Transcript EBB 220/3 FAILURE IN POLYMERS

EBB 220/3
FAILURE IN POLYMERS
DR AZURA A.RASHID
Room 2.19
School of Materials And Mineral Resources Engineering,
Universiti Sains Malaysia, 14300 Nibong Tebal, P. Pinang
Malaysia
Importance of mechanical properties of materials
in engineering

Need to acquire knowledge of the properties of materials 
The correct selection of a material for a given application.

Mechanical properties data were used to predict the response
of materials under mechanical loads.

Expressed in terms of forces which may deform materials or
even cause them to fail completely.

To avoid failure and keep deformation under control so the
individual system components remain functional as parts of a
whole  need a various considerations:

Is stiffness / rigidity important? (i.e. minimum deformation under
a given load)

Is strength essential? (for maximum tolerance of loads before
failure)

The questions we may have to ask are:

What is the nature of the load?

Continuous and uniform or rising steadily:

IMPACT (e.g. hammering action, accidental drop)- Alternating (periodic application
of a force):

FATIGUE (e.g. vibration, rotation in loaded components)

The geometry of the loaded component  can be designed to deal with
these conditions.

The physical nature of the material  has to ensure that the component
can survive in service.

Cost and component weight  when evaluating and selecting materials,
with the use of indices such as:

Modulus-to-density ratio

design for stiffness, in weight-critical applications example: an aircraft

Property-to-cost ratio

design for stiffness and strength where low overall price is important 
example: children’s toys, non-critical parts of home appliances
Fundamental concepts for mechanical properties


Below are some terms we find in dealing with materials in
relation to structural applications:

Stress

Strength

Strain

Stress-strain relationships

Modulus
Concept of deformation:

Deformations can be produced by forces  which cause a body
to be stretched, compressed, twisted or sheared.

These forces can also be combined to produce more complex
types of deformation  for example : flexural.
Unloaded
Cut (Simple
shear)
Stretched
(Tension)
Squeezed
(Compression)
Twisted
(Torsional shear)

Extension by stretching in one direction the simplest type of
deformation that can be used to explain key concepts in
mechanics
Rectangular specimens
subjected to different loads
in tensile mode
Stress

Stress is the force exerted on a body per unit cross sectional area.

By stretching a body using a force (the force is weight), the tensile
stress (in the direction of elongation)

If the force applied is 100 N (Newtons), and the cross sectional area
measures 0.0004 m2 (square metres), the stress becomes

or 250 KN/m2, or 0.25 MN/m2. If the force doubles (200 N), stress will
increase accordingly to 500 kN/m2.

We could also double the level of stress by reducing the cross sectional
area to half of its original value, i.e. to 0.0002 m2.

If the same weights were placed on the rectangular specimens to
cause a contraction in the longitudinal direction  the resulting
stress would be called compressive stress.

The other common type of stress is shear stress.

This relates to the force which distorts rather than extends a body 
example where a solid section is sheared,

Shear forces can also result in failure.
Cylindrical specimen subjected to
simple shear, e.g. during cutting.
Everyday example of shear failure
Strength

Concept of strength  the influence of the cross-sectional area on
the force which ultimately causes the material to fail.

Strength defined  the highest stress that a material can withstand
before it completely fails to perform structurally.

If the applied force is tensile (stretch)  the ultimate stress is
known as tensile strength (i.e., maximum tensile stress that the
material can tolerate).

Others types of strength are related to the mode of the applied force
compressive, shear, torsional and flexural.

Use the following expressions:

A strong material  can withstand a very high force per
unit area before it fails.

A weak material  markedly deteriorates or fails at
relatively low levels of applied forces.
Strain

To understand the effect of specimen size on the amount of
deformation resulting from force use the concept of strain.

Strain  the change in one dimension produced as a result of an
applied force and it is expressed as the ratio of the amount of
deformation to the sample’s original dimension.

In the case of tension,

Strain is often expressed as % – i.e. the strain multiplied by 100.
Assuming the force applied causes the original
length of 0.5 m to extent to a new length of 0.9 m 
then the strain becomes

Stress-strain relationship (below failure conditions)

Materials deform
inelastically.

