IEC 63203-401-1:2023

Wearable electronic devices and technologies - Part 401-1: Devices and systems: functional elements - Evaluation method of the stretchable resistive strain sensor

IEC 63203-401-1:2023 specifies a measurement method of tensile strain for stretchable, resistive strain sensors. This document describes characterization procedures for evaluation of the gauge factor, linearity, response characteristics, and hysteresis of unimodal tension sensors but is not appropriate for assessment of the physical properties of the sensor material such as the elastic modulus, elastic limit, and Poisson's ratio.

Technologies et dispositifs électroniques prêt-à-porter - Partie 401-1 : Dispositifs et systèmes: éléments de fonctionnement – Méthode d’évaluation de la jauge de contrainte extensible de type résistif

L’IEC 63203-401-1:2023 spécifie une méthode de mesure de la contrainte de traction pour les jauges de contrainte extensible de type résistif. Le présent document décrit les procédures de caractérisation pour l’évaluation du coefficient de traction, de la linéarité, des caractéristiques de réponse et de l’hystérésis des jauges de tension unimodales. Toutefois, il n’est pas adéquat pour l’évaluation des propriétés physiques des matériaux de la jauge, tels que le module d’élasticité, la limite d’élasticité et le coefficient de Poisson.

General Information

• Status: Published
• Publication Date: 08-Oct-2023
• ICS: 31.020 - Electronic components in general
• Technical Committee: TC 124 - Wearable electronic devices and technologies
• Drafting Committee: WG 4 - TC 124/WG 4
• Current Stage: PPUB - Publication issued
• Start Date: 09-Oct-2023
• Completion Date: 20-Oct-2023

Overview

IEC 63203-401-1:2023 is an international standard published by the International Electrotechnical Commission (IEC) that specifies a comprehensive evaluation method for stretchable resistive strain sensors used in wearable electronic devices and technologies. This standard outlines precise measurement techniques for tensile strain in stretchable, resistive strain sensors. It focuses on characterization procedures to evaluate critical sensor performance metrics such as the gauge factor, linearity, response characteristics, and hysteresis. However, it explicitly excludes assessment of the intrinsic physical properties of the sensor materials like elastic modulus, elastic limit, and Poisson's ratio.

As wearable electronic technologies evolve, the integration of stretchable sensors plays a vital role in enhancing the flexibility and responsiveness of devices. IEC 63203-401-1:2023 aims to ensure consistent, reliable, and standardized measurement methods that facilitate the development and assessment of high-quality, stretchable strain sensors.

Key Topics

• Measurement Methodology
  Defines standardized procedures for quantifying tensile strain in stretchable resistive sensors, ensuring reproducibility and accuracy in strain sensing evaluation.

• Performance Characterization

  • Gauge Factor: Measurement of sensitivity of the strain sensor relative to applied mechanical strain.
  • Linearity: Assessment of the proportionality between sensor output and applied strain to verify sensor precision.
  • Response Characteristics: Evaluation of dynamic sensor behavior under various strain rates to ensure timely and accurate readings.
  • Hysteresis: Quantification of any lag or difference in sensor output during loading and unloading cycles, critical for sensor reliability.

• Test Environment and Apparatus
  Details about controlled test environments and apparatus setups designed for stretchable sensors, including specimen shape, dimension measurement, and apparatus schematics for two-wire and four-wire measurement methods.

• Test Procedures and Reporting
  Step-by-step instructions covering specimen preparation, test execution, data acquisition, and a structured format for compiling test reports for consistency and transparency.

• Informative Annexes
  Additional guidance on resistance measurement, strain rate effects, response time analysis, and numerical examples of hysteresis calculations to aid in practical testing.

Applications

IEC 63203-401-1:2023 plays a crucial role in advancing wearable electronic technologies by providing a solid framework for evaluating stretchable resistive strain sensors used across multiple applications, including:

• Health Monitoring Devices
  Accurate measurement of strain aids in designing sensors for monitoring physiological movements such as wrist pulse, joint motions, and respiration.

