ISO/DIS 13071:2026(en)

ISO TC 61/SC 13/WG 1

Secretariat: JISC

Date: 2025-10-12

Tensile test of fibre reinforced monolayer flat specimens

© ISO 2026

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Contents

Foreword

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This document was prepared by Technical Committee ISO/TC 61, Plastics, Sub Committee 13, Composites and reinforcement fibres, Working Group 1, Reinforcements and reinforcement products.

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Introduction

Monolayer preimpregnated fibres (prepregs) based on thermoset and thermoplastic matrices are widely used in industry. This standard describes two methods (A and B) for testing the mechanical properties (stiffness, strength, failure strain) of monolayer prepregs in fibre direction.

The primary focus here is on quality assurance for such products. For the determination of the material properties of the processed material in laminates, reference is made to the ISO 527-4 and 527-5 standards.

Tensile test of monolayer flat specimens

This document specifies the test conditions for the determination of the tensile properties of monolayer flat fibre reinforced plastics, based upon the general principles given in ISO 527-1.

The methods are used to investigate the tensile behaviour of the test specimens and for determining the tensile strength, tensile modulus and failure strain under the defined conditions when loaded in fibre direction.

The test method is suitable for use with monolayer materials made of fibre-reinforced thermoset and thermoplastic composites incorporating preimpregnated materials (prepregs). The monolayer has a unidirectional (e.g. tapes) or multidirectional (e.g. fabric) reinforcement. In case of thermoset material, the thermoset matrix needs to be able to transduce shear stresses. So, it is recommended to use specimens in cured state.

The reinforcement fibres covered include glass fibres, carbon fibres, aramid fibres and other similar synthetic or natural fibres.

Two methods are described (A: Reduced force clamping, B: Parallel clamping)

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 cited edition applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

ISO 527‑1:2019, Plastics — Determination of tensile properties — Part 1: General principles

ISO 16012, Plastics — Determination of linear dimensions of test specimens

For the purposes of this document, the following terms and definitions 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

gauge length

L₀

initial distance between the gauge marks on the central part of the test specimen

Note 1 to entry: It is expressed in millimetres (mm).

[SOURCE: ISO 527-1:2019, 3.1]

3.2

thickness

h

smaller initial dimension of the rectangular cross-section in the central part of a test specimen

Note 1 to entry: It is expressed in millimetres (mm).

[SOURCE: ISO 527-1:2019, 3.2]

3.3

width

b

larger initial dimension of the rectangular cross-section in the central part of a test specimen

Note 1 to entry: It is expressed in millimetres (mm).

[SOURCE: ISO 527-1:2019, 3.3]

3.4

cross-section

A

product of initial width (3.3) and thickness (3.2), A = bh, of a test specimen

Note 1 to entry: It is expressed in square millimetres, (mm²).

3.5

test speed

v

rate of separation of the gripping jaws

Note 1 to entry: It is expressed in millimetres per minute (mm/min).

[SOURCE: ISO 527-1:2019, 3.5]

3.6

stress

σ

normal force per unit area of the original cross-section (3.4) within the gauge length

Note 1 to entry: It is expressed in megapascals (MPa).

Note 2 to entry: In order to differentiate from the true stress related to the actual cross-section of the specimen, this stress is frequently called “engineering stress”.

Note 3 to entry: σ for “1"-direction specimens is defined as σ₁ and for "2"-direction specimens as σ₂ (see 3.9 and Figure 2 for definitions of these directions).

[SOURCE: ISO 527-1:2019, 3.6, modified — Domain “<engineering>” and Note 3 to entry has been added.]

3.6.1

strength

σ_m

maximum stress observed during a tensile test

Note 1 to entry: It is expressed in megapascals (MPa).

[SOURCE: ISO 527-1:2019, 3.6.2]

3.7

strain

ε

increase in length per unit original length of the gauge

Note 1 to entry: It is expressed as a dimensionless ratio, or as a percentage (%).

[SOURCE: ISO 527-1:2019, 3.7]

3.7.1

strain at break

ε_b

strain at the last recorded data point before the stress (3.6) is reduced to less than or equal to 10 % of the strength (3.6.1) if the break occurs prior to yielding

Note 1 to entry: It is expressed as a dimensionless ratio, or as a percentage (%).

