EBB 220/3PRINCIPLE OF

VISCO-ELASTICITY

DR AZURA A.RASHIDRoom 2.19

School of Materials And Mineral Resources Engineering,

Universiti Sains Malaysia, 14300 Nibong Tebal, P. Pinang Malaysia

INTRODUCTIONINTRODUCTION

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.

The response of polymeric materials with stress or strain that been applied dependent on :

1. Loading rate

2. Loading time

The differences between materials behaviour are :

1. Elastic materials

2. Viscous materials

3. Visco-elasticity

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

Behaviour of elastic materialBehaviour of elastic material

• Metal or ceramic materials will demonstrate:

1. At low strain in conformity to Hooke’s law

strain is proportional with strain

2. The strain is independent of time

3. Stress with not dependent with loading rate

Ee

E= Elastic modulus

= stress

e strain

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.

Behaviour of viscous materialBehaviour of viscous material

• Materials will demonstrate behaviour:

1. At low strain rate – behave according to the Newtonian relationship

2. Totally dependent with time.

3. Stress being function of strain rate

4. Stress independent of strain

dt

de

= viscosity

de/dt = strain rate

• Behaviour of most polymer is in between behaviour of elastic and viscous materials.

1. At low temperature & high strain rate, Polymer demonstrate elastic behaviour,

2. At high temperature & low strain rate, Polymer demonstrate viscous behaviour

3. At intermediate temperatures & rate of strain

Polymer demonstrate visco-elastic behaviour

Visco elastic behaviourVisco elastic behaviour

• Polymer is called visco- elastic because:

• Showing both behaviour elastic & viscous behaviour

• Instantaneously elastic strain followed by viscous time dependent strain

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

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:

a) creepb) Stress relaxationc) Constant stress rated) Constant strain rate

General time dependent behaviour

General time dependent behaviour

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

(a) Creep(a) Creep

During stress relaxation: Strain is constant Stress decreased slowly with time. For elastic solid the stress is constant

(b) Stress Relaxation(b) Stress Relaxation

The increasing strain with time is not linear. It becoming more steep with:

1. Increasing time

2. Increasing stress rate

(c) Constant stress rate(c) Constant stress 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

(d) Constant strain rate(d) Constant strain rate

It were the general behaviour of polymeric materials and very important in engineering.

It can estimates the strength or the ability to sustained the stress that 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.

Creep phenomenonCreep phenomenon

There were 3 stages of creep:

1. Primer Creep– The slope of strain vs time decreased with time.

2. Secondary creep – Constant strain rate.

3. Tertier creep – the strain rate increased rapidly until rupture (formation of crack, yielding and etc).

3 creep stages3 creep stages

Creep strain,

Rupture

Time, t

Graph for strain curve at constant loading.

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.

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.

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:1. Isometric curve – stress versus time

2. Modulus creep curve – modulus versus time

3. Isochronous curve – stress versus strain

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.

Isometric curveIsometric 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.

Modulus curveModulus 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.

Isochronous curveIsochronous curve

The knowledge of knowing to interpret of creep graphs are useful for materials engineer.

Data from creep graph gives us the information about:

1. The rupture/deformation of the materials

2. Yield and shape change of the materials.

3. Can estimating the life time of the materials

Can choose the materials based on materials applications.

The use of creep graphThe use of creep graph

Can comparing various types of polymeric materials during design because:

The stress for materials were plotted at time for the specific loading being applied.

Isochronous curveIsochronous curve

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.

Example of the problemExample of the problem

From graph creep modulus decreased with increasing time showing the visco-elastic behaviour.

This graph were useful because modulus were needed in engineering deflection.

Modulus curveModulus curve

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.

Example of the applicationExample of the application

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.

Isometric curveIsometric curve

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.

Example of applicationExample of application

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?

Example of the exams questionExample of the exams question

Thank you