ISO 23838:2022

Metallic materials — High strain rate torsion test at room temperature

This document specifies terms and definitions, symbols and designations, principle, apparatus, test piece, procedure, data processing, evaluation of test result, test report and other contents for the torsion test at high strain rates for metallic materials by using torsional split Hopkinson bar (TSHB).

Matériaux métalliques — Essai de torsion à haute vitesse de déformation à température ambiante

General Information

• Status: Published
• Publication Date: 20-Jun-2022
• ICS: 77.040.10 - Mechanical testing of metals
• Technical Committee: ISO/TC 164/SC 2 - Ductility testing
• Drafting Committee: ISO/TC 164/SC 2 - Ductility testing
• Current Stage: 6060 - International Standard published
• Start Date: 21-Jun-2022
• Due Date: 05-Feb-2022
• Completion Date: 21-Jun-2022

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 23838
First edition
2022-06
Metallic materials — High strain rate
torsion test at room temperature
Matériaux métalliques — Essai de torsion à haute vitesse de
déformation à température ambiante
Reference number
ISO 23838:2022(E)
© ISO 2022

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ISO 23838:2022(E)
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© ISO 2022
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
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Published in Switzerland
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ISO 23838:2022(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and designations . 2
5 Principle . 4
6 Apparatus . 5
6.1 Apparatus components . 5
6.2 Loading device . 6
6.3 Bar components . 6
6.4 Data acquisition and recording system . 7
7 Test piece . 7
7.1 Dimensions of test piece . 7
7.2 Measurement of test piece dimensions . 9
8 Procedure .9
8.1 Calibration of the apparatus . 9
8.2 Recording the temperature of the test environment . 10
8.3 Checking the bar alignment . 10
8.4 Mounting test piece . 10
8.5 Loading . 11
8.6 Measuring and recording . 11
9 Data processing .11
9.1 Strain on bars . 11
9.2 Waveform processing . 11
9.2.1 Determination of waveform baseline . 11
9.2.2 Determination of starting points of waves . 11
9.2.3 Synchronization of waves .12
9.2.4 Determination of loading duration of stress wave .12
9.3 Engineering plastic shear strain rate .12
9.4 Engineering plastic shear strain.12
9.5 Engineering plastic shear stress .12
9.6 Engineering plastic shear stress-shear strain curve .12
9.7 Average engineering plastic shear strain rate .12
9.8 Test example . 13
10 Evaluation of test result .13
11 Test report .13
Annex A (informative) Torsional split Hopkinson bar .14
Annex B (informative) Data acquisition and recording system .28
Annex C (informative) Method for determining the starting points of waves .31
Annex D (informative) Example of torsional split Hopkinson bar method .32
Bibliography .36
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ISO 23838:2022(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
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 been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to
the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 164, Mechanical testing of metals,
Subcommittee SC 2, Ductility testing
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.
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ISO 23838:2022(E)
Introduction
In many dynamic events, such as punch forming, metal cutting, and vehicle collision, the metallic
components are susceptible to dynamic impact loading, in which case the maximum strain rate of the
4 −1
order of 10 s can be achieved. During this extreme loading condition, the strength of the material can
be significantly higher than that under quasi-static loading conditions. The shear mechanical properties
of metallic materials, such as yield strength, flow stress and failure strain are essential information for
analysis of shear failure of components, and are also the basic data for construction of constitutive
relations. The shear mechanical properties of many metallic materials depend also on strain rate as
properties under uniaxial load. Therefore, to determine the shear mechanical properties of metallic
materials at high strain rates by torsion test is also of great importance for engineering design,
structural optimization, processing and evaluation of metallic structures. For additional information
see
— ISO 26203-1, and
— ISO 26203-2.
The split Hopkinson (Kolsky) bar is one of the major test methods for measurement of mechanical
2 −1
properties of materials at high strain rates (≥10 s ). It is designed on the base of two assumptions,
namely
a) one-dimensional elastic wave propagation in elastic bars, and
b) uniform distribution of stress–strain along the length of the short test piece.
The fundamental principle is as follows: a small test piece is sandwiched between two long elastic bars,
which are used as loading and measuring devices by means of elastic stress wave propagation. On the
one hand, the propagating waves on elastic bars load dynamically the test piece; on the other hand the
force and displacement measurements of test piece can be calculated by measuring the elastic strain
of the bars through gauges attached to the bars. The torsional split Hopkinson bar apparatus, one kind
of split Hopkinson bar techniques, can provide solutions for dynamic torsional testing problems and is
3 −1
widely used to obtain accurate stress-strain curves at around 10 s .
This document provides test method for the torsional split Hopkinson bar apparatus.
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INTERNATIONAL STANDARD ISO 23838:2022(E)
