Chapter 3

3.0 MECHANICAL PROPERTIES OF
METALS AND FAILURE

Concept of Stress-Strain

(a)
• Figure (a) shows an apparatus that measures the mechanical
properties of metals using applied tensile forces
• Figure (b) was generated from a tensile test performed by an apparatus such as this

on a steel specimen. Data plotted are stress (vertical axis—a measure of applied
force) versus strain (horizontal axis—related to the degree of specimen elongation).
• The mechanical properties of modulus of elasticity (stiffness, E), yield strength , and
tensile strength (TS) are determined as noted on these graphs.

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Video : How Tensile Test being carried out

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Tension Test

• One of the most common
mechanical stress–strain tests is
performed in tension.

• A specimen is deformed, usually
to fracture, with a gradually
increasing tensile load that is
applied uniaxial along the long
axis of a specimen.

• Schematic illustration of how a
tensile load produces an
elongation and positive linear
strain. Dashed lines represent the
shape before deformation ; solid
lines, after deformation.

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Compression Test

• A compression test is conducted in
a manner similar to the tensile
test, except that the force is
compressive and the specimen
contracts along the direction of
the stress

• Schematic illustration of how a
compressive load produces
contraction and a negative linear
strain.

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Shear Test

• F is the load or force imposed parallel to
the upper and lower faces, each of which
has an area of A0. The shear strain is
defined as the tangent of the strain angle ,
as indicated in the figure.

• The units for shear stress and strain are the
same as for their tensile counterparts

• Schematic representation of shear strain

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Torsional Test

• Torsion is a variation of pure shear
in which a structural member is
twisted in the manner

• Torsional forces produce a
rotational motion about the
longitudinal axis of one end of the
member relative to the other end.

• Schematic representation of
torsional deformation

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CHAPTER 3: MECHANICAL PROPERTIES OF MATERIAL AND FAILURE DJJ 30113 MATERIAL SCIENCE AND ENGINEERING

THE ELASTIC AND PLASTIC
DEFORMATION FROM THE STRESS-STRAIN CURVE

STRESS-STRAIN CURVE

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STRESS- STRAIN CURVE

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ELASTIC AND PLASTIC DEFORMATION

• Elastic Deformation: The material returns to
its original shape when the force is removed.

• Plastic Deformation: The material does not
return to its original shape when the force is
removed.

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CHAPTER 3: MECHANICAL PROPERTIES OF MATERIAL AND FAILURE DJJ 30113 MATERIAL SCIENCE AND ENGINEERING

3.1 TENSILE PROPERTIES FOR PLASTIC
DEFORMATION

The most common mechanical properties considered are
yielding and yield strength, tensile strength, ductility,
resilience, toughness and brittleness.

The mechanical properties of metals determine the range of
usefulness of a material and establish the service life that can be
expected.

Materials properties of metal can be improved by heat
treatment, alloying & mechanical work.

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CHAPTER 3: MECHANICAL PROPERTIES OF MATERIAL AND FAILURE DJJ 30113 MATERIAL SCIENCE AND ENGINEERING

3.1 MATERIAL PROPERTIES

3.1.3 Mechanical properties

Types of mechanical properties:

• yielding and yield Strength
• Tensile Strength
• Ductility
• Resilience
• Toughness
• Brittleness

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CHAPTER 3: MECHANICAL PROPERTIES OF MATERIAL AND FAILURE DJJ 30113 MATERIAL SCIENCE AND ENGINEERING

3.2 MECHANICAL PROPERTIES IN MATERIAL

TESTING

The understanding of these mechanical properties will help to
select the most suitable material to produce a product in

industry.

MECHANICAL DEFINITION
PROPERTIES

Yielding and ability to withstand various forces to which it is subjected during a
Yield Strength test or in service (resist deformation).

Tensile is the maximum amount of tensile stress that it can take before
Strength failure, such as breaking or permanent deformation. Tensile
strength specifies the point when a material goes from elastic to
plastic deformation. It is expressed as the minimum tensile stress
(force per unit area) needed to split the material apart.

