Unit ii material properties.

UNIT II MATERIALPROPERTIES
2.1 Mechanical Properties of Engineering
Materials
The mechanical properties of a material are those which affect
the mechanical strength and ability of a material to be molded in
suitable shape. Some of the typical mechanical properties of a material
include:
 Strength
 Toughness
 Hardness
 Hardenability
 Brittleness
 Malleability
 Ductility
 Creep and Slip
 Resilience
 Fatigue

1. Strength
It is the property of a material which opposes the deformation or
breakdown of material in presence of external forces or load.
Materials which we finalize for our engineering products, must have
suitable mechanical strength to be capable to work under different
mechanical forces or loads.
2. Toughness
It is the ability of a material to absorb the energy and gets
plastically deformed without fracturing. Its numerical value is
determined by the amount of energy per unit volume. Its unit is Joule/
m3. Value of toughness of a material can be determined by stress-
strain characteristics of a material. For good toughness, materials
should have good strength as well as ductility.
For example: brittle materials, having good strength but limited
ductility are not tough enough. Conversely, materials having good
ductility but low strength are also not tough enough. Therefore, to be
tough, a material should be capable to withstand both high stress and
strain.
3. Hardness
It is the ability of a material to resist to permanent shape change
due to external stress. There are various measure of hardness – Scratch
Hardness, Indentation Hardness and Rebound Hardness.
1. Scratch Hardness
Scratch Hardness is the ability of materials to the oppose the
scratches to outer surface layer due to external force.
2. Indentation Hardness
It is the ability of materials to oppose the dent due to punch of
external hard and sharp objects.
3. Rebound Hardness
Rebound hardness is also called as dynamic hardness. It is
determined by the height of “bounce” of a diamond tipped
hammer dropped from a fixed height on the material.

4. Hardenability
It is the ability of a material to attain the hardness by heat
treatment processing. It is determined by the depth up to which the
material becomes hard. The SI unit of hardenability is meter (similar
to length). Hardenability of material is inversely proportional to the
weld-ability of material.
5. Brittleness
Brittleness of a material indicates that how easily it gets
fractured when it is subjected to a force or load. When a brittle
material is subjected to a stress it observes very less energy and gets
fractures without significant strain. Brittleness is converse to
ductility of material. Brittleness of material is temperature
dependent. Some metals which are ductile at normal temperature
become brittle at low temperature.
6. Malleability
Malleability is a property of solid materials which indicates that
how easily a material gets deformed under compressive stress.
Malleability is often categorized by the ability of material to be
formed in the form of a thin sheet by hammering or rolling. This
mechanical property is an aspect of plasticity of material. Malleability
of material is temperature dependent. With rise in temperature, the
malleability of material increases.
7. Ductility
Ductility is a property of a solid material which indicates that
how easily a material gets deformed under tensile stress. Ductility is
often categorized by the ability of material to get stretched into a wire
by pulling or drawing. This mechanical property is also an aspect of
plasticity of material and is temperature dependent. With rise in
temperature, the ductility of material increases.

8. Creep and Slip
Creep is the property of a material which indicates the tendency
of material to move slowly and deform permanently under the
influence of external mechanical stress. It results due to long time
exposure to large external mechanical stress with in limit of yielding.
Creep is more severe in material that are subjected to heat for long
time. Slip in material is a plane with high density of atoms.
9. Resilience
Resilience is the ability of material to absorb the energy when it
is deformed elastically by applying stress and release the energy when
stress is removed. Proof resilience is defined as the maximum energy
that can be absorbed without permanent deformation. The modulus of
resilience is defined as the maximum energy that can be absorbed per
unit volume without permanent deformation. It can be determined by
integrating the stress-strain cure from zero to elastic limit. Its unit is
joule/m3.
10. Fatigue
Fatigue is the weakening of material caused by the repeated
loading of the material. When a material is subjected to cyclic loading,
and loading greater than certain threshold value but much below the
strength of material (ultimate tensile strength limit or yield stress
limit), microscopic cracks begin to form at grain boundaries and
interfaces. Eventually the crack reaches to a critical size. This crack
propagates suddenly and the structure gets fractured. The shape of
structure affects the fatigue very much. Square holes and sharp corners
lead to elevated stresses where the fatigue crack initiates.

2. FATIGUE STRENGTH
1. What is Fatigue?
Fatigue can be explained as the weakening of a material due to the
application of fluctuating loads that result in damage to the material’s
structure and eventual failure. The damage starts locally and builds up
over time and can end in acatastrophe.
2. Fatigue Strength
Fatigue strength is the ability of a material to resist fatigue failure.
3. Fatigue Limit
Fatigue limit, denoted by Sf, is defined as the value of stress at
which Nf (number of cycles at which failure occurs) becomes very large.
Nf can be increased by reducing the stress value of the cyclic load.
When keeping the load below the fatigue limit, a part can withstand
a huge number of cycles, usually more than 10 million but up to 500
million.
The difference between fatigue strength and fatigue limit is in
the number of cycles. It is considerably higher with fatigue limit.
Therefore, engineers try to design their parts so that they are kept under
the fatigue limit duringwork.
4. Understanding Fatigue Failure
Fatigue failure is something everyone has encountered while trying
to break a metal wire.
The process includes bending the wire back and forth numerous
times. Each back-and-forth bending is one cycle. When the wire finally
breaks, you can count the number of cycles it took to lead to the initial
crack and final break.
Knowing the number of cycles and the loading stresses gives the
fatigue strength of that material.
If you tried to break the wire just by pulling from both ends, the force
requirements would have been pretty high. At the same time, bending
only requires minuscule forces and arrives at the sameresult.

