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

Outline Introduction Theoretical Experimental Results Modeling and Discussion Conclusion

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 ~ years Operating temperature ~ 60 o C

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

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

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

Screw Pullout PULLOUT FIXTURE LOAD CELL

Screw Relaxation FORCE TORQUE

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

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

Result - Creep Creep compliance at various stress and temperature

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

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

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

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

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/

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

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

Modeling - Time Exponent (n)

Time-Temperature Superposition

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

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

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

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