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Mechanical Properties

Mechanical Properties of Viscoelastic Materials

Stress / Strain Behavior Creep Toughness Reinforcement, Fillers, Modifiers

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Properties and product performance

MaterialsMaterials•PropertiesProperties•AvailabilityAvailability•CostCost

The successful design of a product depends on the synergy of the design, manufacturing and materials

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Why look at polymer chemistry, structures and properties ?What we really want to learn is ….

How to make a plastic part that delivers the required life cycle performance at the best cost and meets regulations

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ProductPerformanceGOAL =

Plastics – from chemistry to performance

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Polymer Polymer ChemistryChemistry

MaterialMaterialmicrostructuremicrostructure

MaterialMaterialpropertiesproperties

ProductProductPerformancePerformance

Intermolecular Attraction Forces

The performance of a plastic part depends on the attraction forces between polymeric chains

These forces increase as chain length increases

These forces are stronger as the chain to chain distance decreases

Force 1/ d & Force n 6

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Bonding Energy & Distance

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Bond energy

Bond length7

Intermolecular Attraction Forces Attraction ForcesForce 1/ d

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Inte

rmole

cula

r fo

rces

Intermolecular distance, d

Intermoleculardistance, d

d

Intermolecular Attraction ForcesForce n

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Inte

rmole

cula

r fo

rces

Degree of polymerization, n

Mechanical Properties & MW

Higher MW means stronger intramolecular interactions which means better mechanical properties

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Intermolecular forces

Molecule length to molecular weight

Differences in Properties

Leathery More soluble Transparent Low shrinkage Tough

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Amorphous Melt Less soluble Opaque High shrinkage Rigid

Crystalline

Mechanical Properties - stiffness

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Amorphousst

iffness

temperature

Tg

Tg – glass transition temperature

Mechanical Properties - stiffness

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Temperature

stiff

ness

Semi crystalline

Tg

Tm – melt temperature

Tm

Mechanical Properties - stiffness

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Tm

Tm

Tg

stiff

ness

Temperature

Amorphous Amorphous plasticplastic

Semi crystalline

plastic

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Physical Physical PropertiesProperties

15%GF Polyester, PBT

1400 nylon 6/6 13%GF

DensityDensity 0.0509 lb/in³ 0.0444 lb/in³

Water Water AbsorptionAbsorption

0.1 % 1.1 %

Linear Linear Mold Mold ShrinkageShrinkage

0.005 in/in 0.006 in/in

Plastic Mechanical Properties

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Secondary Bonds These bonds are physical in nature, there

are no chemical changes happening and they are weaker than chemical bonds

Hydrogen BondsEntanglementVan der Waals

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Increasingstrength

Intermolecular Attraction ForcesThe performance of a plastic part depends on the

attraction forces between polymeric chainsThese forces increase as chain length increasesThese forces are stronger as the chain to chain distance

decreases

Force d & Force n

Elastic Behavior of SolidsStress: Applied force per unit areaStrain: Displacement of sampleStress/Strain = E (Young’s Modulus)Large modulus E : Stiff materialsConstant modulus--linear S/S curve: Hookean material (like most metals or ceramics)

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Stress/Strain CurveLinear Elastic Material

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Types of Forces Pulling on the end: Tensile

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F

F

Types of Forces Rotational: Torsion

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T

T

Types of ForcesPushing and sliding: Shear

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Stress Testing Tensile Test

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AF

L

Stress, = F / A

Strain, = L / L

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Tensile Testing Results Stress vs Strain

For plastics the rate of stress applied affects the material’s response

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Elastic Behavior of SolidsStress: Applied force per unit areaStrain: Displacement of sampleStress/Strain = E (Young’s Modulus)Large modulus E : Stiff materialsConstant modulus--linear S/S curve: Hookean material (like most metals or ceramics)

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Solid MaterialsA Solid can be defined as a state of the material where the

deformation of the part is a function of the load applied to it

= f (force)Elastic behavior - Small deformations then return to

original shapeVirtually all applied energy retained and used to reboundForces typically normalized for sample area

