ISO 20975-1:2023

INTERNATIONAL ISO
STANDARD 20975-1
First edition
2023-01
Fibre-reinforced plastic composites —
Determination of laminate through-
thickness properties —
Part 1:
Direct tension and compression tests
Composites plastiques renforcés de fibres — Détermination des
propriétés dans l'épaisseur d'un composite stratifié —
Partie 1: Essais directs de traction et de compression
Reference number
ISO 20975-1:2023(E)
© ISO 2023

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ISO 20975-1:2023(E)
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© ISO 2023
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Published in Switzerland
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ISO 20975-1:2023(E)
Contents Page
Foreword .v
Introduction . vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 2
4 Principle . 4
5 Apparatus . 4
5.1 Test machine . 4
5.1.1 General . 4
5.1.2 Indicators for load and strain . 5
5.2 Strain gauges and strain acquisition . 5
5.3 Micrometer . 5
5.4 Loading fixtures . 5
5.4.1 General . 5
5.4.2 Method A - Tension loading . . 5
5.4.3 Method B - Compression loading . 5
6 Test specimens . 6
6.1 Shape and dimensions . 6
6.1.1 Type I specimens . 6
6.1.2 Type II specimens . 7
6.1.3 Type III specimens . 8
6.1.4 Non-standard test specimens . 9
6.2 Preparation of test specimens . 10
6.2.1 General . 10
6.2.2 Parallelism . 10
6.2.3 Application of end-loading blocks for Method A — Tension only . 10
6.3 Checking . 10
7 Test specimens .10
8 Conditioning .11
9 Test procedure .11
9.1 Test atmosphere . 11
9.2 Specimen dimensions . 11
9.3 Specimen preparation Method A — Tension only . 11
9.4 Strain recording . 11
9.5 Specimen alignment. 11
9.5.1 Method A – Tension . 11
9.5.2 Method B – Compression .12
9.6 Testing speed, v .12
9.7 Data collection .12
9.8 Maximum load .12
9.9 Failure mode .12
9.10 Test acceptance .12
10 Expression of results .12
10.1 Method A — Tension .12
10.1.1 Tensile strength .12
10.1.2 Modulus of elasticity (chord) in tension . 13
10.1.3 Tensile failure strain . 14
10.2 Method B — Compression . 14
10.2.1 Compressive strength . 14
10.2.2 Modulus of elasticity (chord) in compression . 15
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ISO 20975-1:2023(E)
10.2.3 Compressive failure strain . 15
10.3 Poisson’s ratio – Methods A and B . 15
10.4 Statistical analysis . 16
11 Precision .16
12 Test report .16
Annex A (informative) Precision data .17
Bibliography .19
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ISO 20975-1:2023(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to
the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 61, Plastics, Subcommittee SC 13,
Composites and reinforcement fibres.
A list of all parts in the ISO 20975 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
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ISO 20975-1:2023(E)
Introduction
Through-thickness properties and the methods specified in this document for their measurement are of
interest for engineering specifications and design use. The test geometries recommended are suitable
for testing thermoset and thermoplastic-based fibre-reinforced composites, although some materials
can be difficult to bond to the loading bars when loaded in tension. A through-thickness dimension of
40 mm is recommended.
For the tension method, consideration has been given to the possibility of out-of-plane bending and
end effects influencing the measured tensile properties. Specimen geometries have therefore been
specified on the basis of minimising these effects, promoting failure away from the specimen ends (i.e.
type II and III) and ease of handling (i.e. machining and testing).
For the compression method, consideration has been given to the possibility of Euler buckling and end
effects influencing the measured compressive properties. Specimen geometries have therefore been
specified on the basis of minimising these effects, promoting failure away from the specimen ends (i.e.
type III) and ease of handling (i.e. machining and testing).
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INTERNATIONAL STANDARD ISO 20975-1:2023(E)
Fibre-reinforced plastic composites — Determination of
laminate through-thickness properties —
Part 1:
Direct tension and compression tests
1 Scope
This document specifies methods for determining the through-thickness properties (i.e. strength,
modulus of elasticity, Poisson’s ratio and strain-to-failure) of fibre-reinforced plastic composites using
either rectangular prism and/or waisted block specimens. The methods are suitable for use with a
variety of aligned and non-aligned, continuous, and discontinuous fibre formats, with both thermoset
and thermoplastic matrices, ranging from 20 mm to 40 mm in thickness.
Three specimen types are described in this document. These are:
— Type I - fixed rectangular cross-section along length of specimen. It is the preferred specimen for
determining elastic properties.
— Type II - waisted rectangular cross-section, variable cross-section along length of specimen. It
is only suitable for determining tensile strength values and is the preferred specimen for highly
anisotropic and thermoplastic materials.
— Type III - waisted rectangular cross-section, fixed cross-section along the gauge-length of specimen.
It is used to provide both elastic and strength property data and is the preferred specimen for
generating a full stress-strain response.
Specimen types I and II are also suitable for use with unreinforced plastics but are unsuitable for use
with rigid cellular materials and sandwich structures containing cellular materials.
