ASM Metals HandBook Vol. 8 - Mechanical Testing and Evaluation
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stress-rupture test
See creep-rupture test.
stress-strain curve
See stress-strain diagram.
stress-strain diagram
A graph in which corresponding values of stress and strain are plotted against each other. Values of
stress are usually plotted vertically(ordinates or y-axis) and values of strain horizontally (abscissas or x-
axis). Also known as deformation curve and stress-strain curve.
stress-time curve
See stress-relaxation curve.
stretch-bending test
A simulative test for sheet metal formabilityin which a strip of sheet metal is clamped at its ends in lock
beads and deformed in the center by a punch. Test conditions are chosen so that fracture occurs in the
region of punch contact.
stretcher strains
See Lüders lines.
striation
A fatigue fracture feature, often observed in electron micrographs, that indicates the position of the
crack front after each succeeding cycle of stress. The distance between striations indicates the advance
of the crack front across that crystal during one stress cycle, and a line normal to the striation indicates
the direction of local crack propagation.
Swift cup test
A simulative cupping test in which circular blanks of various diameters are clamped in a die ring and
deep drawn into cups by a flat-bottomed cylindrical punch. Compare with Erichsen cup test and Olsen
cup test.
T
tangent modulus
The slope of the stress-strain curve at any specified stress or strain. See also modulus of elasticity.
temper embrittlement
Brittleness that results when certain steels are held within, or are cooled slowly through, a certain range
of temperature below the transformation range. The brittleness is manifested as an upward shift in
ductile-to-brittle transition temperature but only rarely produces a low value of reduction in area in a
smooth-bar tension test of the embrittled material.
tensile strength
In tension testing, the ratio of maximum load to original cross-sectional area. Also known as ultimate
strength. Compare with yield strength.
tensile stress
A stress that causes two parts of an elastic body, on either side of a typical stress plane, to pull apart.
Contrast with compressive stress.
tensile testing
See tension testing.
tension
The force or load that produces elongation.
tension testing
A method of determining the behavior of materials subjected to uniaxial loading, which tends to stretch
the metal. A longitudinal specimen of known length and diameter is gripped at both ends and stretched
at a slow, controlled rate until rupture occurs. Also known as tensile testing.
testing machine (load-measuring type)
A mechanical device for applying a load (force) to a specimen.
theoretical stress concentration factor
See stress-concentration factor.
thermal fatigue
Fatigue resulting from the presence of temperature gradients that vary with time in such a manner as to
produce cyclic stresses in a structure.
thermocouple
A device for measuring temperature, consisting of lengths of two dissimilar metals or alloys that are
electrically joined at one end and connected to a voltage-measuring instrument at the other end. When
one junction is hotter than the other, a thermal electromotive force is produced that is roughly
proportional to the difference in temperature between the hot and cold junctions.
threshold stress for stress-corrosion cracking (σ
th
)
An experimentally determined critical gross-section stress below which stress-corrosion cracking will
not occur under specified test conditions.
tolerance limits
The extreme values (upper and lower) that define the range of permissible variation in size or other
quality characteristic of a part.
torsion
A twisting deformation of a solid body about an axis in which lines that were initially parallel to the axis
become helices.
torsion test
A test designed to provide data for the calculation of the shear modulus, modulus of rupture in torsion,
and yield strength in shear.
torsional stress
The shear stress on a transverse cross section resulting from a twisting action.
total elongation
A total amount of permanent extension of a test piece broken in a tensile test. See also elongation,
percent.
total-extension-under-load yield strength
See yield strength.
toughness
The ability of a metal to absorb energy and deform plastically before fracturing.
transgranular
Through or across crystals or grains. Also known as intracrystalline or transcrystalline.
transition temperature
(1) An arbitrarily defined temperature that lies within the temperature range in which metal fracture
characteristics (as usually determined by tests of notched specimens) change rapidly, such as from
primarily fibrous (shear)to primarily crystalline (cleavage) fracture. (2) Sometimes used to denote an
arbitrarily defined temperature within a range in which the ductility changes rapidly with temperature.
transverse direction
Literally, the “across” direction, usually signifying a direction or plane perpendicular to the direction of
working. In rolled plate or sheet, the direction across the width is often called long transverse, and the
direction through the thickness, short transverse.
transverse strain
Linear strain in a plane perpendicular to the loading axis of a specimen.
triaxial stress
See principal stress (normal).
tribology
The science and technology concerned with interacting surfaces in relative motion.
true strain
(1) The ratio of the change in dimension, resulting from a given load increment, to the magnitude of the
dimension immediately prior to applying the load increment. (2) In a body subjected to axial force, the
natural logarithm of the ratio of the gage length at the moment of observation to the original gage
length. Also known as natural strain. Compare with engineering strain.
true stress
The value obtained by dividing the load applied to a member at a given instant by the cross-sectional
area over which it acts. Compare with engineering stress.
typical basis
The typical property value is an average value. No statistical assurance is associated with this basis. See
also A-basis, B-basis, and S-basis.
U
ultimate strength
The maximum stress (tensile, compressive, or shear) a material can sustain without fracture, determined
by dividing maximum load by the original cross-sectional area of the specimen. Also known as nominal
strength or maximum strength.
uniaxial strain
Increase (or decrease) in length resulting from a stress acting parallel to the longitudinal axis of the
specimen.
uniaxial stress
See principal stress (normal).
uniform elongation
The elongation at maximum load and immediately preceding the onset of necking in a tension test.
uniform strain
The strain occurring prior to the beginning of localization of strain (necking); the strain to maximum
load in the tension test.
