The Mechanical Properties of
Viscoelastic Materials
-Have a basic understanding of the mechanisms of
creep
-Know about stress exponents and activation energies
and how to obtain them from experimental data
-Be familiar with a particular set-up for experimental
study of the creep characteristics of a polymer material
-Begin to understand why materials need to be designed
to minimize creep
Background
For structural applications of
materials such as bridges, pressure
vessels, ships, and automobiles, the tensile
properties of the metal material set the
criteria for a safe design.
Polymeric materials are being
used more and more in structural
applications, particularly in automobiles
and pressure vessels. New applications
emerge as designers become aware of the
differences in the properties of metals and
polymers and take full advantage of them.
The analyses of structures using metals or plastics
require that the data be available.
Introduction
When material is subjected to a stress that is greater than or
equal to its yield stress, the material deforms plastically.
When the stress is below this level, then in principle it should
only deform elastically
However, provided the temperature is relatively high, plastic
deformation can occur even when the stress is lower than
the yield stress. This deformation is time-dependent and
is known as creep.
Creep occurs faster at higher temperatures.
Normal Trend of Creep Behavior
During loading under a constant stress, the strain often varies
as a function of time in the manner shown above
Purpose of the Experiment
The object of this
experiment is to measure
the tensile properties of a
polymeric material at a
constant strain rate on the
home-made tension testing
machine.
Creep of other metals
As a general rule, creep starts to become significant when the homologous temperature is greater than
0.4. Most metals
do not suffer from creep at room temperature, since they have much higher melting points than solder.
However, creep can still be a major concern when designing metallic components that have to function at
high temperatures.
An example of one such engineering challenge is in the design of turbine blades for use in jet engines.
The blades in these enginescan be exposed to hot gases at up to about
1400°C. They are also under stress, as a result of the high centrifugal forces. These blades must withstand
this environment without excessive creep, which would cause them to strike the turbine enclosure.
Tensile Testing Machine
The experiment is actually performed using a
tensile testing machine.
Teacher
Different
Teacher
Do Not Disturb!
Testing Going ON
Teacher
Before Load- Sample
speciment me asured in 100th
of an inch. Lo =445 parts of
an inch
After Load for a duration of
10 minutes- Sample
speciment demonstrated
effects of plasticity and
elasticity. There was
unnoticeable permanent
deformation after cooling
down from 149.2 F.
Before Load- Sample
speciment me asured in 100th
of an inch. Lo =430 parts of
an inch
After Load for a duration of
30 minutes- Sample
speciment demonstrated
effects of plasticity and
elasticity. There was
noticeable permanent
deformation after cooling
down from 149.2 F.
Creep Behavior
Creep
40
Strain
35
30
25
20
Creep
15
10
5
0
0
500
1000
Time (sec)
1500
2000
What Makes Composite Materials
Very Unique & Supernatural?
Dr. Anastasia Muliana
Ph. D, Structural Engineering and Mechanics,
Georgia Institute of Technology, Atlanta,
Georgia, May 2004
M.S., Civil Engineering, Georgia Institute of
Technology, Atlanta, Georgia, August 1999
B. S., Civil Engineering, Bandung Institute of
Technology, Bandung, Indonesia, October
1997, cum laude
Viscoelastic Analysis of Composite Materials
What does this all mean?
• All materials exhibit some viscoelastic
response.
• Time dependent
• Properties change with increasing or
decreasing temperature
Viscoelastic Phenomena
Characteristics of thermo-mechanical
and long-term behaviors of
multi-layered composite materials
• E-glass/Polyester pultruded composite material tested
(tensile)
• Measuring at 20%, 40%, and 60% ultimate tensile
strength
• Temperature: 0º F to 150 º F
• Off-axis angle (Ɵ) of fiber: 0º, 45º,90º
Creep and Relaxation
ɛ
ϭ
stress
strain
time
time
Creep
ϭ
ɛ
stress
strain
time
Relaxation
time
•
•
•
•
•
•
•
•
•
•
•
•
Classroom Experiment
Composite materials
Viscoelastic behavior
Tensile
Force Applied
Varying Temperatures
Measuring devices
Microsoft Excel
TI-83/TI-84 graphing calculator
TI-CBL 2 System
Dial tool used in machine shops
Thermometer
Digital Clock
Viscoelastic Creep
Creep
Strain (%)
1
0.9
Strain1
0.8
Strain2
0.7
Average
0.6
0.5
0
500
1000
Time (sec)
1500
2000
Acknowledgements & Credits