ISO 14624-1:2023
(Main)Space systems — Safety and compatibility of materials — Part 1: Determination of upward flammability of materials
Space systems — Safety and compatibility of materials — Part 1: Determination of upward flammability of materials
This document specifies a method for the determination of the flammability of aerospace materials by upward flame propagation. This test determines if a material, when exposed to a standard ignition source, will self-extinguish and not transfer burning debris which can ignite adjacent materials.
Systèmes spatiaux — Sécurité et compatibilité des matériaux — Partie 1: Détermination de l'inflammabilité verticale des matériaux
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INTERNATIONAL ISO
STANDARD 14624-1
Second edition
2023-10
Space systems — Safety and
compatibility of materials —
Part 1:
Determination of upward flammability
of materials
Systèmes spatiaux — Sécurité et compatibilité des matériaux —
Partie 1: Détermination de l'inflammabilité verticale des matériaux
Reference number
© ISO 2023
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Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 4
5 Reagents . 4
6 Test system .4
7 Test specimens . 7
7.1 Materials . 7
7.2 Reception and inspection of material . 8
7.3 Preparation of test specimens . 8
8 Procedure .9
8.1 General . 9
8.2 Before testing . 9
8.3 Test . 9
8.3.1 Before ignition of chemical ignitor . 9
8.3.2 After ignition of chemical ignitor . 10
8.4 After testing . 10
9 Accuracy .10
10 Test report .10
10.1 Standard tests . 10
10.2 Non-standard tests . 10
10.3 Good laboratory practice (GLP) . 11
Annex A (informative) Preparation and qualification of chemical ignitors .12
Annex B (informative) Method to determine the flammability thresholds of materials .18
Annex C (informative) Alternate determination with increased precision of upward
limiting oxygen index (ULOI) .21
Annex D (informative) Data sheet for upward flame-propagation oxygen-concentration
flammability threshold test .23
Bibliography .25
iii
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 document 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).
ISO draws attention to the possibility that the implementation of this document may involve the use
of (a) patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed
patent rights in respect thereof. As of the date of publication of this document, ISO had not received
notice of (a) patent(s) which may be required to implement this document. However, implementers are
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expressions related to conformity assessment, as well as information about ISO's adherence to
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www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 20, Aircraft and space vehicles,
Subcommittee SC 14, Space systems and operations.
This second edition cancels and replaces the first edition (ISO 14624-1:2003), which has been technically
revised.
The main changes are as follows:
— updated 6.5 "Bare nickel-chromium wire 0,81 mm (0,5 mm diameter)";
— updated annexes;
— updated the Bibliography.
A list of all parts in the ISO 14624 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.
iv
Introduction
In this document, the following verbal forms are used:
— “shall” indicates a requirement;
— “should” indicates a recommendation;
— “may” indicates a permission;
— “can” indicates a possibility or a capability.
Recommended criteria are, while not mandatory, considered to be of primary importance in providing
serviceable economical and practical designs. Deviations from the recommended criteria may be made
only after careful consideration, extensive testing and thorough service evaluation have shown an
alternative method to be satisfactory.
v
INTERNATIONAL STANDARD ISO 14624-1:2023(E)
Space systems — Safety and compatibility of materials —
Part 1:
Determination of upward flammability of materials
1 Scope
This document specifies a method for the determination of the flammability of aerospace materials
by upward flame propagation. This test determines if a material, when exposed to a standard ignition
source, will self-extinguish and not transfer burning debris which can ignite adjacent materials.
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
ambient conditions
test environment with an oxygen concentration of 20,9 % ± 0,2 %, a pressure of 101,4 kPa ± 5 kPa, and
a temperature of 23 °C ± 5 °C
3.2
burn length
distance from the bottom of the specimen to the farthest evidence of flame consumption damage or
flame attachment point to the test specimen due to flame as determined by visual observation, visual/
physical post-test examination, video of burn, and/or other means
Note 1 to entry: See Table 1 and Figure 1. The burn length distance includes areas of partial or complete
combustion, charring or embrittlement, but does not include heat affected areas which can have been damaged
only by the heat of the flames emanating from sample consumption below. Heat affected areas appear as sooted,
stained, warped or discoloured, or areas where the material has shrunk or melted away from the heat. In some
cases, the flame can propagate beyond the farthest visual evidence of damage to the test specimen. Typical
material burn model depicted in Figure 1, some materials can show multiple flame fronts and/or other variations.
