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 ~ 60oC

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

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. LOAD CELL PULLOUT FIXTURE Screw Pullout

8. TORQUE FORCE Screw Relaxation

9. Results - Viscoelasticity Relaxation modulus and creep compliance as a function of time. Stress relaxation (  ) and creep (  )

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

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

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

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 23oC 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 E(t) =A + Btn Findley’s Law E(t) =Btn Power Law Eqn E(t) =A + B etn E(t) =ER+ (EU+ ER) et/t Where E(t) = Modulus at time t A, B ER & EU = Constant depend on loading conditions n,  = Time exponent

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