www.engineersedge.com/material_science/fatigue_failure.htm

 

v https://en.wikipedia.org/wiki/Conservation_and_restoration_of_ceramic_objects#Physical_degradation

 

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