ISO/DTS 23718

ISO/TC 164
Secretariat: JISC
Date: 2026-06-24xx
Metallic materials — Mechanical testing — Vocabulary
Matériaux métalliques — Essais mécaniques — Vocabulaire

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ii
iii
Contents
Foreword . v
Introduction . vii
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
3.1 General terms . 1
3.2 Terms related to uniaxial testing . 3
3.3 Terms related to ductility testing . 7
3.4 Terms related to hardness testing . 9
3.5 Terms related to fatigue, fracture and toughness testing . 13
Bibliography . 24
Index . 26

iv
Foreword
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bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee has
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ISO/IEC Directives, Part 2 (see www.iso.org/directives).
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www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO /TC 164, Mechanical testing of metals.
This secondfirst edition of ISO/TS 23718 cancels and replaces the first edition (ISO 23718:2007), which has
been technically revised.
The main changes are as follows:
— — The following basic and frequently used terms have been added.

3.1.13.1.1 bending strain
3.2.63.2.6 extension
3.5.1.163.5.1.16 fillet radius
3.5.1.173.5.1.17 force range
3.5.1.243.5.1.24 load train
3.5.1.293.5.1.29 minimum force
3.5.1.323.5.1.32 phase angle
3.5.1.383.5.1.38 stress cycle
3.5.2.63.5.2.6 potential energy
v
3.5.2.113.5.2.11 reference absorbed energy
3.5.2.123.5.2.12 reference test piece
— — In the first edition of the International Standard, the terms to toughness testing (Charpy
impact testing and fracture toughness testing) were specified in 1.5 and the terms to fatigue testing were
specified in 1.6. In this document, the terms to both testing have been integrated into one subclause
(3.5sub-clause (3.5)) because both testing are closely related. The structure of 3.53.5 is as follows:

3.5.13.5.1 Terms commonrelated to fatigue testing
3.5.23.5.2 Terms commonrelated to Charpy impact testing
3.5.33.5.3 Terms commonrelated to fracture toughness testing
— — In accordance with “ISO/IEC Directives, Part 2, 2021”, the editorial modificationmodifications
have been added throughout the document.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
vi
Introduction
This document was prepared to define terms commonrelated to mechanical testing of metals in order to
help avoid contradictions and misunderstandings among them. Included are only those terms regarded as
common to the Technical Committee (TC) and to each Subcommittee (SC).
vii
Metallic materials — Mechanical testing — Vocabulary
1 Scope
This document defines the terminology that is used in mechanical testing of metals and forms a common
basis for standards and general use.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— — ISO Online browsing platform: available at https://www.iso.org/obp
— — IEC Electropedia: available at https://www.electropedia.org/
3.1 General terms
3.1.1 3.1.1
bending strain
difference between the strain at the surface of the test piece and the axial strain
Note 1 to entry: The bending strain varies around the circumference and along the parallel length of the test piece.
3.1.2 3.1.2
crack growth
crack extension
Δa
change in crack length
Note 1 to entry: It is expressed in millimetres.
3.1.3 3.1.3
crack length
a
length of crack measured from load line to crack tip in the CT specimen, from the
perpendicular bisector of the central crack to crack tips in the MT specimen, and from the specimen front
face to crack tip in the bend specimen
3.1.4 3.1.4
crack length
crack size
a
a linear measure of a principal planar dimension of a crack from a reference plane to the crack tip
Note 1 to entry: It is expressed in millimetres.
3.1.5 3.1.5
ductility
ability of a material to deform plastically without fracturing
3.1.6 3.1.6
elastic limit
maximum stress that a material is capable of sustaining without any permanent strain remaining upon
complete release of the stress
3.1.7 3.1.7
force
F
external influence which would cause an unrestrained test piece to accelerate and which induces stresses in
a restrained test piece
3.1.8 3.1.8
mechanical properties
those properties of a material that are associated with elastic and inelastic reaction when force is applied, or
that involve the relationship between stress and strain
3.1.9 3.1.9
mechanical testing
any tests for the determination of mechanical properties
3.1.10 3.1.10
modulus of elasticity
Young's modulus
E
ratio of stress to corresponding strain below the proportional limit
Note 1 to entry: Young’s modulus is the specialspecific case of elastic modulus for normal stress and linear strain.
