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Adherence
The extent to which a coating bonds to a substrate.
Adherence Index
Measure of the adherence of porcelain enamel and ceramic coatings to sheet
metal. (ASTM C-313)
Alpha Rockwell Hardness
Index of the resistance of a plastic to surface penetration by a specified
indenter under specified load applied with a Rockwell hardness tester.
Higher values indicate higher indentation hardness. (ASTM D-785)
Axial Strain
The strain in the direction that the load is applied, or on the same axis
as the applied load.
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Bend Test
Method for measuring ductility of certain materials. There are no standardized
terms for reporting bend test results for broad classes of materials;
rather, terms associated with bend tests apply to specific forms or types
of materials. For example, materials specifications sometimes require
that a specimen be bent to a specified inside diameter (ASTM A-360, steel
products). A bend test for ductility of welds is given in ASTM E-190.
Results of tests of fiberboard are reported by a description of the failure
or photographs. (ASTM D-1037)
Bending Strength
Alternate term for flexural strength. It is most commonly used to describe
flexure properties of cast iron and wood products.
Bond Strength
Stress (tensile load divided by area of bond) required to rupture a bond
formed by an adhesive between two metal blocks. (ASTM D-952)
Break Detector
Feature in many materials testing systems that detects the fracture of
the test specimen. You can set up some systems to perform a user-selected
action when specimen break is sensed.
Break Elongation
The elongation of the specimen to the break point.
Breaking Load
Load which causes fracture in a tensile, compression, flexure or torsion
test. In tensile tests of textiles and yarns, breaking load also is called
breaking strength. In tensile tests of thin sheet materials or materials
in form of small diameter wire it is difficult to distinguish between
breaking load and the maximum load developed, so the latter is considered
the breaking load.
Breaking Strength
Tensile load or force required to rupture textiles (e.g., fibers, yarn)
or leather. It is analogous to breaking load in a tension test. Ordinarily,
breaking strength is reported as lb. or lb/in of width for sheet specimens.
Bulk Modulus of Elasticity
Ratio of stress to change in volume of a material subjected to axial loading.
Related to Modulus of Elasticity (E) and Poisson's Ratio (r) by the following
equation: Bulk Modulus K=E/3(1-2r).
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Cleavage Strength
Tensile load (lb/in of width) required to cause separation of a 1-in.
long metal-to-metal adhesive bond under the conditions set in ASTM D-1062.
Climbing Drum Peel Test
Method for determining peel resistance of adhesive bond between a relatively
flexible and a rigid material. (ASTM D-1781).
Coefficient of Elasticity
An alternate term for modulus of elasticity.
Cohesive Strength
Theoretical stress that causes fracture in tensile test if material exhibits
no plastic deformation.
Complex Modulus
Measure of dynamic mechanical properties of a material, taking into account
energy dissipated as heat during deformation and recovery. It is equal
to the sum of static modulus of a material and its loss modulus. In the
case of shear loading, it is called dynamic modulus.
Compressibility
Extent to which a material is compressed in test for compressibility and
recovery of gasket materials (ASTM F-36). It is usually reported with
recovery.
Compressibility and Recovery Test
Method for measuring behavior of gasket materials under short time compressive
loading at room temperature. ASTM F-36 outlines a standard procedure.
This test is not designed to indicate long term (creep) behavior and should
not be confused with the plastometer test.
Compression-Deflection Test
Nondestructive method for determining relationship between compressive
load and deflection under load for vulcanized rubber. (ASTM D-575)
Compression Fatigue
Ability of rubber to sustain repeated fluctuating compressive loads. (ASTM
D-623)
Compression Set
The extent to which rubber is permanently deformed by a prolonged compressive
load (ASTM D-395). Should not be confused with low temperature compression
set.
Compression Test
Method for determining behavior of materials under crushing loads. Specimen
is compressed, and deformation at various loads is recorded. Compressive
stress and strain are calculated and plotted as a stress-strain diagram
which is used to determine elastic limit, proportional limit, yield point,
yield strength and (for some materials) compressive strength. Standard
compression tests are given in ASTM C-773 (high strength ceramics), ASTM
E-9 (metals), ASTM E-209 (metals at elevated temperatures) and ASTM D-695
(plastics).
Compressive Deformation
Extent to which a material deforms under a crushing load.
Compressive Strength
Maximum stress a material can sustain under crush loading. The compressive
strength of a material that fails by shattering fracture can be defined
within fairly narrow limits as an independent property. However, the compressive
strength of materials that do not shatter in compression must be defined
as the amount of stress required to distort the material an arbitrary
amount. Compressive strength is calculated by dividing the maximum load
by the original cross-sectional area of a specimen in a compression test.
