ISO/DIS 24831:2026(en)

ISO/TC 206/WG 4

Secretariat: JISC

Date: 2025-12-17

Fine ceramics (advanced ceramics, advanced technical ceramics) —Mechanical properties of ceramic matrix composite at ambient temperature in air atmospheric pressure — Determination of leak strength of tubes

© ISO 2026

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Contents

Foreword 3

1 Scope 4

2 Normative references 4

3 Terms and definitions 4

4 Principle 5

5 Significance and Use 5

6 Apparatus 5

6.1 Pressurizing system 5

6.2 Passive seal fixture 6

6.3 Adhesive 6

6.4 Pressurization medium 7

6.5 Strain measurement 7

6.5.1 Strain gauge 7

6.5.2 Digital image correlation (DIC) 7

7 Measurement of test specimen dimensions 7

8 Test specimen dimensions 7

9 Specimen preparation 8

10 Test modes and rates 8

11 Test procedure 8

11.1 Pressure cell 8

11.2 Measurement of test specimen dimensions 8

11.3 Specimen bonding 9

11.4 Setting-up of strain measurement means 9

11.5 Specimen mounting 10

11.6 Measurement 10

11.7 Leak location of test specimen 10

11.8 Test validity 10

11.9 Number of test specimens 10

12 Calculation of results 10

12.1 Hoop tensile stress and strain 10

12.2 Pressure-strain curves 11

12.3 Stress-strain curves 12

12.4 Leak strength 13

13 Statistics 13

14 Test report 14

Annex A (informative) Test setup of leak properties of the tube specimen 1

Bibliography 2

Foreword

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This document was prepared by Technical Committee [or Project Committee] ISO/TC 206, Fine Ceramics.

Fine ceramics (advanced ceramics, advanced technical ceramics) —Mechanical properties of ceramic matrix composite at ambient temperature in air atmospheric pressure — Determination of leak strength of tubes

This document specifies a method of determining the leak strength of ceramic matrix composite (CMC) tubes with fiber-reinforcement at room temperature in air atmospheric pressure. The document provides information on the stress-strain response, such as leak strength, corresponding hoop tensile strain. This document applies primarily to ceramic matrix composite tubes with continuous fiber reinforcement: uni-directional (1-D), bi-directional (2-D), tri-directional (xD, 2<x<3).

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 3611, Geometrical product specifications (GPS) — Dimensional measuring equipment — Design and metrological characteristics of micrometers for external measurements

ISO/IEC 17025, General requirements for the competence of testing and calibration laboratories

ISO 20507, Fine ceramics (advanced ceramics, advanced technical ceramics) —Vocabulary

ASTM E251-92: Test Methods for performance characteristics of metallic bonded resistance strain gauges

ASTM E2208-02: Standard Guide for Evaluating Non-Contacting Optical Strain Measurement Systems

For the purpose of this document, the terms and definitions given in ISO 20507 and the following apply.

ISO and IEC maintain terminological 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

total length

L_t

length including ends of the tube specimen

3.2

calibrated length

L_c

part of the test specimen that has uniform and minimum external diameter

[SOURCE: ISO 21971:2019]

3.3

gauge length

L_g

initial distance between reference points on the test specimen in the calibrated length

[SOURCE: ISO 21971:2019]

3.4

leak pressure

P_l

internal pressure corresponding to the tube specimen when leakage occurs during the test.

3.5

leak strength

σ_l

hoop stress corresponding to leak pressure.

Note 1 to entry: The leak pressure is not enough to cause fiber breakage, therefore, the tube specimen still has a complete integrity geometry.

Closed-end seal fittings incorporating a high-strength adhesive seal against the test specimen tube of specified dimensions are used at room temperature. Both axial and hoop stress act on the tube specimen at the same time during leak properties test of CMC tube specimen. Pressure medium is pumped into the test specimen at a constant volume flow rate. The tube specimen is loaded to leakage and the corresponding test time, internal pressure, and strain data are recorded. The leak strength of the specimen are calculated based on the test data and curves.