3.1.11 3.1.11
Poisson's ratio
v
ratio of transverse strain to the corresponding axial strain resulting from an axial stress below the
proportional limit of the material
3.1.11.1 3.1.11.1
transverse strain
linear strain perpendicular to the applied force
3.1.11.2 3.1.11.2
axial strain
linear strain in the direction of the applied force
3.1.11.3 3.1.11.3
axial stress
stress component in the direction of the applied force
3.1.12 3.1.12
Δ
range
algebraic difference between the maximum and minimum values of a variable
3.1.13 3.1.13
strain
relative deformation caused by a force applied to a test piece
3.1.13.1 3.1.13.1
engineering strain
e
axial strain calculated as the change in length divided by the original length
3.1.13.2 3.1.13.2
linear strain
strain component in a given linear direction
3.1.13.3 3.1.13.3
true strain
ε
natural logarithm of the ratio of an instantaneous length to its original length in the direction of the applied
force up to the onset of necking
3.1.14 3.1.14
stress
localised intensity of forces within a test piece
3.1.14.1 3.1.14.1
engineering stress
S
axial stress calculated on the basis of the original cross-sectional area
3.1.14.2 3.1.14.2
normal stress
stress component perpendicular to a given plane
3.1.14.3 3.1.14.3
true stress
σ
axial stress calculated on the basis of the instantaneous cross-sectional area
3.1.15 3.1.15
test piece
specimen
portion or piece of material to be used for a single test and normally prepared to a predetermined shape and
dimensions
3.1.16 3.1.16
uncertainty of measurement
U, u
parameter, associated with the result of a measurement, that characterizes the dispersion of the values that
could reasonably be attributed to the measurand
Note 1 to entry: See ISO /IEC Guide to the Expression of Uncertainty in Measurement (GUM). 98-3 for more details.
3.2 Terms commonrelated to uniaxial testing
3.2.1 3.2.1
creep curve
curve relating strain and time in a creep test
3.2.2 3.2.2
creep rupture time
t
u
time required for the test piece, maintained at the specified temperature, T, and strained by the specified
tensile stress, to rupture
Note 1 to entry: The symbol tu may have as superscript the specified temperature, T, in Celsius degrees (°C) and as
subscript the initial stress, σo, in newtons per square millimetre (N/mm ).
3.2.2.1 3.2.2.1
creep elongation time
t
fx
time required for a strained test piece to obtain at the specified values of temperature, T, and initial stress,
σ , a specified percentage creep elongation, x
o
3.2.2.2 3.2.2.2
plastic elongation time
t
px
time required to obtain at the specified values of temperature, T, and initial stress, σ , a
o
specified percentage plastic elongation, x
3.2.3 3.2.3
creep strength
stress to cause specified strain in given time during a creep test at a specified constant temperature
3.2.4 3.2.4
creep test
test to measure the change in strain as a function of time with the test piece held at constant temperature
and under constant force or constant stress
3.2.5 3.2.5
elongation
increase in the original gauge length, L , or reference length, L , at any moment during the test
o r
3.2.5.1 3.2.5.1
percentage elongation
A
elongation, expressed as a percentage of the original gauge length or reference length
3.2.5.2 3.2.5.2
percentage creep elongation
A
f
increase in reference length at time t, ΔL , at a specified temperature, expressed as a percentage of the
rt
original reference length, L :
ro
Note 1 to entry: Af may have the specified temperature, T, in Celsius degrees (°C) as superscript and the initial stress,
σ , in megapascals (MPa) and time t in hours (h) as subscript.
o
Note 2 to entry: By convention, the beginning of creep-elongation measurement is the time at which the initial stress,
σo, is applied to the test piece.
