ISO/TS 24858:2026

Technical
Specification
ISO/TS 24858
First edition
Plastics — Conducting materials
2026-10
— Eddy current measurement on
spread carbon fibre tow
Plastiques — Matériaux conducteurs — Mesure par courants de
Foucault sur un câble en fibres de carbone étiré
Reference number
© ISO 2026
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Background — Impedance . 1
5 Specimen . 2
5.1 Sampling .2
5.2 Size .3
6 Apparatus . 3
7 Test conditions . 4
8 Procedure . 4
9 Report of the results . 5
9.1 General .5
9.2 Test environment .5
9.3 Test apparatus.5
9.4 Test conditions .5
9.5 Test results .5
Annex A (informative) Eddy current measurement data for impedance analysis based on the
presence of defects . 6
Bibliography .16

iii
Foreword
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iv
Introduction
As technologies for the commercialization of textile materials have advanced, the development of various
functional materials based on fibres has increased. Conductive fibres are flexible and highly elastic
materials with significant potential for application in next-generation electronic devices, and carbon fibres
are also one of the conductive materials that exhibit electrical conductivity. The homogeneity of a conductive
material is an important factor in determining the performance of the final product, and from a quality
control perspective, it is necessary to have a method to analyse the distribution status of the raw material
before it is processed. In the case of carbon fibres, a conductive material in the form of fibres, it is important
to manage the arrangement of the fibres because the degree to which the filament strands are arranged can
cause performance degradation in localized areas. Optical image analysis is a common method for checking
the homogeneity of filaments in carbon fibres and can be used to analyse the arrangement of filaments on the
surface, but it has limitations in analysing the presence and density of filaments arranged in the thickness
direction. A method for continuously measuring the homogeneity of carbon fibres with large surface areas is
eddy current measurement, which utilizes electrical conductivity properties. An eddy current measurement
is used to detect discontinuities in materials that conduct electricity. This method is a reliable measurement
that can identify materials with low electrical conductivity, such as spread carbon fibre tow, making it
applicable to a wide range of industries. This document specifies a standard method for evaluating the
homogeneity of a spread carbon fibre tow, a conductive fibre, using eddy current measurement.
Annex A gives the eddy current measurement data for impedance analysis based on the presence of defects.

v
Technical Specification ISO/TS 24858:2026(en)
Plastics — Conducting materials — Eddy current
measurement on spread carbon fibre tow
1 Scope
This test method defines test terms and specifies a standard method for evaluating the homogeneity
of electrically conductive spread carbon fibre tow using eddy current measurement. The impedance in
the width direction of the fibre is measured continuously, and the standard deviation of the measured
impedance determines the homogeneity of the conductive material in fibre form.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
spread carbon fibre tow
large bundle of continuous carbon fibre spread thinly and widely in the width direction
3.2
eddy current
electrical currents induced within conductors by a time-varying magnetic field in the conductor
[SOURCE: IEC 62899-202-3:2019, 3.2]
3.3
impedance
value represented by the combined effect of a circuit's resistance, inductance, and capacitance, which
interferes when a voltage is applied to the circuit
3.4
homogeneity
degree of a state of being of a similar kind or of having a uniform structure or composition throughout
4 Background — Impedance
Eddy current instruments work on the principle that an electrically conductive sample is placed in an
electromagnetic field generated by a probe system, which causes an eddy current to flow. Impedance
is the total opposition a circuit presents to the flow of alternating current and is calculated as shown in
Formulae (1) and (2), including resistance, inductive reactance, and capacitive reactance. For eddy current
testing, capacitive reactance is usually not present, so impedance is calculated with Formula (3). As shown

in Figure 1, the impedance value is calculated by the amount of resistance (horizontal line), the amount of
[5]
inductive reactance (vertical line), and the amount of impedance (diagonal line) .
ZRXX (1)
Lc
ZR XX (2)