• Smart Textiles and E-Textiles
  Enables integration of reliable stretchable sensors in fabrics that adapt to the wearer’s movements without compromising accuracy.

• Robotics and Prosthetics
  Facilitates development of compliant sensors that mimic human muscle and skin flexibility, improving control and sensitivity.

• Flexible Consumer Electronics
  Supports quality testing of sensors used in bendable displays and wearable gadgets, ensuring user comfort and device durability.

• Sports and Fitness Technologies
  Helps in producing sensors capable of precise biomechanics monitoring to enhance athletic performance and injury prevention.

By standardizing sensor evaluation, manufacturers and researchers can optimize device design, improve material selection, and ensure consistent sensor performance for end users.

Related Standards

To complement IEC 63203-401-1:2023, the following standards provide broader context and additional testing frameworks for wearable electronics and stretchable sensor materials:

• IEC 62899-202-4:2021 – Printed electronics materials - Measurement methods for conductive ink properties in stretchable printed layers, essential for understanding sensor substrate performance.

• ISO 291:2008 – Plastics - Standard atmospheres for conditioning and testing, important to ensure consistent environmental testing conditions for sensor materials.

• ISO/TS 12901-2:2014 – Nanotechnologies - Occupational risk management related to engineered nanomaterials, providing guidance on safe handling of advanced sensor materials.

These related standards support a holistic approach to the development, testing, and safety of wearable sensor technologies.

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Keywords: IEC 63203-401-1, stretchable resistive strain sensor, wearable electronic devices, tensile strain measurement, gauge factor, sensor linearity, hysteresis evaluation, sensor response characteristics, wearable technology standards, flexible sensors, wearable device testing.

Standards Content (Sample)

IEC 63203-401-1 ®
Edition 1.0 2023-10
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Wearable electronic devices and technologies –
Part 401-1: Devices and systems: functional elements – Evaluation method of
the stretchable resistive strain sensor

Technologies et dispositifs électroniques prêt-à-porter –
Partie 401-1: Dispositifs et systèmes: éléments de fonctionnement – Méthode
d’évaluation de la jauge de contrainte extensible de type résistif
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IEC 63203-401-1 ®
Edition 1.0 2023-10
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Wearable electronic devices and technologies –

Part 401-1: Devices and systems: functional elements – Evaluation method of

the stretchable resistive strain sensor

Technologies et dispositifs électroniques prêt-à-porter –

Partie 401-1: Dispositifs et systèmes: éléments de fonctionnement – Méthode

d’évaluation de la jauge de contrainte extensible de type résistif

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 31.020  ISBN 978-2-8322-7558-0

– 2 – IEC 63203-401-1:2023 © IEC 2023
CONTENTS
FOREWORD . 4
1 Scope . 6
2 Normative references . 6
3 Terms and definitions . 6
3.1 Terms and definitions . 6
3.2 Symbols and abbreviated terms . 7
4 Test environments . 7
5 Test specimen . 8
5.1 Shape of the test specimen of the stretchable strain sensor . 8
5.2 Measurement of dimensions . 8
6 Test method and test apparatus . 9
6.1 General . 9
6.2 Test apparatus and measurement . 9
7 Test procedure . 13
8 Measurement of sensor performance and endurance . 13
8.1 Gauge factor . 13
8.1.1 General . 13
8.1.2 Purpose . 13
8.1.3 Test procedure . 14
8.2 Linearity . 14
8.2.1 Purpose . 14
8.2.2 Test procedure . 14
8.3 Response characteristics . 14
8.3.1 General . 14
8.3.2 Purpose . 15
8.3.3 Test procedure . 15
8.4 Hysteresis . 15
8.4.1 General . 15
8.4.2 Purpose . 15
8.4.3 Test procedure . 15
9 Test report . 16
Annex A (informative) Strain sensor resistance measurement . 17
Annex B (informative) Effects of strain rate on performances of the stretchable strain
sensor . 18
Annex C (informative) Measurement of response time of the stretchable strain sensor . 19
Annex D (normative) Examples of hysteresis calculations . 20
Bibliography . 22

