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Viscoelastic Properties of Wood Fiber Viscoelastic Properties of Wood Fiber Reinforced Polyethylene (WFRP): Stress Reinforced Polyethylene (WFRP): Stress Relaxation, Creep and Threaded JointsRelaxation, Creep and Threaded Joints
Syed Imran FaridProf. J. K. Spelt, Prof. M. T. Kortschot and Prof. J. J. Balatinecz
S. Law and A. Akhtarkhavari
Department of Mechanical & Industrial EngineeringDepartment of Chemical Engineering & Applied ChemistryAll Information in this presentation is the property of University of Toronto and Researchers
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OutlineOutline
Introduction Theoretical Experimental Results Modeling and Discussion Conclusion
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IntroductionIntroduction Wood Fiber Reinforced Polyethylene (WFRP)
Environmental - recycling Economical - cost, availability Mechanical properties - strength, stiffness Processing
Applications Structural application Automotive interior application
Operating condition Service life ~ 10-25 years Operating temperature ~ 60oC
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IntroductionIntroduction
Problem Short and long-term threaded joints performance Long-term viscoelastic properties
Objective To Investigate the Viscoelastic Properties of To Investigate the Viscoelastic Properties of
Wood Fiber Reinforced Polyethylene: Stress Wood Fiber Reinforced Polyethylene: Stress Relaxation, Creep and Threaded JointsRelaxation, Creep and Threaded Joints
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ViscoelasticityViscoelasticity
Time and temperature dependent mechanical properties
Experimental approach Creep Stress Relaxation
Data Reduction Time-Temperature superposition
Modeling Physical models Constitutive equation
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ExperimentalExperimental Short-term joints performance
Pullout force D-6117 Stripping torque and force
Long-term threaded joints performance Clamping force relaxation Tightening torque relaxation
Viscoelastic properties Tensile stress relaxation E-328 Flexural creep D-790
Mechanical properties Tensile experiment D-638 Flexural experiment D-790
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Screw PulloutScrew Pullout
PULLOUT FIXTURE
LOAD CELL
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Screw RelaxationScrew Relaxation
FORCE
TORQUE
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Results - ViscoelasticityResults - Viscoelasticity
Relaxation modulus and creep compliance as a function of time.Stress relaxation ( ) and creep ( )
0
0.5
1
1.5
2
2.5
3
0 50000 100000 150000 200000Time (s)
Mod
ulu
s (G
Pa)
-2
-1
0
1
2
3
4
5
Com
pli
ance
(G
Pa
-1)
23 40 60 Creep 23 40 60
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Result - Stress RelaxationResult - Stress Relaxation
ln(Tensile Modulus) as a function of ln(Time) at 23oC and 0.5% Strain
2
3
4
5
6
7
8
9
10
2 4 6 8 10 12 14ln (Time)
ln (
Ten
sile
Mod
ulu
s)
WFRP LDPE Spruce
Slope = -0.0288
Slope = -0.0487
Slope = -0.0453
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Result - CreepResult - Creep
Creep compliance at various stress and temperature
6.5
7
7.5
8
8.5
2 4 6 8 10 12 14Time (s)
ln (
Fle
xura
l Com
plia
nce
(MP
a-1
)
25%,23 C 30%,40 C 30%,60 C 50%,40 C 50%,60 C
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Results - Fastener PulloutResults - Fastener Pullout
Pullout force for different fastener (a) F vs Fastener (b) F vs engagement Length
0
50
100
150
200
250
I1 I2 S1 S2 S1
Spec
ific
Pul
lout
For
ce (
N)
0
500
1000
1500
2000
2500
3000
3500
Pul
lout
For
ce
Specific Pullout (N/mm)Pullout Force (N)