Two testing modes are covered:
— Method A – Tension
— Method B – Compression
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 291, Plastics — Standard atmospheres for conditioning and testing
ISO 1268 (all parts), Fibre-reinforced plastics — Methods of producing test plates
ISO 2602, Statistical interpretation of test results — Estimation of the mean — Confidence interval
ISO 5893, Rubber and plastics test equipment — Tensile, flexural and compression types (constant rate of
traverse) — Specification
ISO 7500-1, Metallic materials — Calibration and verification of static uniaxial testing machines — Part 1:
Tension/compression testing machines — Calibration and verification of the force-measuring system
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ISO 20975-1:2023(E)
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
tensile stress
σ
33T
tensile force, carried by the test specimen at any particular moment, divided by the initial cross-
sectional area of the specimen at the specimen mid-length
Note 1 to entry: It is expressed in megapascals, MPa.
3.2
tensile failure stress or strength
σ
33TM
tensile stress (3.1) at the moment of failure, or when the load reaches a maximum value
Note 1 to entry: It is expressed in megapascals, MPa.
3.3
tensile failure strain
ε
33TM
through-thickness strain at the tensile failure stress or strength, σ (3.2)
33TM
Note 1 to entry: It is expressed as a dimensionless ratio or in percent, %.
3.4
modulus of elasticity (chord) in tension
E
33T
chord modulus obtained from the ratio of the stress difference (σ” minus σ') and the corresponding
strain difference (ε” = 0,002 5 minus ε' = 0,000 5)
Note 1 to entry: It is expressed in megapascals, MPa.
Note 2 to entry: A ratio of the stress difference (σ” minus σ') and the corresponding strain difference (ε” minus
ε') may be used if failure strain, ε , is less than 0,002 5. Where this is the case, ε”, and the corresponding value
33TM
of σ”, should be taken as the maximum values of strain and stress, respectively, in the linear region of the stress-
strain response.
3.5
compressive stress
σ
33C
compressive force, carried by the test specimen at any particular moment, divided by the initial cross-
sectional area of the specimen at the specimen mid-length
Note 1 to entry: It is expressed in megapascals, MPa.
3.6
compressive failure stress or strength
σ
33CM
compressive stress (3.5) at the moment of failure, or when the compressive load reaches a maximum
value
Note 1 to entry: It is expressed in megapascals, MPa.
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ISO 20975-1:2023(E)
3.7
compressive failure strain
ε
33CM
through-thickness strain at the compressive failure stress or strength, σ (3.6)
33CM
Note 1 to entry: It is expressed as a dimensionless ratio or in percent, %.
3.8
modulus of elasticity (chord) in compression
E
33C
chord modulus obtained from the ratio of the stress difference (σ” minus σ') and the corresponding
strain difference (ε” = 0,002 5 minus ε' = 0,000 5),
Note 1 to entry: It is expressed in megapascals, MPa.
Note 2 to entry: A ratio of the stress difference (σ” minus σ') and the corresponding strain difference (ε” minus
ε') may be used if failure strain, ε , is less than 0,002 5. Where this is the case, ε”, and the corresponding value
33CM
of σ”, should be taken as the maximum values of strain and stress, respectively, in the linear region of the stress-
strain response.
3.9
Poisson’s ratio
ν
negative ratio of the strain, ε , in one of the two axes normal to the direction of loading, to the
n
corresponding strain ε, in the direction of loading, within the initial linear portion of the longitudinal
versus normal strain curve
Note 1 to entry: It is expressed as a dimensionless ratio.
3.10
specimen coordinate axes
1, 2, 3
coordinate axes for the material with the fibres preferentially aligned in one direction
Note 1 to entry: The direction parallel with the fibre axes is defined as the “1” direction, the direction
perpendicular and in the same plane as the “2” direction, and the direction perpendicular to the 1-2 plane as the
“3” direction. For other materials, the “1” direction is normally defined in terms of a feature associated with the
production process, such as the long direction for continuous sheet processes. The “2” direction is perpendicular
to the “1” direction in the same plane, and the “3” direction is perpendicular to the 1-2 plane. Results for
specimens cut parallel with the “3” direction are identified by the subscript “33” (e.g. E ). See Figure 1.
33T
Note 2 to entry: The “1” direction is also referred to as the 0 degree (0°) or longitudinal directional, the “2”
direction as the 90 degree (90°) or transverse direction, and the “3” direction as the through-thickness or out-of-
plane direction.
Note 3 to entry: The specimen height corresponds to the through-thickness or out-of-plane direction dimension.
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ISO 20975-1:2023(E)
Figure 1 — Aligned fibre-reinforced composite plate element showing specimen coordinate
axes
4 Principle
Test specimens are loaded along their major axis at constant speed in tension or compression until
the specimen fractures [for type II (tension only) and III specimens], or for type I specimens until the
specified strain limit or load is reached. The load and strain (or displacement) are measured depending
on the method used.
In tension, the specimens are end-loaded via two metallic (aluminium or stainless steel) loading bars,
adhesively bonded to the end of the specimen.