Metallic materials — High strain rate torsion test at room
temperature
1 Scope
This document specifies terms and definitions, symbols and designations, principle, apparatus, test
piece, procedure, data processing, evaluation of test result, test report and other contents for the
torsion test at high strain rates for metallic materials by using torsional split Hopkinson bar (TSHB).
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 terminological 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
stress wave
strain wave
propagation of disturbance of stress (or strain) in a medium
Note 1 to entry: When a localized mechanical disturbance is applied suddenly into a deformable solid medium,
the disturbance results in the variations of particle velocity, and also the variations of stress and strain states.
The variations or disturbances of the stress and strain states propagate to the other parts of the medium in the
form of waves. The resulting waves in the medium are due to mechanical stress (or strain) effects and, thus,
these waves are called stress wave (or strain) wave.
3.2
elastic stress wave
elastic strain wave
stress wave or strain wave (3.1) propagating in an elastic medium
Note 1 to entry: When loading conditions result in stresses below the yield point of solid medium, the medium
behaves elastically, and consequently the stress wave or strain wave (3.1) is elastic.
3.3
elastic torsional wave
type of propagation of rotation disturbance inducing shear deformation in elastic medium
Note 1 to entry: The direction of particle movement is perpendicular to the wave propagation direction.
3.4
wave front
moving surface which separates the disturbed from the undisturbed part in a medium
3.5
elastic torsional wave velocity
propagation velocity of wave front (3.4) of elastic torsional wave (3.3)
1
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ISO 23838:2022(E)
3.6
split Hopkinson bar
experimental apparatus that utilizes the split-bar system to determine the dynamic stress-strain
curves of materials from the information of stress wave or strain wave (3.1) propagation in bars
Note 1 to entry: In a split Hopkinson bar apparatus a short test piece is sandwiched between the two long elastic
bars, called incident and transmitter bars, by which the test piece is loaded, and force and displacement are
measured.
3.7
TSHB
torsional split Hopkinson bar
kind of split Hopkinson bar (3.6) used for testing materials in torsion
Note 1 to entry: in a torsional split Hopkinson bar (TSHB) apparatus the elastic torsional wave (3.3) propagation
is utilized to measure the shear mechanical properties of materials at high strain rates.
3.8
incident wave
elastic stress wave or elastic strain wave (3.2) generated in the incident bar, propagating towards the
test piece
3.9
reflected wave
elastic stress wave or elastic strain wave (3.2) reflected to the incident bar from the incident bar-test
piece interface
Note 1 to entry: When the incident wave (3.8) propagates till the bar-test piece interface, a part of the incident
wave (3.8) is reflected back into the incident bar.
3.10
transmitted wave
elastic stress wave or elastic strain wave (3.2) transmitted through the transmitter bar-test piece
interface and into the transmitter bar
Note 1 to entry: When the incident wave (3.8) propagates till the bar–test piece interface, a part of the incident
wave (3.8) is reflected back into the incident bar, and a second part of the wave is transmitted through the test
piece to the transmitter bar.
3.11
average engineering plastic strain rate
arithmetic average of the engineering plastic shear strain rate function of time
Note 1 to entry: The arithmetic average value of the engineering plastic shear strain rate function can be found
by calculating the definite integral of the function and dividing the integral value by the time interval for plastic
deformation.
3.12
gauge length
length of thin-wall section of the test piece
4 Symbols and designations
Table 1 — Symbols and designations
Symbol Designation Unit
Distance from the strain gauge location on the incident bar to the bar-test
a mm
1
piece interface
−1
NOTE During the data processing, the unit of shear strain rate and average engineering plastic strain rate is (ms) ; the
−1
resulting expression should be converted to s .
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ISO 23838:2022(E)
Table 1 (continued)
Symbol Designation Unit
Distance from the strain gauge location on the transmitter bar to the bar-test
a mm
2
piece interface
C Velocity of the torsional wave propagation of the elastic bar mm/ms
b
3
ρ Density of the elastic bar g/mm
b
D Diameter of the elastic bar mm
b
L Length of the elastic bar mm
b
G Shear modulus of the elastic bar MPa
b
L Length of the energy storage section mm
E
L Length of the incident bar mm
I
L Length of the transmitter bar mm
T
M
Applied torque in the bar at gauge station N⋅mm
M Torque in the test piece N⋅mm
s
M Torque of the reflected wave N⋅mm
R
M Maximum torque applied on the energy storage section N⋅mm
max
r Radius of the elastic bar mm
b
4
J Polar moment of inertia of the elastic bar mm
b
τ Shear yield strength of the elastic bar material MPa
Y
3
ρ Density of the test piece g/mm
s
G Shear modulus of the test piece MPa
s
D
Diameter of cylindrical flange mm
D Diameter of the circumcircle of regular hexagonal flange mm
1
d Inner diameter of thin-wall section mm
1
d Outer diameter of thin-wall section mm
2
L Total length of the test piece mm
L Flange length of the test piece mm
1
L Gauge length of the test piece mm
s
r Mean radius of the thin-wall of the test piece mm
s
δ Thickness of the thin-wall section of the test piece mm
s
r
Radius at the shoulder of the test piece mm
-1
 
