ISO 23838:2022

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 2022
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Published in Switzerland
ii
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
iii
Foreword
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This document was prepared by Technical Committee ISO/TC 164, Mechanical testing of metals,
Subcommittee SC 2, Ductility testing
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iv
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.
v
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)
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
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 .
Table 1 (continued)
Symbol Designation Unit
Distance from the strain gauge location on the transmitter bar to the bar-test
a mm
piece interface
C Velocity of the torsional wave propagation of the elastic bar mm/ms
b
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
J Polar moment of inertia of the elastic bar mm
b
τ Shear yield strength of the elastic bar material MPa
Y
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
d Inner diameter of thin-wall section mm
d Outer diameter of thin-wall section mm
L Total length of the test piece mm
L Flange length of the test piece mm
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 .
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
T Starting point of the reflected wave ms
T Starting point of the transmitted wave ms
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
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
Gr⋅
bb
τ ()t = et()+et()+et() (3)
[]
s IR T
4r ⋅δ
ss
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).
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.
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
flange length of the test piece diameter of cylindrical flange
D
L
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
Figure 2 — Type-A test piece
Key
total length of the test piece outer diameter of thin-wall section
L
d
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
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
≥10 (4)
δ
s
where
d is the inner diameter of thin-wall section;
is the thickness of the thin-wall section of the test piece;
dd−
δ =
s
d is the outer diameter of thin-wall section.
[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
where
L is the gauge length of the test piece;
s
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) For the measurement of inner diameter and outer diameter of thin-wall section, the resolution of
measuring equipment shall be superior to 0,002 mm.
b) For the measurement of length of thin-wall section, the resolution of measuring equipment shall be
superior to 0,02 mm.
c) For the measurement of thickness of thin-wall section, the resolution of measuring equipment shall
be superior to 0,002 mm.
Measure the inner diameter of the test piece, d , and outer diameter of thin-wall section, d , and
1 2
calculate the mean radius of the thin-wall of the test piece, r , using Formula (6).
s
dd+
r = (6)
s
where
r is the mean radius of the thin-wall of the test piece;
s
d is the inner diameter of thin-wall section;
d is the outer diameter of thin-wall section.
8 Procedure
8.1 Calibration of the apparatus
The output of the strain gauge should be calibrated by applying a known static torsional force to
the strain gauged elastic bar. The strain gauge can be calibrated through checking if the following
relationship is satisfied.
Mr⋅
b
γ = (7)
b
GJ⋅
bb
where
γ is the shear strain on the surface of the bar;
b
M
is the applied static torque in the bar at gauge station;
r is the radius of the elastic bar;
b
G is the shear modulus of the elastic bar;
b
J is the polar moment of inertia of the elastic bar.
b
The velocity of the torsional wave propagation of the elastic bar can be calculated by applying the
theoretical equation with the density and shear modulus of the elastic bar, given as Formula (8):
G
b
C = (8)
b
ρ
b
where
G is the shear modulus of the elastic bar;
b
ρ is the density of the elastic bar.
b
The velocity of the torsional wave propagation of the elastic bar can be also determined using
Formula (9) by measuring the transmission time of the elastic torsional wave in the incident bar alone,
where the incident wave will be reflected into the bar at the free end,
2a
C = (9)
b
Δt
i
where
a
is the distance from the strain gauge location on the incident bar to the bar-test piece interface;
Δt is the time interval between the incident and reflected waves.
i
The physical properties of bars can be calibrated through checking if the elastic torsional wave velocities
measured from the test computed with Formula (9) and theoretically calculated with Formula (8) are
in consistency.
8.2 Recording the temperature of the test environment
The test is carried out at room temperature between 10 °C and 35 °C, unless otherwise specified. The