Table 1 — Burn length assessment characteristics and techniques summary
Burn length / Burn length
Heat affected damage area
Flame consumption damage determination techniques
Complete combustion Sooted Visual observation
Partial combustion Stained Visual post-test examination
Charring Warped Physical post-test examination
Embrittlement Melted away from heat Video of burn
Furthest flame attachment point Shrunk away from heat Other means
Key
1 heat affected damaged area
2 flame consumption damaged area
3 ignitor influence area
A front
B back
C cross section of sample
Figure 1 — Typical material burn depiction, typical sample consumed area and heat affected
damage area
3.3
burn propagation time
time that elapses from ignition of the specimen until vertical flame propagation stops
3.4
chemical ignitor
ignitor validated to strict specifications in order to produce a flame that meets all the requirements for
an ignition source
Note 1 to entry: The requirements for the ignition source are specified in 6.3. See Annex A for one method of
producing a chemical ignitor.
3.5
good laboratory practice
GLP
practice which involves the testing of standard reference materials to verify data accuracy and
repeatability
Note 1 to entry: In addition, the test facility shall successfully demonstrate the ability to obtain repeatable data
when testing a selected material. The authority having jurisdiction shall choose appropriate GLP materials and
shall determine the frequency of testing these materials for its test facilities.
3.6
maximum allowable oxygen concentration
MOC
highest oxygen concentration (volume fraction), in an oxygen/nitrogen atmosphere, at which at least
five specimens pass the acceptance criteria of this test, without a failure, and one or more of the
specimens fail the test if the oxygen concentration is increased by an arbitrary increment, typically 1 %
Note 1 to entry: See Annexes B, C, D and ISO/TS 16697 for the threshold approximation procedure.
3.7
self-extinguish
phenomenon in which the burn length (3.2) on a standard test specimen (3.8) does not exceed 150 mm
3.8
standard test specimen
representative part, taken from a quantity of material or fabricated per required preparation method,
meeting the following minimum dimensions:
a) length = 300 mm
b) width = 60 mm [75 mm for thin-film specimen (3.9)]
c) thickness – use thickness (minimum thickness of the material in its intended use application, but
not to exceed 25 mm, excluding substrate)
Note 1 to entry: The required test specimen exposed width is defined in 6.6.
3.9
thin-film specimen
specimen with a total thickness of less than 0,25 mm
Note 1 to entry: Fabrics or coatings applied to a substrate are not considered thin-film specimens.
3.10
transfer of burning debris
movement of any material from a burning specimen with sufficient energy to ignite adjacent material
Note 1 to entry: The sheet of paper below the test specimen is specified in 6.8.
3.11
upward limiting oxygen index
ULOI
oxygen concentration where approximately 50 % of samples fail the test criteria
3.12
worst-case test configuration
test configuration that simulates worst-case anticipated use conditions including material thickness,
test pressure, and oxygen concentration
Note 1 to entry: Worst-case represents the cumulative effect of multiple factors that increase the probability
that a material is flammable; can involve smallest thickness for use without a substrate or sufficient thickness
to prevent heat loss from a substrate, increased surface area (such as mesh configuration), irregular surface
configuration, increased oxygen concentration, increased pressure, increased temperature, etc. Furthermore,
oxygen concentration (volume fraction) is typically the primary driver of flammability and should be prioritized
over pressure when examining for worst-case conditions.
3.13
worst-case use thickness
material thickness that, for a specific application, makes the material most flammable
Note 1 to entry: Worst-case use thickness can involve the smallest thickness for use without a substrate or
sufficient thickness to prevent heat loss from a substrate and increased surface area (such as mesh configuration),
irregular surface configuration.
4 Principle
An ignition source with specific characteristics is applied for a defined period of time to the lower end
of a standard test specimen of material oriented vertically in a test chamber or fume hood containing
a specific test environment. The post-test burn lengths for at least five standard-sized specimens
are recorded. Materials are considered flammable in the test conditions if at least one standard test
specimen burns more than 150 mm. In addition, the ignited specimens shall not ignite the paper
(produce combustion) below the test samples, which indicates that the transfer of burning debris
has sufficient energy to ignite adjacent materials. If, during a test, the paper used as an indication of
the transfer of burning debris ignites because of burning debris, subsequent burns during the same
material test should be conducted without paper to eliminate burning paper interference.
