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Engineering
Materials
Thermal
Properties
Thermal
conductivity
The thermal conductivity
is the rate of heat transfer through a material
in steady state. It is not easily measured,
especially for materials with low conductivity
but reliable data is readily available for
most common materials.
Thermal
diffusivity
The thermal diffusivity
is a measure of the transient heat flow
through a material.
Specific
heat
The specific heat is a
measure of the amount of energy required
to change the temperature of a given mass
of material. Specific heat is measured by
calorimetry techniques and is usually reported
both as CV, the specific heat
measured at constant pressure, or CP,
the specific heat measured at constant pressure.
Melting
point
The melting point is the
temperature at which a material goes from
the solid to the liquid state at one atmosphere.
The melting temperature is not usually a
design criteria but it offers important
clues to other material properties.
Glass
transition temp
The glass transition temperature,
or Tg is an important property
of polymers. The glass transition temperature
is a temperature range which marks a change
in mechanical behavior. Above the glass
transition temperature a polymer will behave
like a ductile solid or highly viscous liquid.
Below Tg the material will behave
as a brittle solid. Depending on the desired
properties materials may be used both above
and below their glass transition temperature.
Thermal
expansion coefficient
The thermal expansion coefficient
is the amount a material will change in
dimension with a change in temperature.
It is the amount of strain due to thermal
expansion per degree Kelvin expressed in
units of K-1. For isotropic materials
" is the same in all directions, anisotropic
materials have separate "s reported
for each direction which is different.
Thermal
shock resistance
Thermal shock resistance
is a measure of how large a change in temperature
a material can withstand without damage.
Thermal shock resistance is very important
to most high temperature designs. Measurements
of thermal shock resistance are highly subjective
because if is extremely process dependent.
Thermal shock resistance is a complicated
function of heat transfer, geometry and
material properties. The temperature range
and the shape of the part play a key role
in the material's ability to withstand thermal
shock. Tests must be carefully designed
to mimic anticipated service conditions
to accurately asses the thermal shock resistance
of a material.
Creep
resistance
Creep is slow, temperature
aided, time dependent deformation. Creep
is typically a factor in materials above
one third of their absolute melting temperature
or two thirds of their glass transition
temperature. Creep resistance is an important
material property in high temperature design,
but it is difficult to quantify with a single
value. Creep response is a function of many
material and external variables, including
stress and temperature. Often other environmental
factors such as oxidation or corrosion play
a role in the fracture process.
Creep is plotted as strain
vs. time. A typical creep curve shows three
basic regimes. During stage I, the primary
or transient stage, the curve begins at
the initial strain, with a relatively high
slope or strain rate which decreased throughout
stage I until a steady state is reached.
Stage II, the steady state stage, is generally
the longest stage and represents most of
the response. The strain rate again begins
to increase in stage III and rupture at
tR generally follows quickly.
Different applications
call for different creep responses. In situations
where long life is desired minimum creep
rate is the most important material consideration.
Testing through stage II should be sufficient
for determining minimum creep rate. Is not
necessary to proceed all the way to rupture.
For this type of test the longer the test
the more accurate the creep rate will be.
Unfortunately practicality limits most creep
tests to times shorter than would be desirable
for high accuracy.
For short lived applications
such as rocket nozzles the time to failure
may be the only consideration. The main
issue is whether or not the component fails,
not the amount of deformation it may undergo.
For this application creep tests may be
run to completion but without recording
any data but the time to rupture. In this
case temperatures may be elevated above
expected conditions to provide a margin
of safety.
The main objective of a
creep test is to study the effects of temperature
and stress on the minimum creep rate and
the time to rupture. Creep testing is usually
run by placing a sample under a constant
load at a fixed temperature. The data provided
from a complete creep test at a specific
temperature, T, and stress includes three
creep constants: the dimensionless creep
exponent, n, the activation energy Q, and
A, a kinetic factor.
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