Compressive Yield Strength
Stress which causes a material to exhibit a specified deformation. Usually
determined from the stress-strain diagram obtained in a compression test.
See also Yield Strength.
Constant Amplitude
Amplitude of the
command signal during changes in frequency of the signal.
Creep
Deformation that occurs over a period of time when a material is subjected
to constant stress at constant temperature. In metals, creep usually occurs
only at elevated temperatures. Creep at room temperature is more common
in plastic materials and is called cold flow or deformation under load.
Data obtained in a creep test usually is presented as a plot of creep
vs. time with stress and temperature constant. Slope of the curve is creep
rate and end point of the curve is time for rupture. As indicated in the
accompanying diagram, the creep of a material can be divided into three
stages. First stage, or primary creep, starts at a rapid rate and slows
with time. Second stage (secondary) creep has a relatively uniform rate.
Third stage (tertiary) creep has an accelerating creep rate and terminates
by failure of material at time for rupture. See also Stress-Relaxation.
Creep Limit
Alternate term for creep strength.
Creep Rate
Time rate of deformation of a material subject to stress at a constant
temperature. It is the slope of the creep vs. time diagram obtained in
a creep test. Units usually are in/in/hr or % of elongation/hr. Minimum
creep rate is the slope of the portion of the creep vs. time diagram corresponding
to secondary creep.
Creep Recovery
Rate of decrease in deformation that occurs when load is removed after
prolonged application in a creep test. Constant temperature is maintained
to eliminate effects of thermal expansion, and measurements are taken
from time load is zero to eliminate elastic effects.
Creep Rupture Strength
Stress required to cause fracture in a creep test within a specified time.
Alternate term is stress rupture strength.
Creep Strength
Maximum stress required to cause a specified amount of creep in a specified
time. Also used to describe maximum stress that can be generated in a
material at constant temperature under which creep rate decreases with
time. An alternate term is creep limit.
Creep Test
Method for determining creep or stress relaxation behavior. To determine
creep properties, material is subjected to prolonged constant tension
or compression loading at constant temperature. Deformation is recorded
at specified time intervals and a creep vs. time diagram is plotted. Slope
of curve at any point is creep rate. If failure occurs, it terminates
test and time for rupture is recorded. If specimen does not fracture within
test period, creep recovery may be measured. To determine stress relaxation
of material, specimen is deformed a given amount and decrease in stress
over prolonged period of exposure at constant temperature is recorded.
Standard creep testing procedures are detailed in ASTM E-139, ASTM D-2990
and D-2991 (plastics) and ASTM D-2294 (adhesives).
Crush Resistance
Load required to produce fracture in a glass sphere subjected to crush
loading. (ASTM D-1213).
Crushing Load
Maximum compressive force applied during a compression or crushing test.
For materials that do not shatter, crushing load is defined as the force
required to produce a specified type of failure.
Crushing Strength
Compressive load required to cause a crack to form in a sintered metal
powder bearing (ASTM B-438 and B-439). Cold crushing strength of refractory
bricks and shapes is the gross compressive stress required to cause fracture.
(ASTM C-133).
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Deformation Energy
Energy required to deform a material a specified amount. It is the area
under the stress-strain diagram up to a specified strain.
Deformation Under Load
Measure of the ability of rigid plastics to withstand permanent deformation
and the ability of non-rigid plastics to return to original shape after
deformation. Standard test methods for determining both types of deformation
under load are given in ASTM D-621. For rigid plastics, deformation
(which
can be flow or flow and shrinkage) is reported as % change in height
of specimen after 24 hours under a specified load. For non-rigid plastics,
results are reported as % change in height after 3 hours under load and
recovery in the 1-1/2 hour period following removal of the load. Recovery
is % increase in height calculated on basis of original height. See also
Crushing Strength.
De-lamination Strength
Measure of the node-to-node bond strength of honeycomb core materials.
It is equal to the tensile load applied to a honeycomb panel at fracture
divided by its width times its thickness. (ASTM C-363)
Denier
The unit of linear density equal to the mass in grams per 9000 m of fiber,
yarn, or other textile strand.
Dry Strength
Strength of an adhesive joint determined immediately after drying or after
a period of conditioning in a specified atmosphere. (ASTM D-2475)
Ductility
Extent to which a material can sustain plastic deformation without rupture.
Elongation and reduction of area are common indices of ductility.
Dynamic Creep
Creep that occurs under fluctuating load or temperature.
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EASL
Elongation at a specified load.
Eccentricity of Loading
Distance between the actual line of action of compressive or tensile loads
and the line of action that would produce a uniform stress over the cross
section of the specimen.