Failure of any one specimen constitutes failure of the material in that test environment.
Materials shall be tested in the worst-case test configuration. If the worst-case oxygen concentration is
uncertain, determination of the maximum allowable oxygen concentration is recommended.
5 Reagents
5.1 Test gases. The test atmosphere shall consist of a mixture of oxygen and nitrogen, mixed
thoroughly before testing a specimen. These gases can be premixed before introduction of the mixture
into the test chamber, or the oxygen and nitrogen can be introduced separately into the test chamber,
and then mixed inside the test chamber with a test specimen.
Oxygen gases used in test gas mixtures shall be verified to have a minimum purity of 99,5 % and
1)
moisture < 7 µl/l . Nitrogen gases used in test gas mixtures shall be verified to have a minimum purity
of 99,9 % and moisture <11,5 µl/l.
Also, the gas mixture shall be verified for conformity with the specification (including accuracy) for
oxygen concentration to within +1 % −0 %. Pre and post-test gases are analysed for CO and CO and
post test results reported.
6 Test system
6.1 Test chamber, large enough so that complete combustion of the specimen can occur with no
more than a 5 % relative depletion of oxygen concentration. In addition, the test chamber shall not
interfere chemically or physically with the test.
6.2 Measuring devices, properly calibrated.
6.3 Chemical ignition source, meeting the following specifications in ambient conditions:
a) temperature: 1 100 °C ± 90 °C measured using a 0,81 mm (0,5 mm diameter)
exposed tip type K thermocouple;
b) burning duration: 25 s ± 5 s;
c) maximum visible flame height: 65 mm ± 6,5 mm.
1) 1 µl/l = 1 part per million (ppm). The use of “ppm” is deprecated.
Annex A provides a procedure for preparing, certifying and storing chemical ignitors.
NOTE This test method and the corresponding ignition source are designed to evaluate materials for use
in spacecraft habitable environments with typical oxygen concentration ranges (19 % to 50 % volume fraction
of oxygen). When evaluating environments outside of these typical ranges the standard ignition source cannot
be adequate. In these cases, alternative ignition mechanisms can be utilized but test data is reported as a non
ISO 14624-1 standard.
6.4 Power supply, capable of providing 15 A (root mean square), connected to a bare 0,81 mm
diameter nickel-chromium wire (6.5) to initiate the igniter.
6.5 Bare nickel-chromium wire 0,81 mm (0,5 mm diameter), with a nominal resistivity of 2 Ω⋅m
to 2,5 Ω⋅m,3 m and of sufficient length to wrap three equally spaced turns around the chemical igniter.
6.6 Suitable specimen holder, capable of supporting the specimen in the vertical position
6.6.1 Standard test specimen holder (see Figure 2), allowing a minimum of 50 mm of the width
of the specimen to be exposed and extending over the full length of the specimen. The test specimen
exposed width for materials other than thin-film specimens shall be 50 mm to 60 mm.
The bottom of the specimen holder shall be located at least 250 mm from the bottom of the test chamber.
6.6.2 Specimen holder for thin-film specimens, allowing at least 50 mm of the width of the
specimen to be exposed and minimizing shrinkage of the test material away from the flame. The test
specimen exposed width for thin films shall be between 75 mm to 85 mm.
NOTE When mounting thin films for testing, minimized tension across the material reduces test stand
influence. When the influence of the test stand on the results is of concern, large thin films measuring (300 mm x
200 mm × the use thickness) can optionally be used, subject to the approval of the authority having jurisdiction.
Two types of specimen holder can be used:
a) a holder for a standard test specimen (see 6.6.1 and Figure 2), which employs three wing-nut
clamps or one long clamp;
b) a holder employing needle rakes to hold thin film specimens (see Figure 3).
