4. Polymer Failure

8/10/2019 4. Polymer Failure

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EBB 220/3

FAILURE IN POLYMERS

DR AZURA A.RASHIDRoom 2.19

School of Materials And Mineral Resources Engineering,

Universiti Sains Malaysia, 14300 Nibong Tebal, P. Pinang

Malaysia


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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 responseof materials under mechanical loads.

Expressed in terms of forces which may deform materials oreven cause them to fail completely.

To avoid failure and keep deformation under control so theindividual system components remain functional as parts of a

wholeneed a various considerations: Is stiffness / rigidity important? (i.e. minimum deformation under

a given load)

Is strength essential? (for maximum tolerance of loads beforefailure)

Importance of mechanical properties of materialsin engineering


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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 applicationof a force):

FATIGUE (e.g. vibration, rotation in loaded components)

The geometry of the loaded component can be designed to deal withthese conditions.

The physical nature of the material has to ensure that the componentcan 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 applicationsexample: an aircraft

Property-to-cost ratio

design for stiffness and strength where low overall price is important example: childrens toys, non-critical parts of home appliances


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Below are some terms we find in dealing with materials inrelation to structural applications:

Stress

Strength

Strain

Stress-strain relationships

Modulus

Concept of deformation:

Deformations can be produced by forces which cause a bodyto be stretched, compressed, twisted or sheared.

These forces can also be combined to produce more complextypes of deformationfor example : flexural.

Fundamental concepts for mechanical properties


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Unloaded Stretched

(Tension)

Squeezed

(Compression)

Cut (Simple

shear)

Twisted

(Tors ion al sh ear)


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Extension by stretching in one direction the simplest type ofdeformation that can be used to explain key concepts in

mechanics

Rectangular specimens

subjected to different loads

in tensile mode


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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 tensilestress (in the direction of elongation)

If the force applied is 100 N (Newtons), and the cross sectional areameasures 0.0004 m2(square metres), the stress becomes

or 250 KN/m2, or 0.25 MN/m2. If the force doubles (200 N), stress willincrease 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

.

Stress


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If the same weights were placed on the rectangular specimens tocause a contraction in the longitudinal direction the resultingstress would be called compressivestress.

The other common type of stress is shearstress.

This relates to the force which distorts rather than extends a bodyexample where a solid section is sheared,

Shear forces can also result in failure.

Everyday example of shear failureCylindrical specimen subjected to

simple shear, e.g. during cutting.


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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 withstandbefore it completely fails to perform structurally.

If the applied force is tensile (stretch) the ultimate stress isknown as tensile strength (i.e., maximum tensile stress that thematerial can tolerate).

Others types of strength are related to the mode of the applied forcecompressive, shear, torsional and flexural.

Use the following expressions:

A st rong material can withstand a very high force perunit area before it fails.

A weak material markedly deteriorates or fails atrelatively low levels of applied forces.

Strength


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To understand the effect of specimen size on the amount ofdeformation resulting from forceuse the concept of strain.

Strain the change in one dimension produced as a result of anapplied force and it is expressed as the ratio of the amount ofdeformation to the samplesoriginal 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

Strain


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Materials deform elastically orinelastically.

During elastic deformation thestress in a body is directly relatedto the strain, and vice-versa.

When the force is removed (i.e. whenstress becomes zero) then strainreturns to zero.

The plot of stress against strain

produces a straight line the stress can be increased or

decreased, and

stress and strain are alwaysproportional to each other.

Stress-strain relationship (below failure conditions)

Linear elastic stress-

strain relationship


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For ductile materialsincreasing the stress above a certain limit willgive rise to inelastic deformations, known as yield ing.

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 stressor yield po int.

The applied force takes the material

beyond the linear elastic region.

Continued loading causes permanentdeformation.

The amount of permanent

deformation is evident after theforce applied isremoved.


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The relationship between stress and strain is expressed in terms of a propertycalled the Modulus(or Young Modulus).

The linear portion of the stress-strain curve can be used to determine themodulus correspond to the slope of the curve before the yield point, up towhich all deformation is elastic and recoverable.

In other words,

The slope (modulus) at any point in the linear portion of the line gives thesame result.

The modulus denotes stiffness or rigidity for any kind of applied load, i.e.tension, compression or shear.