During elastic deformation  the
stress in a body is directly related
to the strain, and vice-versa.


elastically
or
When the force is removed (i.e. when
stress becomes zero) then strain
returns to zero.
The plot of stress against strain
produces a straight line 

the stress can
decreased, and
be
increased

stress and strain are
proportional to each other.
or
always
Linear elastic stressstrain relationship


For ductile materials  increasing the stress above a certain limit will
give rise to inelastic deformations, known as yielding.

when the stress is removed  the strain does not return to zero (and the
original shape is not fully restored)

some deformation has permanently set in.
The stress level at which this occurs is referred to as the yield stress
or yield point.
•The applied force takes the material
beyond the linear elastic region.
• Continued loading causes permanent
deformation.
The amount of permanent
deformation is evident after the
force applied is removed.
Modulus

The relationship between stress and strain is expressed in terms of a property
called the Modulus (or Young Modulus).

The linear portion of the stress-strain curve can be used to determine the
modulus  correspond to the slope of the curve before the yield point, up to
which all deformation is elastic and recoverable.

In other words,

The slope (modulus)  at any point in the linear portion of the line gives the
same result.

The modulus denotes stiffness or rigidity for any kind of applied load, i.e.
tension, compression or shear.

Stiff materials have a high modulus  the deformation (strain) resulting from the
applied force (stress) is low.

Flexible materials have a low modulus  undergo large deformations with relatively
low applied forces.

Modulus of Elasticity  for materials deformed in tension or compression.

Modulus of Rigidity used to express the resistance to shear or torsion.
Assessment of mechanical properties

The simple tests used to measure mechanical properties
are described in standard test methods.

The most widely used are the ASTM tests  nowadays
these are gradually being replaced by ISO procedures

The most common types of test performed on plastic
materials:
 Tensile
properties
 Flexural properties
 Impact strength
Tensile properties

Tensile properties are determined using dumbbell-shaped specimens.

The type defined in the ASTM D-638 standard is as shown in the diagram
below:

In a tensile experiment the specimen is gripped firmly by mechanical jaws at
the wide portion on either side and extended by means of a tensile testing
machine

The pulling is normally carried out at a constant rate of 0.50, 5.0 and 50
cm/min, depending on the type of plastic being tested.

The low speeds  to test rigid materials;

the higher speeds  to test flexible materials.

Calculated entities:
•
Tensile stress  measured the
force at any time divided by the
original cross sectional area of
the waist portion.
•
Tensile strain  the ratio of the
difference in length between the
length marked by the gauge
marks and the original length,
•
Yield strength sY ultimate
tensile strength (strength value
prior to fracture), st
•
Elastic modulus, E  ultimate
elongation (strain value at
fracture), et
Typical stress-strain curves for a brittle
material (1) and a ductile material (2)
*** Note that in the diagram above yield stress is only specified for the ductile material
as the brittle material fails catastrophically without reaching the yielding conditions.
Flexural properties

Flexural properties are important in assessing the resistance of materials to
bending.

A typical experimental set-up is as the one shown in the schematic below:
Flexural test
experimental set-up

Specimen dimensions may vary but the use of bars with a cross section
measuring 1.27´ 0.32 cm and span of 5.0 cm.

For these standard specimens a loading rate of 0.127 cm (0.05 in/min) is
normally used.
Calculated entities:

The maximum stress caused by
bending is calculated by the
following formula:
where:






S = stress (N/m2)
F = load or force at break or
at yield (N)
L = span of specimen
between supports (m)
b = width (m)
d = thickness (m)

The maximum strain due to
bending (compression and tensile
is estimated by:
where:
 e = strain (dimensionless i.e.,
no units)
 D = deflection at the centre of
the beam (m) – see
schematic below
 d = thickness (m)
 L = specimen’s length of span
between supports (m)
If the load recorded corresponds to the
value at failure occurs  S
corresponds to the flexural strength.
The
flexural modulus from the recorded load (F)
and deflection (D) is:
Impact strength


The energy used by the pendulum hammer to fracture the specimen (see
diagram) is given by the reduction in the height of the hammer in its swing
after fracturing the specimen 
Where:




m = mass of pendulum hammer
g = acceleration due to gravity (9.8 m/s2)
ho = initial height of pendulum hammer (m)
hf = height of the pendulum hammer after fracturing specimen

The specimen geometry is taken into account in terms of the cross-sectional
area which has undergone fracture.

