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Topic 2 Part 1
Polymer PropertiesCharacterization
Mechanical Properties In order to facilitate comparisons with the behaviour of
other classes of materials the approach taken was torefer to standard methods of data presentation such asstress-strain graphs.
It is important to note that when one becomes involved inengineering design with plastics, such graphs are oflimited value.
The reason is that they are the results of relatively short-
term tests and so their use is restricted to quality controland, perhaps, the initial sorting of materials fin terms ofstiffness, strength etc.
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The modulus obtained from a short-term test would notpredict accurately the long-term behaviour of plasticsbecause they are viscoelastic materials.
The viscoelasticity means that quantities such asmodulus, strength, ductility and coefficient of friction aresensitive to straining rate, elapsed time, loading history,temperature, etc.
The manufacturing method used for the plastic product
can create changes in the structure of the material whichhave a pronouced effect on properties.
Therefore, the behaviour of the mouldedproduct is different from the behaviour of amoulded test-piece of the same material.
The time dependent change in thedimensions of a plastic article whensubjected to a constant stress is calledcreep.
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For most traditional materials, theobjective of the design method is todetermine stress values which will notcause fracture.
However, for plastics it is more likely thatexcessive deformation will be the limitingfactor in the selection of working stresses.
Tensile Properties
In typical tensile test, a polymer sample, in theform of a dogbone, is clamped at one end andpulled at a constant rate of elongation at theother clamped end.
The thinner portion of the tensile specimenencourages the sample to fail at the center ofthe bar, where the stress is the highest, and notat the grip sites, where stress concentration mayotherwise result in premature failure.
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The initial length of a central section containedwithin the narrow region of the tensile specimenis called the initial gage length, Lo.
During the deformation, force, F, is measured asa function of elongation at the fixed end bymeans of a transducer. Usually, the tensileresponse is plotted as engineering stress, versus engineering strain, ,
oL
L=
oA
F=
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The Ao is the original (undeformed) cross-sectional areaof the gage region and L is the change in sample gagelength (L-Lo) due to the deformation.
Sample length can be determined from instrumentalsettings of the mechanical-testing instrument or by anextensometer which is a strain gage that is attached tothe gage-length region of the tensile specimen.
Alternately, the stress-strain response of a sample maybe reported in terms of true stress and true strain. Thetrue stress is defined as the ratio of measured force to
the actual cross-sectional area, A, at a given elongation
A
FT=
Since the actual cross-sectional areadecreases as the sample is elongated, the truestress will always be larger than theengineering stress.
Hookes law for an ideal elastic solid provides arelationship between stress and strain fortensile deformation, where the proportionalityfactor, E is called the tensile (or Youngs)modulus,
E =
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Strain-Stress Plot
As shown in the plot, only the initial portionfollows Hooken behaviour.
The point at which stress begins to deviatefrom a linear stress-strain relation is calledthe proportional limit.
At temperatures below Tg, all glassy materials,polymeric as well as low-molecular weightsubstances, have approximately the same valueof modulus (1 GPa).
At first, this modulus slowly decreases withincreasing temperature and then rapidlydecreases in the region of Tg.
The Tg glass transition is the temperature wherethe polymer goes from a hard, glass like state toa rubber like state.
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For low-molecular weight materials, moduluscontinues to fall rapidly with increasingtemperature.
For high-molecular-weight amorphous polymers,modulus drops to a secondary plateau region(approx 1 MPa) called the rubbery plateau.
With further increase in temperature, themodulus again rapidly drops. This point marksthe viscous flow region.
Polymers are typically melt-processed inthis temperature range in which theviscosity is also low.
The appearance of a rubber plateau forhigh-molecular weight polymers is theresult of the formation of entanglements.
Entanglements prevent slippage attemperatures immediately above Tg and,therefore, modulus remains relatively high.
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Above Tg, the entanglements are easily disassociateddue to high kinetic energy and the modulus drops.
