ISO 19375:2024

International
Standard
ISO 19375
First edition
Fibre-reinforced composites —
2024-11
Measurement of interfacial
shear strength by means of a
micromechanical single-fibre pull-
out test
Reference number
© ISO 2024
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ii
Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 3
5 Abbreviated terms, symbols and dimensions . 3
5.1 Symbols .3
5.2 Abbreviated terms .4
6 Apparatus . 5
6.1 Fibre diameter determination .5
6.1.1 General .5
6.1.2 Vibroscopic fibre linear density and diameter test .5
6.1.3 Optical fibre diameter determination .5
6.2 Embedding station .5
6.3 CRE testing machine.6
7 Test specimen . 6
8 Procedure . 8
8.1 Overview .8
8.2 General requirements .9
8.3 Fibre sampling and preparation .10
8.3.1 General .10
8.3.2 Fibre diameter determination . .10
8.3.3 Fibre insertion into the embedding device .10
8.4 Matrix preparation .10
8.4.1 Thermoset matrices .10
8.4.2 Thermoplastic matrices .10
8.4.3 Concrete matrices .10
8.5 Preparation of matrix droplet.10
8.5.1 General .10
8.5.2 Thermoset matrices .11
8.5.3 Thermoplastic matrices .11
8.5.4 Concrete matrices .11
8.6 Fibre positioning, embedding and final forming .11
8.7 Solidification of the test specimen . 12
8.7.1 General . 12
8.7.2 Thermoset matrices . 12
8.7.3 Thermoplastic matrices . 12
8.7.4 Concrete matrices . 13
8.8 Test specimen requirements and final validation . 13
8.9 Post-preparation and conditioning . 13
8.9.1 General . 13
8.9.2 Thermoset matrices . 13
8.9.3 Thermoplastic matrices . 13
8.9.4 Concrete matrices .14
8.10 Pull-out test .14
8.10.1 General .14
8.10.2 Test specimen insertion and clamping .14
8.10.3 Testing conditions . 15
8.10.4 Testing . 15
8.10.5 Validation . 15

iii
9 Evaluation .15
9.1 General . 15
9.2 Measured values and basic evaluation .16
9.2.1 Fibre diameter .16
9.2.2 Embedded length .16
9.2.3 Maximum force and apparent interfacial shear strength .16
9.2.4 Interfacial frictional force and stress .16
9.3 Further evaluations.17
9.3.1 General .17
9.3.2 Local interfacial shear strength derived from the debonding force .17
9.3.3 Alternative determination of local interfacial shear strength .17
9.3.4 Critical energy release rate .17
10 Expression of results . 17
11 Test report . 17
11.1 General .17
11.2 Fibre and matrix preparation .18
11.2.1 Fibre sampling and preparation .18
11.2.2 Matrix preparation .18
11.3 Test specimen preparation .18
11.3.1 Embedding device .18
11.3.2 Matrix droplet preparation .18
11.3.3 Fibre embedding .18
11.3.4 Test specimen solidification .19
11.3.5 Post-preparation and conditioning .19
11.4 Pull-out test and evaluation . .19
11.4.1 CRE testing machine .19
11.4.2 Material parameters for evaluation . 20
11.4.3 Basic evaluation and results . 20
11.4.4 Evaluation of the local interfacial shear strength from the debonding force, if
applied . 20
11.4.5 Alternative evaluation of the local interfacial shear strength, if applied . 20
11.4.6 Evaluation of the critical energy release rate, if applied . 20
11.5 Statistics . 20
Annex A (informative) Images of embedding procedure .21
Annex B (informative) Images of force-displacement curves of pull-out tests .25
Annex C (informative) Exemplary embedding and pull-out settings for thermoset matrices .29
Annex D (informative) Exemplary embedding and pull-out settings for thermoplastic matrices .33
Annex E (informative) Exemplary embedding and pull-out settings for concrete matrices .37
Annex F (informative) Specific material parameters .39
Annex G (informative) Typical test results and precision data .40
Bibliography .44

iv
Foreword
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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).
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This document was prepared by Technical Committee ISO/TC 61, Plastics, Subcommittee SC 13, Composites
and reinforcement fibres.
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.

