ISO 17138:2025
(Main)Fine ceramics (advanced ceramics, advanced technical ceramics) — Mechanical properties of ceramic composites at room temperature — Determination of flexural strength
Fine ceramics (advanced ceramics, advanced technical ceramics) — Mechanical properties of ceramic composites at room temperature — Determination of flexural strength
This document specifies a test method for the determination of the flexural strength of ceramic matrix composite materials with continuous fibre reinforcement, under three-point or four-point bend at room temperature. This document is applicable to all ceramic matrix composites with a continuous fibre reinforcement, unidirectional (1D), bidirectional (2D), and tridirectional xD with (2 x ≤ 3) as defined in ISO 19634, loaded along one principal axis of reinforcement.
Céramiques techniques — Propriétés mécaniques des composites céramiques à température ambiante — Détermination de la résistance en flexion
L'ISO 17138:2014 décrit une méthode permettant de déterminer la résistance en flexion des matériaux composites à matrice céramique et à renforts de fibres continus par un essai de flexion en trois ou quatre points à température ambiante. Cette méthode s'applique à tous les composites à matrice céramique à renforts de fibres continus, unidirectionnels (1D), renforcés dans le plan (2D) et multidirectionnels xD avec (2
General Information
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Standards Content (Sample)
International
Standard
ISO 17138
Second edition
Fine ceramics (advanced ceramics,
2025-11
advanced technical ceramics) —
Mechanical properties of ceramic
composites at room temperature —
Determination of flexural strength
Céramiques techniques — Propriétés mécaniques des composites
céramiques à température ambiante — Détermination de la
résistance en flexion
Reference number
© ISO 2025
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Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Significance and use . 2
5 Principle . 2
6 Apparatus . 2
6.1 Test machine .2
6.2 Test fixture .2
6.3 Data recording system .3
6.4 Dimension measuring devices .4
7 Test specimens . 4
8 Test specimen preparation . 4
8.1 Machining and preparation .4
8.2 Number of test specimens .5
9 Test procedures . 5
9.1 Displacement rate .5
9.2 Measurement of dimensions .5
9.2.1 Test specimen dimensions .5
9.2.2 Distances between supporting rollers.5
9.3 Testing technique .5
9.3.1 Test specimen mounting .5
9.3.2 Measurements .6
9.4 Test validity .6
9.5 Calculation of results .6
9.5.1 General .6
9.5.2 Three-point bend .6
9.5.3 Four-point bend .7
10 Test report . 7
Bibliography . 8
iii
Foreword
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This document was prepared by ISO/TC 206, Fine ceramics, in collaboration with the European Committee
for Standardization (CEN) Technical Committee CEN/TC 184, Advanced technical ceramics, in accordance
with the Agreement on technical cooperation between ISO and CEN (Vienna Agreement).
This second edition cancels and replaces the first edition (ISO 17138:2014) which has been technically
revised.
The main changes are as follows:
— Clause 2 updated;
— Clause 4 on Significance and use added;
— Clause 10 updated;
— editorially revised.
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.
iv
International Standard ISO 17138:2025(en)
Fine ceramics (advanced ceramics, advanced technical
ceramics) — Mechanical properties of ceramic composites at
room temperature — Determination of flexural strength
1 Scope
This document specifies a test method for the determination of the flexural strength of ceramic matrix
composite materials with continuous fibre reinforcement, under three-point or four-point bend at room
temperature. This document is applicable to all ceramic matrix composites with a continuous fibre
reinforcement, unidirectional (1D), bidirectional (2D), and tridirectional xD with (2 < x ≤ 3) as defined in
ISO 19634, loaded along one principal axis of reinforcement.
This document is not intended to be used to obtain absolute values of strength for design purposes.
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 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
ISO 3611, Geometrical product specifications (GPS) — Dimensional measuring equipment — Design and
metrological characteristics of micrometers for external measurements
ISO 19634, Advanced technical ceramics — Ceramic composites — Notations and symbols
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 19634 and the following 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
maximum flexural force
F
m
highest recorded force during a flexural test on the test specimen when tested to failure
3.2
flexural stress
σ
nominal stress on the outer surface of the test specimen, calculated at mid-span
Note 1 to entry: This stress is conventionally calculated according to the beam theory, whose basic assumptions
cannot be met by ceramic matrix composite materials.
3.3
flexural strength
σ
f,m
maximum flexural stress (3.2) or force applied to a test specimen that fractures during a flexural test
4 Significance and use
Flexural tests are used for strength estimation for linear elastic brittle materials, for which tensile testing
presents practical difficulties. They are never used for metallic materials that exhibit plasticity. On materials
that exhibit damage induced non-linear deformations like CMCs, the use of bending tests for strength
estimation is highly questionable, since the equations of elastic beam theory do not apply in the presence
of non-linear deformations. Furthermore, in some cases uncommon facture modes are observed: fracture
initiates from the part of specimens under compressive stresses, or the crack propagates parallel to the
specimen axis. Despite the presence of damage that is not accounted for by the theory, many authors rely
on bending tests and Formulae (1) and (2) of elastic beam theory to evaluate the ceramic matrix composites
they are developing or using.
[1]
The elastic beam theory is based on the following main assumptions :
a) The beam is symmetrical about neutral plane.
b) The beam is a straight bar of homogeneous material.
c) The transverse plane sections remain plane and normal to the longitudinal fibres after bending
(Bernoulli’s assumption).
d) The fixed relationship between stress and strain (Young’s modulus) for the beam material is the same
for tension and compression.
Requirements a), b) and d) are not met on fibre reinforced ceramic matrix composites that exhibit non-linear
deformations associated to damage induced by the tensile stresses. As damage by matrix and interface
cracking proceeds, the composite Young’s modulus decreases in the tensile stressed vol
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