1. Viscoelastic Properties of Wood Fiber Reinforced Polyethylene (WFRP): Stress Relaxation, Creep and Threaded Joints Syed Imran Farid Prof. J. K. Spelt, Prof. M. T. Kortschot and Prof. J. J. Balatinecz S. Law and A. Akhtarkhavari Department of Mechanical & Industrial Engineering Department of Chemical Engineering & Applied Chemistry All Information in this presentation is the property of University of Toronto and Researchers

2. Outline Introduction Theoretical Experimental Results Modeling and Discussion Conclusion

3. Introduction 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 ~ 60 o C

4. Introduction Problem Short and long-term threaded joints performance Long-term viscoelastic properties Objective To Investigate the Viscoelastic Properties of Wood Fiber Reinforced Polyethylene: Stress Relaxation, Creep and Threaded Joints To Investigate the Viscoelastic Properties of Wood Fiber Reinforced Polyethylene: Stress Relaxation, Creep and Threaded Joints

5. Viscoelasticity Time and temperature dependent mechanical properties Experimental approach Creep Stress Relaxation Data Reduction Time-Temperature superposition Modeling Physical models Constitutive equation

6. Experimental 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

7. Screw Pullout PULLOUT FIXTURE LOAD CELL

8. Screw Relaxation FORCE TORQUE

9. Results - Viscoelasticity Relaxation modulus and creep compliance as a function of time. Stress relaxation ( ) and creep ( )

10. Result - Stress Relaxation ln(Tensile Modulus) as a function of ln(Time) at 23 o C and 0.5% Strain Slope = -0.0288 Slope = -0.0487 Slope = -0.0453

11. Result - Creep Creep compliance at various stress and temperature

12. Results - Fastener Pullout Pullout force for different fastener (a) F vs Fastener (b) F vs engagement Length Spruce

13. Threaded Joint - Stripping Fastener stripping experiment (a) torque and force vs time (b) torque vs time

14. Threaded Joints - Relaxation Clamping force relaxation at 23 o C Simple relaxation( ) Retightening after 2 h ( )

15. Threaded Joints - Relaxation 35% 53% Clamping force relaxation as a function of time for Spruce and WFRP

16. Modeling - Phenomenological Where E(t) = Modulus at time t A, B E R & E U = Constant depend on loading conditions n, = Time exponent E(t) =A + Bt n Findleys Law E(t) =Bt n Power Law Eqn E(t) =A + B e t n E(t) =E R + (E U + E R ) e t/

17. Modeling - Viscoelasticity Experimental and calculated values using Power Law model Stress relaxation ( ) & creep ( X +)

18. Modeling - Clamping Force Experimental and calculated values for clamping force relaxation

19. Modeling - Time Exponent (n)

20. Time-Temperature Superposition

21. Modeling - Long-Term creep Long-term flexural creep experiment at 20% UFS

22. Conclusion 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

23. 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

24. Acknowledgement Materials and Manufacturing Ontario Department of Chemical Engineering and Applied Chemistry Faculty of Forestry