v
Introduction
Fibre-reinforced composites have become an indispensable part of modern high-tech applications due to the
excellent tensile properties of the reinforcement fibres incorporated in the composite. This requires that
the loads in a composite need to be distributed evenly to all fibres by means of a matrix. Therefore, a high
interfacial shear strength is required for a good load transfer. Hence, the interfacial shear strength is one of
the key parameters in composite technology.
To characterize the interfacial shear strength, composites with unidirectional aligned fibres are
manufactured. A tensile test is then performed perpendicular to the fibre orientation (transverse tensile
test), or a short beam shear strength test (as defined for instance in ISO 14130 or ASTM D2344) is performed
to measure the apparent interlaminary shear strength (ILSS).
However, the maximum stress found in such macromechanical tests does not only depend on the fibre-matrix
adhesion strength, it is also governed by the following additional factors: the fibre content, orientation,
length, diameter and fibre distribution homogeneity, the pore void of the test specimens, and the mechanical
properties of the fibre and the matrix. To achieve repeatable results for the fibre-matrix adhesion strength
through macromechanical tests, it is necessary to keep a rigid control of the manufacturing process of the
specimen, making the overall testing procedure laborious and difficult to compare across laboratories.
Micromechanical testing techniques have several advantages over the macromechanical methods. By
involving only single fibres in the test, most dependencies on the manufacturing process of the test specimen
listed above are avoided. Whereas macroscopic methods can only determine the apparent interfacial shear
strength, the single-fibre pull-out test as a micromechanical test method also can determine the local
interfacial shear strength, the interfacial frictional stress, and the critical interfacial energy release rate.
Here, the local interfacial shear strength is essential for composites applications with cyclic load, since it
characterizes to which stress a composite can be loaded before the interface between fibre and matrix
is damaged. The critical interfacial energy release rate and the interfacial frictional stress have a strong
impact on the energy absorption of composites, which is important in crash situations as for example in
mobility applications.
This document describes stringent methods for specimen preparation, conditioning, and pull-out testing.
Practical trials have shown that following these procedures leads to minimum variability in the test results.

vi
International Standard ISO 19375:2024(en)
Fibre-reinforced composites — Measurement of interfacial
shear strength by means of a micromechanical single-fibre
pull-out test
1 Scope
This document specifies a test method for determining the interfacial shear strength between a single fibre
and a matrix by means of a pull-out test. The method can be used to measure the critical energy release rate.
The method is applicable to reinforcement fibres, such as carbon fibres, glass fibres, basalt fibres and similar
stiff reinforcement fibres and to thermoset, thermoplastic and fine-grained concrete matrices. It can be used
for polymeric reinforcement fibres and for other inorganic matrices.
It is not applicable to:
a) elastomeric fibres and elastomeric matrices such as rubber;
b) matrices which cure or melt at temperatures above 400 °C;
c) matrices that show a strong tendency to bubble formation or expansion during the sample-preparation
process;
d) foams.
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.
ISO 1973, Textile fibres — Determination of linear density — Gravimetric method and vibroscope method
ISO 2602, Statistical interpretation of test results — Estimation of the mean — Confidence interval
EN 12390-2, Testing hardened concrete — Part 2: Making and curing specimens for strength tests
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
constant-rate-of-extension testing machine
CRE testing machine
tensile testing machine provided with one specimen holder, which is stationary, and one clamp, which moves
with a constant speed throughout the test, the entire testing system being virtually free from deflection
[SOURCE: ISO 5079:2020, 3.10, modified — “clamp” was replaced with “specimen holder“.]

