3 Mechanical and Physical Properties of Material

8/13/2019 3 Mechanical and Physical Properties of Material

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MECHANICAL PROPERTIES OF

Tensile test

Engineering stress Vs Strain True stress Vs Strain

ompress ve es

Shear test

Hardness

Effect of Temperature Vs Properties

Viscosity

Visco-elastic behavior


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Stress-Strain Relationships

Three types of static stresses to which materials canbe subjected:

. -

2. Compressive - tend to squeeze it3. Shear - tend to cause ad acent ortions

of material to slide against each other

Stress-strain curve - basic relationship that

describes mechanical properties for all

three types

Tensile test:


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T ical ro ress of a tensile test: 1 be innin of test no load 2 uniform elon ation

and reduction of cross-sectional area; (3) continued elongation, maximum load

reached; (4) necking begins, load begins to decrease; and (5) fracture. If pieces are

put back together as in (6), final length can be measured

oe

A

F=Engineering Stress

Engineering Strain o

o

L

LL

e

=


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two distinct forms of

behavior:

1. Elastic region prior

to yielding of the

material

2. Plastic region after

yielding of the

-

mater a


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Stress/strain curve


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Elastic Region in Stress-Strain Curve Relationship between stress and strain is linear Material returns to its original length when stress is

removed Hooke's Law: =E e

whereE= modulus of elasticity Eis a measure of the inherent stiffness of a material

As stress increases, a point in the linear relationship is finally

reached when the material begins to yield ie point can e ent e y t e c ange n s ope at t e

upper end of the linear region,beginning of plastic deformation

= , ,

elastic limit

As load is increased be ond Y, elon ation roceeds

at a much faster rate than before, causing the slope of

the curve to change dramatically


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Tensile Strength in Stress-Strain Curve Elongation is accompanied by a uniform reduction incross-sectional area, consistent with maintaining constant vol

, ,

engineering stress at this point is called the tensile strength TSorultimate tensile strength

TS=oA

max

Ductility in Tensile TestAbility of a material to plastically strain without fracture

of LLEL

=

owhereEL = elongation;Lf= specimen length at fracture; and

Lo = original specimen length


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True Stress and True strainTrue Stress : Stress value obtained by dividing the instantaneous

A=

area resisting the load

True strain : Provides a more realistic assessment of "instantaneous

elongation per unit length

L LdL

oL LL

o

n==


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True Stress and True strain curve

= e(1 + e)True strain and engineering strain relation

=

Strain Hardening in Stress-Strain Curve

region until necking means metal is becoming stronger

as strain increases called strain hardenin


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True stress-strain curve in plastic region

plotted on log-log scale

nK=Flow Curve

where K = strength coefficient; and n = strain hardening


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Categories of Stress-Strain RelationshipPerfectly elastic Brittle materials: ceramics,

many cast irons, and

thermosetting polymers

Elastic and Flow curve: K= Y, n = 0

perfectly plastic

heated to sufficiently high

temperatures (above

recrystrallisation point)

Elastic and strain Flow curve: K> Y, n > 0

Most ductile metals behave thisway when cold worked


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Compression Test

cylindrical specimen between two platens

Engineering Comprehensive stresse

A

F=

Engineering Comprehensive stress

o

o

ohhhe =

Shape of plastic region is different

from tensile test because

cross-section increases.

Barreling effect - friction

the true stress-strain relationships are nearly identical

(Kand n) from tensile test data can be applied to

compression operations


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Shear Properties

Shear (a) stress and (b) strain

Application of stresses in opposite directions on either side of a thin element

A

F

=Shear stress defined as

Shear strain defined as =

where = deflection element; and b = distance over which


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Torsion test


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Typical shear stress-strain curve - torsion test

Since cross-sectional area of test

specimen in torsion test does not

change as in tensile and compression,

engineering stress-strain curve for

shear true stress-strain curve

Shear Elastic Stress-Strain Relationship

In the elastic region, the relationship is defined as G=

From tensile strength of most material it is estimatedwhere G = shear mo ulus, or shear mo ulus of elasticity

.

Shear strength (S) : S

0.7(TS)


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(flexural bending test)

2

51 FLTRS

.=

outer fibers of specimen are exceeded

Failure type: cleavage - common with ceramics and metals at low temperatures,

in which se aration rather than sli occurs alon certain cr stallo ra hic lanes


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Fatigue test

Fatigue : cyclic stresses

applied

S/N curve :

S : amplitude of stress

: o o cyc e

Endurance test : max stress

sub ected without fati ue

failure regardless of

number of cycles


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Creep test

material held for long periods. This phenomenon is moredominated at high temperature..