Stif f materials have a high modulus the deformation (strain) resulting from theapplied force (stress) is low.

Flexiblematerials have a low modulus undergo large deformations with relativelylow applied forces.

Modulu s of Elast ic i tyfor materials deformed in tension or compression.

Modulu s of Rig id i tyused to express the resistance to shear or torsion.

Modulus


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The simple tests used to measure mechanical propertiesare 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 plasticmaterials:

Tensile propertiesFlexural properties

Impact strength

Assessment of mechanical properties


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Tensile properties are determined using dumbbell-shaped specimens.

The type defined in the ASTM D-638 standard is as shown in the diagrambelow:

In a tensile experiment the specimen is gripped firmly by mechanical jaws atthe wide portion on either side and extended by means of a tensile testingmachine

The pulling is normally carried out at a constant rate of 0.50, 5.0 and 50cm/min, depending on the type of plastic being tested.

The low speedsto test rigid materials;

the higher speedsto test flexible materials.

Tensile properties


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Typical stress-strain curves for a brittlematerial (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.

Calcu lated ent i t ies:

Tensile stressmeasured theforce at any time divided by the

original cross sectional area of

the waist portion.

Tensile strainthe ratio of thedifference in length between the

length marked by the gauge

marks and the original length,

Yield strengthsYultimatetensile strength (strength value

prior to fracture), st

Elastic modulus, E

ultimateelongation (strain value at

fracture), et


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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:

Specimen dimensions may vary but the use of bars with a cross sectionmeasuring 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) isnormally used.

Flexural testexperimental set-up

Flexural properties


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Calculated entit ies:

The maximum stress caused bybending is calculated by thefollowing formula:

where:

S= stress (N/m2)

F= load or force at break orat yield (N)

L= span of specimenbetween supports (m)

b= width (m)

d= thickness (m)

If the load recorded corresponds to thevalue at failure occurs Scorresponds to the flexural strength.

The maximum strain due tobending (compression and tensileis estimated by:

where:

e= strain (dimensionless i.e.,

no units) D= deflection at the centre of

the beam (m)seeschematic below

d= thickness (m)

L= specimens length of spanbetween supports (m)

The flexural modulusfrom the recorded load (F)and deflection (D) is:


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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 swingafter 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-sectionalarea which has undergone fracture.

The impact strength is defined as the energy divided by the areajoules/m2.

Note:Because the distance from the notch tip to the edge of the specimenis constant, sometimes the impact strength is expressed as the energy tofractureperunit thickness.

Impact strength


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Apparatus to measure impact strength

Charpy test configuration

Izod test configuration


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Permanent deformationsYielding

Mechanical properties at the surface Hardness,Friction, Wear

Special issues in designing with polymers Creepand Stress Relaxation

Factors that determine the resistance of polymeric

components to deformation

Enhancement of the resistance of polymers todeformation

Deformation of polymers


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Yielding is a phenomenon closely related to the onset of permanent

deformation, i.e. an irreversibleprocess. 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 waistregion after the yield point

C: neck propagation ("cold drawing")

D: neck extension and fracture

Yielding of polymers


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In non-crystalline (amorphous) polymers yielding occurs bymolecular uncoiling.

At the yield point a neck forms which is followed by an overalldrop in stress.

At the neck regionthe 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 undera steady loada process known as cold drawing

Any deformation produced beyond the yield point is notrecoverable.

In a crystalline polymer

the unfolding of chains begins in the amorphous regions between thelamellae of the crystals.

this is followed by breaking-up and alignment of crystals


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Alignment of molecular chains in polymer crystals; progress A-D same

as aforementioned


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Points to note:

Yielding is a phenomenon which is

responsible for ductile

deformations,

as opposed to brittlefracture.

the degree of ductility of

a polymeroften

controlled by a numberof variables

Variable Change Typical

effect on

ductility

Temperature

Strain rate

Molecular

weight

Chain

branching

/

Crystallinity

Crosslinking

Particulatefillers

Fibrous

reinforcement


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The deformation behaviour of polymers is time and temperature dependent, specimen may be ductile or brittle, according to the testing conditions: strain rate andtemperature.

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 thetemperature or increasing the strain rate.