Edge Tearing Strength
Measure of the resistance of paper to tearing when folded over a V-notch
beam and loaded in a tensile testing machine. Results are reported in
lb or kg. (ee Tear Resistance).
Elastic Hysteresis
Difference between strain energy required to generate a given stress in
a material and elastic energy at that stress. It is the energy dissipated
as heat in a material in one cycle of dynamic testing. Elastic hysteresis
divided by elastic deformation energy is equal to damping capacity.
Elastic Limit
Greatest stress that can be applied to a material without causing permanent
deformation. For metals and other materials that have a significant straight
line portion in their stress/strain diagram, elastic limit is approximately
equal to proportional limit. For materials that do not exhibit a significant
proportional limit, elastic limit is an arbitrary approximation (the apparent
elastic limit).
Elastic Limit, Apparent
Arbitrary approximation of the elastic limit of materials that do not
have a significant straight line portion on a stress/strain diagram. It
is equal to the stress at which the rate of strain is 50% greater than
at zero stress. It is the stress at the point of tangency between the
stress- Elastic Hysteresis strain curve and the line having a slope, with
respect to the stress axis, 50% greater than the slope of the curve at
the origin.
Elasticity
Ability of a material to return to its original shape when load causing
deformation is removed.
Elongation
Measure of the ductility of a material determined in a tensile test. It
is the increase in gage length (measured after rupture) divided by original
gage length. Higher elongation indicates higher ductility. Elongation
cannot be used to predict behavior of materials subjected to sudden or
repeated loading.
Embrittlement
Reduction in ductility due to physical or chemical changes.
Endurance
Alternate term for fatigue limit. .
Engineering
Stress
Load applied to a specimen in a tension or compression test divided by
the cross-sectional area of the specimen. The change in cross-sectional
area that occurs with increases and decreases in applied load, is disregarded
in computing engineering stress. It is also called conventional stress.
Event Detector
Digital function that looks for and trips on certain
events, such as maximum peak, minimum peak, under-peak, over-peak, and
specimen break. Can perform a number of actions, such as stop, hold,
unload,
transfer control mode, etc. upon trip. It is not used as a safety limit.
Extensometer
Instrument for measuring changes in linear dimensions. Also called a strain
gauge. Frequently based on strain gauge technology.
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Fatigue
Permanent structural change that occurs in a material subjected to fluctuating
stress and strain. However, in the case of glass, fatigue is determined
by long-term static testing and is analogous to stress rupture in other
materials. In general, fatigue failure can occur with stress levels below
the elastic limit.
Fatigue Life
Number of cycles of fluctuating stress and strain of a specified nature
that a material will sustain before failure occurs. Fatigue life is a
function of the magnitude of the fluctuating stress, geometry of the specimen
and test conditions. An S-N diagram is a plot of the fatigue life at various
levels of fluctuating stress. See also Engineering
Stress.
Fatigue Limit
Maximum fluctuating stress a material can endure for an infinite number
of cycles. It is usually determined from an S-N diagram and is equal to
the stress corresponding to the asymptote of the locus of points corresponding
to the fatigue life of a number of fatigue test specimens. An alternate
term is endurance limit.
Fatigue Notch Factor
Ratio of fatigue strength of a specimen with no stress concentration to
fatigue strength of a specimen with a notch or other stress raisers. Fatigue
notch factor is usually lower than the theoretical stress concentration
factor because of stress relief due to plastic deformation. An alternate
term is strength reduction ratio.
Fatigue Ratio
Ratio of fatigue strength or fatigue limit to tensile strength. For many
materials, fatigue ratio may be used to estimate fatigue properties from
data obtained in tension tests.
Fatigue Strength
Magnitude of fluctuating stress required to cause failure in a fatigue
test specimen after a specified number of cycles of loading. Usually determined
directly from the S-N diagram.
Fatigue Strength Reduction Factor
An alternate term for fatigue notch factor.
Fatigue Test
A method for determining the behavior of materials under fluctuating loads.
A specified mean load (which may be zero) and an alternating load are
applied to a specimen and the number of cycles required to produce failure
(fatigue life) is recorded. Generally, the test is repeated with identical
specimens and various fluctuating loads. Loads may be applied axially,
in torsion, or in flexure. Depending on amplitude of the mean and cyclic
load, net stress in the specimen may be in one direction through the loading
cycle, or may reverse direction. Data from fatigue testing often are presented
in an S-N diagram which is a plot of the number of cycles required to
cause failure in a specimen against the amplitude of the cyclical stress
developed. The cyclical stress represented may be stress amplitude, maximum
stress or minimum stress. Each curve in the diagram represents a constant
mean stress. Most fatigue tests are conducted in flexure, rotating beam,
or vibratory type machines. Fatigue testing is generally discussed in
"Manual on Fatigue Testing," ASTM STP 91-A, and "Mechanical
Testing of Materials," A.J. Fenner, Philosophical Library, Inc. ASTM
D-671 details a standard procedure for fatigue testing of plastics in
flexure.