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Mechanical Response as a function of Time

‹#›

F

F

F

time

input

time

displacement

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Elastic Solid Model

‹#›

F

F

k

Spring constantor stiffness

k

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Elastic Solid – Microstructural Behavior

‹#›

The applied force straightens polymer chain segments

FF

The polymer chain segments return back to a more disorder and stable configuration when force is removed

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Viscous Behavior A fluid can be defined as a state of the material where

the RATE of DEFORMATION of is a function of the load applied

d dt = f (force)

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Viscous BehaviorTypically applied to liquids; arises from entanglementFlow Resistance = ViscosityStress causes velocity gradient with time and

distance: Shear Rate Stress = Viscosity * Shear rate, for a Newtonian liquid

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Viscous BehaviorTypically applied to liquids; arises from

entanglementFlow Resistance = ViscosityStress causes velocity gradient with time and

distance: Shear Rate Stress = Viscosity * Shear rate, for a Newtonian

liquid

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Cone and Plate

conepolymer

Stress and Shear Rate

Polymer melt

stationary plate

moving plate

force

Shear stress

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Newtonian Fluid Linear shear-rate with stress:

Slope = viscosity

Example – water

=shear stress=shear rate

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Newtonian / NonNewtonian Non-Newtonian (non-linear) types

Pseudoplastic: Shear-thinning , most plasticsDilatant: Shear-thickening

pseudoplasticpseudoplastic

dilatantdilatant

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Rheometry ExperimentsExperiment H Re-Grind PC Melt Rheology at 550 oF

Effects of Time and TemperatureCompared to other materials, the properties of plastic

are more sensitive to the time (how long) at which they are observed and measured

The properties are also sensitive to the temperature they are being observed and measured

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Elastic Solid – Microstructural BehaviorOne of the most microstructural features of polymers or

plastics is that they try to keep the level of disorder or entropy as high as possible

The preference for high entropy is the driving force for the polymer chains to spring back once the force is removed

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Mechanical Response as a function of Time

Fluid Like Behavior

F

time

time

input response

No recovery

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Viscous Fluid Model

F

F

ddt

C

Viscous DampingConstant

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force force

The polymer chain rub against the nearby chains. This frictional is proportional to the rate of deformation

Heat isgenerated

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The plastic part is subjected to a tensile force

F F

Lo , original length

F F

Lnew = Lo + L

The plastic part is increases its length L, when the force is removed it will not spring back – this a permanent deformation

The plastic part is increases its length L

Lnew = Lo + Lpermanent

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Mechanical Response as a function of Time Viscoelastic Like Behavior

F

time

time

input response

recovery

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Viscoelastic Solid Model

F

(t)

time

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Viscoelastic Solid, Microstructural Behavior

Polymer chain segments are stretched by the force, this is the elastic element of the model

Heat isgenerated

As the Polymer chain segments are stretched there is friction between these chain segments – this is the viscous damping element

When the force is removed, the chains return to the original state – during this motion, there is also friction

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General Viscoelastic Model

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F

CE

KE

CP

KE – elastic stretching of

chain segmentsCE – friction between chain

segments (very small)CP – friction between

complete polymer chains

Maxwell Viscoelastic Model

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F

KE

CP

KE – elastic stretching of

chain segmentsCP – friction between

complete polymer chains

Viscoelastic BehaviorContinuum of liquids and solids is continuum of viscous to

elastic behaviorDisentanglement is time dependent

Elastic and Viscoelastic materials tend to be stiffer at high shear rates (short time)

Viscous properties: energy dissipation in the mass--long range, long time

Elastic properties: Molecular stretching, bending--short range, short time

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Mechanical Response & Intermolecular Forces The same plastic can have the mechanical response of

An Elastic SolidA Viscoelastic SolidA Viscoelastic FluidA Viscous Fluid

The particular mechanical response depends on the intermolecular forces

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Deborah’s NumberThe Deborah number is a dimensionless number, used in

rheology to characterize how "fluid" a material is.It is defined as the ratio of a relaxation time, characterizing

the intrinsic fluidity of a material, and the characteristic time scale of an experiment

The smaller the Deborah number, the more fluid the material appears.