Figure 1 – Shape of a test specimen of the stretchable strain sensor . 8
Figure 2 – Example of stretching test machine . 9
Figure 3 – Schematic drawing of a stretching test machine and two-wire measurement
method . 10
Figure 4 – Schematic drawing of a stretching test machine and four-wire measurement
method . 12
Figure 5 – Linearity measurement of the stretchable strain sensor . 14

Figure A.1 – Changes in electrical resistances when the strain sensor is stretched . 17
Figure B.1 – Examples of the effects of strain rate on performances of the stretchable
strain sensor . 18
Figure C.1 – Examples of the response time of the stretchable strain sensors . 19
Figure D.1 – Example of the hysteresis behaviour of a stretchable strain sensor . 20
Figure D.2 – Example calculation of hysteresis behaviour . 20
Figure D.3 – Calculation of hysteresis behaviour of the stretchable strain sensor . 21
Figure D.4 – Example of calculation of the degree of hysteresis . 21

– 4 – IEC 63203-401-1:2023 © IEC 2023
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
WEARABLE ELECTRONIC DEVICES AND TECHNOLOGIES –

Part 401-1: Devices and systems: functional elements –
Evaluation method of the stretchable resistive strain sensor

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
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rights. IEC shall not be held responsible for identifying any or all such patent rights.
IEC 63203-401-1 has been prepared by IEC technical committee 124: Wearable electronic
devices and technologies. It is an International Standard.
The text of this International Standard is based on the following documents:
Draft Report on voting
124/223/FDIS 124/239/RVD
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.

This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
A list of all parts in the IEC 63203 series, published under the general title Wearable electronic
devices and technologies, can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
• reconfirmed,
• withdrawn, or
• revised.
IMPORTANT – The "colour inside" logo on the cover page of this document indicates
that it contains colours which are considered to be useful for the correct understanding
of its contents. Users should therefore print this document using a colour printer.

– 6 – IEC 63203-401-1:2023 © IEC 2023
WEARABLE ELECTRONIC DEVICES AND TECHNOLOGIES –

Part 401-1: Devices and systems: functional elements –
Evaluation method of the stretchable resistive strain sensor

1 Scope
This part of IEC 63203-401 specifies a measurement method of tensile strain for stretchable,
resistive strain sensors. This document describes characterization procedures for evaluation of
the gauge factor, linearity, response characteristics, and hysteresis of unimodal tension sensors
but is not appropriate for assessment of the physical properties of the sensor material such as
the elastic modulus, elastic limit, and Poisson's ratio.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 62899-202-4:2021, Printed electronics – Part 202-4: Materials – Conductive ink –
Measurement methods for properties of stretchable printed layers (conductive and insulating)
ISO 291:2008, Plastics – Standard atmospheres for conditioning and testing
ISO/TS 12901-2:2014, Nanotechnologies – Occupational risk management applied to
engineered nanomaterials – Part 2: Use of the control banding approach
3 Terms and definitions
3.1 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:
• IEC Electropedia: available at https://www.electropedia.org/
• ISO Online browsing platform: available at https://www.iso.org/obp
3.1.1
stretchable substrate
stretchable material
substrate or material able to recover original size and shape immediately after the removal of
the extending force causing deformation
Note 1 to entry: In this document, the notion of "stretchability" is based on the elasticity of the substrate.
[SOURCE: IEC 63203-101-1:2021, 3.10 [1]]