The impact strength is defined as the energy divided by the area 
joules/m2.

Note: Because the distance from the notch tip to the edge of the specimen
is constant, sometimes the impact strength is expressed as the energy to
fracture per unit thickness.
Charpy test configuration
Apparatus to measure impact strength
Izod test configuration
Deformation of polymers

Permanent deformations  Yielding

Mechanical properties at the surface  Hardness,
Friction, Wear

Special issues in designing with polymers Creep
and Stress Relaxation

Factors that determine the resistance of polymeric
components to deformation

Enhancement of the resistance of polymers to
deformation
Yielding of polymers

Yielding is a phenomenon closely related to the onset of permanent
deformation, i.e. an irreversible process.

This is due to molecular chains unfolding and becoming aligned in
the direction of the applied load.

Yielding under a tensile load is shown below
The progress of the yielding process for a
specimen under tension
•A: prior to loading
•B: onset of necking in the waist
region after the yield point
•C: neck propagation ("cold drawing")
•D: neck extension and fracture

In non-crystalline (amorphous) polymers  yielding occurs by
molecular uncoiling.

At the yield point  a neck forms which is followed by an overall
drop in stress.

At the neck region  the folded chains become aligned.


Macroscopically  because of the thinning down in cross section,
the stress rises locally and any deformation occurs preferentially there.

This helps the neck propagate along the waist of the specimen under
a steady load a process known as cold drawing

Any deformation produced beyond the yield point is not
recoverable.

In a crystalline polymer 

the unfolding of chains begins in the amorphous regions between the
lamellae of the crystals.

this is followed by breaking-up and alignment of crystals
Alignment of molecular chains in polymer crystals; progress A-D same
as aforementioned

Points to note:

Yielding is a 
phenomenon which is
responsible for ductile
deformations,


as opposed to brittle
fracture.
the degree of ductility of
a polymer  often
controlled by a number
of variables
Variable
Change
Typical
effect on
ductility
Temperature


Strain rate


Molecular
weight


Chain
branching

/ 
Crystallinity


Crosslinking


Particulate
fillers


Fibrous
reinforcement



The deformation behaviour of polymers is time and temperature dependent, 
specimen may be ductile or brittle, according to the testing conditions: strain rate and
temperature.

If the temperature is sufficiently high and/or the strain rate is slow enough 



the specimen is ductile and will yield extensively.
The yield stress and stiffness increase and ductility decreases with lowering the
temperature or increasing the strain rate.
Under extreme strain rates, as under impact conditions  specimen may be unable to
undergo cold drawing and become brittle
Tensile stress-strain behaviour at
high strain rate and/or low
temperature(A); low strain rate
and/or high temperature (B)

Highly crosslinked polymers (thermosets) are typically brittle materials  since chain
movement is severely restricted, they do not usually yield, but fail in a brittle manner.
Hardness, Friction & Wear

These three surface-related properties are less frequently dealt with
in theoretical interpretations than fundamental properties such as
modulus, viscoelasticity and yielding,

but they are very important in applications that involve sliding
contact and frictional motions.

Gears, bearings, piston rings and seals are examples of
applications where these properties are of great significance.

The properties are:
 Hardness
 Friction
 Wear
Hardness

Hardness  more appropriately
described as resistance to
abrasion, cutting, machining or
scratching.

Related to fundamental bulk
properties  such as yield
strength and modulus.

Standardized techniques to
measure hardness  based on
the degree of penetration into a
specimen by hard indenters of
conical or spherical shape.
The hardness test
Friction

Friction is the resistance offered by a surface to the relative motion of objects in contact.