Ductility the ability of a metal to withstand elongation or bending before
fracture under tensile force.

Resilience ability of a material to absorb energy when it is deformed
elastically, and release that energy upon unloading.
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CHAPTER 3: MECHANICAL PROPERTIES OF MATERIAL AND FAILURE DJJ 30113 MATERIAL SCIENCE AND ENGINEERING

3.2 MECHANICAL PROPERTIES IN MATERIAL
TESTING

MECHANICAL DEFINITION
PROPERTIES
Toughness ability of a material to absorb energy and plastically deform
without fracturing.
Brittleness Break or shatters without having plastic deformation.

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CHAPTER 3: MECHANICAL PROPERTIES OF MATERIAL AND FAILURE DJJ 30113 MATERIAL SCIENCE AND ENGINEERING

3.3 MATERIAL FAILURE BEHAVIOURS

Failure of materials may have huge costs. Causes included improper materials
selection or processing, the improper design of components, and improper use.

The usual causes of mechanical failure in the component or system are:

• Misuse or abuse
• Assembly errors
• Manufacturing defects
• Improper or inadequate maintenance
• Design errors or design deficiencies
• Improper material or poor selection of materials
• Improper heat treatments

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CHAPTER 3: MECHANICAL PROPERTIES OF MATERIAL AND FAILURE DJJ 30113 MATERIAL SCIENCE AND ENGINEERING

3.3 MATERIAL FAILURE BEHAVIOURS

3.3.1 Creep

Creep occurs under certain load at elevated temperature normally
above 40 % of melting temperature of the material. In other words,
creep is progressive plastic deformation under constant stress with
time. There are 3 stages of creep failure:

a) Primary creep: Creep rate decreases with time due to strain
hardening.

b) Secondary creep: Creep rate is constant due to
simultaneous strain hardening and recovery process.

c) Tertiary creep: Creep rate increases with time leading to
necking and fracture

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CHAPTER 3: MECHANICAL PROPERTIES OF MATERIAL AND FAILURE DJJ 30113 MATERIAL SCIENCE AND ENGINEERING

3.3 MATERIAL FAILURE BEHAVIOURS

3.3.2 Fatigue

• The fatigue behavior of material is usually described by means of S
(stress) vs N (number of cycle) diagram.

• Fatigue is the weakening of a material caused by repeatedly applied
loads or cyclic loading.

• The term fatigue is used because this type of failure normally
occurs after a lengthy period of repeated stress or strain cycling.

• The applied stress may be axial (tension-compression), flexural
(bending), or torsional (twisting) in nature.

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CHAPTER 3: MECHANICAL PROPERTIES OF MATERIAL AND FAILURE DJJ 30113 MATERIAL SCIENCE AND ENGINEERING

3.3 MATERIAL FAILURE BEHAVIOURS

3.3.3 Fracture

• Fracture - material separate or break into two pieces under apply
stress

• There are commonly two type of fracture, which are Brittle Fracture
and Ductile Fracture.

• For brittle fracture, no apparent plastic deformation takes place
before fracture

• while in ductile fracture, extensive plastic deformation (necking)
takes place before fracture.

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CHAPTER 3: MECHANICAL PROPERTIES OF MATERIAL AND FAILURE DJJ 30113 MATERIAL SCIENCE AND ENGINEERING

3.3 MATERIAL FAILURE BEHAVIOURS

3.3.3 Fracture

ductile fracture (necking) brittle fracture

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Video : Ductile and Brittle Fracture Test

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CHAPTER 3: MECHANICAL PROPERTIES OF MATERIAL AND FAILURE DJJ 30113 MATERIAL SCIENCE AND ENGINEERING

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CHAPTER 3: MECHANICAL PROPERTIES OF MATERIAL AND FAILURE DJJ 30113 MATERIAL SCIENCE AND ENGINEERING

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