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The Mechanical Properties of Polymers
Abu Zafar Al Munsur Date: 14/07/2018
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Behavior Of Material Under Mechanical Loads
= Mechanical Properties. Term to address here…
Stress and strain: These are size-independent measures of load
and displacement, respectively.
Elastic behavior: Recoverable Deformation of small magnitude
Plastic behavior: This permanent deformation behavior occurs when
the tensile (or compressive) uniaxial stress reaches σy.
Toughness and ductility: Defining how much energy that a material can
take before failure.
Hardness: The property of being rigid and resistant to pressure;
not easily scratched.
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Types of Stresses/Load-Strain/Displacement
Tensile Compressive Shear Torsion
Stress: = F / Ao (MPa @ N/mm2),
F: is load (N)
A0:cross-sectional area (m2 or mm2)
Strain: = l / lo ( 100 %)
l: change in length
lo: original length (m or mm)
Shear stress: = F / Ao
F is applied parallel to upper and lower faces each having area A0
Shear strain: = tan ( 100 %)
is strain angle
Torsion: like shear.
Load: applied torque, T
Strain: angle of twist, .
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Stress-Strain Behavior
Elastic deformation
Reversible:
( For small strains)
Stress removed material returns to its original size
Plastic deformation
Irreversible:
Stress removed material does not return to its original dimensions.
Yield Strength (y)
The stress at which plastic deformation becomes noticeable (0.2% offset).
P the stress that divides the elastic and plastic behavior of the material.
The Stress - Strain curve
Divided into 2 regions
ELASTIC PLASTIC
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Linear: Elastic Properties • Modulus of Elasticity, E : (also known as Young's modulus)
• Hooke's Law: ut tensio, sic vis ( "as the extension, so the force“)
= E
Linear- elastic
E
Units: E: [GPa] or [psi] : in [Mpa] or [psi] : [m/m or mm/mm] or [in/in]
F
A o d /2
d L /2
Lo w o
Here: The Black Outline is Original,
Green is after application of load
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• Elastic Shear
modulus, G:
t G
g t = G g
Other Elastic Properties
simple
torsion
test
M
M
• Special relations for isotropic materials:
2(1 + n)
E G =
3(1 - 2n)
E K =
• Elastic Bulk
modulus, K:
pressure
test: Init.
vol =Vo.
Vol chg.
= DV
P
P P P = - K
D V V o
P
D V
K V o
E is Modulus of Elasticity
is Poisson’s Ratio
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7
Atomic Demo of Plastic (Permanent) Deformation
• Simple tension test:
stress,
strain,
Elastic+Plastic at larger stress
p
plastic strain
Elastic initially
Adapted from Fig. 6.10(a), Callister & Rethwisch 8e.
permanent (plastic) after load is removed
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Tensile properties: Tensile Stress
• Metals: occurs when noticeable necking starts.
• Polymers: occurs when polymer backbone chains are
aligned and about to break.
Adapted from Fig. 6.11,
Callister 7e.
y
strain
Typical response of a metal
F = fracture or
ultimate
strength
Neck – acts
as stress
concentrator
TS
str
ess
strain
• TS is Maximum stress on stress-strain curve.
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Measure the energy to break a unit volume of material
Approximate by the area under the tensile stress-strain curve.
Tensile Stress :Toughness
Brittle fracture: elastic energy
Ductile fracture: elastic + plastic energy
very small toughness
(unreinforced polymers)
Tensile strain, e
Tensile
stress, s
small toughness (ceramics)
large toughness (metals)
Adapted from Fig. 6.13,
Callister 7e.
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Tensile properties: Ductility
The total elongation of the specimen due to plastic deformation, neglecting the
elastic stretching (the broken ends snap back and separate after failure).
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Tensile Properties: Hardness
Resistance to permanently (plastically) indenting the surface of a product.
Large hardness means:
Resistance to plastic deformation or cracking in compression.
Better wear properties.
e.g.,
Hardened 10
mm sphere
apply known force measure size
of indentation after
removing load
d D Smaller indents
mean larger
hardness.
Increasing hardness
most plastics
brasses Al alloys
easy to machine steels file hard
cutting tools
nitrided steels diamond
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There are three typical classes of polymer stress-strain characteristic
Mechanical Properties of Polymers
Mechanisms of Elastic Deformation, in Amorphous & Semicrystalline Polymers
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General Classes of Materials
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mechanical and thermal aspects. Polymer Testing, Elsevier, 2014, 34, pp.290-298.
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Approaches to get better Mechanical Properties
By cross-linking tensile stress could be increase
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Essentials of Materials Science & Engineering
Second Edition
Authors: Donald R. Askeland & Pradeep P. Fulay
Materials Science and Engineering: An Introduction
Sixth Edition, Author: William D. Callister, Jr.
The Science and Engineering of Materials
Fourth Edition, Authors: Askeland and Phule (Fulay ?)
Introduction to Materials Science for Engineers
Sixth Edition, Author: James F. Shackelford
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Specimen code, wide & length
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Modify those term
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