IntroductionIntroduction

Why ultimate properties?Why ultimate properties?•• For successful product design a knowledge of For successful product design a knowledge of

the behavior of the polymer is importantthe behavior of the polymer is important•• Variation in properties over the entire range Variation in properties over the entire range

of operating conditions should be knownof operating conditions should be known

How do we find ultimate properties?How do we find ultimate properties?

Failure tests are the answerFailure tests are the answerVarious tests are carried out until the material Various tests are carried out until the material fails fails Failure tests carried out with some degree Failure tests carried out with some degree of simulation between test and end useof simulation between test and end use

PropertiesProperties

RigidityRigidityUltimate strengthUltimate strengthToughnessToughnessCreep (long term deformation resistance)Creep (long term deformation resistance)Resistance to thermal degradationResistance to thermal degradation

Ultimate StrengthUltimate Strength

This is the stress at failureThis is the stress at failure

Ultimate StrengthUltimate Strength

It is never the limiting factor for any It is never the limiting factor for any applicationapplication

•• polymer never subjected to a single steady polymer never subjected to a single steady deformation in absence of aggressive deformation in absence of aggressive environmentenvironment

•• failure generally takes place due to failure generally takes place due to repeated stresses, impact, penetrationrepeated stresses, impact, penetrationby sharp objects or propagation of tearby sharp objects or propagation of tear

ToughnessToughness

It is the energy absorbed before failureIt is the energy absorbed before failureIt is the area under the stress strain curveIt is the area under the stress strain curveEnergy may be stored elastically or maybeEnergy may be stored elastically or maybedissipated as heat (as in permanent dissipated as heat (as in permanent deformation of a crystalline material)deformation of a crystalline material)Units Units –– J/mJ/m33. Thus it is the energy required to break . Thus it is the energy required to break

unit volume of materialunit volume of materialTough Tough materials absorb a lot of energy when fractured materials absorb a lot of energy when fractured and and brittlebrittle materials absorb very little energy materials absorb very little energy

Breaking Energy Breaking Energy –– total energy required to cause total energy required to cause rupture. Is a measure of the toughness rupture. Is a measure of the toughness Impact tester used to measure the breaking Impact tester used to measure the breaking energy energy –– a hammer used to apply load to a a hammer used to apply load to a sample until the sample breaks. The energy used sample until the sample breaks. The energy used in breaking the sample is proportional to the in breaking the sample is proportional to the difference in height before and after the break difference in height before and after the break

IzodIzod Test Test –– similar to impact tester except that it similar to impact tester except that it uses a cantilever beam uses a cantilever beam

Impact energy = Impact energy = energyenergy absorbed absorbed = mass of pendulum * g * (= mass of pendulum * g * (hh11 –– hh00))

where g = where g = accelarationaccelaration due to gravity due to gravity

CharpyCharpy Test Test –– simple beam is usedsimple beam is used

CharpyCharpy MachineMachine

CharpyCharpy ChartChart

Brittleness Temperature TestBrittleness Temperature TestPendulum Arm used to break rubber & soft plasticsPendulum Arm used to break rubber & soft plasticsTemperature progressively lowered until the sample Temperature progressively lowered until the sample fracturesfractures

Comparison of ToughnessComparison of Toughness

Tests for strengthTests for strength

DumbbellDumbbell

Advantage Advantage –– Failure will take place at the Failure will take place at the centre and will not be affected by thecentre and will not be affected by thestress concentration at the jawsstress concentration at the jawsDisadvantage Disadvantage –– Accurate measurement ofAccurate measurement of

strain difficultstrain difficult

Ring SpecimenRing SpecimenUsed for rubbery samplesUsed for rubbery samplesRate of strain is uniform Rate of strain is uniform –– thus stress timethus stress timerecord is also stress strain recordrecord is also stress strain recordDisadvantage Disadvantage stress concentration at holder is difficult to avoid even stress concentration at holder is difficult to avoid even when holder is lubricated or rotatedwhen holder is lubricated or rotatedStress needed to deform the ring into an oval masks the Stress needed to deform the ring into an oval masks the stress due to deformation stress due to deformation –– initial portion of the stress initial portion of the stress strain curve maybe distortedstrain curve maybe distorted

