Presentation on theme: "Day 29: Mechanical Behavior of Polymers"— Presentation transcript:
1 Day 29: Mechanical Behavior of Polymers ReviewHow are Properties DefinedIntroduction to ViscoelasticitySimple Material ModelsStrain Rate and Temperature Effects
2 Review Basic definitions: thermoplastic, thermoset, elastomer. Let’s talk about the kind of mechanical behavior seen in polymers.Stiffness, EStrengthDuctilityFactors which can determine the strength of a polymer.
3 Let’s remember some particular polymers PlusDeltaPEPCPSTeflonPPPIPVCPBNylonSBSKevlarABSPMMAEpoxyImportance of fiber. What does it take for a polymer to form fiber?
4 Different Types of Mechanical Behaviors in Polymers A=brittleB=elastic/plasticC=elastomericFocus on this one today
5 Mechanical Properties i.e. stress-strain behavior of polymersbrittle polymerFS of polymer ca. 10% that of metalsplasticelastomerelastic modulus – less than metalAdapted from Fig. 15.1, Callister 7e.Strains – deformations > 1000% possible (for metals, maximum strain ca. 10% or less)
7 T and Strain Rate: Thermoplastics (MPa)• Decreasing T...-- increases E-- increases TS-- decreases %EL• Increasingstrain rate...-- same effectsas decreasing T.20468Data for the4°Csemicrystallinepolymer: PMMA20°C(Plexiglas)40°Cto 1.360°Ce0.10.20.3Adapted from Fig. 15.3, Callister 7e. (Fig is from T.S. Carswell and J.K. Nason, 'Effect of Environmental Conditions on the Mechanical Properties of Organic Plastics", Symposium on Plastics, American Society for Testing and Materials, Philadelphia, PA, 1944.)
8 Effects of Strain Rate and Temperature stressIncreasing strain rateIncreasing tempstrain
11 Time Temp DependencePlastic deformation of polymers involves chain uncoiling and chain slidingIncreasing temperature increases relative space between chains and makes uncoiling easier.Slowing the strain rate means there is more time for chain reconfiguration.
12 Introduction to Viscoelasticity Some features that are observed in polymeric materials that do not seem to be noticeable in metals or ceramicsMechanical properties depend on TemperatureMechanical properties depend on Strain RateCreep (noticed in metals at high temperatures)Stress RelaxationHysteresis
13 CreepTake a tension specimen made from a polymer and and put on a series of constant stresses on it.We observeCreep: Progressive strain (deformation) over time at constant stress (load), usually at high temperatures
14 Creep TestWe instantly load with constant stress for a certain time, and instantly unload.Note that both linear elastic and viscous fluid behaviors are present.Note that there seems to be some residual strain at the end, i.e. the material does not completely recover. There is both elasticity and plasticity.
16 Write down two examples of parts that see constant tensile or bending load.
17 Stress RelaxationThink of a polymer specimen loaded with a constant strain.Note that both linear elastic and viscous fluid behaviors are present.Note that there seems to be some residual stress at the end, i.e. the material does not completely recover. There is both elasticity and plasticity.Stress Relaxation: Progressive loss of stress (load) over time under constant strain (deformation), usually at high temperatures
19 Write down two examples of parts that see constant strain.
20 Time Dependent Deformation • Stress relaxation test:• Data: Large drop in Erfor T > Tg.(amorphouspolystyrene)Adapted from Fig. 15.7, Callister 7e. (Fig is from A.V. Tobolsky, Properties and Structures of Polymers, John Wiley and Sons, Inc., 1960.)1031-1-3560100140180rigid solid(small relax)transitionregionT(°C)TgEr (10s)in MPaviscous liquid(large relax)-- strain to eo and hold.-- observe decrease instress with time.timestraintensile testeos(t)• Relaxation modulus:• Sample Tg(C) values:PE (low density)PE (high density)PVCPSPC- 110- 90+ 87+100+150Selected values from Table 15.2, Callister 7e.
21 Effect of Temperature: Glass Transition Temperature Or why does Garden Hose behave the way it does?
22 Melting vs. Glass Transition Temp. What factors affect Tm and Tg?Both Tm and Tg increase with increasing chain stiffnessChain stiffness increased byBulky sidegroupsPolar groups or sidegroupsDouble bonds or aromatic chain groupsRegularity – effects Tm onlyAdapted from Fig , Callister 7e.
