The Mechanical Properties of The Mechanical Properties of Viscoelastic MaterialsViscoelastic 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

BackgroundBackground 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.

Creep occurs faster at higher temperatures.

IntroductionIntroduction

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.

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

During loading under a constant stress, the strain often varies as a function of time in the manner shown above

Normal Trend of Creep BehaviorNormal Trend of Creep Behavior

Purpose of the ExperimentPurpose 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 metalsAs 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 MachineTensile Testing Machine

Teacher

The experiment is actually performed using a tensile testing machine. 

Different Different

Teacher

Do Not Disturb!Do Not Disturb!Testing Going ON 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

0

5

10

15

20

25

30

35

40

0 500 1000 1500 2000Time (sec)

Str

ain

Creep

What Makes Composite Materials What Makes Composite Materials Very Unique & Supernatural?Very Unique & Supernatural?

Viscoelastic Analysis of Composite Materials

Dr. Anastasia MulianaPh. D, Structural Engineering and Mechanics, Georgia Institute of Technology, Atlanta, Georgia, May 2004M.S., Civil Engineering, Georgia Institute of Technology, Atlanta, Georgia, August 1999B. S., Civil Engineering, Bandung Institute of Technology, Bandung, Indonesia, October 1997, cum laude

What does this all mean?What does this all mean?

Viscoelastic Phenomena

• All materials exhibit some viscoelastic response.

• Time dependent

• Properties change with increasing or decreasing temperature

Characteristics of thermo-mechanicalCharacteristics of thermo-mechanical and long-term behaviors of and long-term behaviors of

multi-layered composite materialsmulti-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

strain

time

ϭ

stress

time

ɛ

ϭɛ

stress strain

Creep

Relaxation

time time

Classroom ExperimentClassroom 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

0.5

0.6

0.7

0.8

0.9

1

0 500 1000 1500 2000

Time (sec)

Stra

in (%

) Strain1

Strain2

Average

Acknowledgements & CreditsAcknowledgements & Credits