NOTE The end fitting incorporating a high-strength adhesive must ensure that there is no leakage of pressurized medium from the seal area during loading.

The micro-sized pores usually generation in process for ceramic matrix composite tubes will degrade its capability to contain internal pressure medium, and increasing the risk of leakage. The nature of micro-sized pores formation mainly depends on deposition temperature, deposition rate and reaction gas composition. The hoop strength of ceramic matrix composite tubes is determined via the "capsule pressurization" method, with the corresponding testing procedures specified in ISO 21971.^[1] Different from hoop strength testing, the leak strength is not enough to cause fiber breakage, and the tube specimen still have a complete integrity geometry after test. Therefore, it is of great significance to establish an effective and reliable test method for leakage strength of ceramic matrix composite tubes to ensure engineering safety. The information obtained from these tests can be used for material development, manufacturing control (quality assurance), material comparison and characterization of tube components.

The maximum medium pressure during test shall not exceed 80% of the rated loading capacity of the pressurizing system. Pressurizing system should be designed with sufficient safety protection device, in case of overpressure, leakage and other dangers, to ensure the safety of staff and equipment (see Annex A).

Special consideration should be given to the following items:

a) Pump, shall be able to apply a continuously increasing and uniform internal pressure to the tube specimen. The pump should not produce a pressure surge with each stroke.

b) Valves, should include the following functions: control, regulation, and safety.

c) Pressure sensor, of adequate capacity, shall be used to monitor system pressure and to record the fluid pressure attained. The accuracy of pressure sensor shall be better than 1 % of the test value.

Passive seal fixture transmits the pressure medium applied by the pressurizing system to the tube specimen. Seal fixture mainly consist of left end fitting, right end fitting and seal bolt. Left end fitting provides bonding area on both the OD and the ID zones of the tube specimen, and there is a channel to induce pressurized medium into the tube specimen. Right end fitting has a vent hole to ensure that all free gases shall be vented prior to test. The seal bolt is used to seal vent hole. An example of the tube specimen closed-end seal fixture is shown in Figure 1. The tube specimen is recommended to fit into the bonding cavity with a thin (~0,1 to 0,2 mm) space for adhesive, providing uniform gluing zone between the tube specimen and the end fitting.^[2] Insufficient bonding surface in the seal fixture will lead to leakage of pressure medium before the tube specimen fails.

Key

1 left end fitting

2 pressure medium channel

3 tube specimen

4 gluing zone (ID)

5 gluing zone (OD)

6 right end fitting

7 vent hole

8 seal area

9 seal bolt

Figure 1 — Tube specimen closed-end seal fixture

NOTE End fittings will be such as to produce nearly 2:1circumferential to axial stress ratio.

The brittle nature of the tube specimen requires particular attention to minimizing crack initiation and fracture during assembling process. The epoxy adhesive is recommended to bond the test specimen into the seal fixture^[3].

NOTE 1 Insufficient bonding surface in the passive seal fixture can cause pressure medium leak before the tube specimen failure.

NOTE 2 The seal fixture can be reused by chemical or thermal approach to remove the adhesive from the seal fixture.

Silicone oil was recommended as the pressurization medium. Industrial liquid lubricants-ISO viscosity classification is specified in ISO 3448.^[4] The viscosity classification of Silicone oil should be below ISO VG 10.

Resistance strain gauge and digital image correlation are used to measure the hoop strain of the tube test specimen. If Poisson's ratio is to be determined, the tube test specimen shall be instrumented to measure strain in both longitudinal and hoop directions at the same time.

Strain gauge method used for measuring tube specimen strain shall satisfy ASTM E251-92. The choice of strain gauge type is very critical. The tube specimen is made of anisotropic fiber-reinforced materials. The strain gauge reading may be influenced by localized strain such as fiber crossovers. Ideally, to eliminate the effect of localized strain on uniaxial strain gauges, three-element strain gauge should be mounted to ensure that maximum principal strain is in the hoop direction.^[5] If strain gauge readings are not influenced by localized strain, strain gauge can be selected according to the geometric dimension of the tube specimen.

