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MSE 321

MECHANICAL BEHAVIOUR OF MATERIALS

Yahya K. Tr

GYTE rencileri tarafndan oaltlmasnda saknca yoktur

Things to Know

Sources : A comprehensive list will be given at the end of the lecture

In Turkish: Malzemelerin yaps ve mekanik davranlar, Kayal ve imenolu, T Kimya Metalurji Fakltesi

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


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


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)


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)


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


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.



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.



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.


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


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.


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


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


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

engi

nee

rin

g

TS s

tres

s

engineering strain

Maximum stress on engineering stress-strain curve.


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.


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.


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)


To Improve Material Properties


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

Rsler, J. , Harders , H., Baeker M., Mechanical Behaviour of Engineering Materials 2007, Springer, ISBN 978-3-540-73446-8


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