Day 29: Mechanical Behavior of Polymers
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Review
How are Properties Defined
Introduction to Viscoelasticity
Simple Material Models
Strain Rate and Temperature Effects
Review
• Basic definitions: thermoplastic, thermoset,
elastomer.
• Let’s talk about the kind of mechanical
behavior seen in polymers.
1. Stiffness, E
2. Strength
3. Ductility
• Factors which can determine the strength of a
polymer.
Mechanical Properties
• i.e. stress-strain behavior of polymers
brittle polymer
FS of polymer ca. 10% that of metals
plastic
elastomer
elastic modulus
– less than metal
Adapted from Fig. 15.1,
Callister 7e.
Strains – deformations > 1000% possible
(for metals, maximum strain ca. 10% or less)
5
Tensile Properties for Polymers
T and Strain Rate: Thermoplastics
• Decreasing T...
-- increases E
-- increases TS
-- decreases %EL
• Increasing
strain rate...
-- same effects
as decreasing T.
(MPa)
80
4°C
60
20°C
40
Data for the
semicrystalline
polymer: PMMA
(Plexiglas)
40°C
20
60°C
0
0
0.1
0.2
e
to 1.3
0.3
Adapted from Fig. 15.3, Callister 7e. (Fig. 15.3 is from T.S. Carswell and
J.K. Nason, 'Effect of Environmental Conditions on the Mechanical
Properties of Organic Plastics", Symposium on Plastics, American Society
for Testing and Materials, Philadelphia, PA, 1944.)
7
Effects of Strain Rate and Temperature
stress
Increasing
strain rate
Increasing temp
strain
Time Temp for Delrin (Strain Rate)
http://www2.dupont.com/Plasti
cs/en_US/assets/downloads/d
esign/230323c.pdf
Time Temp for Delrin (Strain Rate and Temp)
http://www2.dupont.com/Plasti
cs/en_US/assets/downloads/d
esign/230323c.pdf
Time Temp Dependence
• Plastic deformation of polymers involves chain
uncoiling and chain sliding
• Increasing temperature increases relative
space between chains and makes uncoiling
easier.
• Slowing the strain rate means there is more
time for chain reconfiguration.
Introduction to Viscoelasticity
• Some features that are observed in polymeric
materials that do not seem to be noticeable in
metals or ceramics
1. Mechanical properties depend on Temperature
2. Mechanical properties depend on Strain Rate
3. Creep (noticed in metals at high temperatures)
4. Stress Relaxation
5. Hysteresis
Creep
• Take a tension specimen made from a polymer
and and put on a series of constant stresses on it.
• We observe
Creep: Progressive strain (deformation) over time at
constant stress (load), usually at high temperatures
Creep Test
• We instantly load with constant stress for
a certain time, and instantly unload.
Note that both linear
elastic and viscous fluid
behaviors are present.
 Note that there seems to
be some residual strain at
the end, i.e. the material
does not completely
recover. There is both
elasticity and plasticity.

Creep of PEEK
Write down two examples of
parts that see constant tensile or
bending load.
Stress Relaxation
• Think of a polymer specimen loaded with
a constant strain.
Note that both linear
elastic and viscous fluid
behaviors are present.
 Note that there seems to
be some residual stress at
the end, i.e. the material
does not completely
recover. There is both
elasticity and plasticity.

Stress Relaxation: Progressive loss of stress (load) over
time under constant strain (deformation), usually at high
temperatures
Stress Relaxation of Delrin
http://www2.dupont.com/Plasti
cs/en_US/assets/downloads/d
esign/230323c.pdf
Write down two examples of
parts that see constant strain.
Effect of Temperature:
Glass Transition
Temperature
Or why does Garden
Hose behave the way
it does?
Melting vs. Glass Transition Temp.
What factors affect Tm and Tg?
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Both Tm and Tg increase with
increasing chain stiffness
Chain stiffness increased by
1. Bulky sidegroups
2. Polar groups or sidegroups
3. Double bonds or aromatic
chain groups
Regularity – effects Tm only
Adapted from Fig. 15.18,
Callister 7e.
22
Tg and Tm
Hysteresis
• Polymers often don’t load and unload on the
same line on the stress-strain curve.
• The difference in areas under those curves
represents energy loss (often to heat).
• This means that polymers can have inherent
energy damping.
• This means plastic springs may not be as good
an idea as plastic dampers.
Load-Unload Cycle in Nylon
Hysteresis in Delrin
Takeaways
• Yield and Ultimate Strength are defined
differently for polymers.
• Polymers have time and temperature
dependent properties (viscoelasticity)
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Creep
Stress Relaxation
Tg, Tm
Hysteresis