MATERIAL PROPERTIES - MECHANICAL, ELECTRIACAL AND CHEMICAL
PROPERTIES OF MATERIAL
MECHANICAL PROPERTIES
Elasticity/Stiffness
This is a measure of elastic deformation of
a body under stress which is recovered when the stress is released. The ratio
of stress to strain in the elastic region is known as stiffness or modulus of
elasticity (Young’s Modulus). When the stress goes beyond the elastic limit the
material will no longer return completely to its original dimension.
Yield (or Proof Strength)
Stress needed to produce a specified amount
of plastic or permanent deformation. (Usually a 0.2 % change in length)
Ultimate Tensile Strength (UTS)
The maximum stress a material can withstand
before fracture.
Ductility
Hardness
The resistance to abrasion, deformation,
scratching or to indentation by another hard body. This property is important
for wear resistant applications.
Toughness
Fatigue Strength and Endurance Limit
Creep Resistance
The plastic deformation of a material which
occurs as a function of time when the material is subjected to constant stress
below its yield strength. For metals this is associated with high temperature
applications but polymers may exhibit creep at low temperatures.
Brittleness
Ability of a material to break
or shatter without significant deformation when under stress; opposite of
plasticity
Bulk modulus
Ratio of pressure to
volumetric compression (GPa)
Compressive strength
Maximum stress a
material can withstand before compressive failure (MPa)
Creep
The slow and gradual deformation of
an object with respect to time
Viscosity
A fluid's resistance to gradual
deformation by tensile or shear stress; thickness
Yield strength
The stress at which a
material starts to yield plastically (MPa)
Young's modulus
Ratio of linear stress to
linear strain (MPa)
STRESS AND STRAIN DIAGRAM
ELECTRICAL PROPERTIES
Electrical conduction
Materials are classified based on their
electrical propertiesas conductors, semiconductors and
insulators. New to thisgroup is super conductors.
Electrical conductivity of a material is
defined in terms ofease of charge flow through it.Charge that flows comprised of either
electrons, ions,charged holes, and their combinations.
Ohm’s law relates the current and applied
voltage:
V = IR
where V – applied voltage (volts)
I – current (amperes)
R – resistance (ohms)
Material’s electric resistance is NOT an
intrinsic-property i.e. it depends on object geometry.
Electrical resistivity, defined as
follows, is an intrinsicproperty, inverse of which called
conductivity.
Semiconductivity
Electrical properties of semiconductors
are unique, in thesense that their electrical properties are
extremely sensitiveto even minute concentrations of
impurities.
Two kinds of semiconductors – intrinsic
and extrinsic.
For intrinsic semiconductors, their
electrical behavior isbased on inherent electronic structure of
the pure material.
On the other hand, if the electrical
properties are dominatedby impurities, they are called extrinsic
semiconductors.
In semiconductors, the valence and
conduction bands do notoverlap as in metals, but they possess
enough electrons inthe valence band those can be promoted to
the conduction
band at a certain temperature.
Ferro-electricity
Ferro-electricity is defined as the
spontaneous alignment ofelectric dipoles by their mutual interaction
in the absence ofan applied electric field.
It arises from the fact that the local
field increases inproportion to the polarization. Thus,
ferro-electric materialsmust possess permanent dipoles.
Ex.: BaTiO3, Rochelle salt (NaKC4H4O6.4H2O), potassiumdihydrogen phosphate (KH2PO4), potassium niobate(KNbO3).
These materials have extremely high
dielectric constants atrelatively low applied field frequencies.
Thus, capacitorsmade from ferro-electric materials are
smaller thancapacitors made of other dielectric
materials.
Piezo-electricity
Piezo-electricity, or pressure
electricity, is defined aspolarization induced by the application of
external force.
Thus by reversing the direction of
external force, direction of the field can be reversed i.e. the application of an external electric field alters the net dipole length
and causes a dimensional change.
Hence piezo-electric materials are useful
as transducers – devices that convert mechanical stress into
electrical energy and vice versa.
Application for these materials includes
microphones, ultrasonic generators, sonar detectors, and
mechanical strain gauges.
Ex.: Barium titanate, lead titanate, lead
zirconate (PbZrO3), ammoinium dihydrogen phosphate (NH4H2PO4),
and quartz.
Dielectric behavior concept
Dielectric is a material separating two
charged bodies. For a material to be a good dielectric, it must
be an electrical insulator. Dielectric materials are used in
capacitors, devices used to store the electric energy. Electrical conduction in ionic ceramics
Charge can also be conducted via ions -
called ionic conduction. This may occur either in
conjunction with or separately from electronic conduction.
Several types of compounds show
exceptionally high ionic conductivity.
Such phases fall into three broad
categories: halide and chalcogenides of silver and copper; oxides
with ฮฒ-alumina structure; and oxides of fluorite
structure.
Ex.: La2CuO4(Tc = 30 K), YBC compounds – yttrium doped perovskite structure, YBa2Cu3O7 (Tc = 92 K).