Under extreme strain rates, as under impact conditions specimen may be unable toundergo cold drawing and become brittle

Highly crosslinked polymers (thermosets) are typically brittle materials since chainmovement is severely restricted, they do not usually yield, but fail in a brittle manner.

Tensile stress-strain behaviour at

high strain rate and/or low

temperature(A); low strain rate

and/or high temperature (B)


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These three surface-related properties are less frequently dealt within theoretical interpretations than fundamental properties such asmodulus, viscoelasticityand yielding,

but they are very important in applications that involve slidingcontact and frictional motions.

Gears, bearings, piston rings and seals are examples ofapplications where these properties are of great significance.

The properties are:

Hardness

Friction

Wear

Hardness, Friction & Wear
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Hardness more appropriatelydescribed as resistance toabrasion, cutting, machining orscratching.

Related to fundamental bulkproperties such as yieldstrength and modulus.

Standardized techniques to

measure hardness based onthe degree of penetration into aspecimen by hard indenters ofconical or spherical shape.

The hardness test

Hardness


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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 itsresistance 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 frictionabrasion.

In viscoelastic polymers local rises in temperature resulting from shearing at higherloads and sliding velocities cause the coefficient to increase.

In bearing applicationswhere a metal and a thermoplastic are in contact, increases inpressure and the sliding velocity will increase m and limited by the conditions duringservice.

The friction performance of polymers varies extensively, the value of m ranging from 0.2to 0.7 and increasing surface roughness tends to increase friction.

Friction


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Wear occurswhen material is lost from the interface between the contactsurfaces during relative motion.

At low temperaturesprimary mechanism for wear damage is adhesivewear, whereby fine particles are removed from the surface.

Since polymers overheat through frictionmore severe damage can resultas 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 ofinternal lubricants or reinforcing additives

Fibre reinforcements(e.g., glass fabric) and mineral fillers (e.g., calciumcarbonate (CaCO3) may be compounded into the base polymers to improvetheir load-carrying capacitybut can increase friction and give rise to moredetrimental abrasive wear.

Very high molecular weights have a positive effect in reducing wear UHMWPE (Ultra High Molecular Weight Polyethylene).

Wear


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A serious challenge when designing products to bemade from polymeric materials is the prediction ofperformance over long periods of time.

The amount of deformation after short or long term

loading has to be known reasonably accurately inadvance, i.e. at the design stage.

During long term service, creep and stress relaxationare the main deformation mechanisms that can be

cause for concern.

Creep & Stress relaxation


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Creep phenomena are particularly common inpolymers.

Creep occurs when a force is continuously appliedon a component causing it to deform gradually.

For polymers, the delayed response of polymer chains during

deformationscause creep behaviour Deformation stops when the initially folded chains

reach a new equilibrium configuration (i.e. slightlystretched).

This deformation is recoverable after the load isremoved,

but recovery takes place slowly with the chainsretracting by folding back to their initial state.

The rate at which polymers creep depends not onlyon the load, but also on temperature.

In general, a loaded component creeps faster athigher temperatures.

Creep


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If a load is slowly applied to a polymeric bodythe chains in the

polymer have time to unfold and stretch.

There are three main ways of presenting creep data to bepresented as:

1. Creep curvesStrain versusthe logarithm of time elapsed (various

curves at constant load, or stress):

2. Isochronou s curvesStress versusstrain (various curves at constanttime of duration of load):

3. Isom etr ic c urvesStress versusthe logarithm of elapsed time (variouscurves at constant strain values):

Time dependence


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The temperature at which a polymeric body is loaded very important to itsmechanical behaviour.

Low temperaturesimply low internal energy within the molecules.

Polymer chains are less energetic (more sluggish) and also more reluctant to moveunder a force.

Makes it more difficult for them to unfold their ability to undergo large deformationsis suppressed.

In this statepolymers are more likely to resist the applied load and stiffer.

Higher temperaturesthe energy level of chains favours their movement, sounfolding is easier.

A given amount of deformation requires a lower force and a force of a givenmagnitude produces a larger deformation.

Rising temperature and above the glass transition temperature, Tg, solid polymersbecome softer and progress through the rubberystate to finally become a viscousmelt capable of flow.