Angular velocities of the ends of the test piece (ms)
θ , θ
1 2
-1

γ Engineering plastic shear strain rate in the test piece (ms)
s
-1
γ
Engineering shear strain rate (ms)
C Velocity of the torsional wave propagation of the test piece mm/ms
s
γ Engineering plastic shear strain in the test piece -
s
-1
 Average engineering plastic shear strain rate in the test piece (ms)
γ
s
τ Engineering shear stress of the test piece MPa
s
γ
Engineering shear strain -
τ
Engineering shear stress MPa
U
Voltage of channel signal V
−1
NOTE During the data processing, the unit of shear strain rate and average engineering plastic strain rate is (ms) ; the
−1
resulting expression should be converted to s .
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ISO 23838:2022(E)
Table 1 (continued)
Symbol Designation Unit
th
U
Voltage of the j channel signal at the strain calibration, j = 1, 2, …, n V
0j
th
U
Output voltage of the j channel signal, j = 1, 2, …, n V
j
U Bridge voltage V
B
T Starting point of the incident wave ms
1
T Starting point of the reflected wave ms
2
T Starting point of the transmitted wave ms
3
λ Length of the incident wave ms
t
Time ms
T
Load duration of stress wave ms
Time corresponding to the yield strength in engineering shear stress-time
T ms
0
curve
ΔT Sampling interval ms
Δt Rise time of the incident wave ms
Δt Time interval between the incident and reflected waves ms
i
ξ Dummy variable ms
-6
e
Engineering elastic strain 10
th
e
Measured strain value of the j channel, j = 1, 2, …, n -
j
e Strain of incident wave recorded by gauge on the incident bar -
I
e Strain of reflected wave recorded by gauge on the incident bar -
R
e Strain of transmitted wave recorded by gauge on the transmitter bar -
T
γ Measured shear strain of reflected wave on incident bar -
R
γ Shear strain on the surface of the bar -
b
−1
NOTE During the data processing, the unit of shear strain rate and average engineering plastic strain rate is (ms) ; the
−1
resulting expression should be converted to s .
5 Principle
The shear stress-strain characteristics of metallic materials at high strain rates are evaluated by
torsional split Hopkinson bar (TSHB) method, which utilizes two long elastic bars for applying the load
to the test pieces sandwiched between bars, and also for measuring the displacements and loads as
transducers at the test piece ends. The bars remain elastic throughout the test and are long enough so
that the strain signals are recorded before the elastic wave is reflected back from the other end. The
histories of load and deformation in test piece are calculated by one dimensional wave propagation
theory from strain signals obtained by strain gauges mounted on two bars by use of Formulae (1) to
[4]
(3) :
2rC⋅
sb