The wire was subjected to fluctuating loads at the bending point as the
top and bottom part of the wire cross-section were alternately stretched
and compressed. With enough number of load cycles, the wirebreaks.
This is an example of fatigue failure. Thus, it can happen quite easily
even at small loads and a small number of load cycles, depending on the
material.
It is also worth mentioning that fatigue failure does not happen
gradually. It is instantaneous, like a brittle material just breaking into
smaller pieces.
It is also far less predictable than regular failure as there are few
prior indications. These indications are difficult to perceive with the
naked eye. In regular failure, due to excessive loading, necking will be
observed but there is no necking before fatigue breakdown occurs. This
makes it impossible to predict exactly at what point in time a part will
fail due tofatigue.
To prevent such a situation, the parts must be closely inspected and
changed after a recommended number of load cycles. The number of
cycles depends upon the material characteristics and the magnitude of
the load. A higher load means a shorter lifecycle. This can be better
understood using an S-N curve which we will get to shortly.
2.2.5 Failure Process
Fatigue failure happens without warning but on careful
observation, some indications areevident.
1. Formation of the initial micro crack
On continuous exposure of the material to cyclic loading, minuscule
cracks start to develop at high-stress points. These cracks can be
observed by non-destructive testing methods.
Dye penetrant testing for inspection of diesel generator engine
connecting rods during routine overhauls is one such example of crack
detection. If cracks are found, the part is replaced with a new or
reconditioned spare part.

2. Crack propagation
Once a fatigue crack has occurred, it propagates through the part
with every load cycle. While spreading through the material, it will
usually produce striations on thesurface.
Striations are marks on the surface that show the position of the
crack tip. These striations are a tell-tale of the development of a fatigue
crack. The crack propagation is extremely slow when the crack is first
initiated and is referred to as Stage I crack growth.
3. Fracture
When the crack reaches a critical size and the stress intensity at the
point exceeds the fracture toughness of the material, the crack spreads at
a high speed.
This type of fast propagation is known as Stage II crack growth and
it happens in a direction perpendicular to the applied force.
Over time, the material is unable to handle any further loading and
complete fracture occurs. This failure is immediate and can result in
serious consequences for the workers using the machine and the
machinery itself.
2.2.6 Machine Components Prone to Fatigue
Gears

Gears transmit power through their teeth. These teeth deal with high
loads when they are in contact with the other mating gear’s tooth. When
they move away from this position, the stresses are removed. Cyclic
loading occurs as the magnitude of forces on a tooth changes
continuously.
Furthermore, the load is not evenly distributed on the entire length
ofthe teeth. This localization ofload can further exacerbate fatigueissues
2.3 Fracture Toughness
Fracture toughness is an indication of the amount of stress required to
propagate a preexisting flaw. It is a very important material property
since the occurrence of flaws is not completely avoidable in the
processing, fabrication, or service of a material/component. Flaws
may appear as cracks, voids, metallurgical inclusions, weld defects,
design discontinuities, or some combination thereof. Since engineers
can never be totally sure that a material is flaw free, it is common
practice to assume that a flaw of some chosen size will be present in
some number of components and use the linear elastic fracture
mechanics (LEFM) approach to design critical components. This
approach uses the flaw size and features, component geometry,
loading conditions and the material property called fracture toughness
to evaluate the ability of a component containing a flaw to resist
fracture.

A parameter called the stress-intensity factor (K)
is used to determine the fracture toughness of
most materials. A Roman numeral subscript
indicates the mode of fracture and the three modes
of fracture are illustrated in the image to the right.
Mode I fracture is the condition in which the crack
plane is normal to the direction of largest tensile
loading. This is the most commonly encountered
mode and, therefore, for the remainder of the
material we will consider KI
The stress intensity factor is a function of loading,
crack size, and structural geometry. The stress
intensity factor may be represented by the
following equation:
Where: KI is the fracture toughness in
s is the applied stress in MPa or psi
a is the crack length in meters or inches
B
is a crack length and component geometry factor that is
different for each specimen and is dimensionless.
2.3.1 Role of Material Thickness

Specimens having standard proportions but different absolute
size produce different values for KI. This results because the stress
states adjacent to the flaw changes with the specimen thickness (B)
until the thickness exceeds some critical dimension. Once the
thickness exceeds the critical dimension, the value of KI becomes
relatively constant and this value, KIC , is a true material property
which is called the plane-strain fracture toughness. The relationship
between stress intensity, KI, and fracture toughness, KIC, is similar to
the relationship between stress and tensile stress. The stress intensity,
KI, represents the level of “stress” at the tip of the crack and the
fracture toughness, KIC, is the highest value of stress intensity that a
material under very specific (plane-strain) conditions that a material
can withstand without fracture. As the stress intensity factor reaches
the KIC value, unstable fracture occurs. As with a material’s other
mechanical properties, KIC is commonly reported in reference books
and other sources.

