MSE 321
MECHANICAL BEHAVIOUR OF MATERIALS
Yahya K. Tür
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Things to Know
Sources : A comprehensive list will be given at the end of the lecture
In Turkish: Malzemelerin yapısı ve mekanik davranışları, Kayalı ve
Çimenoğlu, İTÜ Kimya Metalurji Fakültesi
Grading : Midterm (35 %) + Final (55 %) + Attendance (10 %)
One A4 sheet hand written notes is permitted in the exams.
Second sheet and photocopied sheets are to be collected.
Course hours: Friday 9:00 -12:00 Office hours: Wednesday 16:00-17:00 Room : MLZ 221, Phone: 605 2640
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Mechanical Behaviour of Materials
Part I Mechanical fundamentals
• Stress and strain relationships of elastic behaviour
• Plastic deformation
Part II Metallurgical fundamentals
• Plastic deformation of single crystals
• Dislocation theory
• Strengthening mechanisms
• Fracture and Fatigue
Part III
•Mechanical behaviour of ceramics
•Mechanical behaviour of polymers.
Objective: Macro and micro mechanical behaviour of materials under the
influence of external forces
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Common States of Stress • Simple tension: cable
A o = cross sectional
area (when unloaded)
F F
o
s = F
A s s
Note: t = M r/Jo here.
o
t = F s
A M
M A o
2R
F s A c
• Torsion (a form of shear): drive shaft Ski lift (photo courtesy
P.M. Anderson)
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OTHER COMMON STRESS STATES (1)
(photo courtesy P.M. Anderson) Canyon Bridge, Los Alamos, NM
A o
Balanced Rock, Arches National Park o
s = F
A
• Simple compression:
Note: compressive structure member (s < 0 here). (photo courtesy P.M. Anderson)
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• Bi-axial tension: • Hydrostatic compression:
Pressurized tank
s < 0 h
(photo courtesy P.M. Anderson)
(photo courtesy P.M. Anderson)
OTHER COMMON STRESS STATES (2)
Fish under water
s z > 0
s q
> 0
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Why failure in materials
• Seven of the Liberty Ships built during
the world war II has broken completely
in two as a result of brittle fractures.
• Over 1000 of approximately 5000
merchant ships built during World War
II had developed cracks of considerable
size by 1946.
Failure of Liberty Ships during services in World War II.
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Why failure in materials
• The bridge building industry did not
pay particular attention to the
possibility of brittle failure until the
failure of Point Pleasant bridge in 1967.
• The bridge collapsed without
warning, costing 46 lives.
Collapse of Point Pleasant suspension bridge, West Virginia, on December 15, 1967.
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Why failure in materials
• The aircraft was used for interisland
transportation for 19 years before failed.
• Failure has been attributed to
multiple-site-damage.
Failed fuselage of the Aloha 737 aircraft in 1988.
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Material property assessments • Hardness :
Micro/Macro hardness tests
• Strength, Ductility (elongation, reduction of area):
Tension tests
• Creep (elevated temperature strength):
Creep tests
• Torsion:
Torsion tests
• Toughness (resistance to failure) :
Impact tests; Fracture toughness tests
• Fatigue:
S-N fatigue tests;Fatigue crack growth tests
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Hardness tests
• Hardness is a property which is a measure of a resistance to permanent or plastic deformation.
increasing hardness
most plastics
brasses Al alloys
easy to machine steels file hard
cutting tools
nitrided steels diamond
• Large hardness means: --resistance to plastic deformation or cracking in compression. --better wear properties.
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Stress-Strain Testing (Tensile Test)
Adapted from Fig. 6.3, Callister 7e. (Fig. 6.3 is taken from H.W. Hayden, W.G. Moffatt, and J. Wulff, The Structure and Properties of Materials, Vol. III, Mechanical Behavior, p. 2, John Wiley and Sons, New York, 1965.)
specimen extensometer
• Typical tensile specimen
Adapted from Fig. 6.2, Callister 7e.
gauge length
• Typical tensile test machine
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• Stress at which noticeable plastic deformation has occurred.
when ep = 0.002
Yield Strength, sy
sy = yield strength
Note: for 50 mm sample
Adapted from Fig. 6.10 (a), Callister 7e.
tensile stress, s
engineering strain, e
sy
e p = 0.002
e = 0.002 = z/z
z = 0.1 mm
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Tensile Strength, TS
• 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.
sy
strain
Typical response of a metal
F = fracture or
ultimate
strength
Neck – acts as stress concentrator e
ngi
nee
rin
g
TS s
tres
s
engineering strain
• Maximum stress on engineering stress-strain curve.
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Impact Tets
• Measure toughness of materials in terms of energy absorption.
• Specimen is impacted by a hammer and the energy absorbed during fracture is measured in Joule.
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Fracture mechanics
• Resistance of materials to crack
propagation (to failure).
• Crack propagation can be
predicted before failure.
• Material will fail when the
stress intensity factor K
reaches the critical value KC.
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Fatigue tests
Material is subjected to a repetitive or
fluctuating stress (cyclic loading) and will fail at
a stress level much lower than that causes
failure in statistic loading.
Stresses in fatigue loading
Parameters: • Fracture life (fatigue strength) • Fatigue crack growth resistance • Paris exponent (m) • Fatigue threshold (Kth)
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To Improve Material Properties
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Main references
• Dieter, G.E., Mechanical metallurgy, 1988, SI metric edition, McGraw-Hill, ISBN 0-07-100406-8.
• Hibbeler, R.C. Mechanics of materials, 2005, SI second edition, Person Prentice Hall, ISBN 0-13-186-638-9.
•Hertzberg, R. W., Deformation and Fracture Mechanics of Engineering Materials, 1995, Wiley; 4 edition, ISBN-13: 978-0471012146
•Callister, W. D. Jr. , Materials Science and Engineering: An Introduction, 2007, John Wiley & Sons, Inc. , ISBN-13: 978-0-471-73696-7
•Meyers, M. A. and Chawla, K. K., Mechanical Behavior of Materials, 2009, Cambridge University Press, ISBN-13 978-0-521-86675-0
•Udomphol, T., Mechanical Metallurgy lecture notes, http://www.sut.ac.th/engineering/Metal/courses/mechmet.html
• Rösler, J. ,· Harders , H., · Baeker M., Mechanical Behaviour of Engineering Materials 2007, Springer, ISBN 978-3-540-73446-8
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