24 HysteresisPolymers often don’t load and unload on the same line on the stress-strain curve.The difference in areas under those curves represents energy loss (often to heat).This means that polymers can have inherent energy damping.This means plastic springs may not be as good an idea as plastic dampers.
25 Sinusoidal Response Tests We have a polymer specimen experiencing a sinusoidal loading.Note that there is a phase shift, and that there is also hysteresis indicating that energy is being dissipated cyclically. This all suggests some simple material models.
28 TakeawaysYield and Ultimate Strength are defined differently for polymers.Polymers have time and temperature dependent properties (viscoelasticity)CreepStress RelaxationTg, TmHysteresis
29 Maxwell ModelHere is an alternative to the simple spring model of linear elasticity. Add a damper. This gives what is called as the Maxwell model.In the limit, it’s a fluid!strainstressStress relaxation is not badCreep not too good!timetime
30 Kelvin-Voigt ModelTry putting the spring and damper in series! This gives the Kelvin-Voigt model.In the limit, it’s a solid!strainstressDoesn’t really show stress relaxation!timetime
31 Shows both creep and stress relaxation! Standard Linear SolidFurther improvement is possible.Shows both creep and stress relaxation!stressstraintime
32 Stress Strain Relationships We can get stress from strain history and strain form stress history through the following heriditary relationships.K is creep modulus, and F is the relaxation modulus.
33 Examples of These Time Dependent Moduli MaterialCreep ModulusRelaxation ModulusMaxwellKelvin VoigtStandard Linear SolidH(t) is the unit step function. d(t) is the Dirac delta function
34 More on the material models Testing needs to be done to fit the parameters of the model to the behavior of an actual material.Note the fact that the history of the material must be recorded to be able to complete the calculations.Some additional complexity. The parameters in the creep modulus and relaxation modulus areTemperature DependentStrain Rate Dependent
35 Summary Very complex behavior! Difficult to model. Great sensitivity to temperature.Great sensitivity to strain rate.
36 Tensile Response: Brittle & Plastic (MPa)fibrillar structurenearfailureInitialNear Failurexbrittle failureonset ofcrystallineregions alignneckingplastic failurexcrystallineregionsslideamorphousregionselongatealigned,cross-linkedcasenetworkedunload/reloadesemi-crystallinecaseStress-strain curves adapted from Fig. 15.1, Callister 7e. Inset figures along plastic response curve adapted from Figs & 15.13, Callister 7e. (Figs & are from J.M. Schultz, Polymer Materials Science, Prentice-Hall, Inc., 1974, pp )
37 Tensile Response: Elastomer Case (MPa)final: chainsare straight,stillcross-linkedxbrittle failureStress-strain curves adapted from Fig. 15.1, Callister 7e. Inset figures along elastomer curve (green) adapted from Fig , Callister 7e. (Fig is from Z.D. Jastrzebski, The Nature and Properties of Engineering Materials, 3rd ed., John Wiley and Sons, 1987.)plastic failurexxelastomerinitial: amorphous chains arekinked, cross-linked.Deformationis reversible!e• Compare to responses of other polymers:-- brittle response (aligned, crosslinked & networked polymer)-- plastic response (semi-crystalline polymers)
38 Thermoplastics vs. Thermosets Callister,Fig. 16.9TMolecular weightTgTmmobileliquidviscousrubbertoughplasticpartiallycrystallinesolid• Thermoplastics:-- little crosslinking-- ductile-- soften w/heating-- polyethylenepolypropylenepolycarbonatepolystyrene• Thermosets:-- large crosslinking(10 to 50% of mers)-- hard and brittle-- do NOT soften w/heating-- vulcanized rubber, epoxies,polyester resin, phenolic resinAdapted from Fig , Callister 7e. (Fig is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 3rd ed., John Wiley and Sons, Inc., 1984.)
39 Predeformation by Drawing • Drawing…(ex: monofilament fishline)-- stretches the polymer prior to use-- aligns chains in the stretching direction• Results of drawing:-- increases the elastic modulus (E) in thestretching direction-- increases the tensile strength (TS) in the-- decreases ductility (%EL)• Annealing after drawing...-- decreases alignment-- reverses effects of drawing.• Compare to cold working in metals!Adapted from Fig , Callister 7e. (Fig is from J.M. Schultz, Polymer Materials Science, Prentice-Hall, Inc., 1974, pp )
Your consent to our cookies if you continue to use this website.