The temperature behaviour of the modulus ofsemicrystalline polymers is qualitatively similar to that ofhigh-molecular weight amorphous polymers except thatthe modulus is typically higher in the secondary plateaudue to the reinforcing effect of crystallites dispersed in an
amorphous rubbery phase at temperature above Tg butbelow Tm. At Tm, the crystallites melt and the modulusdrops in the viscous-flow region.
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Refer to book 192-194 for explanation
Stress-Strain Curve for PMMA
Stress (MPa)
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Different of tensile strength of
machining direction
transverse direction
Flexural Test
Flexural test needed especially forstructural support, eg beam, flooringdecking and etc.
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Flexural Strength (fM)Maximum flexuralstress sustained by the test specimen during abending test.
Some materials that do not break at strains of upto 5 % may give a load deflection curve thatshows a point at which the load does notincrease with an increase in strain, that is, ayield point, Curve B), Y. The flexural strength
may be calculated for these materials by lettingF equal this point, Y.
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Impact Test
Impact tests measure the energy expended upto failure under conditions of rapid loading.
There are a number of different types of impacttests. These include the widely used Izod andCharpy tests in which a hammerlike weightstrikes a specimen and the energy-to-break isdetermined from the loss in the kinetic energy ofthe hammer.
Other variations include the falling ball or darttest, whereby the energy-to-break is determinedfrom the weight of the ball and the height fromwhich it is dropped.
Normally impact strength is calculated as
)/(sectionnotchatArea
breakEnergy tostrengthImpact 2mJ=
)/(sectionnotchatDistance
breakEnergy tostrengthImpact mJ=
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Information obtained from impact tests may beused to determine whether a given plastic hassufficient energy-absorbing properties to beuseful for a particular application, such asplastics for beverage bottles or windowreplacement.
The impact strength will decrease withdecreasing temperature and with increasing rateof deformation.
The presence of defects that act as stressconcentrators will also reduce impact strength.
In order to standardize impact results or to study theeffect of cracks and other defects on impact properties,samples with inscribed notches of specified dimensionsare often used.
Brittle polymers, such as polystyrene, have very lowimpact strengths, while many engineering thermoplastic(e.g. polycarbonate) are very impact resistant.
Generally, amorphous polymers with large bulky
substituent groups and nonlinear backbones are brittle.Unoriented crystalline structure also contributes tobrittleness in polymers whose Tg is above the testingtemperature.
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Brittle polymers can be made to be more impactresistant by dispersing small (< 0.1 m diameter)rubber particles within the polymer matrix, as inthe case of high-impact polystyrene (HIPS) andABS resins.
Good adhesion between the rubbery inclusionsand brittle matrix polymers is important for highimpact resistance and is typically achieved by
grafting the rubber and matrix (glassy) polymers.
Effect of Notch tip radius on Impact Strength
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This can be explained by considering thetotal impact strength as consisting of bothcrack initiation and crack propagationenergy.
When the very sharp notch (0.25 mmradius) is used it may be assumed that theenergy necessary to initiate the crack issmall and the main contribution to theimpact is the propagation energy.
Effect of Test Temperature on Impact Strength
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Creep test
Since the tensile test hasdisadvantages whenused for plastics, creeptests have evolved as thebest method ofmeasuring thedeformation behaviour ofpolymeric materials.
In these tests a constantload is applied to the
material and the variationof strain with time isrecorded.
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Fatigue Testing
Fatigue tests are used to determine the numberof cycles (N) of applied strain at a give level ofstress that a sample can sustain beforecomplete failure.
This number of cycles to failure is called thefatigue life.
The endurance limit is the maximum value ofapplied stress for which failure will not occurirregardless of how many cycles the stress isapplied.
Typically, the value of stress leading to failure ata given N is 20% to 40% of the static tensilestrength.
The fatigue life decreases with increasingfrequency of oscillation and as temperature isdecrease.
Information obtained by means of fatigue tests isextremely important in evaluating engineering
and composite materials considered for loadbearing applications or when frequent periodicstress loading may be encountered, as forexample in a plastic hinge joint.
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