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L06 07 DMA and Polymer Failure
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Transcript of L06 07 DMA and Polymer Failure
2/7/2015
1
POLYMERSIntroductionMicrostructureMechanical PropertiesViscoelastic BehaviorTime Temperature SuperpositionDynamic Mechanical AnalysisPolymer Failure
MEL 844 Designing with New Materials
Department of Mechanical Engineering, IIT Delhi
Dynamic Mechanical Analysis (DMA)
MEL844 2
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Dynamic Mechanical Analysis (DMA)
MEL844 Designing with New Materials 3
๐ธโฒis Storage Modulus๐ธโฒโฒ is Loss Modulus
Kelvin model
๐ธโ = ๐ธ + ๐๐๐ก
Dynamic Mechanical Analysis (DMA)
MEL844 Designing with New Materials 4
๐ ๐ก = ๐ ๐ ๐๐๐๐๐๐ก = ๐๐ cos ๐๐ก
๐ ๐ก = ๐ ๐ ๐๐๐ธโ๐๐๐๐ก
๐ ๐ก = ๐๐ ๐ธโฒ ๐ cos ๐๐ก โ ๐ธโฒโฒ ๐ sin ๐๐ก
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Dynamic Mechanical Analysis (DMA)
MEL844 Designing with New Materials 5
Dynamic Mechanical Analysis (DMA)
MEL844 Designing with New Materials 6
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Strength of Polymers
Strength-limiting processes know in polymers
1. Brittle fracture
2. Cold drawing
3. Shear banding
4. Crazing
5. Viscous flow
MEL844 Designing with New Materials 7
MEL844 Designing with New Materials 8
1. Brittle fracture 2. Cold drawing
โข < 0.75 TG polymers are brittleโข Low toughnessโข Stress concentration like cracks,
notches, sharp section changes are dangerous
โข About 50ยฐC below TG, thermoplastics become plastic
โข Yields at about 0.1 strain, then draws
โข Chains unfold (crystal) or untangle (amorphous)
โข Starts to neck, then spreads throughout
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MEL844 Designing with New Materials 9
3. Crazing 4. Shear bandingโข Visible white streaks on cheap plasticsโข High TG polymers (PS) crazeโข Small crack shaped regions with
drawn polymer (ligaments) that link craze surfaces
โข Crack usually start from a craze and propagates
โข Large plastic strainsโข Finite strain occurs in each shear
bandโข Occurs due to deviatoric stresses,
unlike crazing that occurs due to dilatational stresses
Strength Diagram for Polymers
MEL844 Designing with New Materials 10
PMMA
Strength is less understood than stiffness
At low temperature, there is brittle failure, estimated using fracture mechanics
Depends on strain rateand temperature
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StressโStrain Behavior of Polymers
11MEL844 Designing with New Materials
Variation in stress strain behaviour in different polymers
Temperature Dependent
MEL844 Designing with New Materials 12
Brittle epoxy
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Strain Rate Dependent
MEL844 Designing with New Materials 13
Brittle epoxy
Fatigue Failure
May or may not have endurance limitHigh frequency can lead to excessive heat build up
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Creep Failure
Larson-Miller Theory
๐ฟ๐๐ = ๐ ๐ถ๐๐ + log10 ๐ก๐ = ๐ ๐
LMP - Larson-Miller parameterT - TemperatureC โ constanttr - time to rupture
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Cumulative Creep Damage
๐ก1 โ ๐ก0๐ก๐1+๐ก2 โ ๐ก0๐ก๐2+โฏ =
๐=1
๐๐ก๐ โ ๐ก๐โ1๐ก๐๐
= 1
๐ก๐โ๐ก๐โ1
๐ก๐๐is the increment of creep damage
๐ก๐๐ is critical time at respective load levels
Minerโs Rule