EBB 220/3 PRINCIPLE OF VISCO-ELASTICITY

1. EBB 220/3PRINCIPLE OF 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

2. INTRODUCTION • The differences between the polymeric materials behaviour and materials with totally elastic behaviours are : • Time dependent characteristics • Temperature dependent characteristics • Polymeric materials will shows the properties that dependent on stress & strain  that will influence when the loading being applied.

3. The response of polymeric materials with stress or strain that been applied dependent on : • Loading rate • Loading time • The differences between materials behaviour are : • Elastic materials • Viscous materials • Visco-elasticity

4. Behaviour of elastic material • Elastic behaviour is instantaneous. • The total deformation (or strain) occurs the instant the stress is applied or release. • Upon release of the external stress – the deformation is totally recovered. (deformation is reversible) • The specimens assumes its original deformation

5. Metal or ceramic materials will demonstrate: • At low strain in conformity to Hooke’s law  strain is proportional with strain • The strain is independent of time • Stress with not dependent with loading rate E= Elastic modulus s= stress e= strain

6. Behaviour of viscous material • Deformation or strain is not instantaneously. • In response to an applied stress- deformation is delayed or dependent with time. • This deformation is not reversible or completely recovered after stress is released.

7. Materials will demonstrate behaviour: • At low strain rate – behave according to the Newtonian relationship • Totally dependent with time. • Stress being function of strain rate • Stress independent of strain h= viscosity de/dt = strain rate

8. Visco elastic behaviour • Behaviour of most polymer is in between behaviour of elastic and viscous materials. • At low temperature & high strain rate, • Polymer demonstrate elastic behaviour, • At high temperature & low strain rate, • Polymer demonstrate viscous behaviour • At intermediate temperatures & rate of strain • Polymer demonstrate visco-elastic behaviour

9. Polymer is called visco- elastic because: • Showing both behaviour elastic & viscous behaviour • Instantaneously elastic strain followed by viscous time dependent strain

10. Influence of temperature , the relaxation modulus can be plotted at a fixed time for different temperature

11. General time dependent behaviour • The true mechanical properties that apppriate with time for polymeric materials dependent on  pada types of stress or cycle of strain that been used. • Changes in stress an strain with time (t), can be shown in simple schema of polymer tensile. • It can be categorized based on 4 different deformation behaviour as: • creep • Stress relaxation • Constant stress rate • Constant strain rate

12. (a) Creep • During Creep loading: • A constant load were applied to the specimen at a t = 0, • The strain increased quickly at the beginning but become slowly with time after a long period of deformation. • For elastic solid  the strain rate is constant Constant stress

13. (b) Stress Relaxation • During stress relaxation: • Strain is constant • Stress decreased slowly with time. • For elastic solid  the stress is constant

14. (c) Constant stress rate • The increasing strain with time is not linear. • It becoming more steep with: • Increasing time • Increasing stress rate

15. (d) Constant strain rate • The increasing stress with time is not linear. • The slope of the curve decreased with time • The slope become more steep with the increasing strain rate

16. Creep phenomenon • It were the general behaviour of polymeric materials and very important in engineering. • It can estimates the strength or the ability to sustained the stressthat been applied permanently or constant. • Creep  polymer is stressed at a constant level for a given a time and the strain increases during that time periods. • Creep can be used to estimate the life times of materials • Frequently run at temperatures where thermal degradation is significant  data can be used to estimate of the elevate-temperature life of materials.

17. 3 creep stages • There were 3 stages of creep: • Primer Creep– The slope of strain vs time decreased with time. • Secondary creep – Constant strain rate. • Tertiercreep – the strain rate increased rapidly until rupture (formation of crack, yielding and etc).

18. Creep strain,e Rupture Time, t Graph for strain curve at constant loading.

19. After beginning of strain, specimen will having a slowly shape changes with time until the yielding occur that caused a rupture. • At primer area • Area of early stage of deformation when creep rate is decreased with time (slope of the curve decreased with time). • Polymeric materials having the increased in creep resistance or strain hardening.

20. Secondary area • Area where the creep rate where almost constant • Creep rate were explained by the equilibrium in between strain hardening and the ability to maintain/ retain its shape. • Tertier area • Where creep accelerate and rupture occurred. • Creep happens due to changes in microstructure. • Happen at higher stress for ductile materials. • Decreased in cross-section that make the rupture or creep rate increased rapidly.

21. Creep test normally run in extension/ tension test. (but can be done in shear, compression or flexural test) • Creep rate of polymeric materials were dependent on loading, time and temperatures. • Polymeric components will deformed rapidly at higher temperatures. • Creep results can been shown as: • Isometric curve – stress versus time • Modulus creep curve – modulus versus time • Isochronous curve – stress versus strain

22. Isometric curve • Stress that being applied will dependent on time. • At beginning  stress is higher due to bonding forces between atoms is higher. • After a few moments  slippage between atoms occur and the polymer crystallization rate decreased then the strain were increased with time.

23. Modulus curve • The elasticity of certain materials exists due to the materials decomposition of chain to become more order. • If the measurements is taken in the short periods the decomposition of chain folding had not happened  polymer are more like persistent materials. • This graph is very useful in determination of materials rigidity and persistent  based on the life span of the materials.

24. Isochronous curve • The slope of the graph is equivalent to the modulus Young, E which is the determination the resistance towards the neighbouring separation of the atoms. • Modulus is the rigidity or the resistance of materials towards shapes changes. • The high modulus values  resulting from small strain changes due to the applied stress.

25. The use of creep graph • The knowledge of knowing to interpret of creep graphs are useful for materials engineer. • Data from creep graph gives us the information about: • The rupture/deformation of the materials • Yield and shape change of the materials. • Can estimating the life time of the materials • Can choose the materials based on materials applications.

26. Isochronous curve • Can comparing various types of polymeric materials during design because:  The stress for materials were plotted at time for the specific loading being applied.

27. Example of the problem • One of the engineer has to design rigid structure can sustained the continuous load for 1000 hours with the strain not more than 2 %. • Question: • What is the maximum stress can be allowed? • Solution: • Need to make a comparison from graph strain versus time for different stress for 1000 hours.  strain at different stress can be resolved. • Graph stress versus strain at 1000 hours can be plotted  the maximum stress allowed can be obtained.

28. Modulus curve • From graph  creep modulus decreased with increasing time showing the visco-elastic behaviour. • This graph were useful because modulus were needed in engineering deflection.

29. Example of the application • To chosen the life span of component that being designing at modulus curve  the modulus value is called design modulus. • The stress of the modulus is determine according to the alternative : • If stress being determine  The values should be taken from the modulus curve with the stress value is nearly to the value that needed. • If the stress needed not yet been determine  Need to choose the modulus curve with the conservative stress value and need to be checked before starting the calculation.

30. Isometric curve • With observing materials behaviour during stress relaxation  can estimate the long term materials behaviour. • Materials long term service can be estimate when the certain stress being applied not more that the rupture of the materials.

31. Example of application • For one bottle lid under constant strain for very long period  low stress relaxation is needed. • That bottle lid will fail if the stress decreased instantly. • Time is a the main factor that will influenced the mechanical properties of the bottle lid because : • At very short loading time  higher stresses is needed for particular strain. • At long term loading  lower stresses is needed to get the particular strain.

32. Example of the exams question • What is definition of visco-elasticity? • Please gives the differences between visco-elastic behavior and totally elastic behavior. • Gives the advantages of creep properties in materials engineering?

33. Thank you