De = relaxation time / observation time

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Effects of Time and Temperature – Silly Putty

When heated, it becomes more like a viscous fluidThe longer a load is applied, the more it will act like a

viscous fluid When the temperature is lowered, it becomes more like

a solidThe shorted the load is applied, the more it will act as a

solid

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time

Tension in a part

Relaxation curves

Increasing temps

Initial Tension

to

Length after time to

1

2

3

3

2

1

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Stress Relaxation

Mechanical Response and Intermolecular Forces

The same plastic can have the mechanical response ofAn Elastic SolidA Viscoelastic SolidA Viscoelastic FluidA Viscous Fluid

The particular mechanical response depends on the intermolecular forces

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Mechanical Response and Intermolecular Forces

When the intermolecular forces are very high, the chains are held together tightly

The only possible motion is the stretching and spring back of short chain segments

Therefore the plastic acts as an Elastic Solid

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Mechanical Response and Intermolecular Forces

When the intermolecular forces are not so strong, the chains are held together less tightly and they are more separated

When force is applied, longer segments can stretch and there is friction between these chains – Viscoelastic Solid

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Mechanical Response

The mechanical response a plastic part depends on intermolecular forces % of crystallinity Temperature Hydrogen Bonds Molecular Weight Chain to chain distance Entanglements

Temperature and Mechanical Response Mechanical Response & Tg

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Load Rate & Mechanical Response Stress / Strain Curve

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Increase of Strain rate

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Dynamic Mechanical Analysis

From TA Instruments

Dynamic Mechanical Analysis

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Dynamic Mechanical Analysis

From TA Instruments

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Dynamic Mechanical Analysis

From TA Instruments

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Dynamic Mechanical AnalysisViscous elastic response

From TA Instruments

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Viscous elastic response

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Viscous elastic response

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Dynamic Mechanical Analysis

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CreepSmall, constant load, long timeResults from stretching and uncoiling /disentanglementOpposed by strong intermolecular forces and

crosslinkingIt’s a function Temperature, time, load

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forceforce Heat

Chains flow by each other

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Creep Viscoelastic Model

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(t)

timeCritical time

F

CE

KE

PermanentDeformation –creep

For this load, the viscous motion has started

For this load, there has not been enoughtime to start the viscous motion

Creep – Temperature, Time and Load The critical time is f(temperature, load) The higher the Temp, the lower tcritical – this

is because the higher temp makes the material more viscous like

The higher the Load, the lower tcritical – This is because higher loads can start whole displacement in shorter periods of time

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Creep – time, temp loads

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Impact Strength and ToughnessToughness: Absorb energy without breakingRelated to area under stress/strain curveToughness experiments mostly short time,

e.g., impact strength

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Plastic Toughness The amorphous region can deform more and

absorb more energy

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crystalline

amorphous

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Depends on materialability to absorb energy

Stress/strain curve

Area underneath Stress/strain curve is the measure of impact

strain

stress

Toughness is not StrengthTough: High elongation, low modulus.

High MW

Low Intermolecular StrengthRubber, slight cross linking

Brittle: Low elongation, high modulusCrystallineHigh degree of Xlinked rigidHigh Intermolecular Strength

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Degree of Crosslinking & Toughness

ToughToughStrongStrong

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Small AdditivesGet betweenPolymer chains

This increases d andcan make the degradeproperties

Example – excess of colorant can weaken a plastic partExample – excess of colorant can weaken a plastic part

ReinforcementsDifferent polymer chains are

attracted to the reinforcement

Works as a bridge to attract polymer chains that normally would not interact

This makes the properties better

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Melt Flow Rate(ASTM D1238)

Given a resin's MFR,will the part fill properly?