The term "rubbery" refers to the ability to deform sluggishly, but thedeformations recover when the load is removed.

The term "glassy relates to the hardness, stiffness and brittleness of thepolymer at low temperatures.

Temperature dependence


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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


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Stress relaxation is almost exclusively acharacteristic of polymeric materials and isa consequence of delayed molecularmotions as in creep.

stress relaxation occurs when

deformation (or strain) is constant and

manifested by a reduction in the force(stress) required to maintain a constantdeformation.

Stress Relaxation


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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 ofpolymers to failure

Failure in Polymers


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Failure analysis and prevention important functions to all of theengineering disciplines.

The materials engineer plays a lead role in the analysis offailures, 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 functionssatisfactorily.

Mechanical failure in polymer materialscaused by :

1. Excessive deformation2. Ductile failure

3. Brittle failure

4. Crazing

Modes of Mechanical Failures


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1. Excessive deformation

Very large deformations are possible in low-modulus polymers areable to accommodate large strains before failure.

Such deformations could occur without fracture design features andother considerations might only tolerate deformations to a prescribedceiling value.

The case in rubbery thermoplastics, such as flexible PVC or EVA, forpressurized tubing.

2. Ductile failure

Encountered in materials that are able to undergo large-scaleirreversible plastic deformation under loading, known as yielding,before fracturing.

Yieldingmarks the onset of failure setting the upper limit to stress inservice to be below the yield point is common practice.

Estimate loading conditions likely to cause yielding (yield criteria), inorder to design components with a view to avoid it in service.


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3. Brittle failure

This is a type of failure involves low strains accompanied by negligiblepermanent 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. Effectivestress 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 warningsignal for the ultimate fracture,

Brittle failures can occur without prior warning, except for the formationof crazes, as in glassy thermoplastics.

Because of this design specifications based on fracture strength datatend to be conservative (e.g., will incorporate very large safety margins)with respect to the maximum stress levels allowed relative to the

strength.
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4. Crazing

Crazing is a phenomenon that often occurs in glassy polymersbefore yielding,i.e. for deformation at temperatures below theglass transition.

It occurs ata strain level which is below the level required forbrittle fracture and although undesirable, this type of "failure" is notcatastrophic.

Crazing is often observed in highly strained regions duringbending.

Crazes are made up of microcavities whose surfaces are joined byhighly oriented, or fibrillar, material.

They are initiated near structural discontinuities, such as impurities,

and are collectively visible at the strained surface because theybecome large enough to reflect light.

Crazes are not cracks and can continue to sustain loads after they areformed.

However, they can transform into cracks via the breakage of the fibrils.


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A

short film illustrates tensile tests on plastics. Thetransparent sample is polystyrene and shows the

formation of crazes, as the horizontal lines across

the width of the specimen before fracture.


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Creep rupture is the culmination in the deformation process of creep.

The result of creep is a slow increase in deformation, which ultimately leads tofracture 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 increep are unclearit 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 sustainthe 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. Inother cases, rupture can take place without any signs of warning.

Creep rupture
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Fatigue is a failure processwhich a crack grows as a result of cyclic

loading.

This type of loading involves stresses that alternate between highand low values over time.

The stress values may be entirely positive (tensile), entirely negative(compressive), or a combination of the two (see diagram).

Fatigue failure

Cyclic stress that gives rise to fatigue in materials


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However, the effect of fatigue increases with higher tensile & Cyclic stress thatgives rise to fatigue in materials

Once a crack is initiated it propagates by small steps during the tensileportion of a stress cycle.

The crack grows slowly but steadily up to the point where the remainingarea of the partssection 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 tocause 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 stressedregion 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 ornumber of cycles subject to the mathematical laws of probability.


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The nature of stress in fatigue

The amplitude of the stress the variation in stress between themaximum and minimum values, affects the speed of propagation ofthe crack, because:

it determines the amount by which a crack makes a stepforward during each stress cycle.

higher stress amplitudes with a high positive mean stressdecrease the time, or cycles, to failure.

The frequency of the stress stress alternates between maximumand minimum, also affects the time to failure as it causes the step-

like propagation of the crack to advance more rapidly.


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Parameters in cyclic (alternating) stress

The fatigue in polymers is subject to complications because of

viscoelasticityin polymers.