3.2
crucible
receptacle, made of aluminium or stainless steel, to hold a drop of matrix into which one single fibre is
embedded
3.3
pull-out test
method of pulling out one single embedded fibre from a solidified drop of matrix, carried out with a CRE
testing machine (3.1)
3.4
force-displacement curve
graphical representation of the force recorded during the pull-out test (3.3) over the displacement of the
moving clamp of the CRE testing machine (3.1)
3.5
maximum force
highest force value taken from the force-displacement curve (3.4) appearing just before the complete
debonding of the fibre from the solidified matrix during the pull-out
3.6
debonding force
force at which the fibre starts debonding from the solidified drop of matrix, derived from the force-
displacement curve (3.4)
3.7
interfacial frictional force
force measured after complete debonding, caused by only the friction between the fibre and the solidified
matrix, taken from the force-displacement curve (3.4)
3.8
embedding depth
selected depth to which the embedding device inserts the fibre into the matrix
3.9
embedded length
resulting length of the fibre in contact with the solidified matrix, corresponds to the displacement at which
the fibre completely slips out of the solidified matrix, taken from the force-displacement curve (3.4)
3.10
final forming
process, in which the matrix maximizes the wetting of the fibre and the meniscus reaches a steady state
3.11
solidification
process of transformation of the matrix from liquid to solid
Note 1 to entry: The term solidification includes both the commonly used solidification of thermoplastics by cooling as
well as the curing of thermosets and concrete matrices.
3.12
fixing
first step of the solidification (3.11) of matrices, after which a test specimen can be removed from the
embedding device without the risk that the fibre redirects inside the matrix
3.13
curing
process of solidification (3.11) of thermoset and concrete matrices
3.14
apparent interfacial shear strength
maximum force (3.5) normalized to the contact area between the fibre and the solidified matrix