Fiber Stress
Stress through a point in a part in which stress distribution is not uniform.
For example, the stress in a beam under bending load varies from compression
to tension across the beam. It is more meaningful in determining the properties
of the beam material to consider the maximum stress generated in the outer
fibers of the beam. Similarly, stress in a beam under twist loading is
a maximum in the material furthest from the axis of twist.
Flex Resistance
Ability of foam rubber to sustain repeated compressive loads without damage
to cell structure. (ASTM D-1055)
Flexural Modulus of Elasticity
Alternate term for modulus in bending.
Flexural Strength
Maximum fiber stress developed in a specimen just before it cracks or
breaks in a flexure test. Flexural yield strength is reported instead
of flexural strength for materials that do not crack in the flexure test.
An alternate term is modulus of rupture.
Flexure Test
Method for measuring behavior of materials subjected to simple beam loading.
It is also called a transverse beam test with some materials. Specimen
is supported on two knife edges as a simple beam and load is applied at
its midpoint. Maximum fiber stress and maximum strain are calculated for
increments of load. Results are plotted in a stress-strain diagram, and
maximum fiber stress at failure is flexural strength. Flexural yield strength
is reported for materials that do not crack. Standard test procedures
are given in ASTM D-790 (plastics) and ASTM C-674 (fired whiteware). ASTM
D-797 (elastomers), ASTM A-438 (cast iron) and ASTM D-86 (glass)
Flow Stress
Stress required to cause plastic deformation.
Fracture Stress
True stress generated in a material at fracture.
Fracture Test
Visual test wherein a specimen is fractured and examined for grain size,
case depth, etc.
Fracture Toughness
Ability of a material to resist crack propagation when subjected to shock
load as in an impact test.
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Hardness
Measure of a material's resistance to localized plastic deformation. Most
hardness tests involve indentation, but hardness may be reported as resistance
to scratching (file test), or rebound of a projectile bounced off the
material (scleroscope hardness). Some common measures of indentation hardness
are Brinell Hardness Number, Rockwell Hardness Number, ASTM Hardness Number,
Diamond Pyramid Impact Test Hardness Number, Durometer Hardness, Knoop
Hardness, and Pfund Hardness. A table relating various types of hardness
values of metals is given in ASTM E-140. Hardness often is a good indication
of tensile and wear properties of a material.
Hooke's Law
Stress is directly proportional to strain. Hooke's law assumes perfectly
elastic behavior. It does not take into account plastic or dynamic loss
properties.
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Impact Energy
Energy required to fracture a part subjected to shock loading as in an
impact test. Alternate terms are impact value, impact strength, impact
resistance, and energy absorption.
Impact Strength
Energy required to fracture a specimen subjected to shock loading, as
in an impact test. Alternate terms are impact energy, impact value, impact
resistance and energy absorption. It is an indication of the toughness
of the material.
Impact Test
A method for determining behavior of material subjected to shock loading
in bending, tension, or torsion. The quantity usually measured is the
energy absorbed in breaking the specimen in a single blow, as in the Charpy
Impact Test, Izod Impact Test, and Tension Impact Test. Impact tests also
are performed by subjecting specimens to multiple blows of increasing
intensity, as in the drop ball impact test, and repeated blow impact test.
Impact resilience and scleroscope hardness are determined in nondestructive
impact tests.
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Kink Test
Method for determining ductility of metal wire. A short section of wire
is looped and drawn in tension to produce a kink. Relative ductility is
indicated by the occurrence or non-occurrence of failure and extent to
which kink may be opened up without failure.
Knot Strength
Tenacity of a fiber in which an overhand knot is tied. Knot strength is
a measure of a fiber's sensitivity to compressive and shear stresses.
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LASE
Load At Specified Elongation.
Limits (Operational)
Feature in materials and structural testing systems that suspends motion
or shuts off the system when upper and/or lower bounds of actuator or
crosshead travel, or force or strain, are reached during testing. Correct
setting of operational limits by the operator, prior to testing, will
reduce the risk of damage to test article and system and associated hazard
to the operator.
Linear Density
Mass per unit length.
Load-Deflection Diagram
Plot of load versus corresponding deflection.
Load Protect
See Specimen Protect.
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Mean Stress
Algebraic difference between maximum and minimum stress in one cycle of
fluctuating loading, as in a fatigue test. Tensile stress is considered
positive and compressive stress negative.
