ISO 20504:2006

INTERNATIONAL ISO
STANDARD 20504
First edition
2006-01-15


Fine ceramics (advanced ceramics,
advanced technical ceramics) — Test
method for compressive behaviour of
continuous fibre-reinforced composites
at room temperature
Céramiques techniques — Méthode d'essai de résistance à la
compression des composites renforcés de fibres continues à
température ambiante




Reference number
ISO 20504:2006(E)
©
ISO 2006

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ISO 20504:2006(E)
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ISO 20504:2006(E)
Contents Page
Foreword. iv
1 Scope. 1
2 Normative references. 1
3 Terms and definitions. 1
4 Principle. 3
5 Apparatus. 4
5.1 Test machine. 4
5.2 Load train. 4
5.3 Strain measurement. 4
5.3.1 General. 4
5.3.2 Strain gauges. 4
5.3.3 Extensometry. 5
5.4 Data recording system . 5
5.5 Dimension measuring devices . 5
6 Test specimens. 5
6.1 General. 5
6.2 Compression between platens. 6
6.3 Test specimen used with grips . 7
7 Test specimen preparation . 10
7.1 Machining and preparation. 10
7.2 Number of test specimens. 10
8 Test procedure. 10
8.1 Test mode and rate. 10
8.2 Measurement of test specimen dimensions . 11
8.3 Buckling. 11
8.4 Testing technique . 11
8.4.1 Test specimen mounting . 11
8.4.2 Extensometers. 11
8.4.3 Measurements. 12
8.5 Test validity. 12
9 Calculation of results . 12
9.1 Test specimen origin. 12
9.2 Compressive strength. 12
9.3 Strain at maximum compressive force. 13
9.4 Proportionality ratio or pseudo-elastic modulus, elastic modulus . 13
9.5 Buckling stress. 14
9.6 Rounding of results. 14
9.7 Mean and standard deviation . 14
10 Test report. 15
Annex A (informative) Illustration of elastic modulus . 16
Annex B (normative) Alignment verification . 18
Annex C (normative) Compressive force limits to ensure ‘true’ compressive failure. 20

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ISO 20504:2006(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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
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.
ISO 20504 was prepared by Technical Committee ISO/TC 206, Fine ceramics.

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INTERNATIONAL STANDARD ISO 20504:2006(E)

