Chapter_07.pdf

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Chapter 7
MECHANICAL PROPERTIES
¤ Stress and strain : What are they?
¤ Elastic behavior: When loads are small, how much
deformation occurs? What materials deform least?
¤ Plastic behavior: At what point do dislocations
cause permanent deformation? What
materials are
most resistant to permanent deformation?
¤ Toughness and ductility : What are they and how
do we measure them?
¤ Ceramic Materials: What special provisions/tests are
made for ceramic materials?
Stress & Strain
COMMON STATES OF STRESS
Tension
Compression
Torsion
Shear
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Tensile Stress-Strain
(Engineering)
¤ Tensile stress ( s) :
¤ Tensile strain ( e ) :
F t
d /2
Area, A
L o
w o
d /2
Strain is
always
dimensionless.
d L /2
F t
d L /2
d
s = F t
e =
N/m 2 or lb/in 2
(no unit)
A o
L o
e L = − d L
w o
Original area before loading
(lateral strain)
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PoissonÔs Ratio
e L
compression
d /2
e
L o
tension
w o
- n
d /2
¤ Poisson's ratio ( n) :
d L /2
d L /2
e
n = −
d
(no unit)
L
e
e =
L o
(0.25 ~ 0.35)
e L = − d L
w o
(lateral strain)
* Unless Poisson’s ratio is 1,
there is always volume change.
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