Engineering Polymer

1. DEFENCE ENGINEERING COLLEGE
Metallurgical And Materials Engineering
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Present By:
Weldebrhan Hadush (Maj.) Date April 10 , 2019

2. 2
Content
i. Introduction
ii. Deformation of polymers
iii. Strengthening of polymers
iv. Fast fracture conditions
v. Energy criteria for fast fracture
vi. Creep, recovery and stress relaxation
vii. Creep failure of polymers
viii. Creep modulus of polymers
ix. Creep Resistance of Plastics
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CREEP, RECOVERY AND STRESS RELAXATION

3. 3
Introduction
An understanding of deformation mechanisms of polymers is important
in order to be able to manage the optimal use of these materials, a class of
materials that continues to grow in terms of use in structural applications.
Despite the similarities in ductile and brittle behavior with to metals and
ceramics respectively, elastic and plastic deformation mechanisms in
polymers are quite different.
Plastic deformation in metals and ceramics can be described in terms of
dislocations and slip planes(dislocation movement), whereas polymer
chains must undergo, chains rotate, stretch, slide and disentangle under
load to cause permanent deformation.
The following factors influence the strength of a thermoplastic: average
molecular mass, degree of crystallization, presence of side groups, etc.
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4. 4
Stress–Strain Curves
At a microscopic level, deformation in polymers involves stretching and
rotating of molecular bonds.
More commonly, one distinguishes the deformation mechanisms in
polymers as brittle, ductile (with or without necking), and elastomeric.
Clearly, factors such as the strain rate and temperature affect the shape
of stress—strain curve , much more so in polymer than in ceramics or
metals. This is because the polymers are viscoelastic; that is, their
stress—strain behavior is dependent on time. Temperature and strain rate
have opposite effects.(Chap. 5).
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5. 5
Glassy Polymers
In a manner similar to its occurrence in metals, plastic deformation occurs
inhomogeneously in polymers.
Two forms of inhomogeneous deformation(fracture) are observed in
polymers are shear yielding and crazing.
Shear bands form at about 45◦ to the largest principal stress. The polymeric
molecular chains become oriented within the shear bands without any
accompanying change in volume.
The process of shear band formation can contribute to a polymer’s toughness
because it is an energy-dissipating process.
Shear yielding can take two forms: diffuse shear yielding and localized shear
band formation. In localized shear, the shear is concentrated in thin planar
regions.
The bands form at about 45◦ to the stress axis.
Crazes are narrow zones of highly deformed
polymer containing voids; the zones are oriented
perpendicular to the stress axis. crazing occurs
with an increase in volume.
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Cont’d

7. Example
Describe how the phenomenon of crazing can be exploited to improve the
toughness of a polymer.
Solution:
Craze formation requires energy. Thus, if we increase the number of crazes
nucleated, but do not allow them to grow to fracture, we can improve the
toughness of a polymer.
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8. 8
Semicrystalline Polymers
Semicrystalline polymers containing spherulites show a highly complex
mode of deformation. Characteristically, these materials exhibit a ductile
stress--strain curve with necking.
Neck propagation
in a sheet of linear polyethylene
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9. 9
Viscous Flow
At high temperatures (T ≥ Tg, the glass transition temperature), polymers
undergo a viscous flow. Under these conditions, the stress is related to the
strain rate, rather than the strain.
Thus, where σ is the shear stress, η is the viscosity, and t is the time.
Viscous flow is a thermally activated process. It occurs by molecular motion,
which increases as the temperature increases.
The thermal energy for this is available above the glass transition temperature
Tg. Below Tg, the thermal energy is too low for breaking and re-forming
bonds, and the material does not flow so easily. At very high temperatures, the
viscosity η is given by the Arrhenius-type relationship
where A is a constant, Q is the activation energy, R is the universal
gas constant, and T is the temperature in kelvin.
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10. Cont’d
Unique to most of the polymers is the viscoelasticity – means when an
external force is applied, both elastic and plastic (viscous) deformation occur.
For viscoelastic materials, the rate of strain determines whether the
deformation in elastic or viscous.
The viscoelastic behavior of polymeric materials is dependent on both time
and temperature.
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11. Sometimes it is required to produce polymers which are rigid and resistant
to corrosive chemicals and temperature. Though there are many
strengthening mechanisms of polymers, the following are important.
1. Molecular Weight: The tensile strength of the polymer rises with
increase in molecular weight and reaches the saturation level at some
value of the molecular weight. The tensile strength is related to molecular
weight
2. Cross-linking: The cross-linking restricts the motion of the chains and
increases the strength of the polymer
3. Crystallinity: The crystallinity of the polymer increases strength,
because in the crystalline phase, the intermolecular bonding is more
significant
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12. CREEP, RECOVERY AND STRESS RELAXATION
Creep in polymers
When a plastic material is subjected to a constant load, it deforms
continuously.
The initial strain is roughly predicted by its stress-strain modulus.
The material will continue to deform slowly with time indefinitely or
until rupture or yielding causes failure.
The primary region is the early stage of loading when the creep rate
decreases rapidly with time.
Then it reaches a steady state which is called the secondary creep stage
followed by a rapid increase (tertiary stage) and fracture. This
phenomenon of deformation under load with time is called creep.
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13. Cont’d
All plastics creep to a certain extent. The degree of creep depends on
several factors, such as type of plastic, magnitude of load, temperature and
time.
In this test procedure, the dimensional changes that occur during time under
a constant load are measured.
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Creep curve for plastics, a constant load is applied
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14. Cont’d
If the applied load is released before the creep rupture occurs, an immediate
elastic recovery equal to the elastic deformation, followed by a period of
slow recovery is observed in the figure below.
The material in most cases does not recover to the original shape and a
permanent deformation remains.
The magnitude of the permanent deformation depends on length of time,
amount of stress applied, and temperature.
This is shown in the figure below.
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Creep curve with recovery. A constant load is applied at t0 and removed at t1.