Lc
ZRX (3)
L
where
Z is the complex impedance (ohm);
Z is the magnitude of the impedance (ohm);
R is the resistance (ohm);
X is the inductive reactance (ohm);
L
X is the capacitive reactance (ohm).
C
Key
R resistance
XX- reactance
LC
Z impedance
Z impedance amplitude
ϕ phase angle
Figure 1 — Principle of impedance
5 Specimen
5.1 Sampling
Samples for fibre homogeneity measurements are preferably not impregnated with resin. Randomly take
a sample without twists from a roll or bobbin of product and fix it onto a fibreglass veil using polyvinyl
acetate (PVA) glue. Ensure that the width and density do not change during sample fixation. The purpose of

fixing the sample with PVA glue to the fibreglass veil is to minimize the floating of the sample. Alternatively,
the sample can be fixed by vacuum suction from the bottom of the sample. See ISO 24584.
5.2 Size
The size of the sample should be at least five times larger than the measured diameter of the probe in the
width direction, and a sample longer than 100 mm should be prepared in the length direction.
6 Apparatus
Eddy current testing instruments consists of an oscillator that generates alternating current electricity, a
probe with a coil connected to a direct current power supply, and a voltmeter that can measure the voltage
across the coil.
6.1 Probe
It is recommended to select a probe with an appropriate standard penetration depth based on the thickness
and characteristics of the sample. The standard penetration depth varies with electrical conductivity,
magnetic permeability, and frequency, and the standard penetration depth for eddy current inspection is
calculated by the Formula (4).
  (4)
··f ·
where
δ is the standard penetration depth (millimetre);
π is the ratio of the circumference of a circle to its diameter;
f is the frequency (Hz);
µ is the magnetic permeability (H/m);
σ is the electrical conductivity (micro ohm-centimetre).
6.2 Sample stage
The sample should be placed on top of a sample stage made of a glass plate or insulating material such as
acrylic, PVC, etc. to minimize induced currents due to interference from other objects. The sample stage
should be separated by at least 100 mm depending on the material of the table, especially if it is an electrically
conductive material such as metal. Alternatively, a separate grounding method can be devised to prevent
measurement errors caused by static electricity.

Key
1 probe
2 induction coil
3 sample, conductive materials
4 sample stage, insulating material
5 table
Figure 2 — Apparatus of eddy-current measurement
6.3 Gap
Impedance can be affected by the distance between the sample and the probe. The probe and sample shall
not be in direct contact and the gap shall be kept at a constant distance of not more than 1 mm.
7 Test conditions
The ambient temperature affects impedance of the sample when measuring eddy currents. Therefore, time
should be allowed for the instruments and sample to stabilize to the test temperature before testing, and
it is desirable that the conditions remain as constant as possible during the measurement to increase the
reliability of the measurement.
— Temperature : (23 ± 2) °C
— Humidity : (50 ± 10) %
8 Procedure
1) Place the sample closely attached to the sample stage.
NOTE Mark the position of the sample to be measured with the probe.
2) The probe shall be positioned on the surface of the sample facing vertically, and measured impedance
using eddy current probe.
3) Take at least three measurements per sample under the same conditions to analyse the impedance
standard deviation.
9 Report of the results
9.1 General
The report shall include the following items.
9.2 Test environment
a) Temperature range
b) Humidity
9.3 Test apparatus
a) Type of probe (frequency, diameter, etc.)
b) Material of the sample stage
9.4 Test conditions
a) Gap
b) Number of measurements
9.5 Test results
Homogeneity (No units); Since the homogeneity is expressed as a coefficient of variation value calculated as
a ratio of the mean and standard deviation of the impedance value, there is no unit.
The homogeneity of spread carbon fibre tow can be calculated as the mean and standard deviation of the
impedance values in the width direction (row direction) measured continuously.

Annex A
(informative)
Eddy current measurement data for impedance analysis based on the
presence of defects
A.1 Test sample
— Materials : T700-12K Spread carbon fibre tow
A.2 Test conditions
— Probe: Differential probe (radius 1,0 mm, frequency 27 M
...