2.3.2 Plane-Strain and Plane-Stress
When a material with a crack is loaded in
tension, the materials develop plastic strains as the
yield stress is exceeded in the region near the crack
tip. Material within the crack tip stress field, situated
close to a free surface, can deform laterally (in the z-
direction of the image) because there can be no
stresses normal to the free surface. The state of
stress tends to biaxial and the material fractures in a
characteristic ductile manner, with a 45o shear lip
being formed at each free surface. This condition is
called “plane-stress" and it occurs in relatively thin
bodies where the stress through the thickness
cannot vary appreciably due to the thin section.
However, material away from the free
surfaces of a relatively thick component is not free
to deform laterally as it is constrained by the
surrounding material. The stress state under these
conditions tends to triaxial and there is zero strain
perpendicular to both the stress axis and the
direction of crack propagation when a material is
loaded in tension. This condition is called “plane-
strain” and is found in thick plates. Under plane-
strain conditions, materials behave essentially
elastic until the fracture stress is reached and then
rapid fracture occurs. Since little or no plastic
deformation is noted, this mode fracture is termed
brittle fracture.
Plane Strain - a condition of a
body in which the
displacements of all points in
the body are parallel to agiven
plane, and the values of theses
displacements do not depend
on the distance perpendicular
to the plane
Plane Stress – a condition of a
body in which the state of
stress is such that two of the
principal stresses are always
parallel to a given plane and
are constant in the normal
direction.

2.3.3 Plane-Strain Fracture Toughness Testing
When performing a fracture toughness test, the
most common test specimen configurations are the
single edge notch bend (SENB or three-point bend),
and the compact tension (CT) specimens. From the
above discussion, it is clear that an accurate
determination of the plane-strain fracture toughness
requires a specimen whose thickness exceeds some
critical thickness (B). Testing has shown that plane-
strain conditions generally prevail when:
Where: B
is the minimum thickness that produces a condition
where plastic strain energy at the crack tip in minimal
KIC is the fracture toughness of the material
sy is the yield stress of material
When a material of unknown fracture toughness is tested, a
specimen of full material section thickness is tested or the specimen is
sized based on a prediction of the fracture toughness. If the fracture
toughness value resulting from the test does not satisfy the
requirement of the above equation, the test must be repeated using a
thicker specimen. In addition to this thickness calculation, test
specifications have several other requirements that must be met (such
as the size of the shear lips) before a test can be said to have resulted
in a KIC value.
When a test fails to meet the thickness and other test requirement
that are in place to insure plane-strain condition, the fracture
toughness values produced is given the designation KC. Sometimes it
is not possible to produce a specimen that meets the thickness
requirement. For example when a relatively thin plate product with
high toughness is being tested, it might not be possible to produce a
thicker specimen with plain-strain conditions at the crack tip.

4.Plane-Stress and Transitional-Stress States
For cases where the plastic energy at the crack tip is not negligible,
other fracture mechanics parameters, such as the J integral or R-
curve, can be used to characterize a material. The toughness data
produced by these other tests will be dependent on the thickness of the
product tested and will not be a true material property. However,
plane-strain conditions do not exist in all structural configurations and
using KIC values in the design of relatively thin areas may result in
excess conservatism and a weight or cost penalty. In cases where the
actual stress state is plane-stress or, more generally, some
intermediate- or transitional-stress state, it is more appropriate to use
J integral or R-curve data, which account for slow, stable fracture
(ductile tearing) rather than rapid (brittle) fracture.
5. Uses of Plane-Strain Fracture Toughness
KIC values are used to determine the critical crack length
when a given stress is applied to a component.
Where: sc is the critical applied stress that will cause failure
KIC is the plane-strain fracture toughness
Y is a constant related to the sample's geometry
a
is the crack length for edge cracks
or one half crack length for internal crack
KIC values are used also used to calculate the critical stress value
when a crack of a given length is found in a component.
Where: a
is the crack length for edge cracks
or one half crack length for internal crack
s is the stress applied to the material
KIC is the plane-strain fracture toughness
Y is a constant related to the sample's geometry

2.3.6 Orientation
The fracture toughness of a material commonly varies with grain
direction. Therefore, it is customary to specify specimen and crack
orientations by an ordered pair of grain direction symbols. The first
letter designates the grain direction normal to the crack plane. The
second letter designates the grain direction parallel to the fracture
plane. For flat sections of various products, e.g., plate, extrusions,
forgings, etc., in which the three grain directions are designated (L)
longitudinal, (T) transverse, and (S) short transverse, the six principal
fracture path directions are: L-T, L-S, T-L, T-S, S-L and S-T.

4. Thermal properties
 Physical property of a solid body related to application of heat
energy is defined as a thermal property.
 Thermal properties explain the response of a material to the
application of heat
 Important thermal properties are
o Heat capacity
o Thermal expansion
o Thermal conductivity
o Thermal stresses
1. Heat capacity
 A solid material’s potential energy is stored as its heat energy.
 Temperature of a solid is a measure its potential energy.
 Heat capacity is a property that is indicative of a material’s ability
to absorb heat from the external surroundings
 It is defined as the amount of energy required to produce a unit
temperature rise.
 Mathematically, it is expressed as:
C
dQ
dT
 Where dQ is the energy required to produce a temperature change
equal to dT.
 Heat capacity has units as J/mol-K or Cal/mol-K.
 Heat capacity is not an intrinsic property i.e. It changes with
material volume/mass.
 At low temperatures, vibrational heat contribution of heat
capacity varies with temperature as follows:
Cv AT 3
 The above relation is not valid above a specific temperature
known as Debye temperature. The saturation value is
approximately equal to 3R.