The frictional force opposing movement is described by the formula

The coefficient of friction, m,  is a property of the material which determines its
resistance to sliding action against another surface.

Friction arises from temporary adhesive contacts between the two surfaces

It is overcome through the rupture of these contacts by local plastic deformations.

Compressive yield strength & shear strength of the contacting materials are important in friction
abrasion.

In viscoelastic polymers  local rises in temperature resulting from shearing at higher
loads and sliding velocities cause the coefficient to increase.

In bearing applications  where a metal and a thermoplastic are in contact, increases in
pressure and the sliding velocity will increase m and limited by the conditions during
service.

The friction performance of polymers varies extensively, the value of m ranging from 0.2
to 0.7 and increasing surface roughness tends to increase friction.
Wear

Wear occurs  when material is lost from the interface between the contact
surfaces during relative motion.

At low temperatures  primary mechanism for wear damage is adhesive
wear, whereby fine particles are removed from the surface.

Since polymers overheat through friction  more severe damage can result
as larger volumes of locally melted material can be extracted from the
surface.

Temperature is also expected to adversely affect the wear rates.

High-strength ductile engineering thermoplastics such as nylon and acetal,
offer good wear performance  can be further improved with the addition of
internal lubricants or reinforcing additives

Fibre reinforcements (e.g., glass fabric) and mineral fillers (e.g., calcium
carbonate (CaCO3) may be compounded into the base polymers to improve
their load-carrying capacity  but can increase friction and give rise to more
detrimental abrasive wear.

Very high molecular weights have a positive effect in reducing wear 
UHMWPE (Ultra High Molecular Weight Polyethylene).
Creep & Stress relaxation

A serious challenge when designing products to be
made from polymeric materials is the prediction of
performance over long periods of time.

The amount of deformation after short or long term
loading has to be known reasonably accurately in
advance, i.e. at the design stage.

During long term service, creep and stress relaxation
are the main deformation mechanisms that can be
cause for concern.
Creep

Creep phenomena are particularly common in
polymers.

Creep occurs when a force is continuously applied
on a component  causing it to deform gradually.

For polymers,


the delayed response of polymer chains during
deformations cause creep behaviour

Deformation stops when the initially folded chains
reach a new equilibrium configuration (i.e. slightly
stretched).

This deformation is recoverable after the load is
removed,

but recovery takes place slowly with the chains
retracting by folding back to their initial state.
The rate at which polymers creep depends not only
on the load, but also on temperature.

In general, a loaded component creeps faster at
higher temperatures.
Time dependence

If a load is slowly applied to a polymeric body  the chains in the
polymer have time to unfold and stretch.

There are three main ways of presenting creep data to be
presented as:
1.
Creep curves Strain versus the logarithm of time elapsed (various
curves at constant load, or stress):
2.
Isochronous curves Stress versus strain (various curves at constant
time of duration of load):
3.
Isometric curves Stress versus the logarithm of elapsed time (various
curves at constant strain values):
Temperature dependence

The temperature at which a polymeric body is loaded  very important to its
mechanical behaviour.

Low temperatures  imply low internal energy within the molecules.


Polymer chains are less energetic (more sluggish) and also more reluctant to move
under a force.

Makes it more difficult for them to unfold their ability to undergo large deformations
is suppressed.

In this state  polymers are more likely to resist the applied load and stiffer.
Higher temperatures the energy level of chains favours their movement, so
unfolding is easier.

A given amount of deformation requires a lower force and a force of a given
magnitude produces a larger deformation.

Rising temperature and above the glass transition temperature, Tg, solid polymers
become softer and progress through the rubbery state to finally become a viscous
melt capable of flow.

The term "rubbery"  refers to the ability to deform sluggishly, but the
deformations recover when the load is removed.

