lecture_8

10/12/2010

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Structure and Properties of Materials

Eighth Lecture

Mechanical Properties and Behavior

October 12, 2010ENGR 230


Nahid Abdel Salam

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It is the response of materials

to external forces

Mechanical properties

are expressed in terms of

quantities that are functions of

stress or strain or both stress

and strain.

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Introduction



Nahid Abdel Salam

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The mechanical behavior of materials

is described by their mechanical

properties, which are measured with

idealized, simple tests;

These tests are designed to present

different types of loading conditions;

The properties of a materials

reported in various handbooks are

the results of these tests;

Introduction (cont.)



Nahid Abdel Salam

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The material’s composition,

nature of bonding, crystal

structure, defects, and grain

size have a profound influence

on the strength and ductility

of metallic materials.

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October 12, 2010 ENGR 230


Nahid Abdel Salam

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There are three factors involved in

defining the manner in which the

load is applied:

kind of stress induced;

rate at which the load is applied;

number of times the load is

applied

Types of Loading (stress induced)



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TorsionTension

Compression Bending Shear

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Three groups of the rate of loading

Rate of loading

over a short time

(Static test)

very rapidly

(Dynamic test,

impact)

long-time (Creep

test)



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Number of Times of Loading

Number of times the load is applied

Single application

Repeated application (Fatigue test)



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10/12/2010

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A typical stress-strain diagram for a

ductile metal undergoing tension

October 12, 2010 ENGR 230


Nahid Abdel Salam9

Definitions (V.I)



Nahid Abdel Salam

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You are requested to know precisely the

definitions of the following:

stress, strain, strength, modulus of

elasticity E, shear modulus G,

yield strength, elastic limit,

proportional limit, ultimate tensile

strength UTS, Poisson’s ratio,

stiffness, toughness, ductility.

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Elastic Properties, E and Stiffness



Nahid Abdel Salam

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The modulus of elasticity, Young’s

modulus E = σ / ε in elastic zone

In metallic materials, this modulus is

closely related to the binding forces;

For ceramics, the Young’s modulus

depends on the level of porosity;

Young’s modulus of a composite

depends upon the stiffness and amounts

of the individual components.

Stiffness



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Young’s modulus is a measure of

the Stiffness of a component;

A stiff component, with a high

modulus of elasticity, will show

much smaller changes in

dimensions if the applied stress is

relatively small and, therefore,

causes only elastic deformation.

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Poisson’s Ratio



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μ = εlateral / εlongitudinal

In elastic region μ = 0.25-0.4

In plastic region μ = 0.5

Ductility

Gage Length

Measure of Ductility

Elongation % = (L-Lo)/Lo %

Reduction in Area % = (A-Ao)/Ao%

• Higher elongation indicates higher

ductility.

• The smallest cross section can be

measured at or after fracture.

October 12, 2010

ENGR 230


Nahid Abdel Salam14

10/12/2010

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Toughness



Nahid Abdel Salam

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Modulus of Toughness:

The amount of work per

unit volume of a material

required to carry the

material to failure under

static loading. It equals

the area under stress

strain curve up to

fracture.

Macroscopic deformation of metal

sample under tensile test



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10/12/2010

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Effect of temperature and Strain rate



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