l0
l0 (Tf
T0 )
2. Specific heat
 For comparison of different materials, heat capacity has been
rationalized.
 Specific heat is heat capacity per unit mass. It has units as J/kg-K
or Cal/kg-K.
 With increase of heat energy, dimensional changes may occur.
Hence, two heat capacities are usually defined.
 Heat capacity at constant pressure, Cp, is always higher than heat
capacity at constant volume, Cv.
 Cp is ONLY marginally higher than Cv.
 Heat is absorbed through different mechanisms: lattice vibrations
and electronic contribution.
2. Thermal expansion
 Increase in temperature may cause dimensional changes.
 Linear coefficient of thermal expansion (α) defined as the
change in the dimensions of the material per unit length.
l f Δl
l0ΔT ΔT

 T0 and Tf are the initial and final temperatures (inK)
 l0 and lf are the initial and final dimensions of the materialand
 ε is the strain.
 α has units as (˚C)-1.
 α values:
 for metals 5-25x10-6
 for ceramics 0.5-15x10-6
 for polymers 50-400x10-6
 A volume coefficient of thermal expansion, αv (=3α) is used to
describe the volume change with temperature.
v
Δv
v0
ΔT
 Where Δv and vo are the volume change and the original volume.
 An instrument known as dilatometer is used to measure the
thermal expansion coefficient.
 At microscopic level, because of asymetric nature of the potential
energy trough, changes in dimensions with temperature are due
to change in inter-atomic distance, rather than increase in
vibrational amplitude.

 If a very deep energy trough caused by strong atomic bonding is
characteristic of the material, the atoms separate to a lesser and
the material has low linear coefficient of thermal expansion. This
relationship also suggests that materials having a high melting
temperature – also due to strong atomic bonds – have low thermal
expansion coefficients.
5. Thermal shock
 If the dimensional changes in a material are not uniform, that may
lead to fracture of brittle materials like ceramics. It is known as
thermal shock.
 The capacity of a material to withstand thermal shock is defined
as thermal shock resistance, TRS.
TSR f k
E
 where σf – fracture strength.
 Thermal shock behavior is affected by several factors:
o thermal expansion coefficient – a low value is desired;
o thermal conductivity – a high value is desired;
o elastic modulus – low value is desired;
o fracture strength – high value is desired
 Thermal shock may be prevented by altering external conditions
to the degree that cooling or heating rates are reduced and
temperature gradients across the material are minimized.
 Thermal shock is usually not a problem in most metals because
metals normally have sufficient ductility to permit deformation
rather than fracture.
 However, it is more of a problem in ceramics and glass materials.
It is often necessary to remove thermal stresses in ceramics to
improve their mechanical strength. This is usually accomplished
by an annealing treatment.

6. Thermal conductivity
 Thermal conductivity is ability of a material to transport heat
energy through it from high temperature region to low
temperature region.
 The heat energy, Q, transported across a plane of area A in
presence of a temperature gradient ΔT/Δl is given by
Q kA ΔT
Δl
 where k is the thermal conductivity of the material.
 It has units as W/m.K.
 It is a microstructure sensitive property.
 Its value range
o for metals 20-400
o for ceramics 2-50
o for polymers order of 0.3
Mechanisms - Thermal conductivity
 Heat is transported in two ways – electronic contribution,
vibrational (phonon) contribution.
 In metals, electronic contribution is very high. Thus metals have
higher thermal conductivities. It is same as electrical conduction.
Both conductivities are related through Wiedemann-Franz law:
k
L
T
 where L – Lorentz constant (5.5x10-9 cal.ohm/sec.K2)
 As different contributions to conduction vary with temperature,
the above relation is valid to a limited extension for many metals.
 With increase in temperature, both number of carrier electrons
and contribution of lattice vibrations increase. Thus thermal
conductivity of a metal is expected to increase.
 However, because of greater lattice vibrations, electron mobility
decreases.
 The combined effect of these factors leads to very different
behavior for different metals.

 Eg.: thermal conductivity of iron initially decreases then
increases slightly; thermal conductivity decreases with increase
in temperature for aluminium; while it increases for platinum
7. Thermal stresses
 Stresses due to change in temperature or due to temperature
gradient are termed as
thermal stresses(σthermal).
thermal EΔT
 Thermal stresses in a constrained body will be of compressive
nature if it is heated, and vice versa.
 Engineering materials can be tailored using multi-phase
constituents so that the overall material can show a zero thermal
expansion coefficient.
 Eg.: Zerodur – a glass-ceramic material that consists of 70-
80% crystalline quartz, and the remaining as glassy phase.
 Sodium-zirconium-phosphate (NZP) have a near-
zero thermal expansion coefficient.
5.Magnetic Properties of Engineering
Materials
To finalize the material for an engineering product / application, we
should have the knowledge of magnetic properties of materials. The
magnetic properties of a material are those which determine the ability
of material to be suitable for a particular magnetic Application. Some
of the typical magnetic properties of engineering materials are
listed below-
 Permeability
 Retentivity or Magnetic Hysteresis
 Coercive force
 Reluctance

1. Permeability
It is the property of magnetic material which indicates that how
easily the magnetic flux is build up in the material. Some time is also
called as the magnetic susceptibility of material.
It is determined by the ratio of magnetic flux density to magnetizing
force producing this magnetic flux density. It is denoted by µ.
Hence, μ = B/H.
Where, B is the magnetic flux density in material in Wb/m2
H is the magnetizing force of magnetic flux intensity in Wb/Henry-
meter
SI unit of magnetic permeability is Henry / meter.
Permeability of material is also defined as, μ = μ0 μr
Where, µ0 is the permeability of air or vacuum, and μ0 = 4π × 10-7
Henry/meter and µr is the relative permeability of material. µr = 1 for
air or vacuum.
A material selected for magnetic core in electrical machines
should have high permeability, so that required magnetic flux can be
produced in core by less ampere- turns.
2. Retentivity
When a magnetic material is placed in an external magnetic
field, its grains get oriented in the direction of magnetic field. Which
results in magnetization of material in the direction of external
magnetic field. Now, even after removal of external magnetic field,
some magnetization exists, which is called residual magnetism. This
property of material is called Magnetic retentively of material. A
hysteresis loop or B-H cure of a typical magnetic material is shown in
figure below. Magnetization Br in below hysteresis loop represents the
residual magnetism of material.