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HardnessResistance to permanent indentation Good hardness generally means material is resistant to scratching

and wear

Most tooling used in manufacturing must be hard for scratch andwear resistance

Commonl used for assessin material ro erties because the arequick and convenient

Most well-known hardness tests areBrinell andRockwell

)(22

ibbb DDDD

HB

=


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Effect of Temperature on Properties

Ability of a material to retain hardness at

elevated temperatures


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Effect of Tem erature on Pro erties

Ability of a material to retain hardness at elevated temperatures


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Recrystallization in Metals

Most metals strain harden at room temperature according tothe flow curve (n > 0)

On heatin to sufficientl hi h tem erature and deformed,strain hardening does not occur instead new strain-free grains

form called recrystallization Instead, new grains are formed that are free of strain

The metal behaves as a perfectly plastic material; that is, n= 0

Recr stallization tem erature of a iven metal = aboutone-half its melting point (0.5 Tm) as measured on an absolutetemperature scale

Forming metals at temperatures above recrystallization

temperature is called hot working


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Viscosity Properties Viscosity is the resistance to flow that is characteristic of a

given fluid

gradients are present in the fluid Reciprocal of viscosity isfluidity - the ease with which a

fluid flows

Many manufacturing processes are accomplished on materials

Examples:

Glass is formed in a heated and highly fluid state

Polymers are almost always shaped as thick fluids


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Viscosity can be defined using two parallel plates separated

A fluid fills the space between the two plates.

Shear stress is the frictional force exerted by the fluid per unit area

AF=

from the shear viscosity of the fluid


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Shear Rate

ear stress s re ate to s ear rate ve oc ty gra ent perpen cu ar to

flow direction), which is defined as the change in velocity dv relative to

d dv&

where = shear rate, 1/s; dv = change in velocity, m/s; and dy = change in distance y, m

dy

&

It is the fluid property that defines the relationship between F/A and

dv/d that is

dy

dv

A

F= &=

or

&

=

or

where = a constant of proportionality - coefficient of viscosity, Pa-s

Viscosity of a fluid is the ratio of shear stress to shear rate during flow

,

For non-Newtonian fluids, it is not


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Viscosity of Polymers and Flow Rate

Viscosity of a thermoplastic polymer melt is not constant

It is affected by flow rate

s e av or s non- ew on an

A fluid that exhibits this decreasing viscosity with increasing shear rateis calledpseudoplastic

This complicates analysis of polymer shaping processes such as

injection molding


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Viscoelastic Behavior (Combination of viscosity and

e as c yMaterial property that determines the strain it experiences when

subjected to combinations of stress and temperature over time

Viscoelastic Behavior of Pol mers: Sha e Memor

It remembers


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Types of BondMechanical Pro erties for

Engineering materialsStress strain

Tens e

Compression

Bending

Polymers

ViscosityVisco-elasticity


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ysica properties o

dimensions, tolerance andsurfaces


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Thin s to be covered Volumetric and Melting properties

Thermal Properties

Mass diffusion

Electric Properties

Electrochemical Processes Dimension, tolerance related

Surface texture and integrity


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Volumetric and MeltingProperties

Pro erties related to the volume of solids

Density atomic number, atomic radius, Packingfactor

Strength to weight ratio - criteria

Thermal expansion

ens y w empera ure

Co-efficient of thermal expansion length ratio -

Melting point


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Specific heat

Thermal diffusivity

- Volumetric specific heat


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Mass diffusionMovement of atoms or molecules within a material or across

a boundary between two materials in contact

Ficks first law : dm/dt = -D (dc/dt) A

D = diffusion co-eff of the metal


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Electrical Properties Solid electrons bring movement

Liquid ions

Resistivity

Resistivity changes with temperature property

Dielectric property - non conductor apply to DC source Superconductor and semi conductor

resistivity between conductor and insulator


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Electrochemical rocessesFaradays law :

*

Q = It

F = Faradays constantM = molar massZ = valency number of ions

At electrodes followin reactions can ha en Deposition or dissolution can be done Gas can be released

Dimensions tolerance


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Dimensions, tolerance

component specified on the part drawing

dimension.

Surface exterior boundary ofan an ob ect with its surroundin


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Sur ace technolo Characteristics of surface

Surface texture roughness, waviness, lay patternof surface texture

Surface integrity Surface texture and

subsurface changes


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En ineerin materials Metals

Alloys, Ferrous and Non ferrous,superalloys

Ceramics

Pol er Thermo plastic, Thermo set, Elastomer

reinforcement


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Introduction General properties of metals High stiffness and strength, Toughness, Good electrical and

erma proper es

Metal categories starting form Cast, wrought and powdered metal

Metal classification

Ferrous and non ferrous

Alloy is a metal composed of two or more elements

where at least 1 is metallic

Solid Solution Substitutional and intermetallic hase

Intermediate phases (phase any homogeneous mass of material)

S lid l ti (SS) ll


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Solid solution (SS) alloys

SS - solvent base element metal

Substitutional SS Hume-Rothery rules

1. Atomic radius of elements 15%

Interstitial (SS) - atomicradius of solute atom must be

-2. Lattic types must be same

3. Lower valence metal solvent

4. Chemical affinity should be less

Intermediate phase : The amount of dissolving element exceedsthe solid solubility forms second phase formChemical composition and crystal structure are different

Two types : Intermetallic and metallic compound (metal

and non metal)

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3 Mechanical and Physical Properties of Material