3.7.2

strain at strength

ε_m

strain at which the strength (3.6.1) is reached

Note 1 to entry: It is expressed as a dimensionless ratio, or as a percentage (%).

[SOURCE: ISO 527-1:2019, 3.7.3]

3.8

tensile modulus

modulus of elasticity under tension

E_t

slope of the stress/strain curve σ(ε) in the interval between the two strains ε₁ = 0,05 % and ε₂ = 0,25 %

Note 1 to entry: It is expressed in megapascals (MPa).

Note 2 to entry: It may be calculated either as the chord modulus or as the slope of a linear least-squares regression line in this interval.

[SOURCE: ISO 527-1:2019, 3.9]

Key

X strain, ε

Y stress, σ

a slope E

Fig. 1 — Stress-strain curve

Two types of test specimen are specified for use with this document.

Fig. 2 — a) Type A and type B2 specimen b) type B1 specimen

Table 1 — Dimensions of type A and type B specimens

Type A is a rectangular strip specimen with a typical length of 800 - 900 mm (depending on the gripping tool) for usage in a reduced force clamping device.

Type B is a rectangular strip specimen with a length of 250 mm for use in a direct clamping device. Type B1 has bonded, co-cured or unbonded end tabs and Type B2 is tested without end tabs.

The preferred width of all specimen types is 25 mm, but smaller width or widths of up to 50 mm may be used. If material is produced in smaller width or the above tolerances cannot be maintained during cutting, this width is also possible

The free length is defined as the distance between last contact points between specimens and upper and lower tool, respectively.

The test speed can be adapted if valid failure occurs, and material does not exhibit significant strain rate sensitivity.

The thickness of the specimens is the production thickness of the monolayer material (typically between 0,1 and 0,3 mm.

Note 1 to entry: Based on the longer load introduction area of method A the testing speed of 6 mm/min results in comparable strain rates to 2 mm/min in method B

Methods, which allow high cutting accuracy and minimise specimen deformation shall be applied. It has to be ensured that the cutting is executed parallel to the fibres. Some recommended methods are described in ANNEX B.

Ideally, the specimens should be free of twist and should have mutually perpendicular pairs of parallel surfaces. The surfaces and edges shall be free from defects that may influence the test results like scratches, pits, sink marks, splits along the fibre and flash.

The specimens shall be checked for conformity with these requirements by visual observation or with micrometer callipers.

Specimens showing observed or measured departure from one or more of these requirements shall be rejected. If non-conforming specimens are tested, report the reasons.

5.1.1 General

The machine shall comply with ISO 7500-1 and ISO 9513, and meet the specifications given in 5.1.2 to 5.1.6.

5.1.2 Test speeds

The tensile-testing machine shall be capable of maintaining the test speeds below 10 mm/min within a tolerance of 20 %.

5.1.3 Grips

Grips for holding the test specimen shall be attached to the machine so that the major axis of the test specimen coincides with the direction of extension through the centre line of the grip assembly. The test specimen shall be held such that slip relative to the gripping jaws is prevented. The gripping system shall not cause premature fracture at the jaws or squashing of the specimen in the grips.

This method introduces the force smoothly into the specimen and avoids failure of specimens in the clamping region.

Note 1 to entry: Method A is recommended for material with high failure stress or if clamping failure occurs in method B

According to ISO 527-1.

A monolayer specimen is placed in a reduced clamping force device and clamped before applying tension.

Fig. 3 — Reduced force clamping device a) Overall b) Detail

Table 2 — Dimensions of apparatus / clamping device

The specimen is wrapped around a curved apparatus before it is fixed between clamping jaws. Vertical alignment of the specimens is mandatory. Starting at the section near the test area (α =0°), the fixture shall have an outer radius of R₀ up to l_arc > 15 mm and shall be tangential to the testing direction. In the following wrapping length the radius may decrease constantly and without any steps. The minimum of the radius at the end is 25 mm. The wrapping angle α shall reach at least180°. The edge at the entry area should be free of burrs.