3.1.2
gauge factor
G
F
ratio of the change in electrical resistance divided by the original resistance (R , resistance in
o
the undeformed configuration) to engineering strain (e) along the axis of the stretching
(RR – ) / R
OO
Note 1 to entry: Gauge factor is expressed as G = , where R is the initial resistance in the
F O
e
undeformed configuration, R is the electrical resistance in the deformed configuration, and e is the tensile strain.
[SOURCE: IEC 62047-22:2014, 3.1.1 [2] modified – "along the axis of the stretching" has been
added to the definition, and, in the note to entry, "where R is the electrical resistance in the
is the initial resistance in the
deformed configuration" has been replaced with "where R
O
undeformed configuration, R is the electrical resistance in the deformed configuration, and e is
the tensile strain".]
3.1.3
gauge length
length of the strain-sensitive section of a stretchable strain sensor in the direction of the
measurement axis
3.2 Symbols and abbreviated terms
Symbol Unit Description
a mm Width of the resistive thin film
h µm Thickness of the resistive thin film
l
mm Gauge length of the sensor
l
mm Length of the stretchable resistive thin film
4 Test environments
The tests shall be performed under constant temperature and humidity conditions. As
environmental conditions, including temperature and humidity, can affect the electrical and
mechanical properties of resistive materials and substrates, the testing temperature and
humidity shall be monitored during testing. Fluctuations in temperature during a test shall be
kept to less than ±2 °C. Thus, when testing such materials, the change in relative humidity (RH)
in the testing laboratory shall be kept to less than ±10 % RH. The recommended temperature
and relative humidity are 23 °C ± 2 °C and relative humidity of 50 ± 10 %, respectively,
conforming to standard atmosphere class 2 specified in ISO 291. Whenever testing is
conducted, the environmental conditions shall be recorded.

– 8 – IEC 63203-401-1:2023 © IEC 2023
5 Test specimen
5.1 Shape of the test specimen of the stretchable strain sensor
The shape of the stretchable strain sensor to be tested shall be rectangular or square. The test
specimen has a stretchable substrate, and the resistive thin film is deposited on this stretchable
substrate or on a single layer of resistive composite materials. Figure 1 presents the basic
shape of the test specimen including pads or metal layer for electrical connection. For the pad,
the thin metal film or coating is deposited or fabricated on the resistive sensor layer. The film
materials with low stiffness are fabricated as thin as possible (for example, less than a few
micrometers in thickness) so that the stretching of the resistive sensor material is not restricted
due to the pad material. Some resistive composite sensor materials can be sensitive to the
deposition process. Thus, there will be a possibility that the resistive sensor materials can be
damaged during the deposition process. In this case, metal clips can be used for electrical
connection instead of metal pads. The length of the stretchable resistive thin film l can be the
same as the length of the stretchable substrate. The width of stretchable substrate is more than
or equal to the width of the resistive thin film.
For uniform strain distribution, a rectangular strip shape is recommended. Since the change in
electrical resistance is related to the strain, the electrical resistance is measured in a region of
nearly uniform strain within the gauge length. The gauge length is chosen as the region where
the strain of the stretchable sensor layer is uniform over the cross-sectional area.

Key
1 Stretchable strain sensor material 2 Stretchable substrate
3 Pad for electrical connection

Figure 1 – Shape of a test specimen of the stretchable strain sensor
5.2 Measurement of dimensions
The length of the stretchable strain sensor, in particular the gauge length of the sensor shall be
measured accurately, because the dimension of the length can be used to determine the
mechanical and electrical properties of the strain sensor. Each test specimen should be
measured directly. The test specimens' dimensions shall be specified within the maximum error
of ±5 %.
6 Test method and test apparatus
6.1 General
The test is performed by applying a tensile load to a test specimen. The tensile strain induced
by the tensile load shall be uniform in a pre-defined gauge section in the elastic region of the
substrate or the thin resistive material. To measure the change in electrical resistance along
with the change in mechanical strain, the section of gauge shall be selected carefully. The
gauge section used to measure the mechanical strain shall be coincident with or scalable to the
section used to measure the electrical resistance.
6.2 Test apparatus and measurement
The stretching test machine includes the grips to hold a test sample, the actuating motor which
regulates moving distance and moving speed while testing. Stretching shall be applied along
the tensile axis of the test sample to avoid the bending or twisting of the test piece. An example
of a stretching test machine is shown in Figure 2.