2.5.3 Coercive Force
Due to retentivity of material, even after removal of external
magnetic field some magnetization exists in material. This magnetism
is called residual magnetism of material. To remove this residual
magnetization, we have to apply some external magnetic field in
opposite direction. This external magnetic motive force (ATs)
required to overcome the residual magnetism is called “coercive
force” of material. In above hysteresis loop, – Hc represents the
coerciveforce.
The material having large value of residual magnetization and
coercive force are called magnetically hard materials. The material
having very low vale of residual magnetization and coercive force are
called magnetically soft materials.

2.5.4 Reluctance
It is a property of magnetic material which resists to buildup of
magnetic flux in material. It is denoted by R. Its unit is “Ampere-
turns / Wb”.
Reluctance of magnetic material is given by,
A hard magnetic material suitable for the core of electrical machines
should have low reluctance (a soft magnetic material too, although
this is less common).
6.Electrical Properties of Engineering
Materials
To finalize the material for an engineering product / application, we
should have the knowledge of Electrical properties of materials.
The Electrical properties of a material are those which determine
ability of material to be suitable for a particular Electrical Engineering
Application. Some of the typical Electrical properties of
engineering materials are listed below-
 Resistivity
 Conductivity
 Temperature coefficient of Resistance
 Permittivity
 Thermoelectricity
2.6.1 Resistivity
It the property of material which resists the flow of electric
current through material. It is the reciprocal of conductivity.
It is dented by ‘ρ’. Resistivity of a material of a conductor can be
determined as below
Where, ‘R’ is the resistance of conductor in Ω.

‘A’ is the cross sectional area of conductor in m2
‘l’ is the length of the conductor in meter SI unit of resistivity of is
Ω¦-meter. Resistivity of some materials is listed below
Sl. No. Element Resistivity at 20oC in Ω – m
1 Silver 1.59 × 10-8
2 Copper 1.7 × 10-8
3 Gold 2.44 × 10-8
4 Aluminum 2.82 × 10-8
5 Tungsten 5.6 × 10-8
6 Iron 1.0 × 10-7
7 Platinum 1.1 × 10-7
8 Lead 2.2 × 10-7
9 Manganin 4.82 × 10-7
10 Constantan 4.9 × 10-7
11 Mercury 9.8 × 10-7
12 Carbon (Graphite) 3.5 × 10-5
13 Germanium 4.6 × 10-1

14 Silicon 6.4 × 102
15 Glass 1010 to 1014
16 Quartz (fused) 7.5 × 1017
2.6.2 Conductivity
It is the property of material with allow the flow of electric
current through material. It is a parameter which indicates that how
easily electric current can flow through the material. It is denoted by
‘σ’. Conductivity of material is the reciprocal of resistivity.
Conductivity of material can be determined by,
Its SI unit is 1/(Ω-meter) or ℧/meter.
Sl. No. Material Dielectric Strength [KV(max.)/cm]
1 Air 30
2 Porcelain 80
3 Paraffin Wax 120
4 Transformer oil 160
5 Bakelite 220
6 Rubber 280
7 Paper 500

8 Teflon 600
9 Glass 1200
10 Mica 2000
Sl. No. Element
Temperature Coefficient of Resistance
in /oC
1 Manganin 0.00002
2 Constantan 0.00017
3 Nichrome 0.0004
4 Mercury 0.0009
5 Silver 0.0038
6 Copper 0.00386
7 Annealed copper 0.000393
8 Platinum 0.003927
9 Aluminum 0.00429
10 Carbon (Graphite) – 0.0005
11 Germanium – 0.05

3. Dielectric Strength
It is the property of material which indicates the ability of
material to withstand at high voltages. Generally it is specified for
insulating material to represent their operating voltage. A material
having high dielectric strength can withstand at high voltages.
Generally, it is represented in the unit of KV/cm. Dielectric strength
of some insulating materials are listed below-
4. Temperature Coefficient of Resistance
The temperature coefficient of resistance of a material indicates
the change in resistance of material with change in temperature.
Resistance of conductor changes with change of temperature. The
rise in resistance of a material with rise in temperature depends on
following things,
1. R2 – R1 ∝ R1
2. R2 – R1 ∝ t2 – t1
3. Property of material ofconductor.
o
Where, R1 is the resistance of conductor at temperature of t1 Cand
o
R2 is the resistance of conductor at temperature of t2 C.
Hence, from above, R2 – R1 ∝ R1 (t2 – t1)
Or, R2 – R1 = α1 R1 (t2 – t1) ⇒ R2 = R1 [1 + α1 (t2 – t1)]
Where, α1 is temperature coefficient of resistance of material at
o o
temperature of t1 C. Its unit is / C. Temperature coefficient of
resistance of material is also depends on temperature. emperature
coefficient of some materials are listed below,
2.6.5 Thermoelectricity
If the junction, formed by joining to two metals, is heated, a
small voltage in the range of millivolt is produced. This effect is called
thermoelectricity or thermoelectric effect. This effect forms the basis
of operation of thermocouples and some temperature based
transducers. This effect can be used to generate electricity, to measure
the temperature and to measure the change is temperature of objects.
12 Silicon – 0.07