The term "glassy“  relates to the hardness, stiffness and brittleness of the
polymer at low temperatures.
• The diagram below describes the variation of the deformability of
polymers over a wide range of temperatures:
Typical effect of temperature on the deformability (reverse of
stiffness / rigidity) of a polymer
Stress Relaxation

Stress relaxation is almost exclusively a
characteristic of polymeric materials and is
a consequence of delayed molecular
motions as in creep.

stress relaxation occurs when
 deformation
 manifested
(or strain) is constant and
by a reduction in the force
(stress) required to maintain a constant
deformation.
Failure in Polymers
1.
Modes of mechanical failure
2.
Types of mechanical failure: Creep
Rupture, Fatigue, Impact
3.
Factors that determine the mode of
failure of polymers
4.
Enhancement of the resistance of
polymers to failure
Modes of Mechanical Failures
• Failure analysis and prevention  important functions to all of the
engineering disciplines.
• The materials engineer  plays a lead role in the analysis of
failures, whether a component or product fails in service or if
failure occurs in manufacturing or during production processing.
• Must determine the cause of failure to prevent future occurrence,
and/or to improve the performance of the device, component or
structure.
• Failure in a product implies  the product no longer functions
satisfactorily.
• Mechanical failure in polymer materials  caused by :
1. Excessive deformation
2. Ductile failure
3. Brittle failure
4. Crazing
1.
Excessive deformation

Very large deformations are possible in low-modulus polymers  are
able to accommodate large strains before failure.

Such deformations could occur without fracture  design features and
other considerations might only tolerate deformations to a prescribed
ceiling value.

The case in rubbery thermoplastics, such as flexible PVC or EVA, for
pressurized tubing.
2.
Ductile failure

Encountered in materials that are able to undergo large-scale
irreversible plastic deformation under loading, known as yielding,
before fracturing.

Yielding marks the onset of failure  setting the upper limit to stress in
service to be below the yield point is common practice.

Estimate loading conditions likely to cause yielding (yield criteria), in
order to design components with a view to avoid it in service.
3.
Brittle failure

This is a type of failure  involves low strains accompanied by negligible
permanent deformation and is frequently characterized by "clean"
fracture surfaces.

It occurs in  components that contain geometrical discontinuities that
act as stress concentrations.

These physical features  the effect of locally raising stress. Effective
stress concentrating discontinuities are usually in the form of




cracks,
badly distributed or
oversized additive particulates,
impurities etc.

Contrary to ductile failures  plastic deformation provides a warning
signal for the ultimate fracture,

Brittle failures can occur without prior warning, except for the formation
of crazes, as in glassy thermoplastics.

Because of this  design specifications based on fracture strength data
tend to be conservative (e.g., will incorporate very large safety margins)
with respect to the maximum stress levels allowed relative to the
strength.
4.
Crazing

Crazing is a phenomenon that often occurs in glassy polymers
before yielding, i.e. for deformation at temperatures below the
glass transition.

It occurs at  a strain level which is below the level required for
brittle fracture and although undesirable, this type of "failure" is not
catastrophic.

Crazing is often observed in highly strained regions during
bending.

Crazes are made up of microcavities whose surfaces are joined by
highly oriented, or fibrillar, material.

They are initiated near structural discontinuities, such as impurities,
and are collectively visible at the strained surface because they
become large enough to reflect light.

Crazes are not cracks and can continue to sustain loads after they are
formed.

However, they can transform into cracks via the breakage of the fibrils.
A short film illustrates tensile tests on plastics. The
transparent sample is polystyrene and shows the
formation of crazes, as the horizontal lines across
the width of the specimen before fracture.
Types of Failures
 Because of the viscoelastic character of polymers  no failure can
be described
 entirely ductile or
 entirely brittle.
The proportion of each type of fracture involved in polymer failure
depends on many factors:
the speed (and time) of loading and
the temperature of the sample.
The type of stress, for instance, whether static or dynamic
(fluctuating), determine the mode of failure.
Below are links to the most common of rupture:
Creep Rupture
Fatigue Failure
Impact Failure
Creep rupture

Creep rupture is the culmination in the deformation process of creep.

The result of creep is a slow increase in deformation, which ultimately leads to
fracture when the polymer chains can no longer accommodate the load.





The level of stress,
the service temperature,
the component geometry,
the nature of the material and
any defects induced by the fabrication process
** are all decisive factors in determining the time taken for fracture to occur.