(a) Schematic drawing of stretching test machine (top view)

(b) Photograph of example of a stretching test machine (top view)
Key
1 Stretchable strain sensor 2 Stretchable substrate
3 Grip 4 Actuating motor
5 Actuating direction
Figure 2 – Example of stretching test machine

– 10 – IEC 63203-401-1:2023 © IEC 2023
A vertical stretching test machine can be used if the deformation of the stretchable sensor due
to its own weight can be ignored. The electrical measurement circuit can use two-wire or
four-wire methods depending on the magnitude of the electrical resistance of the sensor. For a
sensor that has an electrical resistance greater than 1 kΩ, a two-wire method can be used for
ease of measurement. For a sensor that has an electrical resistance 1 kΩ or less, the four-wire
method shall be used to eliminate contact- and lead-wire resistance.
Figure 3 shows the schematic drawing of the two-wire method and test machine setup. The test
machine consists of grips, load cell (or strain measurement sensor), motor or actuator. Prepare
a stretchable specimen with a gauge length longer than the distance between the grips. The
resistance measurement setup consists of a constant-current source, a voltmeter, metal clips
or electrodes. The clips that are made of rust-resistant metal or that have a surface treatment
(such as gold plating) to prevent rust shall be used.

Key
1 Stretchable sensor material 2 Stretchable substrate
3 Grip 4 Motor or actuator
5 Pad 6 Constant current source
7 Voltmeter 8 Metal electrode (or metal clip)
9 Actuating direction
Figure 3 – Schematic drawing of a stretching test machine
and two-wire measurement method
Figure 4 shows the schematic drawing of the four-wire method and test machine setup. The
resistance measurement setup consists of a constant-current source, a voltmeter, metal
electrodes or metal clips (voltage electrode), and metal electrodes (current electrode).
Therefore, the constant-current source pad and voltmeter pad for electrical connection are
required at each side of the sample. The fabrication of pads is explained in 5.1. Figure 4 a)
illustrates the initial test setup, and Figure 4 b) illustrates the test setup during stretching. The
examples of sensor resistance or changes in resistance during stretching test are illustrated in
Annex A.
The quality-certified rulers or mechanical extensometer are used to measure the stretched
gauge length (l ). Several optical methods using laser, interference light, camera, and image
s
systems such as digital image correlation (DIC) systems can be utilized to measure the
stretched gauge length. Several methods are available to measure resistance changes of the
sensor with sufficient resolution and accuracy.

During stretching, there will be slippage or friction between the pad and metal electrode or metal
clip which will cause the distortion of the electrical resistance value of the sensor. In this case,
more stable electrical connection such as soldering of electrical wire to the pad shall be used.
In particular, the continuous cyclic stretching test shall require stable electrical connection.
Figure 4 c) illustrates an example of the test setup during stretching.
Figure 4 d) illustrates another example of test setup. This test setup and test method are
described in detail and shall be as described in IEC 62899-202-4:2021. When this test method
is used, it is noted that the stress is not relatively uniform across the sample in lateral direction.
The test machine shall use the gripping system which holds the test sample evenly and firmly
to avoid the slippage of the sample during the stretching test. The gripping system shall not
cause premature damage, failure and scratch in the sample. For highly stretchable samples,
including elastomers, over-tightening of the gripping is also to be avoided to prevent
concentrating the local stress, scratching, or damaging the substrate in the area that is clamped.
The sample shall not twist or bend during stretching.