6.7 Scale, attached to one side of the specimen holder, for measurement of the burn length.
6.8 Sheet of paper, mounted horizontally approximately 200 mm below the specimen holder, but
50 mm above the bottom of the test chamber, centred directly below the specimen. The paper shall
be supported by a non-flammable, non-conducting material. The paper shall have the following
characteristics:
a) dimensions: (200 mm ± 50 mm) × (300 mm ± 50 mm);
2 2
b) surface density: between 200 g/m and 300 g/m ;
c) type: chemical wood processed;
d) colour: uniformly white;
e) condition: clean, free from dirt spots, oil spots and foreign matter (lint,
fuzz, etc.), free from holes, tears, cuts, folds and scuff marks, and
containing no splices.
The sheet of paper is used to assess if burning debris from the specimen can cause ignition of adjacent
materials.
Key
1 specimen
2 scale
3 ignitor
4 nickel-chrome wire
a
The ignitor is placed inside the coil.
Figure 2 — Standard test specimen holder
Key
1 specimen
2 scale
3 ignitor
4 needle
Figure 3 — Needle rake mount for thin-film specimens
7 Test specimens
7.1 Materials
The minimum quantities and dimensions of materials required to perform each test properly are
summarized in Table 2. Actual test configurations and material quantities for material forms other
than those listed (e.g. O-rings and other seals) shall be established and approved by the responsible
procuring activity/user materials organization.
Table 2 — Minimum quantities of materials required for testing for each atmosphere
Form of material Minimum quantity
Sheets 6 specimens measuring 300 mm × 65 mm × the required thickness
Thin films 6 specimens measuring 300 mm × 75 mm × the required thickness
When the influence of the test stand on the results is of concern, large thin films meas-
uring 300 mm × 200 mm × the required thickness can optionally be used, subject to the
approval of the authority having jurisdiction.
Coatings Sufficient to cover, at the required thickness, 6 substrates measuring 300 mm × 65 mm
Foams 6 specimens measuring 300 mm × 65 mm × the required thickness
Insulated wires 2 m in length
As a minimum, all materials used in testing shall meet or exceed user specifications.
Material and configured-system characteristics can be significantly compromised by sources of
contamination, such as exposure to solvents, cleaning agents, abnormal temperatures, variations in
humidity, environmental pollutants, particulates and handling. It is important that exposure of test
material(s) to these and other contamination sources be sufficiently controlled to minimize variation in
test results.
7.2 Reception and inspection of material
7.2.1 Receive and visually inspect the test material: when received, it shall be accompanied by proper
identification and its thickness shall correspond to the specified worst-case use thickness. Any flaws
shall be noted. Specimens should have been cleaned and dried to the end-use specifications prior to
receipt at the test facility.
7.2.2 If required, prepare specimens to the proper dimensions, including worst-case use thickness.
7.2.3 If specimens are received with obvious contamination, clean them. All cleaning methods shall
be approved by the test requester prior to use. Surface contamination should be removed by washing
with deionized water and mild detergent, rinsing with deionized water, and drying with filtered
nitrogen gas. As a minimum, particulates on the surfaces of solid porous specimens should be removed
with filtered nitrogen gas.
7.2.4 After preparation and/or cleaning at the test facility, inspect the specimens to ensure that they
are at the specified worst-case use thickness. Any flaws and any residual contamination shall be noted.
If the flaws result from specimen preparation at the test facility, new specimens shall be prepared.
Specimens shall be weighed and individually identified.
7.3 Preparation of test specimens
7.3.1 Sheet and other bulk-form specimens shall be cut to 300 mm × 65 mm in the worst-case use
thickness. Materials, configurations and components that cannot be prepared to these requirements
shall be tested in the configuration as purchased. Non-standard test specimens shall be mounted in the
chamber in a manner that does not inhibit flame propagation.
Test specimens prepared from bulk materials shall be prepared in a manner that is representative of
the material’s end usage configuration. Inherent variability in non-homogenous bulk materials should
be considered when determining test specimen preparation methods.
7.3.2 Materials that require a cure shall be prepared in the worst-case use thickness and cured
according to the requester's instructions. The specimens shall be cut to 300 mm × 65 mm.
7.3.3 Coatings or any viscous material that cannot be mounted for testing without a substrate shall
be applied to the end-use substrate material in the worst-case use thickness. Aluminium substrates
approximately 0,5 mm thick can be used if the end-use substrate is unknown or inappropriate. Any
requested cures shall be performed. The specimens shall measure 300 mm × 65 mm. Coatings, films or
adhesive backed tapes proposed for use on non-metallic surfaces shall be applied to the proposed non-
metallic surface, in the worst-case use thickness.