V
variance
A measure of the squared dispersion of observed values or measurements expressed as a function of the
sum of the squared deviations from the population mean or sample average.
verification
Checking or testing an instrument to ensure conformance with a specification.
verified loading range
For testing machines, the range of indicated loads for which the testing machine gives results within the
permissible variation specified.
vernier
A short auxiliary scale that slides along the main instrument scale to permit more accurate fractional
reading of the least main division of the main scale. See also least count.
Vicat softening point
The temperature at which a flat-ended needle of 1 mm
2
(0.0015 in.
2
) circular or square cross section will
penetrate a thermoplastic specimen to a depth of 1 mm (0.040 in.) under a specified load, using a
uniform rate of temperature rise.
Vickers hardness number (HV)
A number related to the applied load and the surface area of the permanent impression made by a
square-based pyramidal diamond indenter having included face angles of 136°, computed from the
equation:
(Eq 1)
where P is applied load, kgf; d is mean diagonal of the impression, mm; and α is face angle of diamond,
136°.
Vickers hardness test
An indentation hardness test employing a 136° diamond pyramid indenter (Vickers) and variable loads,
enabling the use of one hardness scale for all ranges of hardness—from very soft lead to tungsten
carbide. Also know as diamond pyramid hardness test.
viscoelasticity
A property involving a combination of elastic and viscous behavior. A material having this property is
considered to combine the features of a perfectly elastic solid and a perfect fluid. A phenomenon of
time-dependent, in addition to elastic, deformation (or recovery) in response to load.
volumetric modulus of elasticity
See bulk modulus of elasticity.
W
Wallner lines
A distinct pattern of intersecting sets of parallel lines, usually producing a set of V-shaped lines,
sometimes observed when viewing brittle fracture surfaces at high magnification in an electron
microscope. Wallner lines are attributed to interaction between a shock wave and a brittle crack front
propagating at high velocity. Sometimes Wallner lines are misinterpreted as fatigue striations.
wear
Damage to a solid surface, generally involving progressive loss of material, due to relative motion
between that surface and a contacting surface or substance.
wear rate
The rate of material removal or dimensional change due to wear per unit of exposure parameter—for
example, quantity of material removed(mass, volume, thickness) in unit distance of sliding or unit time.
welding
In tribology, the bonding between metallic surfaces in direct contact, at any temperature.
Widmanstätten structure
A structure characterized by a geometrical pattern resulting from the formation of a new phase along
certain crystallographic planes of the parent solid solution. The orientation of the lattice in the new
phase is related crystallographically to the orientation of the lattice in the parent phase. The structure is
readily produced in many alloys by appropriate heat treatment.
workability
See formability.
work hardening
See strain hardening.
wrap-around bend
The bend obtained when a specimen is wrapped in a closed helix around a cylindrical mandrel. This
term is sometimes applied to a semiguided bend of 180° or less.
wrinkling
A wavy condition obtained in deep drawing of sheet metal, in the area of the metal between the edge of
the flange and the draw radius. Wrinkling may also occur in other forming operations when unbalanced
compressive forces are set up.
Y
yielding
Evidence of plastic deformation in structural materials. Also known as plastic flow or creep. See also
flow.
yield point
The first stress in a material, usually less than the maximum attainable stress, at which an increase in
strain occurs without an increase in stress. Only certain metals—those which exhibit a localized,
heterogeneous type of transition from elastic to plastic deformation—produce a yield point. If there is a
decrease in stress after yielding, a distinction may be made between upper and lower yield points. The
load at which a sudden drop in the flow curve occurs is called the upper yield point. The constant load
shown on the flow curve is the lower yield point.
yield-point elongation
During discontinuous yielding, the amount of strain measured from the onset of yielding to the
beginning of strain hardening.
yield strength
The stress at which a material exhibits a specified deviation from proportionality of stress and strain. An
offset of 0.2% is used for many metals. Compare with tensile strength.
yield stress
The stress level of highly ductile materials, such as structural steels, at which large strains take place
without further increase in stress.
Young's modulus
A term used synonymously with modulus of elasticity.The ratio of tensile or compressive stresses to the
resulting strain.
Z
zero time
The time when the given loading or constraint conditions are initially obtained in creep or stress-
relaxation tests, respectively.
Glossary of Terms
Selected References
• Compilation of ASTM Standard Definitions, 8th ed., ASTM, 1994
• H.E. Davis, G.E. Troxell, and G.F.W. Hauck, The Testing of Engineering Materials, 4th ed., McGraw
Hill, 1982
• J.R. Davis, Ed., ASM Materials Engineering Dictionary, ASM International, 1992
• G.E. Dieter, Mechanical Metallurgy, 2nd ed., McGraw Hill, New York, 1976
• Glossary of Metallurgical Terms and Engineering Tables, American Society for Metals, 1979
• D.N. Lapedes, Ed., Dictionary of Scientific and Technical Terms, 2nd ed., McGraw Hill, 1974
• A.D. Merriman, A Dictionary of Metallurgy, Pitman Publishing, London, 1958
• “Metal Test Methods and Analytical Procedures,” Annual Book of ASTM Standards, Vol 03.01 and
03.02, ASTM, 1984
• J.G. Tweeddale, Mechanical Properties of Metals, American Elsevier, 1964