2.7 Optical Properties of General Engineering
Material:
Optical property deals with the response of a material against
exposure to electromagnetic radiations, especially to visible light.
When light falls on a material, several processes such as reflection,
refraction, absorption, scattering etc.
1. Refraction:
When light photons are transmitted through a material, they
causes polarization of the electrons in the material and by interacting
with the polarized materials, photons lose some of their energy. As a
result of this, the speed of light is reduced and the beam of light
changes direction.
2. Reflection:
When a beam of photons strikes a material, some of the light is
scattered at the interface between that we media even if both are
transparent. Reflectivity, R, is a measure of fraction of incident light
which is reflected at the interface.
3. Absorption:
When a light beam is striked on a material surface, portion of the
incident beam that is not reflected by the material is either absorbed
or transmitted through the material. The fraction of beam that is
absorbed is related to the thickness of the materials and the manner in
which the photons interact with the material’s structure.
4. Rayleigh scattering:
Here photon interacts with the electron orbiting around an atom
and is deflected without any change in photon energy. This is more
vital for high atomic number atoms and low photon energies. Ex. Blue
colour in the sunlight gets scattered more than other colors in the
visible spectrum and thus making sky look blue.

a. Tyndall Effect:
Here scattering occur form particles much larger than the
wavelength of light Ex. cloud look white
b. Compton Scattering:
In this incident photon knocks out an electron from the atom
losing some of its energy during the process.
5. Transmission:
The fraction of beam that is not reflected or absorbed is
transmitted through the material. Thus the fraction of light that is
transmitted through a transparent material depends on the losses
incurred by absorption and reflection.
Thus, R + A + T = 1
where R = reflectivity,
A = absorptivity, and
T = transitivity
6. Thermal Emission:
When a material is heated electrons are excited to higher energy
levels generally in the outer energy levels where the electrons are less
strongly bound to the nucleus. These excited electrons, upon returning
back to the ground state, release photons in process termed as thermal
emission.
By measuring the intensity of a narrow band of the emitted
wavelengths with a pyrometer, material’s temperature can be
estimated.
7. Electro-Optic Effect:
The behaviour of a material in which its optical isotropic nature
changes to anisotropic nature on application of an electric field. This
effect is seen in LiNbO3, LiTiO3 etc.

8. Photoelectric Effect:
Phenomenon in which the ejection of electrons from a metal
surface takes place, when the metal surface is illuminated by light or
any other radiation of suitable frequency (or wavelength). Several
devices such as phototube, solar cell, fire alarm etc. work on this effect
(principle).
9. Photo Emissivity:
Phenomenon of emission of electrons from a metal cathode,
when exposed to light or any other radiations.
10. Brightness:
Power emitted by a source per unit area per unit solid angle.
Photo Conductivity- Phenomenon of increase in conductivity
of a semi-conductor due to excess carriers arisen from optical
luminescence.
Optical Properties of Non-Metals:
i.These materials may be transparent, translucent, or opaque.
Therefore, they exhibit different optical properties such as reflection,
refraction, absorption and transmission. The phenomenon of
refraction is more dominant in them.
ii.The non-metals which are transparent are generally coloured due
to light absorption and remission in the visible region by them.
Absorption of light occurs due to: Electronic polarization.
iii.Excitation of electrons from filled valence band to empty state
within conduction band, and Wide band gaps in dielectric materials.
iv.The non-metallic transparent materials transmit light due to net
energy formed by absorption and reflection processes.

Optical Properties of Metals:
i.In metals, the valence band is partially filled and so there are large
number of quasi continuous vacant energy levels available within the
valence band. When light is incident on metals the valence electrons
absorb all frequencies of visible light and get excited to vacant states
inside the valence band (intra-band transitions). This result in the
opacity of metals.
ii.The total absorption of light by the metal surface is within a very
thin outer layer of less than 0.1 jam. The excited electrons return back
to lower energy states thereby causing emission of radiation from the
surface of the metal in the form of visible light of the same
wavelength. This emitted light which appears as the reflected light is
the cause of the lustrous appearance of metals.
iii.In copper, inter-band transitions occur for energies greater than 2.2
eV i.e. the photons of energy greater than 2.2 eV are strongly
absorbed. This energy corresponds to wavelength below 5625 Å. This
means that the radiation in the blue-violet range is absorbed. This is
reason for the reddish-orange colour of copper.
iv.In silver and aluminium, there is no absorption in the full range of
visible radiation. So, the re-emission occurs over the entire
wavelength range of the visible spectrum due to which the white
colour of these metals exist.
v.Gold appears yellow because there is absorption in green portion
and reflection in yellow and red region.
Optical Properties of Semiconductors:
i. Intrinsic semiconductors at low temperatures have a
completely filled valence band and an empty conduction
band. So no intra-band transitions can occur in
semiconductors.