In compression, specimens are end-loaded between two hardened steel parallel platens. Failure of the
specimens (type III only) occurs in local shear.
The procedures are performed using specimens, which can be either machined from flat areas of
products, machined from semi-finished products such as mouldings, laminates and extruded or cast
sheet, or made from test panels manufactured in accordance with ISO 1268.
The methods specify preferred dimensions for each type of specimen. Tests which are carried out on
specimens of other dimensions or on specimens which are prepared under different conditions can
produce results which are not comparable. Other factors, such as the speed of testing and the condition
of the specimens can influence results. Consequently, when comparative data are required, these
factors shall be carefully controlled and recorded.
5 Apparatus
5.1 Test machine
5.1.1 General
The test machine shall be in accordance with ISO 5893 as appropriate. The testing machine shall be
capable of maintaining the required speed of testing (see 9.6).
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ISO 20975-1:2023(E)
5.1.2 Indicators for load and strain
The error for the indicated load shall not exceed ±1 % of applied full load, according to ISO 7500-1, and
the error for the indicated strains shall not exceed ±2 %.
5.2 Strain gauges and strain acquisition
Strain shall only be determined by means of strain gauges. For maximum accuracy, the strain shall be
measured on all four faces of the specimen (types I and III only). The active length of the strain gauge
shall not be more than 2 mm. The gauges, surface preparation and bonding agents shall be chosen to
give adequate performance on the subject materials, and suitable strain-recording equipment shall be
employed. Strain-recording equipment should have 8 channels, of which 4 channels should be used to
measure the axial strain on 4 surfaces and 4 channels should be used to measure the transverse strain
on those surfaces.
5.3 Micrometer
Micrometer, or equivalent, reading to less than or equal to 0,01 mm, shall be used to determine the
cross-sectional dimensions of the specimen at the specimen mid-length.
The shape of the anvils shall be suitable for the surface being measured (i.e. flat faces for flat, and
hemispherical faces for irregular surfaces).
5.4 Loading fixtures
5.4.1 General
The loading arrangement shall load the specimen so that the requirement on allowable specimen
bending in 9.10 is achieved. For tensile tests the use of hydraulic grips is recommended. The fixture in
use shall be identified in accordance with Clause 12.
The main design points for all test methods and specimen types are alignment (initial and throughout
the test) and the prevention of failure at the end of the specimen.
[1]
NOTE The use of hydraulic grips has been shown to provide a means of achieving acceptable and
repeatable alignment (see 9.10).
5.4.2 Method A - Tension loading
The tensile loading bars shall be in accordance with, or similar to, that shown in Figure 2. Load is
applied direct to the end of the specimen via adhesively bonded loading bars. The loading bars shall
be manufactured from either aluminium or stainless steel and the contact surfaces shall be flat and
parallel to within ±0,01 mm.
Figure 2 — Schematic of Method A — Tension loading arrangement
5.4.3 Method B - Compression loading
Compression loading shall be between hardened surfaces on the test machine platens, or a die set (see
Figure 3). The contact surfaces shall be flat and parallel to within ±0,01 mm.
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ISO 20975-1:2023(E)
Key
1 die set
2 hardened and ground loading plate
Figure 3 — Schematic of Method B — Compression loading
6 Test specimens
6.1 Shape and dimensions
6.1.1 Type I specimens
The specimens shall be straight sided and of rectangular cross-section (see Figure 4) with the
dimensions given in Table 1.
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ISO 20975-1:2023(E)
Dimensions in millimetres
Figure 4 — Schematic of type I specimen
Table 1 — Dimensions for type I specimens
Dimensions in millimetres
Overall height, h Width, a Depth, b
40 ± 1 16 ± 0,1 16 ± 0,1
NOTE Requirements for specimen quality and parallelism of specimen are given in 6.2.2.
6.1.2 Type II specimens
The specimens shall have a circular profile extending along the entire gauge-length of the specimen
(see Figure 5). The radius of curvature, R, of the gauge-length shall be 30 mm. The cross-section along
the entire specimen length is square but varies in cross-sectional area. The dimensions are given in
Table 2.
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ISO 20975-1:2023(E)
Dimensions in millimetres
Figure 5 — Schematic of type II specimen
Table 2 — Dimensions for type II specimens
Dimensions in millimetres
Parameter Dimensions
Overall height, h 40 ± 1
Gauge-length, L 32 ± 0,25
Mid-section width, a 16 ± 0,1
Mid-section depth, b 16 ± 0,1
Base-section width 25 ± 0,1
Base-section depth 25 ± 0,1
NOTE The reduction in cross-sectional area promotes failure at the specimen mid-thickness and away from
the specimen ends.
6.1.3 Type III specimens
The specimens shall have a constant rectangular cross-section extending along the entire gauge-length
of the specimen (see Figure 6). The dimensions are given in Table 3.
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ISO 20975-1:2023(E)
Dimensions in millimetres
Figure 6 — Schematic of type III specimen
Table 3 — Dimensions for type III specimens
Dimensions in millimetres
Parameter Dimensions
Overall height, h 40 ± 1
Gauge-length,
...