a) Initial test setup using four-wire method

b) Test setup during stretching using four-wire method

– 12 – IEC 63203-401-1:2023 © IEC 2023

c) Example of test setup during stretching using four-wire method and stable electrical connection

d) Other example of test setup during stretching using four-wire method and stable electrical connection
Key
1 Stretchable sensor material 2 Stretchable substrate
3 Grip 4 Motor or actuator
5 Pad 6 Constant current source
7 Voltmeter 8 Metal electrode (or metal clip)
9 Actuating direction l Stretched gauge length
s
Figure 4 – Schematic drawing of a stretching test
machine and four-wire measurement method

7 Test procedure
The test procedure is as follows.
1) Mount the sensor to be tested on the test apparatus. Centre the sensor symmetrically on
the constant-stress area and align it with the longitudinal centre line of the actuating
direction of the test apparatus.
2) Measure the initial electrical resistance of the sensor.
3) The sensor is stretched with the increasing tensile strain to a designated tensile strain.
Then, the sensor's electrical resistance is measured simultaneously with no time delay.
Record the strain and electrical resistance.
4) The test is performed under a constant strain speed that depends on the material of the
–1
sensor and the sensor's actual application. The strain rate will range from 0,01 min to
–1
1 min to avoid the possible external effects on the sensor performances caused by
stretching speed or strain-rate, depending on the sensor material and the actual usage
condition of the strain sensor. The examples of the relative resistance change of the strain
sensors at different strain rates are illustrated in Annex B in which the strain rate affects the
sensor performances. When the speed is not specified, use the lowest speed.
5) This operation is repeated, increasing the tensile strain until the resistance value reaches a
level that is unacceptable in the actual usage condition of the strain sensor. The test will be
stopped at this point.
6) Unload the sensor when electrical failure occurs in the sensor or when fracture or damage
occurs in the resistive thin film or the substrate. The test will also be terminated when
electrical failure occurs in the sensor or when fracture or damage occurs in the resistive thin
film or the substrate.
8 Measurement of sensor performance and endurance
8.1 Gauge factor
8.1.1 General
The gauge factor is the ratio of relative change in electrical resistance to the mechanical strain.
It is determined from the average of the slope of the straight line between the measurement
points in the graph of relative change in electrical resistance to nominal tensile strain. The
measurement of the gauge factor of the stretchable strain sensor can be similar to that of the
conventional metal-foil strain gauge except for the amount of strain applied. The gauge factor
may vary with the strain due to the nonlinear characteristics of the resistive material and the
elastomer. In addition, it should be noted that the change in resistance with strain is not due
solely to the dimensional changes in the resistive sensor but that the resistivity of the resistive
material also changes with the strain. In this case, the characteristics of the gauge factor with
the strain shall be reported.
NOTE Details on measuring the thermal characteristics of the strain sensor can be found in ASTM E251-92 [3] .
8.1.2 Purpose
The purpose of this method is to measure the gauge factor of the stretchable strain sensor.
___________
Numbers in square brackets refer to the Bibliography.

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8.1.3 Test procedure
The test procedure to measure the gauge factor of the strain sensor is basically the same as
the test procedure described in 6.1 and 7 except that the strain sensor is stretched up to a
designated operating strain range of the stretchable strain sensor. The gauge factor can be
calculated as the ratio of the change in electrical resistance of the strain sensor to the change
in length (strain) along the axis of the stretching.
8.2 Linearity
8.2.1 Purpose
Linearity of the stretchable strain sensor refers to the relationship between the relative change
of the electrical signal, such as electrical resistance, and applied strain as shown in Figure 5.
The closeness of the calibration curve to a specified straight line shows the linearity of a sensor.
Its degree of resemblance to a straight line describes how linear a sensor's output is. A
stretchable sensor with good linearity can reliably produce sensor signals even without rigorous
calibration over a wide range of tensile strain.
8.2.2 Test procedure
Measure the electrical resistance outputs when the stretchable strain sensor is stretched within
measuring stretchability range including end-point. Five measurements at least should be taken
throughout the possible range of measurement. These points are then recorded on a graph as
shown in Figure 5. An attempt is made to fit a straight line through these points. The point which
deviates most from the simple straight line will be used to specify the "linearity" of the
stretchable strain sensor. The degree to which the points lie away from the straight line of best
fit is called the linearity error of the strain sensor.