This causes damping of the alternating load, a process which itselfcreates 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, andso stored heat causes the temperature in the material to rise.

At sufficiently high temperatures the polymer may overheat and fail notthrough fatigue but rather through creep or heat softening,

whereby the modulus decreases to the extent that the material isunsuitable for its intended use.
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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 brittlefracture.

Is achieved is through the transfer of the energy of impact to

defects in the structurethen grow rapidly. Accidental occurrence of impact makes resistance to this type of

abuse an important oneespecially for materials used incritical applications.

Impact strengthis the typical parameter quoted in order tocharacterize resistance to impact.

However the conditions under which impact is experienced arecrucial to the relevance of this data.

Impact failure


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Variable Change Typical effect onductility

Temperature

Strain rate (speed of

impact)

Amount/size of notch-

like defects

Mass of impacting

body

Fibrous reinforcement

In general, resistance to fracture through impact is affected by the following:


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Factors relevant to the ductility of polymers have the same effect on impactresistance the time and temperature dependenceof polymers limit theability of chains to "give"under impact (very high strain rate) conditions byundergoing compensating molecular motion.

An important exception to the ductility and impact toughness isuse of fibrereinforcement in composites, where impact strength is improved.

the energy of impact is expended on diverting the crack along the fibre-matrix interface.

Although some debonding of fibres occurs in the processcatastrophicfailure is largely prevented.

The factors that increase the possibility of embrittlementlead to decreasesin impact strength.

The presence of notcheslowers the energy requirements of fracture byhighly concentrating the stress of impact locallystress

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 thepolyamide family, polymethyl methacrylate (acrylic) significantlyaffected by the notch condition and are often described as notch

sensitive.
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The following factors affect polymer fracture behavior adversely by

promoting the brittle type of mechanism:

1. Loading Conditions

2. Environmental

3. Material structure aspects

1. Loading conditions Very fast loadingas in the case of impacts

Triaxial ity o f s tress:the development of stresses in moredirections 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.

Factors that affect the mode of polymer Failure


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2. Environmental Low temperature

can bring a transition in fracture mode from ductile to brittle experiencedby a material when the temperature falls below a point known as theductile-brittle transition temperature, TDB.

Deterioration of physical properties as a result of chemical changes tomolecular structure through

Oxidat ion: reactions with substances such as oxidizing acids and watermoisture

Weathering:the combined effect of exposure to u.v. radiation and oxygen

Degradat iondue to exposure to excessive heat, particularly in the presenceof oxygen

Environm ental Stress Cracking :ingress to defect sites within the material ofnormally non-aggressive liquids (mostly organic) that promote fracture at lowlevels of stress and over short periods of time.

3. Material structure aspects

Discont inuous microst ruc turearising from the presence of: particulate additives

crystallinity in the polymer

Molecular weigh t toughness generally increases with molecularweight.


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To minimize the risk of catastrophic failure a material needs to be tough aswell as ductile.

The mechanical design has a role in avoiding the incorporation of featuresthat promote the likelihood of brittle fracture.

The following guidelines to identify the steps to enhance the failureresistance of polymers in service:

1. Design considerations2. Material Selection

3. Material Modification

1. 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

Improving the resistance of polymers to failures


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2. Material Selection

Should be based structural aspects affecting failure, as well as physicaland chemical issues arising from the use of polymers in a particularenvironment such as the effect of temperature, oxidants and aggressive

liquids.

Given that the most important properties affecting resistance to brittlefracture are toughnessand 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 bymeasuring the area under load-extension curves obtained in tensiletests which are carried out to failure (see schematic).

Energy absorption values derived from impact tests

Energy absorbed during extension


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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 tippossible to produce toughened thermoplastic polymers by theincorporation 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 rubbermodification depends on:

the rubber existing as well dispersed discrete particles

the interfacial adhesion between the thermoplastic matrix and the rubberbeing at an optimum level (i.e., neither too strong nor too weak)

the glass transition temperature of the rubber phase lower than theservice temperature.

Rubber toughening works by lowering the average yield stress, itfacilitates the occurrence of plastic deformation.


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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 ductiledeformation

Example of the exams question


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Date post:	02-Jun-2018
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4. Polymer Failure

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