Transcript Slide 1
EBB 220/3 PRINCIPLE 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
INTRODUCTION
The differences materials between behaviour and totally elastic behaviours are : the polymeric materials with
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
dependent on : that been applied
1.
Loading rate 2.
Loading time
The differences behaviour are : between materials
1.
Elastic materials 2.
Viscous materials 3.
Visco-elasticity
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
•
1.
Metal or ceramic materials will demonstrate: 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
Behaviour of viscous material
Deformation instantaneously.
or strain is not
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.
•
1.
Materials will demonstrate behaviour: At low strain rate – behave according to the Newtonian relationship 2.
3.
Totally dependent with time.
Stress being function of strain rate 4.
Stress independent of strain
de dt
= viscosity
de/dt
= strain rate
Visco elastic behaviour
• 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
behaviour
demonstrate
visco-elastic
• •
Polymer is called visco- elastic because:
Showing both behaviour
behaviour elastic & viscous
• Instantaneously elastic strain viscous time dependent strain followed by
Influence of temperature , the relaxation modulus can be plotted at a fixed time for different temperature
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
deformation behaviour as
:
a) b) c) d) creep Stress relaxation Constant stress rate Constant strain rate
4 different
(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
(b) Stress Relaxation
During stress relaxation: Strain is constant Stress decreased slowly with time.
For elastic solid the stress is constant
(c) Constant stress rate
The increasing strain with time is not linear.
It becoming more steep with: 1.
2.
Increasing time Increasing stress rate
• • •
(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
Creep phenomenon
It were the general behaviour of polymeric materials and very important in engineering.
It can
estimates the strength sustained the stress
constant.
or
the ability to
that been applied permanently or
Creep
polymer is stressed at a constant level for a given a time and the strain increases during that time periods.
Creep
materials can be used to
estimate the life times
of Frequently
run at temperatures
degradation is significant where thermal data can be used to estimate of the
elevate-temperature life
of materials.
3 creep stages
There were 3 stages of creep:
1.
Primer Creep
– The slope of strain vs time decreased with time.
2.
Secondary creep
rate.
– Constant strain
3.
Tertier creep
– the strain rate increased rapidly until rupture (formation of crack, yielding and etc).
Creep strain,
e
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 increased in materials creep strain hardening.
having resistance the or
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
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 occur and the slippage between atoms polymer crystallization rate decreased then the strain were increased with time.
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 decomposition of chain folding had not happened the 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
.
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.
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: 1.
The rupture/deformation of the materials 2.
Yield and materials.
shape change of the 3.
Can estimating the life time of the materials Can choose the materials based on materials applications.
Isochronous curve
Can comparing polymeric because: various materials during types of design The stress for materials were plotted at time for the specific loading being applied.
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.
Modulus curve
From graph increasing behaviour.
time creep modulus decreased with showing the visco-elastic This graph were useful because modulus were needed in engineering deflection.
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 checked before starting the calculation.
to be
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.
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 needed for particular strain.
higher stresses is At long term loading get the particular strain.
lower stresses is needed to
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?