has several definitions depending on
the material type and application. Before
choosing a material based on its published
or measured strength it is important
to understand the manner in which strength
is defined and how it is measured. When
designing for strength, material class
and mode of loading are important considerations.
the most common measure of strength
is the yield strength. For most polymers
it is more convenient to measure the
failure strength, the stress at the
point where the stress strain curve
becomes obviously non-linear. Strength,
for ceramics however, is more difficult
to define. Failure in ceramics is highly
dependent on the mode of loading. The
typical failure strength in compression
is fifteen times the failure strength
in tension. The more common reported
value is the compressive failure strength.
limit is the highest stress at which
all deformation strains are fully recoverable.
For most materials and applications
this can be considered the practical
limit to the maximum stress a component
can withstand and still function as
designed. Beyond the elastic limit permanent
strains are likely to deform the material
to the point where its function is impaired.
limit is the highest stress at which
stress is linearly proportional to strain.
This is the same as the elastic limit
for most materials. Some materials may
show a slight deviation from proportionality
while still under recoverable strain.
In these cases the proportional limit
is preferred as a maximum stress level
because deformation becomes less predictable
strength is the minimum stress which
produces permanent plastic deformation.
This is perhaps the most common material
property reported for structural materials
because of the ease and relative accuracy
of its measurement. The yield strength
is usually defined at a specific amount
of plastic strain, or offset, which
may vary by material and or specification.
The offset is the amount that the stress-strain
curve deviates from the linear elastic
line. The most common offset for structural
metals is 0.2%.
tensile strength is an engineering value
calculated by dividing the maximum load
on a material experienced during a tensile
test by the initial cross section of
the test sample. When viewed in light
of the other tensile test data the ultimate
tensile strength helps to provide a
good indication of a material's toughness
but is not by itself a useful design
limit. Conversely this can be construed
as the minimum stress that is necessary
to ensure the failure of a material.
fracture strength is the load at fracture
divided by the cross sectional area
of the sample. Like the ultimate tensile
strength the true fracture strength
can help an engineer to predict the
behavior of the material but is not
itself a practical strength limit. Because
the tensile test seeks to standardize
variables such as specimen geometry,
strain rate and uniformity of stress
it can be considered a kind of best
case scenario of failure.
is a measure of how much deformation
or strain a material can withstand before
breaking. The most common measure of
ductility is the percentage of change
in length of a tensile sample after
breaking. This is generally reported
as % El or percent elongation. The R.A.
or reduction of area of the sample also
gives some indication of ductility.
describes a material's resistance to
fracture. It is often expressed in terms
of the amount of energy a material can
absorb before fracture. Tough materials
can absorb a considerable amount of
energy before fracture while brittle
materials absorb very little. Neither
strong materials such as glass or very
ductile materials such as taffy can
absorb large amounts of energy before
failure. Toughness is not a single property
but rather a combination of strength
of a material can be related to the
total area under its stress-strain curve.
A comparison of the relative magnitudes
of the yield strength, ultimate tensile
strength and percent elongation of different
material will give a good indication
of their relative toughness. Materials
with high yield strength and high ductility
have high toughness. Integrated stress-strain
data is not readily available for most
materials so other test methods have
been devised to help quantify toughness.
The most common test for toughness is
the Charpy impact test.
materials the toughness is strongly
dependent on crystal structure. Face
centered cubic materials are typically
ductile while hexagonal close packed
materials tend to be brittle. Body centered
cubic materials often display dramatic
variation in the mode of failure with
temperature. In many materials the toughness
is temperature dependent. Generally
materials are more brittle at lower
temperatures and more ductile at higher
temperatures. The temperature at which
the transition takes place is known
as the DBTT, or ductile to brittle transition
temperature. The DBTT is measured by
performing a series of Charpy impact
tests at various temperatures to determine
the ranges of brittle and ductile behavior.
Use of alloys below their transition
temperature is avoided due to the risk
of catastrophic failure.
fatigue ratio f is the ratio of the
stress required to cause failure after
a specific number of cycles to the yield
stress of a material. Fatigue tests
are generally run through 107
or 108 cycles. A high fatigue
ratio indicates materials which are
more susceptible to crack growth during
coefficient is an other important material
parameter in cyclic loading. It is the
fraction of mechanical energy lost in
a stress strain cycle. The loss coefficient
for each material is a function of the
frequency of the cycle. A high loss
coefficient can be desirable for damping
vibrations while a low loss coefficient
transmits energy more efficiently. The
loss coefficient is also an important
factor in resisting fatigue failure.
If the loss coefficient is too high,
cyclic loading will dissipate energy
into the material leading to fatigue