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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

ElasticityIt 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.Percentage of elongation =

The percentage of reduction =

100legnth Original

length in Increase

100area sectional cross Original

area sectional crossin Decrease

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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 cross-section.

• 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 StrengthIt is the value of strength of the material which corresponds to plastic range and rupture. It is also termed as ultimate strength.

Tensile StrengthTensile strength is the ultimate strength in tension and corresponding to the maximum load.

Tensile strength = areationalcrossOriginal

LoadTensileMaximum

sec

The tensile stress is expressed in N/m2

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Compressive strengthThe compressive strength of a metal is the value of load applied to break it off by crushing.

Compressive strength = areationalcrossOriginal

LoadeCompressivMaximum

sec

The compressive stress is expressed in N/m2.

Shear StrengthThe shear strength of a metal is the value of load applied tangentially to shear it off across the resisting section.

Shear strength =areationalcrossOriginal

LoadgentialMaximum

sec

tan

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Shear StrengthThe shear strength of a metal is the value of load applied tangentially to shear it off across the resisting section.

Shear strength =areationalcrossOriginal

LoadgentialMaximum

sec

tan

Bending StrengthBending strength of a metal is the value of load which can break the metal by bending it across the resisting section.

Bending stress =areationalcrossOriginal

LoadBendingMaximum

sec

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Torsional StrengthTorsional Strength of a metal is the value of load applied to break the metal by twisting across the resisting section.

Torsional strength =areationalcrossOriginal

LoadtwistingMaximum

sec

This is expressed in N/m2.BrittlenessIt 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.