0 5 10 15 20
Screw I Screw II
Spruce
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Threaded Joint - StrippingThreaded Joint - Stripping
Fastener stripping experiment (a) torque and force vs time (b) torque vs time
0 5 10Time (s)
I-1 I-2 S-1 S-2
0
500
1000
1500
2000
2500
3000
0 5 10Time (s)
Cla
mp
ing
For
ce (
N)
0
1
2
3
4
5
Dri
ving
Tor
que
(N/M
)
Load Torque
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Threaded Joints - RelaxationThreaded Joints - Relaxation
Clamping force relaxation at 23oC Simple relaxation( ) Retightening after 2 h ( )
0
400
800
1200
1600
2000
2400
0 50000 100000 150000 200000Time (s)
Cla
mpi
ng F
orce
(N
)
0
200
400
600
800
1000
1200
1400
16000.33Fpo 0.33Fpo 0.50Fpo 0.50Fpo
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Threaded Joints - RelaxationThreaded Joints - Relaxation
35%
53%
Clamping force relaxation as a function of time for Spruce and WFRP
0
200
400
600
800
1000
1200
0 50000 100000 150000 200000
Time (s)
Cla
mpi
ng F
orce
(N
)
Spruce WFRP
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Modeling - PhenomenologicalModeling - Phenomenological
Where
E(t) = Modulus at time t
A, B ER & EU = Constant depend on loading conditions
n, = Time exponent
E(t) =A + Btn Findley’s Law
E(t) =Btn Power Law Eqn
E(t) =A + B etn
E(t) =ER+ (EU+ ER) et/
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Modeling - ViscoelasticityModeling - Viscoelasticity
Experimental and calculated values using Power Law modelStress relaxation ( ) & creep ( X +)
5
6
7
8
9
2 4 6 8 10 12 14ln (Time)
Ln
(Ten
sile
Mod
ulus
)
3
4
5
6
7
8
9
ln (
Cre
ep C
ompl
ianc
e)
23C 40C 50C 23C 40C 60C Power Law
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Modeling - Clamping ForceModeling - Clamping Force
Experimental and calculated values for clamping force relaxation
5
5.5
6
6.5
7
7.5
2 4 6 8 10 12 14ln (Time)
ln (
Cla
mpi
ng F
orce
)
0.17Fpo 0.33Fpo 0.50 Fpo Calculated
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Modeling - Time Exponent (n)Modeling - Time Exponent (n)
0
0.02
0.04
0.06
0.08
0.1
0.12
290 295 300 305 310 315 320 325 330 335
Temperature (K)
Tim
e E
xpon
ent
Stress Relaxation Creep Clamping Force
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Time-Temperature SuperpositionTime-Temperature Superposition
0
200
400
600
800
1000
1200
-10 -5 0 5 10 15
ln (Time)
Ten
sile
Mod
ulus
50 C 40 C 23 C0
200
400
600
800
1000
1200
2 4 6 8 10 12 14
ln (Time)
Ten
sile
Mod
ulus
(M
Pa)
23 C 40 C 50 C
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Modeling - Long-Term creep Modeling - Long-Term creep
Long-term flexural creep experiment at 20% UFS
0
0.5
1
1.5
2
2.5
0.0E+00 5.0E+06 1.0E+07 1.5E+07 2.0E+07 2.5E+07
Time (s)
Stra
in (
%)
Strain
Calculated
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ConclusionConclusion Viscoelastic behavior was mainly controlled by matrix Higher dependence on temperature and loading conditions
than spruce Proposed model was in good agreement with experimental
data Modeling tertiary creep was not possible using Power Law Master curve was plotted and good superposition was
observed
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Conclusion – Cont’Conclusion – Cont’ Power Law model satisfactorily predicted long-term
creep Fastener pullout load was comparable than pullout
load in spruce Fastener load relaxation was higher in WFRP than in
spruce Retightening of screw results in memory effects and
lower relaxation was observed
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AcknowledgementAcknowledgement
Materials and Manufacturing Ontario Department of Chemical Engineering and
Applied Chemistry Faculty of Forestry