3.15
interfacial frictional stress
interfacial frictional force (3.7) normalized to the contact area between the fibre and the solidified matrix
3.16
local interfacial shear strength
debonding force (3.6) related to the contact area of the interface between the fibre and the solidified matrix,
without the impact of the friction between the fibre and the matrix
3.17
critical interfacial energy release rate
interfacial toughness
calculated as a function of the crack length (energy-based method), taking the deformation of the fibre and
matrix during the pull-out into account
4 Principle
A single fibre is embedded into a drop of matrix at a defined temperature to form a test specimen for the
pull-out test by means of an embedding station. If required, the embedding can take place in an inert gas
atmosphere. Embedding depth and speed are variable. After embedding, the test specimen is solidified, for
instance by heating up to a curing temperature (for example for thermoset matrices) or cooling down to a
suitable temperature for matrix solidification (for example for thermoplastic matrices).
After complete solidification and conditioning, the test specimen is transferred into a CRE testing machine
for pull-out testing.
During the pull-out test, certain values are measured along the force-displacement curve respectively
calculated from these measured values.
5 Abbreviated terms, symbols and dimensions
5.1 Symbols
Table 1 lists symbols used throughout this document.
Table 1 — Test characteristics, symbols, and dimensions
Characteristic Matrix Symbol Unit
Material parameter
ρ 3
Fibre density — g/cm
Fibre linear density — dtex
ρ
l
Fibre diameter — µm
d
f
Fibre breaking force — f cN
B
Melting temperature TP °C
T
M
Solidification temperature TP T °C
S
Testing procedure
Heating rate to embedding temperature TS, TP r K/min
TE
Time at embedding temperature TS, TP s
t
TE
Embedding temperature TS, TP T °C
E
Embedding depth TS, TP, C µm
l
ED
Embedding speed TS, TP, C mm/min
v
E
Forming time TS, TP, C t s
F
TTabablele 1 1 ((ccoonnttiinnueuedd))
Characteristic Matrix Symbol Unit
Curing temperature TS, C T °C
C
Heating rate to curing temperature TS, C K/min
r
TC
Time at curing temperature TS, C t s
TC
Below-solidification-temperature TP °C
T
BS
Cooling rate to below-solidification-temperature TP K/min
r
TBS
Final cooling rate TS, TP r K/min
TF
Final withdrawal temperature TS, TP °C
T
F
Testing speed for pull-out test All V mm/min
P
Evaluation
Maximum force All F cN
max
Debonding force All cN
F
d
Interfacial frictional force All F cN
b
Embedded length All µm
l
e
Apparent interfacial shear strength All MPa
τ
app
Interfacial frictional stress All τ MPa
f
Local interfacial shear strength All MPa
τ
d
Alternative local interfacial shear strength All τ MPa
d,alt
Shear-lag parameter All β —
Residual thermal stresses All MPa
τ
T
Critical energy release rate All G J/m
ic
5.2 Abbreviated terms
Table 2 lists abbreviated terms used throughout this document.
Table 2 — Materials and abbreviated terms
Material Abbreviation
Fibres
Carbon fibre CF
Recycled carbon fibre rCF
Glass fibre GF
Natural fibre NF
Matrices
Thermoset TS
Thermoplastic TP
Concrete Concrete
Polypropylene PP
Polyamide 6 PA6
Polyether ether ketone PEEK
Polyether block amide PEBA
Polyurethane PU
TTabablele 2 2 ((ccoonnttiinnueuedd))
Material Abbreviation
Polycarbonate PC
Epoxy EP
Vinylester VE
6 Apparatus
6.1 Fibre diameter determination
6.1.1 General
The diameter of each individual fibre shall be determined close to the position where the fibre is embedded
into the matrix. The following methods may be used for the determination of the individual fibre diameter.
NOTE For the sake of simplicity, all calculations within this document assume a circular fibre cross section.
6.1.2 Vibroscopic fibre linear density and diameter test
This method uses a device measuring the linear density according to the vibroscopic principle, which shall be
in accordance with ISO 1973. Here, a fibre with a known pretension is excited to its fundamental resonance
frequency at a known vibrating length.
For glass fibres, due to a possible impact of the bending stiffness to the resonance curve, a vibrating length
of at least 50 mm and a pretension of 3 cN/tex are strongly recommended. For carbon fibres, a vibrating
length of minimum 25 mm and a pretension of 1 cN/tex are commonly used.
Based on the selected pretension and vibrating length and on the measured resonance frequency, the linear
density ρ is calculated following the vibrating string equation. Employing the known fibre density ρ
l
(specified by the fibre producer), the fibre diameter d is calculated according to Formula (1):
f
4⋅ρ
l
d =⋅10 (1)
f
π⋅ρ
where
d is the fibre diameter, in micrometres (µm);
f
ρ is the fibre linear density, in decitex (dtex, g/1 000 m);
l
3)
ρ
is the fibre density, in gram per square centimetres (g/cm .
The determination of the fibre diameter is required to calculate the contact area between the fibre and
the matrix.
6.1.3 Optical fibre diameter determination
This method uses an optical assembly to measure the individual fibre diameter directly. Possible principles
are a laser-scan micrometre, a laser diffractometer, an optical microscope, or an appropriate device
according to ISO 11567.
6.2 Embedding station
This method uses a device for producing test specimens for the pull-out test, by embedding a single fibre
into a drop of matrix placed in a crucible. A single fibre with a minimum length of 5 mm is placed into a
cannula, after insertion the fibre protrudes out of the cannula. The protruding end is embedded into the