Fine ceramics (advanced ceramics, advanced technical
ceramics) — Test method for compressive behaviour of
continuous fibre-reinforced composites at room temperature
1 Scope
This International Standard describes procedures for determination of the compressive behaviour of ceramic
matrix composite materials with continuous fibre reinforcement at room temperature. This method applies to
all ceramic matrix composites with a continuous fibre reinforcement, uni-directional (1D), bi-directional (2D)
and tri-directional (xD, with 2 < x u 3), tested along one principal axis of reinforcement. This method may also
be applied to carbon-fibre-reinforced carbon matrix composites (also known as: carbon/carbon or C/C). Two
cases of testing are distinguished: compression between platens and compression using grips.
2 Normative references
The following referenced documents are indispensable for the application 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 — Verification of static uniaxial testing machines — Part 1:
Tension/compression testing machines — Verification and calibration of the force-measuring system
ISO 3611, Micrometer callipers for external measurements
ISO 9513, Metallic materials — Calibration of extensometers used in uniaxial testing
ISO 14126, Fibre-reinforced plastic composites — Determination of compressive properties in the in-plane
direction
ASTM E1012, Standard Practice for Verification of Test Frame and Specimen Alignment Under Tensile and
Compressive Axial Force Application
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
gauge section
part of the test specimen which has uniform and minimum cross-sectional area
3.2
gauge section length
l
length of the gauge section
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ISO 20504:2006(E)
3.3
initial gauge length
L
o
initial distance between reference points on the test specimen in the gauge section before initiation of the test
3.4
final gauge length
L
f
final distance between reference points on the test specimen in the gauge section at the completion of the test
3.5
initial cross-sectional area
A
o
initial area of the gauge section’s cross-section
3.6
longitudinal deformation
∆L
change (contraction) of the initial gauge due to the application of a uniaxial compressive force
NOTE The longitudinal deformation corresponding to the maximum force should be denoted as ∆L .
c,m
3.7
compressive strain
ε
relative change in the gauge length defined as the ratio ∆L/L
o
NOTE The compressive strain corresponding to the maximum force is denoted as ε .
c,m
3.8
compressive force
F
c
uniaxial compressive force applied to a test specimen
3.9
maximum compressive force
F
c,m
greatest uniaxial compressive force applied to a test specimen when tested to failure
3.10
compressive stress
σ
compressive force supported by the test specimen at any time in the test divided by the initial cross-sectional
area such that σ = F /A
c o
3.11
compressive strength
S
c,m
greatest compressive stress applied to a test specimen when tested to failure
3.12
proportionality ratio or pseudo-elastic modulus
E
p
slope of the linear region of the stress-strain curve, if any
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ISO 20504:2006(E)
NOTE Examination of the stress-strain curves for ceramic matrix composites allows definition of the following cases:
⎯ Material with a linear region in the stress-strain curve.
For ceramic matrix composites that have a mechanical behaviour characterised by a linear region, the proportionality
ratio E is defined as:
p
σ −σ
21
E σσ, = (1)
()
p1 2
ε −ε
21
where (ε , σ ) and (ε , σ ) lie near the lower and the upper limits of the linear region of the stress-strain curve (see
1 1 2 2
Figures A.1 and A.2).
⎯ Material with non-linear region in the stress-strain curve. In this case only, stress-strain couples can be determined at
specified stresses or specified strains.
3.13
elastic modulus
E
proportionality ratio or pseudo-elastic modulus, in the special case where the linearity starts near the origin
See Figure A.2.
3.14
axial strain
average of the longitudinal strain measured at the surface of the test specimen at specified locations
See Annex B.
3.15
bending strain
difference between the longitudinal strain at a given longitudinal location on the test specimen surface and the
axial strain at the same location
See Annex B.
3.16
buckling force
critical axially applied force at which an initially straight column assumes a curved shape
3.17
critical buckling stress
critical axial compressive stress at which an initially straight column assumes a curved shape
4 Principle
A test specimen of specified dimensions is loaded in compression. The compression test is usually performed
at a constant cross-head displacement rate or at a constant deformation rate.
NOTE Constant force rate is only allowed in the case of linear stress-strain behaviour up to failure.
For cross-head displacement tests, a constant rate is recommended when the test is conducted to failure.
The force and longitudinal deformation are measured and recorded simultaneously.
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ISO 20504:2006(E)
5 Apparatus
5.1 Test machine
The machine shall be equipped with a system for measuring the force applied to the test specimen that shall
conform to grade 1 or better in accordance with ISO 7500-1.
5.2 Load train
The load train is composed of movable and fixed cross-heads, the loading rods and the grips or platens. Load
train couplers may additionally be used to connect the grips or platens to the loading rods.
The load train shall align the test specimen axis with the direction of force application without introducing
bending or torsion in the test specimen. The misalignment of the test specimen shall be verified and
documented in accordance with the procedure described in Annex B. The maximum percent bending shall not
−6
exceed 5 % at an average axial strain of 500 × 10 .
There are two alternative means of force application:
a) Compression platens are connected to the force transducer and the moving cross-head. The parallelism
of these platens shall be better than 0,01 mm, in the loading area and the faces of the platens shall be
perpendicular to the force application direction.
NOTE 1  The use of platens is not recommended for compression testing of 1D and 2D materials with small
thicknesses because of buckling.
NOTE 2  A compliant interlayer material (composed only of paper or cardboard), between the test specimen and
platens, can be used for testing macroscopically inhomogeneous materials to ensure uniform contact pressure.
When the dimensions of the test specimen are such that buckling may occur, it is recommended to use
antibuckling devices similar to those described in ISO 14126. These devices should not introduce
parasitic stresses (i.e. stresses other than the uniform, axial stress) during loading of the test specimen.
b) Grips are used to clamp and load the test specimen. The grip design shall prevent the test specimen from
slipping and the grips shall align the test specimen axis with that of the applied force.
Alignment shall be verified and documented in accordance with, for example, the procedure described in
Annex B.
5.3 Strain measurement
5.3.1 General
For continuous measurement of the longitudinal deformation as a function of the applied force, either strain
gauges or a suitable extensometer may be used. Use an extensometer that meets the requirements of at least
class 1 in ISO 9513. Measurement of longitudinal deformation over a length as long as possible within the
gauge section length of the test specimen is recommended.
5.3.2 Strain gauges
Strain gauges are used for the verification of the alignment on the test specimen. They may also be used to
determine longitudinal deformation during testing. In both cases, the length of the strain gauges shall be such
that the readings are not affected by local features on the surface of the specimen, such as fibre crossovers.
Unless it can be shown that strain gauge readings are not unduly influenced by localized strain events, such
as fibre crossovers, strain gauges should be not less than 9 mm to 12 mm in length for the longitudinal
direction and not less than 6 mm in length for the transverse direction. The strain gauges, surface preparation
and bonding agents/adhesives should be chosen to provide adequate performance on the subject materials.
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ISO 20504:2006(E)
Suitable strain-conditioning and recording equipment should be used. Care shall be taken to ensure that the
strain gauge readings are not influenced by the surface preparation and the adhesive used.
5.3.3 Extensometry
The linearity tolerance of the extensometer shall be less than 0,15 % of the extensometer range used.
Extensometers shall meet the requirements of at least class 1 in accordance with ISO 9513.
Types of commonly used extensometers are described in 5.3.3.1 and 5.3.3.2.
5.3.3.1 Mechanical extensometer
For a mechanical extensometer, the gauge length corresponds to the longitudinal distance between the two
locations where the extensometer contacts the test specimen. Mounting of the extensometer to the test
specimen shall prevent slippage of the extensometer at the contact points and shall not initiate failure under
the contact points. Any extensometer contact forces shall not introduce bending greater than that allowed
in 5.2.
5.3.3.2 Electro-optical extensometer
Electro-optical measurements of strain require reference marks on the test specimen. For this purpose,
fiducial marks such as rods or flags are attached to the test specimen surface perpendicular to the longitudinal
axis of the test specimen. The gauge length corresponds to the longitudinal distance between the two fiducial
marks.
NOTE The use of integral flags as part of the test specimen geometry is not recommended, because of stress
concentrations induced by such features.
5.4 Data recording system
A calibrated recorder may be used to record force-deformation curves. The use of a digital data recording
system combined with an analog recorder is recommended.
5.5 Dimension measuring devices
Devices used for measuring linear dimensions of the test specimen shall be accurate to ± 0,1 mm.
Micrometers shall be in accordance with ISO 3611.
6 Test specimens
6.1 General
The choice of test specimen geometry depends on several parameters:
⎯ the nature of the material and of the reinforcement structure;
⎯ the type of testing system.
The ratio between the length of the test specimen subject to buckling and the thickness of the test specimen,
in addition to the stiffness of the material, will influence the resistance of the test specimen to buckling.
If buckling occurs, it may be necessary to modify the dimensions of the test specimen or alternatively to use
an antibuckling device (e.g. fixed lateral guides pressed against the test specimen so as to freely allow
longitudinal motion while simultaneously suppressing transverse motion).
The volume in the gauge length shall be representative of the material.
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ISO 20504:2006(E)
Two types of test specimens can be distinguished.
a) As-fabricated test specimens, where only the length and the width are machined to the specified size. In
this case, the two faces of the test specimen may present irregular surfaces while the two edges present
regular machined surfaces.
b) Machined test specimens, where the length and the width, as well as the two faces of the test specimen,
have been machined and present regular machined surfaces.
Tolerance on the thickness dimension only applies to machined test specimens. For as-fabricated test
specimens, the difference in thickness out of three measurements (at the centre and at each end of the gauge
section length) should not exceed 5 % of the average of the three measurements.
6.2 Compression between platens
The test specimen geometry and/or compliant interlayers may be adapted in order to avoid buckling and
damage at the edges due to contact forces.
Type 1 is commonly used and is illustrated in Figure 1. Recommended dimensions are given in Table 1.