DIC method used for measuring tube specimen strain shall satisfy ASTM E2208-02. The gauge length can be selected as real requirement, and the value of the gauge length must be reported. The high-contrast patterns are recommended to be coated on the surface of tube specimen, which is beneficial to achieve a higher resolution. Once the specimen leaks, the test results are not accurate due to the decrease of image quality.

Micrometers used for measuring the dimensions of the tube specimen shall satisfy ISO 3611. Micrometers and other devices used for measuring linear dimensions shall be accurate and precise to at least one-half the smallest unit to which the individual dimension is required to be measured. For the purposes of this test method, measure the wall thickness and outer diameter of the gauge section of each test specimen to within 0,02 mm, thereby requiring dimension measuring devices with accuracies of 0,01 mm.^[1] Measurements of inner diameters of tube specimens can be done by using suitable instruments such as “3 points internal micrometer” or an “inside micrometer rod-type”. X-ray computed tomography has been used to make dimensional measurements on tube specimens^[6].

If the leak location is easily to be observed, the wall thickness of the tube specimen can be measured at the leak location after testing.

Although the diameter and the wall thickness of the tube specimen mainly depend on the application, experience has shown that successful tests are commonly achieved within the following ranges of relative dimension^[7]:

a) The ratio between the external diameter and wall thickness (D_o/H) is commonly selected in the range from 5 to 30.

b) The gauge length should be commonly selected to keep the ratio (L_g/D_o) between 2 and 3 with a minimum recommended value of 30 mm.

Example of a straight-sided tube specimen is shown in Figure 2. Dimensional requirements for an acceptable specimen are contained in Table

Figure 2 — Example of a straight-sided tube specimen

Table 1 — Dimensional requirements for straight-side tube test specimen

Dimensions in millimeters

Sample cutting location and quantity should satisfy the relevant technical requirements. The brittle nature of the tube specimen requires particular attention to minimize crack initiation and fracture during cutting. There are currently no standardized surface preparation/machining methods for ceramic matrix composite tube specimens. Sample preparation has been recommended in ISO 21971 and ASTM C1863-18^[7,1]. The diamond wire cutting machine is recommended for cutting tube specimen.

Flow control mode, that pressure medium is pumped into the test specimen at a constant volume flow rate, is used as test mode. To prevent the sample tube from bursting due to an overly rapid pressure increase, it is recommended that the volume flow rate be kept below 5 ml/min. Alternately, leakage of the tube specimen is recommended occur more than 120 s after the start of the test. Once the tube specimen leaks, the internal pressure increase rate will decrease, therefore the leak strength can be obtained from the curvature change of pressure-strain curve of the tube specimen.

NOTE Pressure control mode, or where the pressure medium is increased at a constant rate, is not suitable for leak strength measurement, since the internal pressure and strain of the tube specimen still keep a nearly linear proportional relationship, even leaks already occurred, and the leak strength cannot be obtained from the pressure-strain curve of the test specimen.

Calibrate the machine and zero the pressure sensor.

The external and internal diameters of the tube specimen shall be measured before testing. At least three position points shall be selected equidistantly along the axial within the tube specimen gauge section, and the external and internal diameters of the tube specimen are measured at each position point with equally circumferential spaced (60° apart). In some cases, it is not possible to determine the external and internal diameters with a micrometer, measurements can be performed by the optical profilometry method. The arithmetic mean of the measured results is used to calculate the hoop tensile stress.

Tube specimen and sealing fixtures are connected by high-strength adhesive. A holding device should be used to ensure concentric alignment of axes of the two end seal fixtures and the tube specimen.

The selected strain measurement shall be installed and/or calibrated without any pressure applied to the tube specimen.