iii.
ii. Radiation of low frequencies, i.e. infrared radiation are not
absorbed and that’s why semiconductors are transparent to
infra-red radiation.
The energy gaps in semiconductors are in the range of 0.5 –
3eV. So inter-band absorption occurs for radiation in this
range which corresponds to near infra-red and visible range,
this is responsible for the opacity of semiconductors.
8. Environmental Properties
1. Dry Corrosion
Dry corrosion is the chemical reaction of a solid surface with dry
gases. Typically a metal, M reacts with oxygen and forms a surface
layer of the oxide.
b
aM
2
O2 M a Ob
If the oxide is protective, forming a continuous film without
cracks over the surface, the reaction slows down with time, as the
oxygen cannot pass through this layer. The oxidation rate mainly
depends on the characteristic of this oxide layer, which can be
described by the Pilling-Bedworth ratio.
AO M
P-B Ratio
aA
AO ist the molecular weight of the metaloxyde, AM ist he atomic
weight of the metal,
ρO and ρM are the correspondingdensities.
If the Pilling-Bedworth ratio is smaller than 1, so the volume of the
oxide is smaller than the oxidized metal, than the oxide layer is
porous and does not protect the surface. If the P-B Ratio is greater
than 1, non-porous and protective oxide layer forms. If this ratio is
higher than 2 or 3 the layer breaks and is not protective any more.
M O

2. Flammability
Flammability is a materials ability to suppress combustion. The
number given as flammability corresponds to a relative rating system,
thus using it almost only for comparisons is reasonable.
3. Wet corrosion
(Corrosion caused by a reaction of metal with water, brine, acids
and alkalis) is much more complicated and cannot be defined by
simple relations. It is more usual to scale the resistance by relative
values.
Corrosion is effective in fresh water, organic solvents, sea water,
strong acid, strong alkalis, UV, weak acid and weak alkalis.
4. Corrosion Properties
Corrosion involves the deterioration of a material as it reacts with its
environment. Corrosion is the primary means by which metals
deteriorate. Corrosion literally consumes the material reducing load
carrying capability and causing stress concentrations. Corrosion is
often a major part of maintenance cost and corrosion prevention is
vital in many designs. Corrosion is not expressed in terms of a
design property value like other properties but rather in more
qualitative terms such as a material is immune, resistant, susceptible
or very susceptible to corrosion.

The corrosion process is usually
electrochemical in nature, having the
essential features of a battery.
Corrosion is a natural process that
commonly occurs because unstable
materials, such as refined metals want
to return to a more stable compound.
For example, some metals, such as
gold and silver, can be found in the
earth in their natural, metallic state
and they have little tendency to
corrode. Iron is a moderately active
metal and corrodes readily in the
presence of water. The natural state of
iron is iron oxide and the most
common iron ore is Hematite with a
chemical composition of Fe203. Rust,
the most common corrosion product of
iron, also has a chemical composition
of Fe2O3.
The difficulty in terms of energy
required to extract metals from their
ores is directly related to the ensuing
tendency to corrode and release this
energy. The electromotive force
series (See table) is a ranking of metals with respect to their inherent
reactivity. The most noble metal is at the top and has the highest
positive electrochemical potential. The most active metal is at the
bottom and has the most negative electrochemical potential.
Note that aluminum, as indicated by its position in the series, is a
relatively reactive metal; among structural metals, only beryllium
and magnesium are more reactive. Aluminum owes its excellent
corrosion resistance to the barrier oxide film that is bonded strongly
to the surface and if damaged reforms immediately in most
environments. On a surface freshly abraded and exposed to air, the
Partial Electromotive Force Series
Standard Potential
Electrode Reaction
(at 25oC), V-SHE
Au3+ + 3e- -> Au 1.498
Pd2+ + 2e- ->Pd 0.987
Hg2+ + 2e- ->Hg 0.854
Ag+ + e- ->Au 0.799
Cu+ + e- ->Cu 0.521
Cu2+ + 2e- ->Cu 0.337
2H+ + 2e- ->H2 0.000 (Ref.)
Pb2+ + 2e- ->Pb -0.126
Sn2+ + 2e- ->Sn -0.136
Ni2+ + 2e- ->Ni -0.250
Co2+ + 2e- -> Co -0.277
Cd2+ + 2e- ->Cd -0.403
Fe2+ + 2e- ->Fe -0.440
Cr3+ + 3e- ->Cr -0.744
Cr2+ + 2e- ->Cr -0.910
Zn2+ + 2e- ->Zn -0.763
Mn2+ + 2e- ->Mn -1.180
Ti2+ + 2e- ->Ti -1.630
Al3+ + 3e- ->Al -1.662
Be2+ + 2e- ->Be -1.850
Mg2+ + 2e- -> Mg -2.363
Li+ + e- ->Li -3.050

protective film is only 10 Angstroms thick but highly effective at
protecting the metal from corrosion.
Corrosion involve two chemical processes…oxidation and reduction.
Oxidation is the process of stripping electrons from an atom and
reduction occurs when an electron is added to an atom. The oxidation
process takes place at an area known as the anode. At the anode,
positively charged atoms leave the solid surface and enter into an
electrolyte as ions. The ions leave their corresponding negative charge
in the form of electrons in the metal which travel to the location of the
cathode through a conductive path. At the cathode, the corresponding
reduction reaction takes place and consumes the free electrons. The
electrical balance of the circuit is restored at the cathode when the
electrons react with neutralizing positive ions, such as hydrogen ions,
in the electrolyte. From this description, it can be seen that there are
four essential components that are needed for a corrosion reaction to
proceed. These components are an anode, a cathode, an electrolyte
with oxidizing species, and some direct electrical connection between
the anode and cathode. Although atmospheric air is the most
common environmental electrolyte, natural waters, such as seawater
rain, as well as man-made solutions, are the environments most
frequently associated with corrosion problems.
A typical situation might involve
a piece of metal that has anodic
and cathodic regions on the same
surface. If the surface becomes
wet, corrosion may take place
through ionic exchange in the
surface water layer between the
anode and cathode. Electron
exchange will take place through
the bulk metal. Corrosion will
proceed at the anodic site
according to a reaction such as
M → M++ +2e-