Although the precise details of the failure mechanism that precedes rupture in
creep are unclear  it is known that locally,

stress reaches high enough levels for microcracks to form.

These propagate in a slow stable manner, gradually reducing their ability to sustain
the load.

It is worth noting that the ultimate failure in creep may be preceded by shear yielding,
i.e. the creation of a neck, or by crazing.

These are good indicators that failure is in progress and that fracture is following. In
other cases, rupture can take place without any signs of warning.
Fatigue failure

Fatigue is a failure process which a crack grows as a result of cyclic
loading.

This type of loading involves  stresses that alternate between high
and low values over time.

The stress values may be entirely positive (tensile), entirely negative
(compressive), or a combination of the two (see diagram).
Cyclic stress that gives rise to fatigue in materials

However, the effect of fatigue increases with higher tensile & Cyclic stress that
gives rise to fatigue in materials

Once a crack is initiated  it propagates by small steps during the tensile
portion of a stress cycle.

The crack grows slowly but steadily up to the point where the remaining
area of the part’s section is unable to support the load.

The subsequent failure is invariably brittle.

Failure prediction

The stresses involved in fatigue are  much lower than the value required to
cause outright failure.

Final failure is only possible by cumulative damage.

The initial crack from which the damage starts is either



pre-existing (i.e., mechanically generated or fabrication imperfection) or
initiated by high local stress at weak regions in the material.
A suitably large flaw or weak enough region lies in an adequately stressed
region of loaded components may vary according to




flaw density (number of flaws per unit volume)
component size
batch
other factors which make the prediction of fatigue failure in terms of time or
number of cycles subject to the mathematical laws of probability.

The nature of stress in fatigue

The amplitude of the stress  the variation in stress between the
maximum and minimum values, affects the speed of propagation of
the crack, because:


it determines the amount by which a crack makes a step
forward during each stress cycle.

higher stress amplitudes with a high positive mean stress
decrease the time, or cycles, to failure.
The frequency of the stress  stress alternates between maximum
and minimum, also affects the time to failure as it causes the steplike propagation of the crack to advance more rapidly.

Parameters in cyclic (alternating) stress

The fatigue in polymers is subject to complications because of
viscoelasticity in polymers.

This causes damping of the alternating load, a process which itself
creates heat.

This heat is dissipated with difficulty because of the generally low
thermal conductivity of the polymers.

The rate of heat production due to an increase in stress amplitude

and/or frequency becomes lower than the rate of heat dissipation, and
so stored heat causes the temperature in the material to rise.

At sufficiently high temperatures the polymer may overheat and fail not
through fatigue but rather through creep or heat softening,

whereby the modulus decreases to the extent that the material is
unsuitable for its intended use.
Impact failure

The type of loading that constitutes an impact is what could be
described as a "knock" or "blow",

a force applied very fast, capable of causing failure by brittle
fracture.

Is achieved is through the transfer of the energy of impact to
defects in the structure  then grow rapidly.

Accidental occurrence of impact makes resistance to this type of
abuse an important one  especially for materials used in
critical applications.

Impact strength is the typical parameter quoted in order to
characterize resistance to impact.

However the conditions under which impact is experienced are
crucial to the relevance of this data.
In general, resistance to fracture through impact is affected by the following:
Variable
Change
Typical effect on
ductility




Amount/size of notchlike defects


Mass of impacting
body


Fibrous reinforcement


Temperature
Strain rate (speed of
impact)

Factors relevant to the ductility of polymers have the same effect on impact
resistance  the time and temperature dependence of polymers limit the
ability of chains to "give" under impact (very high strain rate) conditions by
undergoing compensating molecular motion.

An important exception to the ductility and impact toughness is  use of fibre
reinforcement in composites, where impact strength is improved.


the energy of impact is expended on diverting the crack along the fibrematrix interface.

Although some debonding of fibres occurs in the process catastrophic
failure is largely prevented.
The factors that increase the possibility of embrittlement  lead to decreases
in impact strength.