The rough surface of the tube specimen usually requires some preparation prior to strain gauge bonding. The basic steps shall include solvent degreasing, abrading or filling and cleaning. For simplicity, at least one strain gauge should be mounted on the upper, middle and down position of the tube specimen to measure hoop or axial strain as shown in Figure 4. The strain gauge planes should be separated by 3/4 L_g, where L_g is the length of the designated gauge section. In addition, care must be taken to select the strain gauge planes to be symmetrical about the longitudinal midpoint of the gauge section. The strain gauges should be allowed to equilibrate under power for at least 30 min prior to conducting the test.

If the DIC method is used, the camera should be focused on the suitable area of the tube specimen within gauge section. Illumination should be adjusted to achieve the brightest possible image without any reflections.

An example of strain gauge and DIC placement on the gauge section plane of the tube specimen is shown on Figure 3.

Key

1 tube test specimen

2 test area of digital CCD camera

3 strain gauge

Figure 3 — Illustration of strain gauges and DIC placement on gauge section of the tube specimen

The tube specimen assembly shall be mounted on the pressure system, and ensure the location of the tube specimen is suitable to strain measurement.

All free gas shall be vented from the pressure system and the tube specimen prior to test. The internal pressure of the tube specimen is continuously applied through a pressure medium. The volume flow rate is recommended to 1-5 mL/min. Both internal pressure hoop strain and test time of the tube specimen will be recorded by data acquisition system.

High speed framing cameras can be used to monitor the leak location of the tube specimen. There maybe one or more leakage positions in the tube specimen.

If the pressure medium leaks at the bounding position of seal fixtures, the results should be discarded and the test should be repeated with a new specimen.

For leak strength measurement, at least 5 valid test results shall be obtained for the purpose of estimating a mean value. If material cost or test specimen availability limit the number of possible tests, fewer tests can be conducted to determine an indication of material properties.

The strain gauges are applied to the external surface of the tube sample. The stress at the external wall is used for the construction of the stress-strain curves.

For the assumption of isotropic, homogeneous, linear elastic material calculate the hoop tensile stress at the external wall using Formula (1):

(1)

where

σ_h is the hoop tensile stress in megapascals (MPa)

P_i is the internal pressure, in megapascals (MPa)

R_o is the outer radius of the tube specimen, in millimeters (mm)

R_i is the inner radius of the tube specimen, in millimeters (mm)

NOTE The hoop tensile stress on the external surface of the tube specimen is calculated assuming the tube to be isotropic. The relation is applicable for CMC tube specimen since the attainable deformation levels remain very low.

The hoop tensile strain can be determined by strain gauges or the DIC method on the external surface of the tube specimen.

NOTE The strain of the tube specimen is very small, therefore calculating the hoop tensile strain by measuring the change in radius maybe not accurate.

The typical stress-strain curve of the tube specimen is shown in Figure 4. The leakage process of the tube specimen can be divided into three stage, — I: the non-leakage stage, the pressure and strain of the tube specimen have a nearly linear proportional relationship. The elastic deformation of the tube specimen will not cause the pressure medium to leak during pressurization;— II: the transition stage from non-leakage to leakage, the pressure-strain curve of the tube specimen is not linear. The cracks are formed and propagated in the matrix, which cause a small leakage occured during pressurization;— III: the leakage stage, the pressure is no longer increasing, the pressure medium is leak from the tube specimen during pressurization obviously.

Key

internal pressure, in megapascals (MPa);

strain in hoop direction, in millimetres / millimetres (mm/mm).

Figure 4 — The pressure-strain curve of the tube specimen

“Curvature extremum method” is used to derive the leak pressure from the pressure-strain curve, as shown in Figure 5. The curvature of the pressure-strain curve is calculated by Formula (1), and the curvature extremum point is selected by Formula (2). The internal pressure corresponding to the extreme point of curvature is the leak pressure.

(1)

(2)

where

curvature of i-th point of pressure-strain curve;

the first order difference of normalized internal pressure with respect to test time at i-th point;

the first order difference of normalized hoop strain with respect to test time at i-th point;

the second order difference of normalized internal pressure with respect to test time at i-th point;

the second order difference of normalized hoop strain with respect to test time at i-th point;

curvature extremum point.