where M is a metal atom. The resulting metal cations (M++) are
available at the metal surface to become corrosion products such as
oxides, hydroxides, etc. The liberated electrons travel through the
bulk metal (or another low resistance electrical connection) to the
cathode, where they are consumed by cathodic reactions such as
2H+ + 2e- → H2
The basic principles of corrosion that were just covered, generally
apply to all corrosion situation except certain types of high
temperature corrosion. However, the process of corrosion can be
very straightforward but is often very complex due to variety of
variable that can contribute to the process. A few of these variable
are the composition of the material acting in the corrosion cell, the
heat treatment and stress state of the materials, the composition of
the electrolyte, the distance between the anode and the cathode,
temperature, protective oxides and coating, etc.
5. Types of Corrosion
Corrosion is commonly classified based on the appearance of
the corroded material. The classifications used vary slightly from
reference to reference but there is generally considered to be eight
different forms of corrosion. There forms are:
Uniform or general – corrosion that is distributed more or less
uniformly over a surface.
Localized – corrosion that is confined to small area. Localized
corrosion often occurs due to a concentrated cell. A concentrated cell
is an electrolytic cell in which the electromotive force is caused by a
concentration of some components in the electrolyte. This difference
leads to the formation of distinct anode and cathode regions.
 Pitting – corrosion that is confined to small areas and take the
form of cavities on a surface.
 Crevice – corrosion occurring at locations where easy access to
the bulk environment is prevented, such as the mating surfaces
of two components.

 Filiform – Corrosion that occurs under some coatings in the
form of randomly distributed threadlike filaments.
Intergranular – preferential corrosion at or along the grain
boundaries of a metal.
 Exfoliation – a specific
form of corrosion that
travels along grain
boundaries parallel to
the surface of the part
causing lifting and
flaking at the surface.
The corrosion products
expand between the
uncorroded layers of metal to produce a look that resembles
pages of a book. Exfoliation corrosion is associated with sheet,
plate and extruded products and usually initiates at unpainted
or unsealed edges or holes of susceptible metals.
Galvanic – corrosion associated primarily with the electrical
coupling of materials with significantly different electrochemical
potentials.
Environmental Cracking – brittle fracture of a normally ductile
material that occurs partially due to the corrosive effect of an
environment.
 Corrosion fatigue – fatigue cracking that is characterized by
uncharacteristically short initiation time and/or growth rate due
to the damage of corrosion or buildup of corrosion products.
 High temperature hydrogen attack – the loss of strength and
ductility of steel due to a high temperature reaction of absorbed
hydrogen with carbides. The result of the reaction is
decarburization and internal fissuring.
 Hydrogen Embrittlement – the loss of ductility of a metal
resulting from absorption of hydrogen.
 Liquid metal cracking – cracking caused by contact with a
liquid metal.

 Stress corrosion – cracking of a metal due to the combined
action of corrosion and a residual or applied tensile stress.
Erosion corrosion – a corrosion reaction accelerated by the
relative movement of a corrosive fluid and a metal surface.
Fretting corrosion – damage at the interface of two contacting
surfaces under load but capable of some relative motion. The
damage is accelerated by movement at the interface that
mechanically abraded the surface and exposes fresh material to
corrosive attack.
Dealloying – the selective corrosion of one or more components of
a solid solution alloy.
 Dezincification – corrosion resulting in the selective removal
of zinc from copper-zinc alloys.
9. Availability of Materials
Materials engineers and purchasing agents become frustrated in
trying to obtain materials that have a limited number of producers or
a limited production volume. Such frustration can be particularly
high when a small amount of material is needed to finish a job or
replace a failed piece. Some excerpts of that document are used here.
 Industry Dynamics: Metals companies are undergoing
what can only be described as wrenching change. The
competitive landscape is dramatically changing thanks to the
following drivers: (reference)
o Industry consolidation
o Globalization
o Over capacity
o Price erosion

 Best Practices:
The industry leaders are turning challenges into
competitive advantage and seeking areas where
technology can deliver needed improvements.
 Managing more complex supply chains:
More complex supply chains are emerging as a
consequence of the industry consolidation. Opportunities to
benefit include: lower costs, faster response to customers,
flexible product sourcing and more efficient distribution
strategies.
 Consolidating disparate systems:
Metals companies have built sophisticated information
systems to support their operations. The undeniable strengths
of the existing infrastructure can be leveraged while reining in
costs and complexity. New business models that make
decisions about customers and suppliers simple and as effective
are now possible.
 Expediting order processing:
Understanding customer needs while managing
metallurgical and mill capabilities reduces overall processing
time, a key element toward gaining market share.
 Innovative business processes:
Redefining supply chain networks to maximize efficiency
and adopting new business processes such as build-to-stock /
finish-to-order allows metals companies to achieve extremely
competitive lead times.
 Consolidating MRO spending:
As an asset-intensive industry, keeping expensive

facilities running is critical. Addressing the challenge of
maximizing equipment uptime while reducing parts inventory
uncover major benefits.
 Extended value chain:
All too often, efficiency stops at the edges of the company.
Streamlining cross-enterprise processes is the next great frontier
for reducing costs, speeding operations and deliveries to create
value for customers and shareholders.

Unit ii material properties.

Unit ii material properties.

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Unit ii material properties.