Minimum Bend Radius
Minimum radius to which a sheet or wire can be bent to a specified angle
without failure.
Modulus
Alternate term for modulus of elasticity, often used in connection with
rubber.
Modulus in Bending
Ratio of maximum fiber stress to maximum strain, within elastic limit
of stress-strain diagram obtained in flexure test. Alternate term is flexural
modulus of elasticity.
Modulus of Elasticity
Rate of change of strain as a function of stress. The slope of the straight
line portion of a stress-strain diagram. Tangent modulus of elasticity
is the slope of the stress-strain diagram at any point. Secant modulus
of elasticity is stress divided by strain at any given value of stress
or strain. It also is called stress-strain ratio.
Tangent and secant modulus of elasticity are equal, up to the proportional limit of a material. Depending on the type of loading represented by the stress-strain diagram, modulus of elasticity may be reported as: compressive modulus of elasticity (or modulus of elasticity in compression); flexural modulus of elasticity (or modulus of elasticity in flexure); shear modulus of elasticity (or modulus of elasticity in shear); tensile modulus of elasticity (or modulus of elasticity in tension); or torsional modulus of elasticity (or modulus of elasticity in torsion). Modulus of elasticity may be determined by dynamic testing, where it can be derived from complex modulus. Modulus used alone generally refers to tensile modulus of elasticity. Shear modulus is almost always equal to torsional modulus and both are called modulus of rigidity. Moduli of elasticity in tension and compression are approximately equal and are known as Young's modulus.
Modulus of Rigidity
Rate of change of strain as a function of stress in a specimen subjected
to shear or torsion loading. It is the modulus of elasticity determined
in a torsion test. Alternate terms are modulus of elasticity in torsion
and modulus of elasticity in shear. Apparent modulus of rigidity is a
measure of the stiffness of plastics measured in a torsion test (ASTM
D-1043). It is "apparent" because the specimen may be deflected
past its proportional limit and the value calculated may not represent
the true modulus of elasticity within the elastic limit of the material.
Modulus of Rupture
Ultimate strength determined in a flexure or torsion test. In a flexure
test, modulus of rupture in bending is the maximum fiber stress at failure.
In a torsion test, modulus of rupture in torsion is the maximum shear
stress in the extreme fiber of a circular member at failure. Alternate
terms are flexural strength and torsional strength.
Modulus of Strain Hardening
Alternate term for rate of strain hardening.
Modulus of Toughness
The work done on a unit volume of material as a simple tensile force
is gradually increased from zero to the value causing rupture is defined
as the Modulus of Toughness. This may be calculated as the entire area
under the stress-strain curve from the origin to rupture. Toughness
of
a material is its ability to absorb energy in the plastic range of the
material.
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Necking
Localized reduction of cross-sectional area of a specimen under tensile
load. It is disregarded in calculating engineering stress but is taken
into account in determining true stress.
Nominal Stress
Stress calculated on the basis of the net cross section of a specimen
without taking into account the effect of geometric discontinuities such
as holes, grooves, fillets, etc.
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Offset Yield Strength
Arbitrary approximation of elastic limit. It is the stress that corresponds
to the point of intersection of a stress-strain diagram and a line parallel
to the straight line portion of the diagram. Offset refers to the distance
between the origin of the stress-strain diagram, and the point of intersection
of the parallel line and the 0 stress axis. Offset is expressed in terms
of strain (often 0.2%).
Operating Stress
Stress imposed on a part in service.
Overstressing
Application of high fluctuating loads at the beginning of a fatigue test
and lower loads toward the end. It is a means for speeding up a fatigue
test.
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Peel Resistance
Torque required to separate an adhesive and adhere in the climbing drum
peel test (ASTM D-1781). It is a measure of bond strength.
Peel Strength
Measure of the strength of an adhesive bond. It is the average load per
unit width of bond line required to part bonded materials where the angle
of separation is 180 degrees and separation rate is 6 in/min. (ASTM D-903)
Plastic Deformation
Deformation that remains after the load causing it is removed. It is the
permanent part of the deformation beyond the elastic limit of a material.
It also is called plastic strain and plastic flow.
Plasticity
Tendency of a material to remain deformed, after reduction of the deforming
stress, to a value equal to or less than its yield strength.
Plasticity Number
Index of the compressibility of rubber at elevated temperatures. Equal
to 100 times the height of a standard specimen, after a 3 to 10 minute
compression by a 5 kg load. (ASTM D-926)
Plastic Strain Ratio
Plastic strain ratio, r, is the ratio of the true width strain to the
true thickness strain.
Proof Stress
Stress that will cause a specified permanent deformation.