Key

ε strain in hoop direction, in millimetres/millimetres (mm/mm);

internal pressurize, in megapascals (MPa);

curvature of pressure-strain curve;

leak pressure, in megapascals (MPa);

curvature extremum point.

Figure 5 — The method to obtain leak pressure of the tube specimen

When the first stage of the stress-strain curve is non-linear, high speed framing cameras shall be used to monitor the leak location of the tube specimen. The σ_l of the tube specimen corresponding to the leak location of the tube specimen was observed.

The typical stress-strain curve of the tube specimen is shown in Figure 6.

Key

σ stress in hoop direction, in megapascals (MPa);

ε strain in hoop direction, in millimetres / millimetres (mm/mm);

σ_l leak strength, in megapascals (MPa);

leak strain, in in millimetres / millimetres (mm/mm).

Figure 6 — The stress-strain curve of the tube specimen

In engineering, the maximum allowable leak stress shall be the leak strength divided by the safety factor.

Calculate the leak strength using Formula (3):

(3)

where

σ_l leak strength, in megapascals (MPa);

P_l leak pressure, in megapascals (MPa);

R_o outer radius of the tube specimen, in millimeters (mm);

R_i inner radius of the tube specimen, in millimeters (mm).

For each series of tests the mean, standard deviation, and coefficient of variation for each measured value according to Formulae (4), (5) and (6):

— mean

(4)

— standard deviation

(5)

— coefficient of variation

V = (6)

where

X is the measured value

n is the number of valid tests

The test report shall be prepared in accordance to ISO/IEC 17025:2017, and shall at least include the following information:

a) the name and address of the testing establishment;

b) the date of the test, report identification, number, operator and signatory;

c) a reference to this ISO document;

d) a description of the equipment used;

e) a complete identification of the tested specimen (manufacturer, type, batch, date of receipt, etc.);

f) the gauge length of each test specimen;

g) the external diameter, internal diameter, wall thickness of each specimen;

h) the type of test specimen gripping and end closure;

i) the type of strain measurement equipment;

j) the type of pressurizing system;

k) the volume flow rate;

l) the testing time;

m) the individual values and average value of leak strength, hoop elastic modulus, and hoop tensile strain;

n) the number of tests and valid results;

o) details of any aspect of experimental procedures which might influence the results;

p) any deviations from the procedure;

q) any unusual features observed.

1. 
   (informative)
   
   Test setup of leak properties of the tube specimen

Key

1 data acquisition system

2 pressure system

3 pressure pipeline

4 sample chamber

5 observation window

6 high speed framing camera

7 fixed station

8 tube specimen close-end seal fixture

Figure A.1 — Schematic diagram of test setup for determining leak properties of the tube specimen

Bibliography

[1] ISO 21971, Fine ceramics (advanced ceramics, advanced technical ceramics) — Mechanical properties of ceramic composites at ambient temperature in air atmospheric pressure — Determination of hoop tensile properties of tubes

[2] ISO 20323, Fine ceramics (advanced ceramics, advanced technical ceramics) — Mechanical properties of ceramic composites at ambient temperature in air atmospheric pressure — Determination of tensile properties of tubes

[3] K Shapovalov, G. M. Jacobsen, L Alva, Strength of SiC_f-SiC_m composite tube under uniaxial and multiaxial loading, Journal of Nuclear Materials, 500 (2018) 280-294

[4] ISO 3448, Industrial liquid lubricants — ISO viscosity classification

[5] ASTM C1819, Standard test method for hoop tensile strength of continuous fiber-reinforced advanced ceramic composite tubular test specimens at ambient temperature using elastomeric inserts

[6] ASTM C1863, Standard test method for hoop tensile strength of continuous fiber-reinforced advanced ceramic composite tubular test specimens at ambient temperature using direct pressurization

[7] Deck, C. P., Jacobsen, G. M., Sgeeder, J., “Characterization of SiC-SiC Composites for Accident Tolerant Fuel Cladding,’’ Journal of nuclear Materials, Vol 466, 2015, pp. 667-681