matrix, embedding depth and speed are variable. During embedding and solidification, the fibre can be held
with low tension in the cannula by means of a holding air flow such that it can follow a possible shrinkage of
the matrix.
The device is equipped with a heating system for the crucible, allowing temperature changes at rates of
0,1 K/min to 99,9 K/min up to a maximum temperature of 400 °C. The heat transfer is given by the contact
between the heating system and the bottom side of the crucible. The actual temperature at the top of the
matrix might be lower than the displayed actual heater temperature, due to the geometry of the setup.
Temperature, temperature rates and duration of heating (time at temperature) can be adjusted.
The device is equipped with an active cooling system which can be enabled, and which rate can be adjusted.
The maximum cooling shall allow the device to cool down from 400 °C to 40 °C within 6,5 min (from 100 °C
to 40 °C within 3,5 min).
The device is equipped with a triaxial linear table to allow positioning of the fibre relative to the matrix, and
a microscope to observe and control the positioning. The fibre can be moved along its axis for embedding
using an additional z-axis of the embedding device.
The device is equipped with a system to flush the embedding zone with an inert gas to avoid oxidation of
the matrix.
6.3 CRE testing machine
This method uses a device for pull-out testing, equipped with an upper holder for the test specimen mounted
to the force-measuring system, and a moveable lower draw-off clamp to pull the single fibre out of the matrix.
The device shall be equipped with a high-precision load cell with a minimum force range higher than the
fibre breaking force to be tested and a resolution of 1 µN or better. The distance between the draw-off clamp
and the upper specimen holder can be freely adjusted, the testing speed can be adjusted to values between
0,1 mm/min to 100 mm/min.
The draw-off clamp consists of one stationary and one moveable clamp jaw with adjustable clamping
pressure. The surface of the clamp jaws in contact with the specimen shall be made of a material to provide
correct grip without damaging the fibre, thereby avoiding slippage and jaw breaks.
A high-precision displacement measurement sensor with a resolution of 0,1 µm or better is required. The
device is resistant against deformation under load, with a maximum deflection of 0,02 mm/N.
The lateral position of the draw-off clamp is adjustable to match the position of the fibre axis. In this way, the
fibre is clamped at minimum shear force even at very low distances between matrix and clamp. Adjustment
is carried out by means of a camera with microscopic optics.
During the pull-out test, the displacement of the draw-off clamp and the resulting force is recorded as a
force-displacement curve.
7 Test specimen
The test specimen consists of a rotationally symmetric metallic crucible, a drop of a matrix placed into the
upper recess of the crucible, and a fibre embedded into the matrix. Figure 1 shows the physical dimensions
of the crucible that shall be used for the preparation of the test specimen.

Dimensions in millimetres
Figure 1 — Test specimen, crucible
Figure 2 shows a schematic lateral section through the test specimen.
Key
1 crucible with recess
2 matrix drop
3 embedded fibre (dashed line = embedded section)
Figure 2 — Test specimen, lateral section
An appropriate embedding device to create the test specimen is described in 6.2.
After the test specimen has been produced and solidified, it is transferred to the CRE testing machine.
Figure 3 shows schematic lateral sections through the test specimen, mounted into the upper test specimen
holder and the lower draw-off clamp of the CRE testing machine, described in 6.3.

a)  Draw-off clamp adjustment b)  Draw-off clamp open c)  Draw-off clamp closed
Key
1 crucible with recess 6 stationary jaw of draw-off clamp
2 matrix drop 7 free fibre length
3 embedded fibre (dashed line = embedded section) 8 lateral + vertical adjustment
4 test specimen holder = measuring clamp 9 closing direction of movable clamp jaw
5 movable jaw of draw-off clamp 10 draw-off direction
Figure 3 — Test specimen in CRE testing machine, lateral section
8 Procedure
8.1 Overview
Fibre embedding and matrix solidification are carried out using complex temperature profiles, described
in 8.2 to 8.10. Figure 4 and Figure 5 schematically illustrate those temperature profiles for thermoset
respectively thermoplastic matrices.
Key
T temperature
t time
1 heating to and holding embedding temperature for degassing (heating rate to embedding temperature, time at
embedding temperature, embedding temperature)
2 positioning, embedding and final forming (embedding temperature, forming time)

