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EBB 220/3 MODEL FOR VISCO-ELASTICITY

EBB 220/3 MODEL FOR VISCO-ELASTICITY. DR AZURA A.RASHID Room 2.19 School of Materials And Mineral Resources Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, P. Pinang Malaysia. INTRODUCTION. It is difficult to predict the creep and stress relaxation for polymeric materials.

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EBB 220/3 MODEL FOR VISCO-ELASTICITY

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  1. EBB 220/3MODEL FORVISCO-ELASTICITY DR AZURA A.RASHID Room 2.19 School of Materials And Mineral Resources Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, P. Pinang Malaysia

  2. INTRODUCTION • It is difficult to predict the creep and stress relaxation for polymeric materials. • It is easier to predict the behaviour of polymeric materials with the assumption  it behaves as linear viscoelastic behaviour. • Deformation of polymeric materials can be divided to two components: • Elastic component – Hooke’s law • Viscous component – Newton’s law • Deformation of polymeric materials  combination of Hooke’s law and Newton’s law.

  3. Hooke’s law & Newton’s Law • The behaviour of linear elastic were given by Hooke’s law: or • The behaviour of linear viscous were given by Newton’s Law: E= Elastic modulus s= Stress e=strain de/dt = strain rate ds/dt = stress rate h= viscosity ** This equation only applicable at low strain

  4. Mechanical Model • Methods that used to predict the behaviour of visco-elasticity. • They consist of a combination of between elastic behaviour and viscous behaviour. • Two basic elements that been used in this model: • Elastic spring with modulus which follows Hooke’s law • Viscous dashpots with viscosity h which follows Newton’s law. • The models are used to explain the phenomena creep and stress relaxation of polymers involved with different combination of this two basic elements.

  5. STRESS RELAXATION CREEP Constant strain is applied  the stress relaxes as function of time Constant stress is applied  the strain relaxes as function of time

  6. The common mechanical model that use to explain the viscoelastic phenomena are: • Maxwell • Spring and dashpot align in series • Voigt • Spring and dashpot align in parallel • Standard linear solid • One Maxwell model and one spring align in parallel.

  7. Maxwell Model • Maxwell model consist of spring and dashpot in series and was developed to explain the mechanical behaviour on tar. • On the application of stress, the strain in each elements are additive. • The total strain is the sum of strain in spring & dashpot. The stress each elements endures is the same. Elastic spring Viscous dashpot

  8. Overall stress s, overall strain e in the system is given by: es = strain in spring and ed = strain in dashpot dashpot • Because the elements were in series the stress is the same for all elements, • Equations for spring and dashpot can be written as: and

  9. For Maxwell model, the strain rate is given as • The accuracy of prediction the mechanical behaviour of Maxwell model can be confirm. • In creep case, the stress at s = s0maka ds/dt = 0. The equations can be written as: • Maxwell model can predict the Newtonian behaviour  the strain is predict to increased with time

  10. The behavior of Maxwell model during creep loading (constant stress, s0 strain is predicted to increased linearly with time . This is not the viscoelastic behaviour of polymeric materials  de/dt decreased with time

  11. May be this model is useful to predict the behaviour of polymeric materials during stress relaxation. • In this case, the strain is constant e=e0 applied to the system given de/dt =0 • then • Integration at t=0s= s0 given  so= earlier stress

  12. The term h/Eis constant for Maxwell model and sometimes can be refered as time relaxation, t0 written as: • The exponential decreased in stress can be predicted  give a better representation of polymeric materials behaviour. • Stress were predicted completely relaxed with time period  it is not the normal case for polymer

  13. Voigt Model • Can also known as the Kelvin model. • It consists of a spring and dashpot in parallel. • In application of strain, the stress of each element is additive, and the strain in each element is the same. Elastic spring Viscous dashpot

  14. The parallel arrangement of spring and dashpot gives the strain e are the same for the system given by: es = strain in spring and ed = strain in dashpot • Because the elements in parallel  stress s din every elements are additive and the overall stress are • Equation for spring and dahpot can be written as: and

  15. For Voigt model, the strain rate are • The accuracy of prediction the mechanical behaviour of Voigt model can be confirm. • In creep case, stress is s = soso ds/dt = 0. Theequation can be written as: • The simple differential equation given by:

  16. Constant ratio h/Ecan be replace with time relaxation, t0. • Changes in strain with time for Voigt model that having creep are given by: Figure shows polymer behavior under creep deformation strain rate decreased with time e so /.E and t=

  17. Voigt model fails to predict the stress relaxation behaviour of polymer • When the strain is constant at e0 and dan de/dt= 0 the equation shows:  The linear response is shown in the figure: or Behavior of Voigt model at different loading  Stress relaxation

  18. Standard linear solid • As shown: • Maxwell model can accurately predict the phenomenon stress relaxation to a first approximation. • Voigt Model can accurately predict the phenomenon creep to a first approximation. • Standard linear solid modelwas developed to combined the Maxwell and Voigt model  to describe both creep & stress relaxation to a first approximation.

  19. Elastic spring • In consist  one Maxwell elements inparallelwith a spring. • The presence on this second spring will stop the tendency of Maxwell element undergoing viscous flow during creep loading but will still allow the stress relaxation to occur Viscous dashpot

  20. Summary • There were a lots of attempts to discover more complex model that can give a good approximation to predict viscoelastic behaviour of polymeric materials. • When the elements used is increased mathematicalcan be more complex. • It can be emphasis that mechanical models can only gives mathematical representations for mechanical behaviour only  it not much help to predict the behaviour of viscoelasticity at molecular level.

  21. Boltzman superposition principle • Linear viscoelastic theory is Boltzman superposition principle. • It is the first mathematical statement of linear viscoelastic behaviour that allows the state of stress or strain in a viscoelastic body to determine  from a knowledge of it’s entire deformation history. • This principle can be used to predict the overall creep and stress relaxation of polymeric materials

  22. Botzmann proposed that: • The creep in a specimen is a function of it’s entire loading history • Each loading step makes an independent contribution to the final deformation • Overall deformation  algebraic sum of each contribution

  23. Illustrating the Boltzman superposition principle

  24. Example of the exams question • What is the purpose of mechanical model in visco-elasticity theories? • Gives a brief description how the chosen mechanical model can be used to estimate the creep or stress relaxation behavior for polymeric materials?

  25. Thank you

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