test temperature may be recorded if needed. Tests carried out under controlled conditions should be
[2]
conducted at a temperature of (23 ± 5) °C conforming to ISO 26203-1 .
8.3 Checking the bar alignment
Check the alignment of bars with the method specified in A.5.4.
8.4 Mounting test piece
Connect coaxially test piece between the incident and transmitter bars, prevent any relative movement
between the test piece flange and the bars during the loading.
When a type-A test piece is selected, the test piece should be glued to the ends of bars, for example
with epoxy adhesive, ensure high bonding strength to apply torsional force on the test piece in order to
prevent any relative movement between the test piece flange and the bars during the loading.
8.5 Loading
Firstly, install an unbroken, notched bolt into the jaws of the vise to ready the clamping mechanism.
Secondly, apply clamping pressure to the incident bar, and employ the rotary actuator to apply a
predetermined rotation to the free end of the incident bar in order to elastically store a torque in the
energy storage section. Then, instantaneously release the torque, causing the notched bolt to fracture
and the clamping jaws to release the incident bar. A torsional stress wave then initiates from clamping
point, and propagates towards the test piece, dynamically loading the test piece in torsion.
8.6 Measuring and recording
Measure and record the following information:
a) dimensions, elastic torsional wave velocity and shear modulus of the elastic bars;
b) strain gauge parameters such as dimensions, sensitivity ratio, resistance value and the position of
strain gauge on the bars;
c) status of the apparatus, including test circuit and strain calibration values;
d) preloaded shear strain of the energy storage section;
e) original waveform from bars.
9 Data processing
9.1 Strain on bars
Strain on bars shall be calculated according to Formula (10):
et()=Ut()/U (10)
jj 0 j
where
th
e is the measured strain value of the j channel, j = 1, 2, …, n;
j
th
U is the output voltage of the j channel signal, j = 1, 2, …, n;
j
th
U is the voltage of the j channel signal at the strain calibration, j = 1, 2, …, n;
0j
t
is time.
9.2 Waveform processing
9.2.1 Determination of waveform baseline
Attach two strain gauges symmetrically on the bar surface across from each other, take the average
value of the two strain signals for the data processing.
Take the average value of data in the straight section before the rising of the wave as the waveform
baseline. During the data processing, reset waveform baselines of waves to zero.
9.2.2 Determination of starting points of waves
Recommended method is specified in Annex C for the determination of the starting points of incident,
reflected and transmitted waves. The determined starting points shall be on corresponding waveform
baseline and near the rising stage.
9.2.3 Synchronization of waves
Shift the incident wave, reflected wave and transmitted wave calculated from Formula (10) at the three
locations to the same staring point (e.g. T or zero time), so that the calculations of strain functions
with synchronized time can be conveniently operated as Formulae (1), (2) and (3). Check the dynamic
equilibrium of the test piece using the shifted waves. The test piece is regarded as being in dynamic
equilibrium if the waves satisfy Formula (11):
0,9 ee≤≤+1ee,1 (11)
TI RT
where
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
9.2.4 Determination of loading duration of stress wave
After shifting the incident, reflected and transmitted signals to the starting point (see Annex C for
specifications), determine the intersection of platform and falling edge of incident signal, take a time
point corresponding to the intersection as the ending time of loading duration of stress wave.
When the fracture of a test piece occurs, determine the intersecting point of flat portion after rise time
and steep dropping edge of transmitted signal. Take the corresponding time point to the intersection as
the ending time of loading duration of stress wave.
9.3 Engineering plastic shear strain rate
Calculate the engineering plastic shear strain rate according to the formulae in the Annex A.
9.4 Engineering plastic shear strain
Calculate the engineering plastic shear strain according to the formulae in the Annex A.
9.5 Engineering plastic shear stress
Calculate the engineering plastic shear stress according to the formulae in the Annex A.
9.6 Engineering plastic shear stress-shear strain curve
Use the engineering shear stress obtained in 9.5 and engineering shear strain obtained in 9.4 to obtain
the engineering shear stress-shear strain curve.
9.7 Average engineering plastic shear strain rate
The average engineering plastic shear strain rate is obtained by calculating the arithmetic average of
the engineering strain rate function of time from 9.3 by Formula (12):
T