a)  Test specimen b)  Example of anti-buckling guides
Key
1 loading anvil 4 O-ring
2 specimen 5 frame
3 lateral support 6 unsupported length
Figure 1 — Compression test specimen (type 1) used between platens and anti-buckling guides
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ISO 20504:2006(E)
Table 1 — Dimensions for compression test specimen (type 1) used between platens
Dimensions in millimetres
Parameter 1D, 2D, xD Tolerance
l, gauge section length W 15 ± 0,5
l , total length W 1,5 × l mm ± 0,5
t
d, cylindrical or square-section side length or diameter W 8 ± 0,2
Parallelism of machined parts 0,05 N/A
Perpendicularity of machined parts 0,05 N/A
Concentricity of machined parts 0,05 N/A
Type 2 is cylindrical in shape and is not used as frequently as type 1. It is illustrated in Figure 2 and
recommended dimensions are given in Table 2.

Figure 2 — Compression test specimen (type 2) used between platens
Table 2 — Dimensions for compression test specimen (type 2) used between platens
Dimensions in millimetres
Parameter 1D, 2D, xD Tolerance
l, gauge section length W 10 ± 0,5
d, cylindrical or square-section W 10 ± 0,2
Parallelism of machined parts 0,05 N/A
Perpendicularity of machined parts 0,05 N/A
NOTE This test specimen is mainly used when the thickness of the part is not sufficient to machine a test specimen of
type 1.
6.3 Test specimen used with grips
For these types of test specimens, the total length l depends on the gripping system. These types of test
t
specimens allow testing of thin test specimens without using an anti-buckling device. It is, however, necessary
to verify that the chosen l/h ratio does not lead to buckling.
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ISO 20504:2006(E)
Type 3 is represented in Figure 3 and recommended dimensions are given in Table 3.