Proportional Limit
Highest stress at which stress is directly proportional to strain. It
is the highest stress at which the curve in a stress-strain diagram is
a straight line. Proportional limit is equal to elastic limit for many
metals.
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Rate of Strain Hardening
Rate of change of true stress as a function of true strain in a material
undergoing plastic deformation. An alternate term is modulus of strain
hardening.
Rationalization
The method of adding a self-ID and auto-calibration feature to the transducer.
Recovery
Index of a material's ability to recover from deformation in the compressibility
and recovery test (ASTM F-36), the deformation under load test (ASTM
D-621)
and the plastometer test (ASTM D-926). In the compressibility and recovery
test, it usually is reported with compressibility and given as %.
It is
calculated by dividing the difference between recovered thickness and
thickness under load, by the difference between original thickness
and
thickness under load. In the deformation under load test, it indicates
the extent to which a nonrigid plastic recovers from prolonged compressive
deformation at an elevated temperature. It is given as %, and is calculated
by dividing the difference between height recovered 1-1/2 hours after
load is removed and height after three hours of loading, by the change
in height under load. In the plastometer test, it indicates the extent
to which an elastomer recovers from compressive loading at an elevated
temperature. It is equal to plasticity number minus recovered height.
Recovery Test
Method for measuring compressibility and recovery of gasket and seal materials.
(ASTM F-36)
Reduction of Area
Measure of the ductility of metals obtained in a tensile test. It is the
difference between original cross sectional area of a specimen and the
area of its smallest cross section after testing. It is usually ex-pressed
as % decrease in original cross section. The smallest cross section can
be measured at or after fracture. For metals, it usually is measured after
fracture and for plastics and elastomers, it is measured at fracture.
Relative Modulus
Ratio of the modulus of a rubber at a given temperature to its modulus
at 73° F. It is determined in the Gehman torsional test.
Relaxation
Rate of reduction of stress in a material due to creep. An alternate term
is stress relaxation.
Residual Elongation
Measure of ductility of plastics. It is the elongation of a plastic specimen
measured 1 minute after rupture in a tensile test.
Rupture Resistance
Indication of ability of rubber to withstand tensile loading. It is the
load required to rupture a rubber specimen under conditions set out in
ASTM D-530.
Rupture Strength
Nominal stress developed in a material at rupture. It is not necessarily
equal to ultimate strength. And, since necking is not taken into account
in determining rupture strength, it seldom indicates true stress at rupture.
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S-N Diagram
Plot of stress (S) against the number of cycles (N) required to cause
failure of similar specimens in a fatigue test. Data for each curve on
an S-N diagram are obtained by determining fatigue life of a number of
specimens subjected to various amounts of fluctuating stress. The stress
axis can represent stress amplitude, maximum stress or minimum stress.
A log scale is almost always used for the N scale and sometimes for the
S scale. See also Relative Modulus.
Secant Modulus of Elasticity
Ratio of stress to strain at any point on curve in a stress-strain diagram.
It is the slope of a line from the origin to any point on a stress-strain
curve.
Set Point
Arithmetic mean of the excursions of the controlling waveform, i.e.,
the algebraic sum of the positive and negative amplitudes of the waveform.
It is roughly equivalent to "mean level" on
earlier ServoCon ALPHA Servohydraulic Testing Systems.
Shear Modulus of Elasticity
Tangent or secant modulus of elasticity of a material subjected to shear
loading. Alternate terms are modulus of rigidity and modulus of elasticity
in shear. Also, shear modulus of elasticity usually is equal to torsional
modulus of elasticity. A method for determining shear modulus of elasticity
of structural materials by means of a twisting test is given in ASTM E-143.
A method for determining shear modulus of structural adhesives is given
in ASTM E-229.
Shear Strength
Maximum shear stress that can be sustained by a material before rupture.
It is the ultimate strength of a material subjected to shear loading.
It can be determined in a torsion test where it is equal to torsional
strength. The shear strength of a plastic is the maximum load required
to shear a specimen in such a manner that the resulting pieces are completely
clear of each other. It is reported in psi based on the area of the sheared
edge (ASTM D-732). The shear strength of a structural adhesive is the
maximum shear stress in the adhesive prior to failure under torsional
loading (ASTM E-229). Methods for determining shear strength of timber
are given in ASTM D-143 and ASTM D-198.
Specimen Protect
Feature in many materials testing systems that limits the maximum force
applied to the test article. When Specimen Protect is enabled, the actuator
or crosshead moves automatically to ensure the force on the test article
remains within the pre-set bounds. It is often used to protect specimens
or components during set-up, prior to the setting of operational limits.