The presence of notches lowers the energy requirements of fracture by
highly concentrating the stress of impact locally   stress
concentrations.

The size and shape of the notch (i.e., whether blunt or sharp) is critical 
in determining the impact strength obtained from tests.

Polymers such as rigid PVC, polycarbonate, some members of the
polyamide family, polymethyl methacrylate (acrylic) significantly
affected by the notch condition and are often described as notch
sensitive.
Factors that affect the mode of polymer Failure

The following factors affect polymer fracture behavior adversely by
promoting the brittle type of mechanism:
1.
2.
3.
Loading Conditions
Environmental
Material structure aspects
1. Loading conditions


Very fast loading – as in the case of impacts
Triaxiality of stress: the development of stresses in more
directions relative to the one from which a load is applied

triaxial stresses promote brittle failure in materials.

this 3D type of stress system appears at discontinuities
(stress concentrations) within a component.
2. Environmental

Low temperature

can bring a transition in fracture mode from ductile to brittle  experienced
by a material when the temperature falls below a point known as the
ductile-brittle transition temperature, TDB.

Deterioration of physical properties  as a result of chemical changes to
molecular structure through

Oxidation: reactions with substances such as oxidizing acids and water
moisture

Weathering: the combined effect of exposure to u.v. radiation and oxygen

Degradation due to exposure to excessive heat, particularly in the presence
of oxygen

Environmental Stress Cracking: ingress to defect sites within the material of
normally non-aggressive liquids (mostly organic) that promote fracture at low
levels of stress and over short periods of time.
3. Material structure aspects

Discontinuous microstructure  arising from the presence of:

particulate additives

crystallinity in the polymer

Molecular weight  toughness generally increases with molecular
weight.
Improving the resistance of polymers to failures

To minimize the risk of catastrophic failure  a material needs to be tough as
well as ductile.

The mechanical design  has a role in avoiding the incorporation of features
that promote the likelihood of brittle fracture.

The following guidelines  to identify the steps to enhance the failure
resistance of polymers in service:
1.
2.
3.
1.
Design considerations
Material Selection
Material Modification
Design considerations


Design for a particular set of stress conditions anticipated in service 
example:

attention to section thicknesses, and

utilisation of material data obtained under conditions relevant to service
(creep, fatigue, impact)
Elimination of the majority of stress-concentrating design features
abrupt changes in section, holes, notches
2.
Material Selection

Should be based structural aspects affecting failure, as well as physical
and chemical issues arising from the use of polymers in a particular
environment such as the effect of temperature, oxidants and aggressive
liquids.

Given that the most important properties affecting resistance to brittle
fracture are toughness and ductility,

key material data to be used in design in order to minimise the likelihood of
brittle fracture should include:

ductility indicators (e.g., energy absorption values obtained directly by
measuring the area under load-extension curves obtained in tensile
tests which are carried out to failure (see schematic).

Energy absorption values derived from impact tests
Energy absorbed during extension
3.
Material Modification

Toughening through microstructural modification of thermoplastics

Based on the principle that the energy which contributes to brittle
fracture can be dissipated by localized yielding ahead of the crack tip
 possible to produce toughened thermoplastic polymers by the
incorporation of a partially compatible rubbery phase.

This is typically accomplished:
(a) at the polymerisation stage by copolymerisation, and by
(b) direct blending (e.g. mixing acrylic rubber with PVC or with PBT.

The success of the toughening of thermoplastics by rubber
modification depends on:

the rubber existing as well dispersed discrete particles

the interfacial adhesion between the thermoplastic matrix and the rubber
being at an optimum level (i.e., neither too strong nor too weak)

the glass transition temperature of the rubber phase  lower than the
service temperature.

Rubber toughening works  by lowering the average yield stress, it
facilitates the occurrence of plastic deformation.
Example of the exams question

The failures of polymeric materials can be affect
by a few factors. Discuss two of this factors.
failure?

There are a few types of failures in polymeric
materials such as creep rupture, fatigue and
impact. Based on your understanding, discuss
two of this mechanical failures and how this
failures can be describe as brittle or ductile
deformation