3 curing (heating rate to curing temperature, time at curing temperature, curing temperature; divided into several
steps, if required)
4 cooling (if inside the embedding device: final cooling rate, final withdrawal temperature)
5 optional test specimen extraction from the embedding device after fixing
Figure 4 — Typical temperature profile for thermoset matrices
Key
T temperature
t time
1 heating to and holding of the embedding temperature (heating rate to embedding temperature, time at embedding
temperature, embedding temperature)
2 positioning, embedding and final forming (embedding temperature, forming time)
3 cooling down below-solidification-temperature (cooling rate to below-solidification-temperature, below-
solidification-temperature)
4 final cooling (final cooling rate, final withdrawal temperature)
Figure 5 — Typical temperature profile for thermoplastic matrices
After solidification and conditioning, the test specimen is transferred to the CRE testing machine for pull-
out testing.
8.2 General requirements
A glove shall be used during all following steps to avoid any influence from contamination.
The embedding depth for a test series can be determined by iterative pre-tests, where the maximum pull-
out force is investigated as a function of the embedding depth, following Clause 8. The embedding depth
shall be chosen such that the maximum pull-out force is close to but does not exceed the fibre breaking force
(for example measured as described in ISO 5079 or ISO 11566). The resulting embedded length (refer to
9.2.2) shall be a least 4 times the fibre diameter.
In case the embedded length is much smaller than the embedding depth, this can be an indicator for irregular
wetting, and the processing parameters should be reconsidered.
For a comparative test of similar fibre-matrix combinations, the same embedding depth shall be chosen for
test specimen preparation and, in case one of the further evaluations in 9.3 is applied, the embedded length
shall be similar.
A sufficient number of test specimen shall be prepared to ensure solid average values of the evaluated
parameters. After testing, invalid tests shall be eliminated, assuring that the sample size is not below 15
valid pull-out tests, unless otherwise agreed-on between the interested parties. For more information on
invalid results, please refer to 8.10 and Annex B.

8.3 Fibre sampling and preparation
8.3.1 General
Fibres are drawn from a fibre sample. Typical sample forms are rovings, fabrics, flocks (for example natural
fibres) and chopped fibres (including recycled fibres).
Special care shall be taken to assure that a representative set of fibres is drawn from different sections
of the sample, for example by selecting different filaments from a roving or selecting fibres from different
roving sections. Care shall be taken not to damage the fibres by stretching or bending. The minimum fibre
length shall be sufficient to assure a fibre diameter determination (see 6.1).
8.3.2 Fibre diameter determination
The diameter of each individual fibre test specimen shall be determined as described in 6.1. Diameter
determination shall be carried out on the fibre section which is embedded into the matrix.
8.3.3 Fibre insertion into the embedding device
The fibre is inserted into the cannula of the embedding device. The fibre shall be cut by means of scissors to
protrude by a length of 2 mm to 5 mm from the cannula (preferred length: 2 mm to 3 mm). The fibre is held
in place by means of the holding air.
8.4 Matrix preparation
8.4.1 Thermoset matrices
The matrix shall be prepared in accordance with the manufacturer’s instructions, for example regarding
the mixing ratio, vacuum/evacuation, the mixing procedure (manual mixing, magnetic stirring) and the
processing time. A bubble-free matrix is required, for example by vacuum application. Any deviation from
the manufacturer´s instruction shall be indicated in the test report.
8.4.2 Thermoplastic matrices
The matrix shall be prepared in accordance with the manufacturer’s instructions, for example regarding
the required drying, to achieve a defined, not degraded condition. Any deviation from the manufacturer´s
instruction shall be indicated in the test report.
8.4.3 Concrete matrices
The matrix shall be prepared in accordance with the manufacturer’s instructions, for example regarding
the required mixing ratio (for example cement, water, filler and further additives). Any deviation from the
manufacturer´s instruction, for example to adjust the viscosity, shall be indicated in the test report.
8.5 Preparation of matrix droplet
8.5.1 General
A crucible is placed into the embedding device described in 6.2. In general, an aluminium crucible is
recommended for most thermoset
...