Figure 5 – Linearity measurement of the stretchable strain sensor
8.3 Response characteristics
8.3.1 General
The response time is how fast the sensor responds to external strain. Response characteristics
determine how quickly the stretchable strain sensor moves toward a steady-state response.
The response characteristics of the stretchable strain sensor are important when the strain
sensor is used to monitor human motion or activity. Since a response delay exists in all
elastomer-based stretchable strain sensors due to the viscoelastic nature of elastomers,
measuring the stretchable sensor's response time is recommended. The response time required
for a stretchable sensor is dependent on the sensor's application.

8.3.2 Purpose
The purpose of this method is to measure the response characteristics of the stretchable strain
sensor.
8.3.3 Test procedure
a) The response time of the stretchable sensor is calculated as the time span between
stretching and the point in time when the sensor output signal rises to the 90 % of maximum
response to the applied strain. Therefore, the exact definition of the response time used
should be defined and reported. Examples of the measurement of the response time for
various stretchable strain sensors are illustrated in Annex C.
b) As the stretchable strain sensor is stretched, the corresponding transient electrical signals,
such as electrical resistances of the strain sensor over time, are continuously recorded.
c) To measure the response time of the strain sensor, the resistance measurement methods
with the data acquisition systems and computer are used. Various data acquisition systems
and instruments such as high-speed digital oscilloscope and digital multimeter also can be
used.
d) While the sensor is at rest, the strain is increased to the lowest possible strain level with a
maximum speed and then released back to the initial state. The time required for the
stretch/release process is different because the stretch/release process is a quasi-transient
process that requires more time for large elongations, which means that with increasing
elongations, the residual time also increases. It is appropriate to set the minimum
satisfactory elongation with maximum speed (for example, 1 mm deformation length and
100 mm/min strain rate). It is recommended to determine the stretching elongation or strain
within a range in which the non-linearity and hysteresis characteristics of the sensor are
minimized. The stretching strain and speed can be dependent on the sensor's application.
8.4 Hysteresis
8.4.1 General
Hysteresis and recovery become important when stretchable strain sensors are used to sense
dynamic load application, including in skin-mountable and wearable applications. Hysteresis
represents the history dependence of the stretchable sensor under mechanical deformation.
Large hysteresis behaviour in the sensors leads to the sensing performance of the sensors
being irreversible upon dynamic loadings. Typically, nanocomposite-based strain sensors that
use metallic particles, Ag nanowire, carbon-based materials, and elastomer composite are
known to exhibit longer recovery times due to the friction force between the fillers and the
elastomer matrices. Therefore, the measurement of the hysteresis and the recovery
characteristics of the stretchable sensors is recommended. When handling nano-materials such
as nanoparticles, nanopowders, nanofibers, nanotubes, and nanowires, the risks associated
with occupational exposure to nano-material or prevention of any possible adverse effects on
workers' health shall be controlled in accordance with ISO/TS 12901-2:2014.
8.4.2 Purpose
The purpose of this method is to measure the hysteresis of the stretchable strain sensor.
8.4.3 Test procedure
a) To characterize the hysteresis behaviour of the stretchable strain sensor, the strain sensor
is subjected to a stretching-releasing cycle with a targeted maximum strain.
b) Place the stretchable strain sensor in the tensile-testing machine.
c) Stretch the strain sensor to a designated strain and record the electrical resistance value
and applied strain. The stretching speed or strain rate is described in Clause 7.
d) Repeat the increment of strain with continuous measurement of the electrical resistance up
to a targeted maximum strain.
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e) After the strain sensor is stretched to a maximum strain, release the strain sensor to a
designated strain. Repeat the reduction of the strain down to 0 % strain, that is, initial state.
f) There can be several methods to calculate the hysteresis value. Therefore, the designated
strain and the exact definition of the hysteresis value used should be defined and described
in the test reports. Examples of hysteresis calculation and hysteresis behaviour of various
stretchable strain sensors are illustrated in Annex D.
9 Test report
The test report shall contain the following information.
1) Sensor material and substrate material
2) Sensor dimensions and the method used to measure them
3) Number of sensors tested
4) Description of the testing apparatus
5) Strain rate applied
6) Sensor performance, including
– Stretchability or maximum allowable amount of strain
– Gauge factor and linearity
– Response and hysteresis characteristics
– Definition and measurement methods of the response time and hysteresis value