Specimen Protect only functions in position control mode.
Splitting Resistance
Measure of the ability of felt to withstand tearing. It is the load required
to rupture a slit felt specimen by gripping lips of the cut in jaws and
pulling them apart (ASTM D-461). An alternate term is tear resistance.
Spring-back
Degree to which a material returns to its original shape after deformation.
In plastics and elastomers, it is also called recovery.
Stiffness
Measure of resistance of plastics to bending. It includes both plastic
and elastic behavior, so it is an apparent value of elastic modulus rather
than a true value. (ASTM D-747)
Strain
Change per unit length in a linear dimension of a part or specimen, usually
expressed in % Strain, as used with most mechanical tests, is based on
original length of the specimen. True or natural strain is based on instantaneous
length, and is equal to: ln X l lo , where l is instantaneous length and
lo is original length of the specimen. Shear strain is the change in angle
between two lines originally at right angles.
Strain Energy
Measure of energy absorption characteristics of a material under load
up to fracture. It is equal to the area under the stress-strain curve,
and is a measure of the toughness of a material. See also Splitting
Resistance.
Strain Hardening Exponent
Measure of increase in hardness and strength caused by plastic deformation.
It is related to true stress and true strain by the equation:
s = s0d h where s is true stress, s0 is true stress at unit strain, d is true strain and h is strain hardening exponent.
Strain Point
Temperature at which internal stress in glass is substantially relieved
in about 1 hour. (ASTM C-336)
Strain Rate
Time rate of elongation.
Strain Relaxation
Alternate term for creep of rubber.
Strength Reduction Ratio
Alternate term for fatigue notch factor.
Stress
Load on a specimen divided by the area through which it acts. As used
with most mechanical tests, stress is based on original cross-sectional
area without taking into account changes in area due to applied load.
This sometimes is called conventional or engineering stress. True stress
is equal to the load divided by the instantaneous cross-sectional area
through which it acts.
Stress Amplitude
One-half the range of fluctuating stress developed in a specimen in a
fatigue test. Stress amplitude often is used to construct an S-N
diagram.
Stress Concentration Factor
Ratio of the greatest stress in the area of a notch or other stress raiser
to the corresponding nominal stress. It is a theoretical indication of
the effect of stress concentrators on mechanical behavior. Stress concentration
factor usually is higher than the empirical fatigue notch factor or strength
reduction ratio, because it does not take into account stress relief due
to local plastic deformation.
Stress Ratio
Ratio of minimum stress to maximum stress in one cycle of loading in a
fatigue test. Tensile stresses are considered positive and compressive
stresses negative.
Stress Relaxation
Decrease in stress in a material subjected to prolonged constant strain
at a constant temperature. Stress relaxation behavior is determined in
a creep test. Data often is presented in the form of a stress vs. time
plot. Stress relaxation rate is the slope of the curve at any point.
Stress Rupture Strength
Alternate term for creep strength.
Stress-Strain Ratio
Stress divided by strain at any load or deflection. Below the elastic
limit of a material, it is equal to tangent modulus of elasticity. An
alternate term is the secant modulus of elasticity.
Stripping Strength
Alternate term for peel strength.
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Tangent Modulus of Elasticity
The instantaneous rate of change of stress as a function of strain. It
is the slope at any point on a stress-strain diagram.
Tear Length
Measure of the drawability of sheet metal. Two small parallel slots are
cut in the edge of the sheet to form a tab which is gripped and torn from
the sheet. The variation in length of tabs torn in different directions
is an indication of crystal orientation in the sheet (tabs torn in the
direction of orientation are longer). The degree of orientation is an
indication of difficulty to be expected in drawing the sheet to uniform
shapes.
Tear Resistance
Measure of the ability of sheet or film materials to resist tearing. For
paper, it is the force required to tear a single ply of paper after the
tear has been started. Three standard methods are available for determining
tear resistance of plastic films: ASTM D-1004 details a method for determining
tear resistance at low rates of loading; a test in ASTM D-1922 measures
the force required to propagate a precut slit across a sheet specimen;
and ASTM D-1038 gives a method for determining tear propagation resistance
that is recommended for specification acceptance testing only. Tear resistance
of rubber is the force required to tear a 1 inch thick specimen under
the conditions outlined in ASTM D-624. Tear resistance of textiles is
the force required to propagate a single-rip tongue-type tear (starting
from a cut) by means of a falling pendulum apparatus. (ASTM D-1424)
Tearing Strength
Tensile force required to rupture a pre-slit woven fabric specimen under
the conditions outlined in ASTM D-2261 and ASTM D-2262. Edge tearing strength
of paper is the force required to tear a specimen folded over a V-notch
and loaded in a tensile test machine.