3.2.5.3 3.2.5.3
percentage elongation after creep rupture
A
u
permanent increase of the original reference length, L , after rupture (L − L ), expressed as a percentage
ro ru ro
of the original reference length, L :
ro
Note 1 to entry: Au may have the specified temperature, T, in Celsius degrees (°C) as superscript, and the initial stress,
σo, in megapascals (MPa) as subscript.
3.2.5.4 3.2.5.4
percentage initial plastic elongation
A
i
non-proportional increase of the original reference length, L , due to the application of the test force
ro
3.2.6 3.2.6
extension
increase in the extensometer gauge length, at any moment during the test
3.2.7 3.2.7
extensometer
device for measuring extension and transversal dimensional changes
3.2.8 3.2.8
gauge length
L
length of the section of the test piece on which elongation is measured
3.2.8.1 3.2.8.1
extensometer gauge length
L
e
length of the parallel portion of the test piece used for the measurement of extension by means of an
extensometer
Note 1 to entry: In some cases, L = L
e o.
3.2.8.2 3.2.8.2
final gauge length
L
u
gauge length after fracture of the test piece
3.2.8.3 3.2.8.3
original gauge length
L
O
gauge length before application of force
3.2.9 3.2.9
initial stress
σ
o
applied force divided by the original cross-sectional area, S , of the test piece
o
3.2.10 3.2.10
maximum force
F
m
maximum force which the test piece withstands
during the test
3.2.11 3.2.11
maximum force
F
m
maximum force which the test piece withstands during
the test after the beginning of discontinuous yielding
3.2.12 3.2.12
parallel length
L
c
parallel length of the reduced section of the test piece
3.2.13 3.2.13
proof strength, non-proportional extension
R
p
stress at which the plastic extension is equal to a specified percentage of the extensometer gauge length, L
e
Note 1 to entry: The symbol used is followed by a suffix giving the prescribed percentage, for example Rp0,2.
3.2.14 3.2.14
proportional limit
greatest stress which a material is capable of sustaining without any deviation from proportionality of stress
to strain
Note 1 to entry: The proportional limit depends on the level of observation used to record the data or results of the
test.
3.2.15 3.2.15
percentage reduction of area
Z
Z
u
maximum change in cross-sectional area which has occurred during the test (S − S ), expressed as a
o u
percentage of the original cross--sectional area, S
o
3.2.16
3.2.16
reference length
L
r
base length used for the calculation of elongation
3.2.163.2.17 3.2.17
stress-strain curve
curve representing the relation between nominal stress and corresponding strain of the parallel portion of
the test piece throughout the whole process of tensile testing
3.2.173.2.18 3.2.18
tensile strength
R
m
stress corresponding to the maximum force, F
m
Note 1 to entry: It is calculated as the ratio between the maximum force during a tensile test carried to rupture and
the original cross-sectional area of the specimen.
3.2.183.2.19 3.2.19
tensile test
L
e
test that involves straining a test piece by tensile force, generally to fracture, for the purpose of determining
one or more of the tensile properties
3.2.193.2.20 3.2.20
yield strength
exhibiting a yield phenomenon〉> stress corresponding to the point reached during the
test at which plastic deformation occurs without any increase in the force
3.2.19.13.2.20.1 3.2.20.1
lower yield strength
R
eL
lowest value of stress during plastic yielding, ignoring any initial transient effects
3.2.19.23.2.20.2 3.2.20.2
upper yield strength
R
eH
maximum value of stress prior to the first decrease in force
3.3 Terms commonrelated to ductility testing
3.3.1 3.3.1
bend test
test which consists in submitting a test piece to plastic deformation by bending until a specified angle of
bend is reached
Note 1 to entry: The absence of visible cracks is considered as evidence that the test piece withstood the bend test.