15. Stress relaxation, constant strain
Stress relaxation is defined as a gradual decrease in stress with time
under a constant deformation or strain.
This behavior of polymer is studied by applying a constant deformation to
the specimen and measuring the stress required to maintain that strain as a
function of time.
The quantity η/E is referred to as the relaxation time τ
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Stress relaxation of plastics.

16. Creep failure of polymers
It is the culmination in the deformation process of creep.
The fracture strengths of polymers are low relative to ceramics and metals.
The fracture mode in thermosetting polymers (heavily crosslinked
networks) is typically brittle.
For thermoplastic polymers, both ductile and brittle modes are possible.
Reduced temperature, increased strain rate, sharp notches, are some
factors that can influence a brittle fracture.
One phenomenon that occurs in thermoplastics is crazing, very localized
plastic deformation and formation of micro voids and febrile bridges.
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17. Creep modulus of polymers
The creep modulus EC is used when designing polymers against creep.
Creep modulus is used to provide an estimation of the deformation during
the life of the structure.
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The visco-elastic nature of polymers reduces the rate of creep while being
loaded and allows for a small amount of reverse creep upon unloading.
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18. Creep Resistance of Plastics
Creep resistance can be defined as a material's ability to resist any kind of
distortion when under a load over an extended period of time.
For optimum performance and maximum lifetime, engineering plastics,
which are subjected to long-term loading, should have a high creep
resistance (i.e. low plastic deformation under load).
The glass temperature of a polymer (TG ) increases with degree of cross
linking; heavily cross linked polymers (e.g. epoxies) are more creep
resistant at room temperature than those that are less cross linked (e.g.
polyethylene).
The viscosity of polymers above TG increase with the molecular weight, so
that the rate of creep thereby is reduced by having a high molecular weight.
Finally crystalline polymers (e,g HDPE than LDPE& MDPE)
Composite containing continuous fibers because much of the load is now
carried by the fibers which, being very strong, do not creep at all.
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19. 19
“The future can not be predicted,
but it can
be made !”
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20. Example
The activation energy for stress relaxation in a polymer is 50 kJ/mol. The relaxation time at
25 ◦C is 90 days. What is the relaxation time at 125 ◦C?
Solution:
1/τ = E/η= A exp(−Q /RT ),
(1/τ25)/(1/τ125) = exp(−Q /R298)/ exp(−Q /R398),
τ125 = τ25 exp[(Q /R)(1/398 − 1/298)],
τ125 = 90 exp[((50 × 103)/8.314)(1/398 − 1/298)],
= 90 × 6.4 × 10−3,
= 0.57 day
Example
A nylon cord, used to tie a sack, has an initial stress of 5 MPa. If the relaxation time for this
cord is 180 days, in how many days will the stress reduce to 1 MPa?
Solution:
σ = σ exp(−t/τ ),
1 MPa = 5 exp (−t/180),
t = −180 ln1/5,
t = 290 days.
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Deformation of Polymers
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21. 21
Deformation of Polymers
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22. 22
Load is applied instantaneously at time ta and released at tr
a. Load versus time , for the load time cycle
b. Strain versus time responses are for totally elastic behaviors
c. Viscoelastic behaviors
d. Viscous behaviors
Cont’d
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