γ ()t = et()−et()−et() (1)
[]
s IR T
rL⋅
bs
t
2rC⋅
sb
γξ()t = []ee()− ()ξξ−e () dξ (2)
s IR T
∫
rL⋅
bs
0
3
Gr⋅
bb
τ ()t = et()+et()+et() (3)
[]
s IR T
2
4r ⋅δ
ss
4
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ISO 23838:2022(E)
where
γ is the engineering plastic shear strain in the test piece;
s
τ is the engineering shear stress of the test piece;
s
e is the strain of incident wave recorded by gauge on the incident bar;
I
e is the strain of reflected wave recorded by gauge on the incident bar;
R
e is the strain of transmitted wave recorded by gauge on the transmitter bar;
T
r
is the mean radius of the thin-wall of the test piece;
s
r is the radius of the elastic bar;
b
L is the gauge length of the test piece;
s
δ is the thickness of the thin-wall section of the test piece;
s
C is the velocity of the torsional wave propagation of the elastic bar;
b
G is the shear modulus of the elastic bar;
b
t
is time;
ξ
is dummy variable.
6 Apparatus
6.1 Apparatus components
The TSHB apparatus consists of three major components: loading device (rotary actuator, energy
storage section and clamp), bar components (incident bar and transmitter bar), and data acquisition
and recording system (strain gauge, amplifier and data recorder) (see Figure 1, the stored-torque TSHB
for example).
5
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ISO 23838:2022(E)
Key
1 rotary actuator
2 energy storage section
3 strain gauge
4 clamp
5 incident bar
6 bearing
7 test piece
8 transmitter bar
9 supporting frame
10 amplifier
11 data recorder
Figure 1 — Schematic of torsional split Hopkinson bar apparatus
6.2 Loading device
The loading device is used for generating the incident wave by means of explosives, or sudden release of a
stored torque, or impact, etc. In stored-torque TSHB, the incident wave is initiated by the instantaneous
release of a torque, which is elastically stored previously in a section of the incident bar between the
clamp and the turning end. The loading device in stored-torque TSHB apparatus consists of three major
components:
a) a rotary actuator fastened to free end of the incident bar, by which the external torque is applied;
b) an energy storage section, the segment of the incident bar for storing torsional elastic strain energy;
c) a clamp with a quick releasing mechanism.
6.3 Bar components
The bar components in TSHB consist of an incident bar, a transmitter bar and some bearings. By using
long elastic bars, the incident strain signal should be recorded before the elastic wave is reflected
back from bar-test piece interface, i.e. the incident and the reflected waves are recorded separately.
The reflected strain should be recorded before the wave is reflected back again from the other end of
incident bar, and transmitted strain should be recorded before the wave is reflected back from the other
end of transmitter bar (see Annex A). Consequently, the strain signals on the bars can be measured
without being disturbed by the wave interaction.
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ISO 23838:2022(E)
6.4 Data acquisition and recording system
The data acquisition and recording system consists of strain gauge, amplifier and data recorder such
as oscilloscopes (see Annex B). The testing data is acquired with the use of strain gauges mounted
on the incident and transmitter bars in conjunction with an oscilloscope. The frequency response
of all instruments in the system shall be selected to ensure that all recorded data are not negatively
influenced by the frequency response of any individual components. Signal conditioning amplifiers are
usually employed to maximize precision in the obtained strain measurements. The minimum frequency
response for amplifier shall be not lower than 100 kHz, the minimum resolution of measured data for
digital data recorders shall be not less than10 bits, and the sampling frequency of data recorder should
be not lower than 1 MHz. It is recommended that frequency response for amplifier is on the order of
[2]
500 kHz conforming to ISO 26203-1 .
7 Test piece
7.1 Dimensions of test piece
a) The test pieces used in the torsional testing are short and thin-wall tubes with integral flanges.
Two types of geometric configurations are recommended:
1) type-A, tubular test piece with cylindrical flanges (see Figure 2), and
2) type-B, tubular test piece with hexagonal flanges (see Figure 3).
The type-A test piece is glued to the ends of bars with high strength adhesive, for example with epoxy
adhesive. The type-B test piece is connected to the ends of bars by mechanical means using hexagonal
flanges with matching sockets at the ends of bars.
Key
total length of the test piece outer diameter of thin-wall section
L
d
2
flange length of the test piece diameter of cylindrical flange
D
L
1
gauge length of the test piece radius at the shoulder of the test piece
r
L
s
a
inner diameter of thin-wall section Others.
d
1
Figure 2 — Type-A test piece
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ISO 23838:2022(E)
Key
total length of the test piece outer diameter of thin-wall section
L
d
2
flange length of the test piece diameter of the circumcircle of regular hexagonal flange
L D
1 1
gauge length of the test piece radius at the shoulder of the test piece
r
L
s
a
inner diameter of thin-wall section Others.
d
1
Figure 3 — Type-B test piece
b) The dimensions of test piece are determined by the following requirements:
1) Diameter-to-thickness ratio of test piece in the thin-wall section shall comply with the following
rule shown in Formula (4):
d
1
≥10 (4)
δ
s
where
d is the inner diameter of thin-wall section;
1
is the thickness of the thin-wall section of the test piece;
dd−
21
δ =
s
2
d is the outer diameter of thin-wall section.
2
[8][9]
2) The gauge length of the test piece, L , shall comply with the following rule in Formula (5) and
s
shall not be less than 2,5 mm.
Ct⋅Δ
s
L < (5)
s
5
where
L is the gauge length of the test piece;
s
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ISO 23838:2022(E)
is the velocity of the torsional wave propagation of the test piece;
G
s
C =
s
ρ
s
G is shear modulus of the test piece;
s
ρ is the density of the test piece;
s
Δt
is the rise time of the incident wave.
The rise time of incident wave can be determined from difference between time points corresponding
to the maximum strain and the starting point of incident wave (See Annex C).
3) The radius at the shoulder of test piece shall be small enough so that the total length of thin-wall
section could be considered as the original gauge length. The radius of the shoulder of test piece
should be equal or less than 0,5 mm.
7.2 Measurement of test piece dimensions
The test piece dimensions shall be measured and recorded before tests. Selection of the measuring
equipment shall meet the following requirements for resolution, and the equipment shall be periodically
calibrated.
a)
...