3.3.2 3.3.2
bend test of tube
bending a straight tube in full section around a grooved former of a specified radius, r, until the angle of
bend, α, reaches the value specified in the relevant product standard
3.3.3 3.3.3
degree of planar anisotropy
coefficient calculated using the formula,

3.3.4
3.3.4
drift-expanding test of tube
expansion of the end of the test piece cut from the tube, by means of a conical mandrel, until the maximum
outside diameter reaches the value specified in the relevant product standard
3.3.5 3.3.5
earing test
test in which cylindrical cups are formed from circular blanks taken from metal sheets or strips, and the
height of any earing produced by this process is measured
3.3.6 3.3.6
Erichsen cupping index
IE
measured depth of the cup in the Erichsen cupping test
3.3.7 3.3.7
Erichsen cupping test
test which consists in forming a cup shape by pressing a punch with a spherical end against a clamped test
piece between a blank holder and a die until a through crack appears
3.3.8 3.3.8
flattening test of tube
test which consists in flattening the end of a tube or a test piece of specified length, cut from a tube in a
direction perpendicular to the longitudinal axis of the tube, until the distance between platens measured
under load in the direction of flattening reaches a value specified in the relevant product standard
3.3.9 3.3.9
flanging test of tubes
forming a flange on the end of a test piece cut from the tube, in a plane perpendicular to the axis of the tube,
until the external diameter of the flange reaches the value specified in the relevant product standard
3.3.10 3.3.10
formability
capability of a material to be formed into a required form without the occurrence of fracture, localized
thinning, or wrinkling
3.3.11 3.3.11
formability test
test to compare formability of materials by using forming limits determined by a process in which a test
piece is shaped by deformation working analogous to actual forming until cracking occurs, by a test tool with
standardized form and dimension
3.3.12 3.3.12
forming-limit diagram
FLD
a forming-limit curve, the extent to which the material can be formed by drawing,
stretching or any combination of drawing and stretching
3.3.13 3.3.13
plastic strain ratio
r
e
ratio of the true width strain and true thickness strain in a test piece that has been submitted to uniaxial
tensile stress
3.3.14 3.3.14
ring-expanding test of tubes
expanding a ring cut from the end of a tube, over a conical mandrel until fracture, or until the expansion of
the test piece reaches the value specified in the relevant product standard
3.3.15 3.3.15
strain hardening exponent
n
exponent of the true strain in the mathematical equation relating the true stress σ to the true strain ε
Note 1 to entry: During uniaxial application of a force: . .
3.3.16 3.3.16
torsion test of wire
test to examine the number of times of twisting rotation, aspect of fracture surface, state of torsion, etc. at
the time of fracture, where both ends of the test piece are gripped tightly with the specified free length
between grips and one end is rotated while stretching, so as not to show deflection
3.3.16.1 3.3.16.1
simple torsion test of wire
test which consists of twisting a test piece of wire around its own axis in one direction
3.3.16.2 3.3.16.2
reverse torsion test of wire
test which consists of a twist through 360° in one direction followed by another twist through 360° in the
opposite direction
3.3.17 3.3.17
wrapping test of wire
test to examine the state of occurrence of breaking, flaws, etc. by wrapping a test piece tightly around a
mandrel of a specified diameter until it reaches the specified number of turns
3.4 Terms commonrelated to hardness testing
3.4.1 3.4.1
Brinell hardness
HBW
measure of a material's resistance to permanent indentation when a test force is applied through a
hardmetal ball indenter
Note 1 to entry: It is given by:
HBW = 0,102 × [test force (N)/surface area of permanent indentation (mm )].
Note 2 to entry: The indentation is assumed to retain the shape of the ball, and its surface area is calculated from the
mean indentation diameter and the ball diameter.
3.4.2 3.4.2
direct verification
process for determining whether critical components of a machine (such as the maximum error in the
applied force, the measurement of indentation depth or size, the indenter geometry, and testing cycle
parameters) are within specified tolerances or not.