7.3.4 Non-adhesive-backed hook-and-loop tapes shall be tested in the “as received” condition, unless
otherwise noted by the requester. Adhesive-backed hook-and-loop tapes shall be applied to a 300 mm ×
65 mm × 0,5 mm aluminium substrate. To ensure ignition of the tape backing, as opposed to the hooks
or loops only, approximately 13 mm of the test specimen shall extend beyond the substrate.
7.3.5 Semi-solid materials shall be applied to a suitable non-combustible substrate such as glass-fibre
cloth. Semi-solids shall be applied to the substrate as a uniform coat.
7.3.6 Liquid materials shall be tested using an appropriate standard test for determining the flash
point of liquids.
7.3.7 Thin-film specimens shall be cut to 300 mm × 75 mm. Optionally, large thin films measuring
300 mm × 200 mm can be used, subject to the approval of the authority having jurisdiction, to reduce
material shrinkage away from the flame.
8 Procedure
8.1 General
This test method shall be performed per the procedure specified in 8.2 to 8.4.
WARNING — Burning of materials can produce smoke and toxic gases and residues, which can
affect the health of operators. Clear test area of smoke and fumes by suitable means. Ensure
appropriate safety precautions are taken at all times.
8.2 Before testing
Before testing, observe the vertical surfaces of the test specimens and record any pertinent observations,
such as imperfections, discolorations. Also record all other pertinent information (including oxygen
concentration, specimen identification, and specimen pre-test mass and size). Photograph a typical
pretest.
Maintain and verify the test system to be visibly clean, and all measuring devices to be in proper
calibration.
The exposed centre section of standard-sized specimens shall be 50 mm wide. Do not overly stretched
or tighten specimens, which would cause lines of horizontal stress. Mount thin films with 10 mm of
slack in the width to allow for shrinkage.
Place the top point of the ignitor surface 6 mm ± 1 mm below the lower edge of the specimen parallel to
the lower edge of the specimen and centred along the plane of the front surface of the specimen. Take
care when testing materials on substrates to ensure that the ignitor is centred along the plane of the
front surface of the specimen and not that of the substrate.
Position a sheet of paper centred 200 mm below the test specimen. Hold paper in place by a non-
flammable, non-conducting screen material.
Video record all specimens during testing.
8.3 Test
8.3.1 Before ignition of chemical ignitor
Test five replicate test specimens.
Subject all specimens to vacuum for no less than 1 min, but no more than 3 min before introduction of
the proper test atmosphere.
If vacuum exposure damages the test material, purging of the test chamber or gas supplementation can
be used to achieve the required test atmosphere.
Expose specimens to the proper test atmosphere for a minimum of 3 min before ignition.
Prior to ignition, ensure that a homogenous mixture is achieved within the chamber.
Measure, verify and record the percentage oxygen concentration and the total pressure for each
individual specimen.
Activate the recording device prior to activating the chemical ignitor. Activate the chemical ignitor.
8.3.2 After ignition of chemical ignitor
Immediately upon ignition of the chemical ignitor, turn off the power to the nickel-chromium wire.
Activate the timing device used to measure the burn propagation time once specimen ignition is
observed.
Retract ignitor from the test specimen once the ignitor extinguishes.
Observe the vertical surfaces of the specimen and record any pertinent observations. Record any flame
jets or sparks emanating from the sample during the test. Observe the paper below the sample for any
indications of ignition. Observe and record flames emanating from the paper below the specimen. Take
post-test photographs.
8.4 After testing
After the test, record the final oxygen concentration, the burn length, and the post-test mass, and any
abnormal occurrences. Take post-test photographs.
9 Accuracy
Measurements shall be made within the following tolerance limits:
a) absolute pressure: ±1 %;
b) oxygen concentration: ±0,5 %;
c) specimen width: ±5 %;
d) specimen length: ±20 %;
e) specimen mass: ±1 %;
f) burn length: ±10 mm.
10 Test report
10.1 Standard tests
The test report shall include details of the specimen identification, the specimen preparation, the
specimen configuration, the test conditions, observations from the test, as well as pre and post-test
photographs. Proper reporting of the test observations, such as the burn length, the transfer of burning
debris, and other observations (especially of unusual behaviour) is critical. The test report shall be
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