By properly engineering the point
defects, it is possible to convert ceramics into semiconductors. Ex.:
Indium tin oxide
Electrical conduction in polymers
Polymers are, in general, insulators.
They can be made conductors in two ways: (1) introducing an
additive to the polymer to improve conductivity, and (2)
creating polymers
with inherent conductivity.
(1) Adding ionic compound or Introducing
conductive fillers such as carbon black.
(2) Inherent conductivity by doping.
Ex.: polyparaphynylene, polypyrole,
polyaniline, acetalpolymers.
Some other polymers such as
polyphthaocyanine can be cross-linked by special curing processes to
raise its conductivity.
Specific gravity is the ratio of density of
a substance compared to the density of fresh water at 4°C (39° F). At this
temperature the density of water is at its greatest value and equal 1 g/mL. Since specific gravity is a ratio, so it has no units. An object will float in
water if its density is less than the density of water and sink if its density
is greater that that of water. Similarly, an object with specific gravity less
than 1 will float and those with a specific gravity greater than one will sink.
Specific gravity values for a few common substances are: Au, 19.3; mercury,
13.6; alcohol, 0.7893; benzene, 0.8786. Note that since water has a density of
1 g/cm3, the specific gravity is the same as the density of the material
measured in g/cm3.
When heat is added to most materials, the
average amplitude of the atoms' vibrating within the material increases. This,
in turn, increases the separation between the atoms causing the material to
expand. If the material does not go through a phase change, the expansion can
be easily related to the temperature change. The linear coefficient of thermal
expansion ( a) describes the relative change in length of a material per degree
temperature change. As shown in the following equation, a is the ratio of
change in length ( Dl) to the total starting length (li) and change in
temperature ( DT).
Magnetic permeability or simply
permeability is the ease with which a material can be magnetized. It is a
constant of proportionality that exists between magnetic induction and magnetic
field intensity. This constant is equal to approximately 1.257 x 10-6 Henry per
meter (H/m) in free space (a vacuum). In other materials it can be much
different, often substantially greater than the free-space value, which is
symbolized ยต0.
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.
Chemical properties
Phase Transformation Temperatures
When temperature rises and pressure is held
constant, a typical substance changes from solid to liquid and then to vapor.
Transitions from solid to liquid, from liquid to vapor, from vapor to solid and
visa versa are called phase transformations or transitions. Since some substances
have several crystal forms, technically there can also be solid to another
solid form phase transformation.
Phase transitions from solid to liquid, and
from liquid to vapor absorb heat.
The phase transition temperature where a
solid changes to a liquid is called the melting point. The temperature at which
the vapor pressure of a liquid equals 1 atm (101.3 kPa) is called the boiling
point. Some materials, such as many polymers, do not go simply from a solid to
a liquid with increasing temperature. Instead, at some temperature below the
melting point, they start to lose their crystalline structure but the molecules
remain linked in chains, which results in a soft and pliable material.
The
temperature at which a solid, glassy material begins to soften and flow is
called the glass transition temperature.
Density
Mass can be thinly distributed as in a
pillow, or tightly packed as in a block of lead. The space the mass occupies is
its volume, and the mass per unit of volume is its density.
Mass (m) is a fundamental measure of the
amount of matter. Weight (w) is a measure of the force exerted by a mass and
this force is force is produced by the acceleration of gravity. Therefore, on
the surface of the earth, the mass of an object is determined by dividing the weight
of an object by 9.8 m/s2 (the acceleration of gravity on the surface of the
earth). Since we are typically comparing things on the surface of the earth,
the weight of an object is commonly used rather than calculating its mass.
Specific gravity
Thermal conductivity
Thermal conductivity (ฮป) is the intrinsic
property of a material which relates its ability to conduct heat. Heat transfer
by conduction involves transfer of energy within a material without any motion
of the material as a whole. Conduction takes place when a temperature gradient
exists in a solid (or stationary fluid) medium. Conductive heat flow occurs in
the direction of decreasing temperature because higher temperature equates to
higher molecular energy or more molecular movement. Energy is transferred from
the more energetic to the less energetic molecules when neighboring molecules
collide.
Thermal expansion
Magnetic Permeability
Materials that cause the lines of flux to
move farther apart, resulting in a decrease in magnetic flux density compared
with a vacuum, are called diamagnetic. Materials that concentrate magnetic flux
by a factor of more than one but less than or equal to ten are called
paramagnetic; materials that concentrate the flux by a factor of more than ten
are called ferromagnetic. The permeability factors of some substances change
with rising or falling temperature, or with the intensity of the applied
magnetic field.
Electrical cunductivity
Ellectrical
conductivity is a measure of how well a material accommodates the movement of
an electric charge. It is the ratio of the current density to the electric
field strength. Its SI derived unit is the Siemens per meter, but conductivity
values are often reported as percent IACS. IACS is an acronym for International
Annealed Copper Standard, which was established by the 1913 International
Electrochemical Commission.
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