• Test conditions are not real world processing conditions

• Different weights and temperatures used for different resins

- Comparison of different resins is not 1 to 1

- Only relative comparisons are possible

• Single-Point Data vs. Rheological Curves

Len Czuba

August 2006

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Melt Flow Rate

Test Apparatus:

Resin

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Melt Flow Index# grams of flow per 10 minutes

Weighted Plunger

Barrel

MoltenPellets

Extrudate

Orifice

Heater Band

Dynisco LMI 4000Len Czuba

August 2006

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Tensile Strength(ASTM D638)

• Cross-head speed not standardized

• Specimen thickness can be anything up to 0.55"

• Specimen gating not standardized

• How did the specimen fail:

- Ductile ?

- Brittle ?

Len Czuba

August 2006

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Tensile Strength(ASTM D638)

•Ultimate tensile strength

•Tensile modulus

•Tensile elongation

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Tensile Strength

Test Apparatus:

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Impact ResistanceASTM D256

Is this a relevant impact testfor your device?

• Five Different Methods - Izod (Methods A, C, and D)

- Charpy (Method B)

- Unnotched (Method E)

• Cannot correlate results from different methods

• Specimen toughness highly dependent on notch size

• Specimen preparation not standardized

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Impact Resistance

Test Apparatus:

Len Czuba

August 2006

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Impact Resistance

Test Specimen:

Len Czuba

August 2006

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Summary

Elastic, Viscoelastic, ViscousStretch/bend vs entanglementTensile, Compressive, Flexural, Torsional, ShearStress/strain performanceStrength/toughnessEffect of modification on properties

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Summary Plastic materials behave as elastic solids, viscous fluid or

a combination of both The mechanical behavior of plastics depends on factors

such as:Intermolecular forcesTemperatureTime load is applied

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SummaryThere are many important mechanical properties that

must be considered for processing and use, such asTensile strengthImpact ResistanceCreepUse TemperatureProcessing Temperature

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Design ExampleReduce Costs of the

system without reducing quality or compromising safety

This part works mostly in bending

= Mc / I old

Load and Material Interaction Normally, we want the material property to

be higher than the value actually applied to the material – example yield stress material yield stress > applied stress Safety Factor = material yield / load

Caution – loads and property values are probabilistic not deterministic

Load and Material Interaction

Failureprobability

Load ave = 6000, std dev = 1000PET ave = 12000, std dev = 1000

Example - Material Properties & Loads

Load ave = 6000, std dev = 2000PET ave = 12000, std dev = 1000

Solution 1 - use the same part for another application

Load ave = 6000, std dev =1000PET ave = 12000, std dev 1500

Solution 2 - get cheaper materials

Load ave = 6000, std dev = 1000Regrind PET ave = 9500, std dev = 1200

Solution 3 - get cheaper materials, use regrind

25% regrind 75% virgin

Solution 4 - get cheaper materials, use regrind + virgin

Solution 4 - get cheaper materials, use regrind + virgin

Solution 4 - get cheaper materials, use regrind + virgin

Solution 5 - Redesign Part The stress depends on the MC / I

where C - is the distance from the neutral axis & I is the moment of inertia of area

We can redesign the part to reduce the C / I ratio so that even with the if M is the same, we reduce the stress

Solution 5 - Redesign Part Existing cross section New cross section

= Mc / I new

Possible SolutionsAdd a great % of virgin material, or use 100 % virgin

Good Quality ControlIncreases Solid Waste ProblemIncreases Production Costs, not added

valueNo new jobs are created by importing

resin

Possible SolutionsIncrease the use of regrind and redesign the part

Reduces Solid Waste VolumeReduces CostsCan generate more jobs in PRReduces production time (material sources

are closer by )

Design SummaryMost recycled materials will have lower properties than

virgin materialsVirgin material can be combined with recycled material

to improve propertiesIncreasing the use of regrind reduces costs

Design SummaryThe best way to take advantage of the low cost of

recycled and compensate for the lower performance is by redesign of the part

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