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

OutlineOutline

Introduction Theoretical Experimental Results Modeling and Discussion Conclusion

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

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

ViscoelasticityViscoelasticity Time and temperature dependent mechanical

properties Experimental approach

Creep Stress Relaxation

Data Reduction Time-Temperature superposition

Modeling Physical models Constitutive equation

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

Screw PulloutScrew Pullout

PULLOUT FIXTURE

LOAD CELL

Screw RelaxationScrew Relaxation

FORCE

TORQUE

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

ulus

(GP

a)

-2

-1

0

1

2

3

4

5

Com

plia

nce

(GP

a-1

)

23 40 60 Creep 23 40 60

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 (T

ensi

le M

odul

us)

WFRP LDPE Spruce

Slope = -0.0288

Slope = -0.0487

Slope = -0.0453

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 (F

lexu

ral C

ompl

ianc

e (M

Pa

-1)

25%,23 C 30%,40 C 30%,60 C 50%,40 C 50%,60 C

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

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

mpi

ng F

orce

(N)

0

1

2

3

4

5

Dri

ving

Tor

que

(N/M

)

Load Torque

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

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 200000Time (s)

Cla

mpi

ng F

orce

(N)

Spruce WFRP

Modeling - PhenomenologicalModeling - Phenomenological

WhereE(t) = Modulus at time tA, 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/

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 (C

reep

Com

plia

nce)

23C 40C 50C 23C 40C 60C Power Law

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 (C

lam

ping

For

ce)

0.17Fpo 0.33Fpo 0.50 Fpo Calculated

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 335Temperature (K)

Tim

e E

xpon

ent

Stress Relaxation Creep Clamping Force

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

(MP

a)

23 C 40 C 50 C

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 (%

)

StrainCalculated

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

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

AcknowledgementAcknowledgement

Materials and Manufacturing Ontario Department of Chemical Engineering and

Applied Chemistry Faculty of Forestry