mech properties

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Chapter 6 - 1 ISSUES TO ADDRESS... Stress and strain: What are they and why are they used instead of load and deformation? Elastic behavior: When loads are small, how much deformation occurs? What materials deform least? Plastic behavior: At what point does permanent deformation occur? What materials are most resistant to permanent deformation? Toughness and ductility: What are they and how do we measure them? Chapter 6: Mechanical Properties

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Chapter 6they used instead of load and deformation?
• Elastic behavior: When loads are small, how much
deformation occurs? What materials deform least?
• Plastic behavior: At what point does permanent
deformation occur? What materials are most
resistant to permanent deformation?
do we measure them?
(photo courtesy P.M. Anderson)
o
s
s < 0
• Hooke's Law:
Chapter 6 -
Mechanical Properties
Slope of stress strain plot (which is proportional to the elastic modulus) depends on bond strength of metal
Adapted from Fig. 6.7,
Callister 7e.
of aligned
carbon (CFRE),
Plastic (Permanent) Deformation
• Simple tension test:
engineering stress, s
engineering strain, e
Callister 7e.
Chapter 6 -
occurred.
= 0.002 = z/z
Callister 7e.
tensile stress,
Callister 7e.
a = annealed
Nylon 6,6
in tension, fracture usually occurs before yield.
H
DPE
PP
humid
dry
PC
PET
• Polymers: occurs when polymer backbone chains are
aligned and about to break.
Adapted from Fig. 6.11, Callister 7e.
• Maximum stress on engineering stress-strain curve.
y
strain
F = fracture or
Callister 7e.
a = annealed
Adapted from Fig. 6.13, Callister 7e.
Ductility
100
x
A
A
A
RA
• Approximate by the area under the stress-strain
curve.
Toughness
very small toughness
Chapter 6 -
Resilience, Ur
Energy stored best in elastic region
If we assume a linear stress-strain curve this simplifies to
Adapted from Fig. 6.15, Callister 7e.
y
y
r
2
1
U
e
s
Chapter 6 -
• Large hardness means:
compression.
True stress
True Strain
Chapter 6 -
• An increase in sy due to plastic deformation.
e
T
s
T
shows a linear relation between stress and strain.
To minimize deformation, select a material with a
large elastic modulus (E or G).
• Toughness: The energy needed to break a unit
volume of material.
Summary
behavior occurs when the tensile (or compressive)
uniaxial stress reaches sy.