Trouser Tear testTrouser Tear test

Trouser Tear (ASTM D 1938)Trouser Tear (ASTM D 1938)——The Trouser Tear The Trouser Tear test measures primarily crack propagation. Force is test measures primarily crack propagation. Force is applied to the specimen in the same direction as the applied to the specimen in the same direction as the separating jaws of the test equipment. The Trouser separating jaws of the test equipment. The Trouser Tear test gives a better measure of tear strength because Tear test gives a better measure of tear strength because the sample legs are much wider helping to minimize the sample legs are much wider helping to minimize sample elongation during testing. If the sample breaks sample elongation during testing. If the sample breaks across one of the legs of the specimen, rather than across one of the legs of the specimen, rather than between the legs, the test is no longer measuring the between the legs, the test is no longer measuring the resistance to tear propagation. resistance to tear propagation.

AdvantageAdvantage –– Stress to propagate tear stays at Stress to propagate tear stays at constant value. This makes it easy to estimate constant value. This makes it easy to estimate the tear propagated than some others in which the tear propagated than some others in which the tear propagates so rapidly that only a peak the tear propagates so rapidly that only a peak stress can be read.stress can be read.

Creep FailureCreep FailureCreep is development of additional strains in a Creep is development of additional strains in a material over time material over time Creep is most prevalent under high stresses and Creep is most prevalent under high stresses and temperatures, and is not necessarily a failure temperatures, and is not necessarily a failure mode mode Creep test gives information on long term Creep test gives information on long term dimensional stability of a load dimensional stability of a load –– bearing elementbearing elementWhen combined with temperature the test When combined with temperature the test measures deflection temperaturemeasures deflection temperature

Test:Test: Material is subjected to prolonged Material is subjected to prolonged constant tension or compression loading at constant tension or compression loading at constant elevated temperature. Deformation is constant elevated temperature. Deformation is recorded at specified time intervals and a creep recorded at specified time intervals and a creep vs. time diagram is plotted. Slope of curve at any vs. time diagram is plotted. Slope of curve at any point is point is creep ratecreep rate. If failure occurs, it terminates . If failure occurs, it terminates the test and the time for rupture is recorded. If the test and the time for rupture is recorded. If specimen does not fracture within the test specimen does not fracture within the test period, period, creep recoverycreep recovery may be measured.may be measured.

Creep CurvesCreep Curves

FatigueFatigueFatigue is a process by which a material is weakened by Fatigue is a process by which a material is weakened by cyclic loadingcyclic loadingFatigue testing gives much better data to predict the service Fatigue testing gives much better data to predict the service life of materials life of materials Fatigue testing can be thought of as simply applying cyclic Fatigue testing can be thought of as simply applying cyclic loading to the test specimen to understand how it will loading to the test specimen to understand how it will perform under similar conditions in actual use. The load perform under similar conditions in actual use. The load application can either be a repeated application of a fixed application can either be a repeated application of a fixed load or simulation of the service loads. The load application load or simulation of the service loads. The load application may be repeated millions of times and up to several may be repeated millions of times and up to several hundred times per second hundred times per second

Fatigue StrengthFatigue Strength

WLF EquationWLF EquationIt gives the effect of temperature on viscosityIt gives the effect of temperature on viscosity

If a change in the material property with temperature arises witIf a change in the material property with temperature arises with h change in the viscosity then it is possible to apply the WLF equchange in the viscosity then it is possible to apply the WLF equationatione.g.e.g.

•• Stress Relaxation Stress Relaxation –– The force required to maintain a fixed strain at a The force required to maintain a fixed strain at a constant temperature will decay with time owing to decrease in constant temperature will decay with time owing to decrease in viscosity of molecules. A measure of the stress relaxation is thviscosity of molecules. A measure of the stress relaxation is the e relaxation time i.e. time taken by the material to relax to 1/e relaxation time i.e. time taken by the material to relax to 1/e of its of its stress on application of strain stress on application of strain

)(6.51)(44.17

log10g

g

T

T

TTTT

g−+

−−=

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

ηη

WLF Equation (contd.)WLF Equation (contd.)