Figure 3 — Compression test specimen (type 3) for use with grips
Table 3 — Dimensions of compression test specimen (type 3) for use with grips
Dimensions in millimetres
Parameter 1D, 2D, xD Tolerance
Long length
l , total test specimen length W 1,5 × l mm ± 0,5
t
l, gauge section length W 15 ± 0,5
h, thickness W 3 ± 0,2
b , width in the gauge section length W 8 ± 0,2
1
b , width b = αb with α: 1,2 to 2 ± 0,2
2 2 1
r, radius W 30 ± 2
Plane parallelism of machined parts 0,05 N/A
Short length
l , total test specimen length W 1,5 × l mm ± 0,5
t
l, gauge section length u 15 ± 0,5
h, thickness W 3 ± 0,2
b , width in the gauge section length W 8 ± 0,2
1
b , width b = αb with α: 1,2 to 2 ± 0,2
2 2 1
r, radius W 30 ± 2
Plane parallelism of machined parts 0,05 N/A
NOTE This type of test specimen is recommended, if buckling occurs, using type 3 test specimen dimensions given in this
table (for long lengths). With this type of test specimen, it is very difficult to obtain strain measurements.

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ISO 20504:2006(E)
Type 4 is a straight-sided test specimen. It is represented in Figure 4 and the dimensions are given in Table 4.

Figure 4 — Compression test specimen for use with grips (straight-sided, without tabs)
Table 4 — Dimensions of compression test specimen (type 4) for use with grips
(straight-sided, without tabs)
Dimensions in millimetres

Parameter 1D, 2D, xD Tolerance
l , total length W 100 ± 0,5
t
l, gauge section length (ungripped) W 40 ± 0,2
h, thickness W 3 ± 0,2
b, width W 10 ± 0,2
Plane parallelism of machined part 0,05 N/A
NOTE This test specimen is easy to machine and, although its use allows the determination of elastic modulus; it should
not be used for strength measurement.
Type 5 is a straight-sided test specimen equipped with end tabs that are either metallic or polymeric
composite, and bonded or cured onto the test specimen. The dimensions are given in Table 5 and the test
specimen is illustrated in Figure 5. This type of test specimen is mainly used for 1D, 2D and xD (with 2 < x < 3)
materials.
Table 5 — Dimensions of compression test specimen (type 5) for use with grips
(straight-sided with end tabs)
Dimensions in millimetres

Parameter 1D, 2D, xD Tolerance
l , total length W 100 ± 0,5
t
l, gauge section length W 40 ± 0,2
End tab length W 30 ± 0,2
h, thickness W 3 ± 0,2
b, width W 10 ± 0,2
Plane parallelism of machined parts and of tab faces 0,05 N/A
NOTE The thickness of the end tabs is generally between 1 mm and 3 mm.
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ISO 20504:2006(E)

a
End-tab length.
Figure 5 — Straight-sided test specimen with end tabs
Type 6 test specimens are used for elevated temperature tests. All test specimens designed for elevated
temperature testing can be used for room temperature testing.
NOTE It is customary to obtain results at room temperature when testing a material at elevated temperature and, to
do so, the same type of test specimen is used. However, the costs of test specimens for elevated temperature tests are
generally much greater than test specimens used for room temperature tests. Therefore, elevated-temperature test
specimen types are not used when only room-temperature properties are required.
7 Test specimen preparation
7.1 Machining and preparation
When extracting test specimens from as-fabricated plates of material, care shall be taken to align the test
specimen axis with the desired fibre-related loading axis.
Machining parameters that avoid damage to the material shall be established and documented. These
parameters shall be adhered to during test specimen preparation.
7.2 Number of test specimens
At least five valid test results (see 8.5) are required.
If statistical evaluation of the test results is required, the number of test specimens should be chosen using
accepted statistical procedures and guidelines.
8 Test procedure
8.1 Test mode and rate
Test modes may involve force, displacement, or strain control. Test rates should be sufficiently rapid so as to
complete the test in the range 10 s to 30 s, thereby obtaining the maximum possible compressive strength at
fracture of the material. However, test rates may also be used to evaluate rate effects. In all cases, report the
test mode and rate.
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ISO 20504:2006(E)
−6 −6 −1
Strain rates on the order of 50 × 10 to 500 × 10 s , stress rates on the order of 35-50 MPa/s, and cross-
head displacement rates on the order of 0,001 to 0,05 mm/s are recommended to minimize environmental
effects when testing in ambient air.
8.2 Measurement of test specimen dimensions
The cross-sectional area shall be determined at the longitudinal centre of the test specimen and at each end
of the gauge section length. The arithmetic means of the measurements shall be used for calculations.
Necessary dimensions to calculate cross-sectional area shall be measured with an accuracy of ± 0,01 mm.
8.3 Buckling
During a compression test, the test specimen may be susceptible to
...