Transcript LECTURE-I - Srm University

LECTURE-I
 Introduction
 Some important definitions
 Stress-strain relation for
different engineering materials
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Introduction
•
The mechanical properties of materials, their strength, rigidity and
ductility are of vital important. The important mechanical properties of
materials are elasticity, plasticity, strength, ductility, hardness, brittleness,
toughness, stiffness, resilience, fatigue, creep, etc…
Important of mechanical properties of various materials
•
It provides a basis for predicting the behavior of a material under various
load conditions.
•
It is helpful in making a right selection of a material for every component
of a machine or a structure for various types of load and service
conditions.
•
It helps to decide whether a particular manufacturing process is suitable
for shaping the material or not, or vice-versa.
•
It also informs in what respect the various mechanical properties of a
material will get affected by different mechanical processes or operations
on a material.
•
It is helpful in safe designing, of the shape and size of various metal
parts for a given set of service conditions.
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Some Important Definitions
Isotropy
• A body is said to be isotropic if its physical properties are
not dependent upon the direction in the body along
which they are measured.
• Ex:Aluminum steels and cast ions
Anisotropy
• A body is said to Anisotropic if its physical properties are
varied with the direction in a body along which the
properties are measured
• Ex: Various composite materials, wood and laminated
plastics
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Some Important Definitions
Elasticity
It is the property of a material which enables it to regain its original
shape and size after deformation with in the elastic limit.
This property is always desirable in metals used in machine tools
and other structural constituents.
Plasticity
• It is the ability of materials to be permanently deformed even after
the load is removed
• This property of a material is of importance in deciding
manufacturing processes like forming, shaping, extruding operations
etc.
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Ductility
It is defend as the property of a metal by virtue of which it can be draw
into elongated before rupture takes plac.
It is measured by the percentage of elongation and the percentage of
.
reduction in area before rupture of test piece
Increasein lengt h
 100
Percentage of elongation =
Originallegnt h
The percentage of reduction =
Decreasein crosssectionalarea
 100
Originalcrosssectionalarea
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Stress-strain curve
• The elastic behavior of a material can be studied by plotting a curve
between the stress along the x axis and the corresponding strain
along the y axis. This curve is called stress-strain curve.
•
•
•
•
elastic limit
permanent set
yield point
creeping
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Strength
•
It is defined as the capacity of a material to with stand, once the
load is applied. It is expressed as force per unit area of crosssection.
•
Depending upon the value of stress, the strengths of a metals
can be elastic or plastic
•
Depending upon the nature of stress, the strength of a metal can
be tensile, compressive, shear, bending and torsional.
Elastic Strength
•
It is the value of strength corresponding to transition from elastic
to plastic range, i.e., when material changes its behaviors from
elastic range to plastic range.
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Plastic Strength
It is the value of strength of the material which corresponds to plastic
range and rupture. It is also termed as ultimate strength.
Tensile Strength
Tensile strength is the ultimate strength in tension and corresponding
to the maximum load.
Tensile strength
=
Maximum Tensile Load
Original cross sec tional area
The tensile stress is expressed in N/m2
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Compressive strength
The compressive strength of a metal is the value of load
applied to break it off by crushing.
Compressive strength
Maxim um Com pressive Load
=
Original cross sec tional area
The compressive stress is expressed in N/m2.
Shear Strength
The shear strength of a metal is the value of load applied
tangentially to shear it off across the resisting section.
Maxim um tan gential Load
Shear strength =
Original cross sec tional area
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Shear Strength
The shear strength of a metal is the value of load applied tangentially to
shear it off across the resisting section.
Shear strength =
Maxim um tan gential Load
Original cross sec tional area
Bending Strength
Bending strength of a metal is the value of load which can break the metal
by bending it across the resisting section.
Bending stress =
Maxim um Bending Load
Original cross sec tional area
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Torsional Strength
Torsional Strength of a metal is the value of load applied to break
the metal by twisting across the resisting section.
Maximum twisting Load
Torsional strength =
Original cross sec tional area
This is expressed in N/m2.
Brittleness
It may be defined as the property of a metal by which it will
fracture without any appreciable deformation.
Ex: cast iron, glass and concrete
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Some Important Definitions
Toughness
•
It may be defined as the property of a metal by virtue
of which it can absorb maximum energy before fracture
takes place.
Stiffness
•
This may be defined as the property of a metal by
virtue of which it resists deformation. Modulus of rigidity
is the measure of stiffness.
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Some Important Definitions
Resilience
•
Resilience is the property of a material by virtue of
which it stores energy and resists shocks or impacts
Endurance
•
The endurance is the property of a material by virtue
of which it can withstand varying stresses or repeated
application of stress.
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Stress-Strain Relation for Different Engineering
Materials
• The stress and strain relation can be studied by drawing a graph or
curve by taking strain along the x axis and the corresponding stress
along the y axis. This curve is called stress- strain curve.
For ferrous metal
• From the stress-strain diagram for different types of steel and
wrought iron the strength of the ferrous metals depends up on
carbon content.
• The proportion of carbon does not have an appreciable effect on
young’s modulus of elasticity during any hardening process.
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Stress-Strain Relation for Different Engineering
Materials
Stress- Strain curve for ferrous metals
Stress Strain curve for non - ferrous metals
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Non-ferrous metal
• The elastic properties of non-ferrous metals vary to a considerable
extent, depending upon the method of working and their
compositions in the case of alloys.
• The early portion of the stress-strain diagram for most of the metals
is never quite straight line, but the yield point is well define.
• Brittle materials show little or no permanent deformation prior to
fracture. Brittle behavior is exhibited by some metals and ceramics
like magnesium oxide .
• The small elongation prior to fracture means that the materials gives
no indication of impending fracture and brittle fracture. It is often
accompanied by loud noise.
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Saline Features of stress-strain relation
• The properties of ductile metals can be explained with
the help of stress-strain curves.
• Higher yield point will represents greater hardness of the
metals.
• A higher value of maximum stress point will represent a
stronger metal.
• The distance from the ordinates of the load point (or)
breaking stress will indicate the toughness and
brittleness of the metal. The shorter the distance then
the metal is more brittle.
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