Tenacity
The tensile stress expressed as force per unit linear density of an unstrained
specimen.
Tensile Strength
Ultimate strength of a material subjected to tensile loading. It is the
maximum stress developed in a material in a tensile test.
Tensile Impact Test
Method for determining energy required to fracture a specimen under shock
tensile loading (ASTM D-1822). Also known as Tension Impact Test.
Tensile Test
Method for determining behavior of materials under axial stretch loading.
Data from test are used to determine elastic limit, elongation, modulus
of elasticity, proportional limit, reduction in area, tensile strength,
yield point, yield strength and other tensile properties. Tensile tests
at elevated temperatures provide creep data. Procedures for tensile tests
of metals are given in ASTM E-8. Methods for tensile tests of plastics
are outlined in ASTM D-638, ASTM D-2289 (high strain rates), and ASTM
D-882 (thin sheets). ASTM D-2343 outlines a method for tensile testing
of glass fibers; ASTM D-897, adhesives; ASTM D-412, vulcanized rubber.
Also known as tension test.
Tension Set
Extent to which vulcanized rubber is permanently deformed after being
stretched a specified amount for a short time. It is expressed as a %
of the original length or distance between gage marks (ASTM D-412).
Tex
The unit of linear density equal to the mass in grams per 1000 m of fiber,
yarn, or other textile strand.
Time for Rupture
Time required to rupture specimen under constant stress and temperature
in a creep test.
Torsion Test
Method for determining behavior of materials subjected to twisting loads.
Data from torsion test is used to construct a stress-strain diagram and
to determine elastic limit torsional modulus of elasticity, modulus of
rupture in torsion, and torsional strength. Shear properties are often
determined in a torsion test. (ASTM E-143)
Torsional Deformation
Angular displacement of specimen caused by a specified torque in torsion
test. It is equal to the angular twist (radians) divided by the gage length
(in.).
Torsional Modulus of Elasticity
Modulus of elasticity of material subjected to twist loading. It is approximately
equal to shear modulus and also is called modulus of rigidity.
Torsional Strain
Strain corresponding to a specified torque in the torsion test. It is
equal to torsional deformation multiplied by the radius of the specimen.
Torsional Strength
Measure of the ability of a material to withstand a twisting load. It
is the ultimate strength of a material subjected to torsional loading,
and is the maximum torsional stress that a material sustains before rupture.
Alternate terms are modulus of rupture and shear strength.
Toughness
Toughness is the resistance of a material to fracture or break. It is
usually measured in units of energy.
True Strain
Instantaneous % of change in length of specimen in mechanical test. It
is equal to the natural logarithm of the ratio of length at any instant
to original length.
True Stress
Applied load divided by actual area of the cross section through which
load operates. It takes into account the change in cross section that
occurs with changing load.
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Ultimate Elongation
Alternate term for elongation of material at rupture under tensile loading.
Ultimate Strength
Highest engineering stress developed in material before rupture. Normally,
changes in area due to changing load and necking are disregarded in determining
ultimate strength.
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Wet Strength
Breaking strength of paper saturated with water. Also, the strength of
an adhesive bond after immersion in water.
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Yield Point
Stress at which strain increases without accompanying increase in stress.
Only a few materials (notably steel) have a yield point, and generally
only under tension loading.
Yield Point Elongation
In materials that exhibit a yield point, the Yield Point Elongation (YPE)
is the difference between the elongation of the specimen at the start
and at the finish of discontinuous yielding (the area in which an increase
in strain occurs without an increase in stress).
Yield Strength
Indication of maximum stress that can be developed in a material without
causing plastic deformation. It is the stress at which a material exhibits
a specified permanent deformation and is a practical approximation of
elastic limit. Offset yield strength is determined from a stress-strain
diagram. It is the stress corresponding to the intersection of the stress-strain
curve, and a line parallel to its straight line portion offset by a specified
strain. Offset for metals is usually specified as 0.2%, i.e., the intersection
of the offset line and the 0-stress axis is at 0.2% strain. Offset for
plastics is usually 2%.
Yield Strength Elongation
Strain corresponding to yield strength of material. It is an indication
of ductility.
Yield Value
Stress in an adhesive joint at which a marked increase in deformation
occurs without an increase in load.
Young's Modulus
Alternate term for modulus of elasticity in tension or compression.
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Zero Suppression
Servohydraulic testing systems have a zero suppression feature that shifts
the absolute zero of the command waveform to an offset or "apparent"
zero. It is used to improve resolution when using a waveform that is small
in relation to the full scale range in use. It is also used when the actuator
is offset from its normal or absolute zero to accommodate large grips
or long specimens.
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