Annex A
(informative)
Strain sensor resistance measurement
Figure A.1 shows examples of stretchability test results of a stretchable resistive strain sensor
using a tensile test machine. The stretchable strain sensor was made of Ag flakes and a polymer
binder and printed on the polyurethane stretchable substrate. The stretching test was performed
using a tensile test at a low speed of 0,1 mm/s. The change in the electrical resistance of the
strain sensor was measured during the test using the two-wire method. Three kinds of the
resistive strain sensor were evaluated and compared. In Figure A.1 b), the change in the
resistance of the strain sensor was expressed as ΔR (= R – R )/R , where R and R are the
o o o
resistances of the strain sensor before and after testing, respectively. The electrical resistance
of the sensors gradually increases as tensile strain increases, and then at a certain point, the
resistance increases sharply for some sensors.

a) b)
Figure A.1 – Changes in electrical resistances
when the strain sensor is stretched

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Annex B
(informative)
Effects of strain rate on performances
of the stretchable strain sensor
Figure B.1 shows two examples of the resistance change of the stretchable strain sensor for
different strain rates. Figure B.1 a) shows the relative resistance change of the stretchable
strain sensor for different strain rates with a strain of 50 %. The applied rate was in the range
of 5 mm/min to 25 mm/min [4]. The relative resistance ΔR (= R – R )/R was measured where
o o
R and R are the resistances of the strain sensor before and after testing, respectively. As the
o
strain rate increases from 5 mm/min to 20 mm/min, the resistance of the strain sensor reduces.
Figure B.1 b) also shows the effect of the strain rate on performances of the stretchable strain
sensor [5]. For different strain rates, the stretchable strain sensor shows the different resistance
behaviour during stretching.
a) b)
Figure B.1 – Examples of the effects of strain rate
on performances of the stretchable strain sensor

Annex C
(informative)
Measurement of response time
of the stretchable strain sensor
Figure C.1 shows two examples of the response time of the stretchable resistive strain sensor
tested using a tensile test machine. The stretchable strain sensor in Figure C.1 a) was made of
Ag ink and polymer binder [6]. The relative resistance ΔR (= R – R )/R was measured with time
o o
where R and R are the resistances of the strain sensor before and after testing, respectively.
o
The response time of this strain sensor is 70 ms. Figure C.1 b) shows another example of the
response time of the stretchable strain sensor made of the carbon nanotube and
polydimethylsiloxane (PDMS) binder [7]. The response time of this strain sensor is 87 ms.
Obviously, the sensor with the faster response time is better.

a) b)
Figure C.1 – Examples of the response time
of the stretchable strain sensors

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Annex D
(normative)
Examples of hysteresis calculations
There can be several methods to calculate the hysteresis behaviour of the sensors. Figure D.1
shows an example of the hysteresis behaviour of a stretchable resistive strain sensor.

Figure D.1 – Example of the hysteresis behaviour
of a stretchable strain sensor
In many cases, the main portion of the hysteresis curve is not a simple straight line. Non-
linearity and sampling error tend to make the
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