WLF = Williams, WLF = Williams, LandelLandel and Ferry and Ferry

)(6.51)(44.17

log10g

g

T

T

TTTT

g−+

−−=⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

θθ

Fracture of Glassy PolymersFracture of Glassy Polymers

Two mechanisms are involvedTwo mechanisms are involved•• Shear Bands caused due to shear deformation. These Shear Bands caused due to shear deformation. These

bands have very little void volumebands have very little void volume•• Crazes Crazes –– these are fine cracks at right angles to the these are fine cracks at right angles to the

applied stress. They are narrow zones of highly applied stress. They are narrow zones of highly deformed polymer. A craze can contain 20deformed polymer. A craze can contain 20--90% voids 90% voids the rest being fibrils which are threadlike elements that the rest being fibrils which are threadlike elements that make up the structure of fibers make up the structure of fibers

•• Unlike actual cracks, crazes and shear bands are capable Unlike actual cracks, crazes and shear bands are capable of supporting stresses because of the oriented polymer of supporting stresses because of the oriented polymer involvedinvolved

•• Critical Stress Critical Stress

KKICIC= Plane strain fracture toughness= Plane strain fracture toughnessY = constant to accommodate the geometryY = constant to accommodate the geometryc = crack lengthc = crack lengthif if σσff<<σσcc then fracture will not occur then fracture will not occur

cYK IC

c .πσ =

•• Fracture toughness can be increased by various Fracture toughness can be increased by various inclusions e.g. rigid fibers or particlesinclusions e.g. rigid fibers or particles

•• These can spread the applied force over a larger These can spread the applied force over a larger zonezone

•• Heterophase systems (partly crystalline and Heterophase systems (partly crystalline and partly amorphous) are used when toughness is a partly amorphous) are used when toughness is a major criterionmajor criterion

Fibers Fibers

•• They exhibit viscoelastic behavior like They exhibit viscoelastic behavior like amorphous polymersamorphous polymers

•• Viscoelasticity: A combination of viscous and Viscoelasticity: A combination of viscous and elastic properties in a material with the relative elastic properties in a material with the relative contribution of each being dependent on time, contribution of each being dependent on time, temperature, stress and strain rate temperature, stress and strain rate

Crystalline fibers have high melting temperature Crystalline fibers have high melting temperature due to a polar structure e.g. rayon, nylon, due to a polar structure e.g. rayon, nylon, polyesters, acrylics, cotton, wool and silk contain polyesters, acrylics, cotton, wool and silk contain ester, amide or hydroxyl groups that can form ester, amide or hydroxyl groups that can form hydrogen bondshydrogen bondsMoisture and heat will have a large effect on the Moisture and heat will have a large effect on the physical properties of fibers physical properties of fibers

RubberRubber

Two types of rubberTwo types of rubber–– natural and styrene natural and styrene –– butadiene butadiene rubberrubberNatural rubber crystallizes on stretching even though it Natural rubber crystallizes on stretching even though it is above its melting point and the crystallites melt on is above its melting point and the crystallites melt on release of stress. It is release of stress. It is self self –– reinforcingreinforcing i.e. it is i.e. it is stronger at highly stressed pointstronger at highly stressed pointAddition of fillers (e.g. carbon black) increases the Addition of fillers (e.g. carbon black) increases the tensile strength only slightly for self tensile strength only slightly for self –– reinforcing reinforcing polymers. But tear strength and abrasion resistance are polymers. But tear strength and abrasion resistance are very much improved very much improved

Rubber (contd.)Rubber (contd.)For styrene For styrene –– butadiene rubber addition of fillers butadiene rubber addition of fillers like carbon black increases the tensile strength like carbon black increases the tensile strength more than for natural rubbermore than for natural rubber

CompositesComposites

Composite materials are combination of Composite materials are combination of materialsmaterialsThey are made by combining two or more They are made by combining two or more materials in such a way that the resulting materials in such a way that the resulting material has certain desired properties e.g. glass material has certain desired properties e.g. glass fiber reinforced plastics (GRP)fiber reinforced plastics (GRP)Polymers when combined with glass fibers result Polymers when combined with glass fibers result in Polymer Matrix Composites (PMC)in Polymer Matrix Composites (PMC)

Composites (contd.)Composites (contd.)

Composites (contd.)Composites (contd.)

Composites (contd.)Composites (contd.)