γξ= γ ()dξ (12)
ss
∫
TT−
T
where
is the average engineering plastic shear strain rate in the test piece;
γ
s
is the engineering plastic shear strain rate in the test piece;

γ
is the time corresponding to the yield strength in engineering shear stress-time curve;
T
is the load duration of stress wave;
T
is dummy variable.
ξ
9.8 Test example
Annex D shows an example of testing the engineering shear stress-shear strain curve of metallic
materials at high strain rates by using the torsional split Hopkinson bar at room temperature.
10 Evaluation of test result
The following cases may influence the evaluation of material properties, and interface slippage or test
piece buckling means an invalid test, and a retest or a suitable interpretation of the test data should be
considered:
a) there is any relative slip at the bar-test piece interface;
b) the buckling of the test piece occurs at gauge section of test piece;
Meanwhile, it is recommended to compare the deformation of the test piece calculated by the TSHB
method with the deformation of the test piece measured by the optical techniques to confirm test result.
11 Test report
The test report should contain items selected from the following:
a) a reference to this document, i.e. ISO 23838:2022;
b) specified materials, if known (names of tested materials and sources);
c) test method used (strain-measuring method, and type of load cell, etc.);
d) identification of the test piece;
e) geometry and dimensions of the test piece;
f) environmental conditions of the test;
g) instruments (amplifier, oscilloscope, etc.);
h) confirmation of dynamic equilibrium;
i) measured properties and results (i.e. engineering stress-strain curve with average strain rate);
j) personnel performing the test, the review and the approval.
Annex A
(informative)
Torsional split Hopkinson bar
A.1 Principle of TSHB method
The original split Hopkinson bar testing method was proposed to measure the compressive mechanical
[5]
behaviour of materials at high strain rates . Because of its simplicity, some modified Hopkinson bar
apparatuses were developed by many investigators for measuring tensile or torsional properties by
[2][6][7]
loading test pieces in uniaxial tension or torsion .
The TSHB apparatus is made up of two collinear bars that are made of the same material and having
the same diameter. The bars are applied as wave-guides and the dynamic loading unit, and also as the
measuring unit. To conduct a test, a torsional wave has to be generated in the incident bar. There are
several ways to initiate the torsional wave in the incident bar, for instance the way of a sudden release
of a stored torque and the way by explosives. In the stored-torque TSHB, the torque is generated by
first tightening the clamp and then turning the end of the bar, and the loading wave in the incident bar
is produced by the release of the stored torque. Once upon release of the clamp, the torsional pulse
front propagates down the bar toward the test piece, and simultaneously a torsional pulse of equal
magnitude propagates from the clamp toward the loading pulley and is reflected back in the incident
[4]
bar, forming an entire incident wave with wave front and unloading stage .
The incident wave propagates down the incident bar towards the test piece, dynamically loading the test
piece in torsion. Once the incident wave reaches the test piece, some portion of the wave is transmitted
through the test piece and into the transmitter bar, while the remainder of the wave is reflected back
to the incident bar, as shown in Figure A.1 (for example the stored-torque TSHB). The stress waves can
be recorded as incident, transmitted and reflected pulses using strain gauges mounted on the bars. The
typical incident, transmitted and reflected signals obtained by strain gauges are shown in Figure A.2.
Key
X distance
Y time
1 incident wave
2 reflected wave
3 transmitted wave
Figure A.1 — Schematic of a stored-torque TSHB and its wave characteristic diagram
Key
X time (ms)
-6
Y engineering elastic strain (10 )
1 incident wave
2 transmitted wave
3 reflected wave
Figure A.2 — Typical incident, transmitted and reflected signals obtained by strain gauges
Under stress wave loading, the end of the incident bar where the test piece is attached rotates with an
 
angular velocity θ that is much larger than the angular velocity θ of the other end of the test piece
1 2
that is connected to the transmitter bar, as shown in Figure A.3. The difference in the angular velocity
between the test piece ends causes the test piece to deform in shear at a high strain rate.
Key
1 incident bar
2 test piece
3 transmitter bar
angular velocities of the end of the test piece

θ
angular velocities of the end of the test piece

θ
gauge length of the test piece
L
s
Figure A.3 — Angular velocities of the ends of the test piece

The shear strain rate in the test piece as a function of time, γ ()t , is determined from the difference in
s
the angular velocity between the ends of the test piece as given by Formula (A.1):
r
s


γθ()t=  ()tt−θ () (A.1)
s 1 2
 
L
s
where
γ is the engineering plastic shear strain rate in the test piece;
s

is angular velocities of the end of the test piece;
θ

is angular velocities of the end of the test piece;
θ
r is the mean radius of the thin-wall of the test piece;
s
L is the gauge length of the test piece.
s
If the test piece is designed as outlined in this standard, the deformation of the test piece will be uniform
[5]
since it is short and the time it takes for the wave to propagate through is very short . If the deformation
is uniform, the torque at both ends is equal and the test piece is in force equilibrium. The difference in

the angular velocity θθ()tt− () can be determined from the torque of the reflected wave given by
 
Formula (A.2):
2[]−Mt()
R

θθtt− = (A.2)
() ()
ρ ⋅J
sb
where

is angular velocities of the end of the test piece;
θ

is angular velocities of the end of the test piece;
θ
M is the torque of the reflected wave;
R
ρ
is the density of the test piece;
s
J is the polar moment of inertia of the elastic bar;
b
t
is time.
The torque, M, at a gauge station is related to the measured shear strain on the surface of the bar, γ , by
b
Formula (A.3):
GJ⋅
bb
M = γ (A.3)
b
r
b
where
M
is the torque at a gauge station of the bar;
γ is the shear strain on the surface of th
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