Definition of Properties of materials .
Stress is the mechanical force or energy that produces deformation or fracture .
Stress is the ratio between the applied force and the cross-sectional area where the aplied force is acting - and can be expressed as
σ = F / A (1)whereσ = stress (N/m2, Pa, psi)F = applied force (N, lb)A = cross-sectional area (m2, in2)
Strain The
ratio of extension to original length is called strain it has no units as it is
a ratio of two lengths measured in metres.
strain = strain it has no units
DL =extension measured in metres
L = original length measured in metres
Ratio of stress and strain
L = the limit of proportionality, Hooke’s law applies up to
this point.
E = elastic limit, beyond this point the material is
permanently stretch and it will not go back to its original length. Elastic
behaviour is when a material returns to its original length, plastic behaviour
is when the stretched material does not return to its original length.
Y = yield point, beyond this point small increases in force
give much big increases in length.
B = breaking point / breaking stress, the material breaks at
this point.
Stress-Strain graph for a brittle
material (like glass)
Tensile or Ultimate strength is the highest stress a
material can sustain without rupture .
Yield strength is the highest stress that a material can
sustain without undergoing plastic ( permanent ) deformation .
Compression strength is the maximum stress a material can
sustain under crush loading .
Elasticity is the ability of a material to return to its
original shape when a load causing deformation is removed from it.
Hooke's Law
One property of elasticity is that it takes about twice as
much force to stretch a spring twice as far. That linear dependence of
displacement upon stretching force is called Hooke's law which can be expressed
as
Fs = -k dL
where
Fs = force in the spring (N)
k = spring constant (N/m)
dL = elongation of the spring (m
Modulus of elasticity is the ratio of stress to elastic
strain , it is the measure of the stiffness of a material .
Young's modulus can be expressed
as
E = stress / strain
= (F / A) / (dL /
L)
where
E = Young's modulus (N/m2) (lb/in2, psi)
Ductility is the
extent to which a material can sustain plastic deformation without rupture. .
Toughness is the ability of a material to absorb energy without
rupturing .
Impact strength is a measure of a material,s toughness . The impact strength describes the ability of a material to absorb shock and impact energy without breaking. The impact strength is calculated as the ratio of impact absorption to test specimen cross-section.
It
is expressed as the foot – pounds of energy or joules required to fracture a specimen
subject to shock loading.
Hardness is a measure
of a materials resistance to localized plastic deformation . The
characteristics generally associated with hardness are resistance to
deformation ( Brinell , Rockwell , Knoop , Vickers ) resistance to scratching (
Moh,s scale ) and resistance to wear and
abrasion .
Fatigue strength is the ability of a material to withstand
mechanical load or forces applied in a repetitive or cyclic fashion . It can be
expressed by the stress required to cause failure in a test specimen after a specified number of cycles of loading
.
Endurance limit is the stress level at which the material
will not fail under an unlimited number of cycles of loading .
Maximum service temperature is a rough indicator of the material,s heat resistance . The
maximum temperature at which a material can be used continuously over a long
period of time and retain its room temperature properties .
Creep and creep strength is resistance to plastic flow ( or permanent deformation ) under a load over a
period of time . The creep strength of a material can be expressed as the
maximum stress required to cause a specified amount of creep in a specified
time .
Thermal Conductivity - k - is the quantity of heat
transmitted due to an unit temperature gradient, in unit time under steady
conditions in a direction normal to a surface of the unit area. Thermal
Conductivity - k - is used in the Fourier's equation.
The linear thermal expansion coefficient is the ratio change
in length per degree temperature to length.
Electrical
conductivity or specific conductivity is the measure of a material's ability to
conduct electric current. Conductivity is the reciprocal (inverse) of
electrical resistivity.
Electrical conductivity is defined as the ratio of the
current density to the electric field strength and can be expressed as
σ = J / E
where
σ = electrical conductivity (1/ohms m, 1/Ω m, siemens/m,
S/m, mho/m)
J = current density (amps/m2)
E = electric field strength (volts/m)
One siemens - S - is equal to the reciprocal of one ohm and
is also referred to as one mho.
Conductors are materials with loosely attached valence
electrons - electrons can drift freely between the atoms
Insulators have structures where the electrons are bound to
the atoms by ionic or covalent bonds - almost no current can flow
Semiconductors are insulating materials where
the bonds can be broken with applied voltage - electrons can be released and
moved from one vacated valence
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