3.4.3 3.4.3
hardness
resistance of a material to deformation, particularly permanent deformation, by indentation or scratching
3.4.4 3.4.4
hardness calibration machine
machine used for the calibration of hardness reference blocks, usually differing from a hardness testing
machine by having tighter tolerances on certain parameters, such as the maximum error in the applied force,
measurement of indentation depth or size, indenter geometry, and testing-cycle parameters
3.4.5 3.4.5
hardness testing machine
an indentation testing machine used for performing indentation hardness tests
3.4.6 3.4.6
indentation
impression made in the surface of the material by the indenter in an indentation test
3.4.7 3.4.7
indentation hardness
H
IT
specified measure of the mean indentation pressure required to induce plastic deformation in a material
indented by a specified indenter of specified geometry when a test force is applied under specified
conditions during a specified testing cycle
3.4.8 3.4.8
indentation hardness test
indentation test, carried out using a hardness testing machine, to measure the hardness of a material
3.4.9 3.4.9
indentation modulus
E
IT
estimate of the average isotropic Young’s modulus of the test piece calculated from the plane strain
indentation modulus
Note 1 to entry: It is given by:
2 2 *
EIT = (1 − ν ν ) EIT where ν is the Poisson’s ratio of the test material.
3.4.10 3.4.10
indentation test
test, carried out using an indentation testing machine, to measure a specified parameter of a material by
pressing, with a specified force, an indenter of specified form into the surface of the material under specified
conditions during a specified testing cycle
3.4.11 3.4.11
indentation testing machine
machine, which has been verified both directly and indirectly, used for performing indentation tests to
measure, for example, hardness and/or elastic modulus
3.4.12 3.4.12
indenter
body with a hard tip, typically of diamond, hardmetal (tungsten carbide alloy), or, in special cases, steel, of
specified geometry through which the test force is applied in the course of an indentation test
3.4.13 3.4.13
indenter area function
table or mathematical function describing the evolution of specified area of indentation as a function of
indentation depth, obtained either by direct measurement or indirectly by applying specified calculations to
indentation results obtained from reference blocks
Note 1 to entry: Two types of area function are currently used – projected area, Ap, and surface area, As.
3.4.14 3.4.14
indirect verification
process for determining the performance of a machine by means of making measurements on reference
blocks
3.4.15 3.4.15
instrument frame compliance
C
f
measurable compliance (displacement measured by the instrument as a result of a force being applied by the
instrument) of the frame of an indentation testing machine
3.4.16 3.4.16
instrumented indentation testing machine
indentation testing machine that is instrumented to give measurements of any or all of displacement, force,
and time, at points throughout the testing cycle
3.4.17 3.4.17
Knoop hardness
HK
measure of a material's resistance to permanent indentation when a test force is applied through a diamond
rhombic-based pyramidal indenter
Note 1 to entry: It is given by:
HK = 0,102 × [test force (N)/projected area of permanent indentation (mm )].
Note 2 to entry: The indentation is assumed to retain the shape of an indenter of perfect geometry, and its projected
area is calculated from the long diagonal length.
3.4.18 3.4.18
Martens hardness
HM
measure of a material's resistance to both permanent and recoverable indentation when an increasing test
force is applied through a diamond pyramidal indenter (either square-based or triangular-based)
Note 1 to entry: It is given by:
HM = test force (N)/surface area of indenter (mm ) projecting into the material beyond the original surface plane.
Note 2 to entry: The surface area of the indenter projecting into the material is calculated from the depth of
indentation and the indenter area function.
3.4.19 3.4.19
plane strain indentation modulus
E *
IT
measure of the equivalent isotropic average plane strain elastic modulus of a material obtained by an
indentation test
*
Note 1 to entry: E is calculated from a specified measure of the indentation contact stiffness during force removal;
IT
the calculation uses a specified contact mechanics model and requires a knowledge of the instrument frame compliance
and the indenter area function.
3.4.20 3.4.20
reference block
reference material in block form, with a certified indentation value, primarily used for the indirect
verification of indentation testing machines
3.4.21 3.4.21
Rockwell hardness
HR
measure of a material's resistance to permanent indentation when a test force is applied through a
hardmetal or steel ball indenter or, for certain scales, a spheroconical diamond indenter
Note 1 to entry: It is given by:

where
N and S are constants for a given Rockwell hardness scale, and;
h (mm) is th
...