Note 1 to entry: The apparatus can also start with R₀ at a position with a slightly negative l_arc (α below 0°) to avoid an edge at the contact point between the specimen and the grip. Be aware to measure the free length L in this case between the position l_arc =0 of the upper and lower apparatus.

According to ISO 527-1.

A monolayer specimen is clamped between two parallel clamps before applying tension.

Note 1 to entry: A balanced combination of clamping pressure, clamping surface roughness, use of tabs and choice of their nature is mandatory to avoid clamping induced failure by introducing severe stress peaks.

Fig. 4 — Direct clamping device

Table 3 — Dimensions of apparatus / clamping device

The apparatus shall conform to ISO 527-1:2019, Clause 5:

The apparatus and the specimen must be correctly aligned.

Test atmosphere in accordance with ISO 527-1:2019, 9.1.

Conditioning shall be in accordance with ISO 527-1:2019, Clause 8.

The measurement of the specimen dimensions shall conform to ISO 527-1:2019, 9.2, except for the procedure for thickness measurement, as described as follows:

A micrometre or equivalent (see ISO 16012: 2015, 5.5) has to be used, that shall provide a resolution of 0,001 mm and an accuracy of 0,004 mm or better. To generate comparable results, the type of micrometer is fixed to a flat tipped anvil type with a plate diameter between 6,0 and 6,5 mm.

Fig. 5 — Thickness measurement of monolayer specimens (unscaled)

For both methods, at least three measurements of the thickness (h) and the width (b) shall be performed along the free length of the specimen acc. to fig. 6. At every measurement location the cross-section shall be calculated. For the calculation of the modulus, stress and strength, the mean value of the measured cross-sections shall be used and the standard deviation shall be calculated.

Fig. 6 — Positions for measuring the thickness and width of the specimens

Note 1 to entry: This standard is not intended to produce design values of the tested materials. One must be aware that this measurement method in combination with the low tape thickness could lead to overestimation and underestimation of the thickness and stress values, respectively (see fig. 5).

Note 2 to entry: It is important to note how the width, thickness and respectively cross-section vary at the measuring points. If there are significant differences, this should be taken into account when considering the stress and strength values.

A minimum of five valid test specimens shall be tested. The number of measurements may be more than five if greater precision of the mean value is required. It is possible to evaluate this by means of the confidence interval (95 % probability, see ISO 2602).

Place the test specimen in the grips, taking care to align the longitudinal axis of the test specimen with the axis of the testing machine. Tighten the grips evenly and firmly to avoid slippage of the test specimen and movement of the grips during the test. Gripping pressure shall not cause fracture or squashing of the test specimen

When using test method A (reduced clamping force), unacceptable clamping conditions (overpressure or inappropriate clamping surface roughness) can be detected if the specimens fails initially in the clamping region. These tests are invalid. The test results must be suspended from the campaign and the test has to be repeated.

NOTE 1: Sometimes the initial failure occurs in the free length and because of elastic behaviour of the specimen a second failure occurs in the edge of the clamping area. If this happens the test can be evaluated as a valid test.

The rough gripping surfaces must be prevented from damaging the outer fibres. This can be achieved by using clamps with sufficiently low surface roughness. This is important especially for type B2 specimens. If damage in the outer fibres is observed consider using bonded or unbonded tabs (type B1)

Note 1 to entry: Correctly adjusted stops can help to centre the specimens easily in the apparatus

Note 2 to entry: Unacceptable clamping conditions can often be avoided by adjusting the gripping force or pressure (e.g. via torque or manometer depending on gripping system used) so that it does not cause fracture or crushing of the test specimen. Misalignment of specimen can also be a reason for the invalid failure described.

For test method B (Direct clamping) failure in the clamping is often not avoidable. So invalid tests cannot be detected by clamping failure mode. Clamping failure can be avoided by using Method A (Reduced force clamping) as the force in the specimen is strongly reduced before the clamping region.

The prestress shall conform to ISO 527-1:2019, 9.4.

The strain measurement setting shall conform to ISO 527-1:2019, 9.5. Measure the gauge length to an accuracy of 1 % or better. The interaction between the local measurement system and the surface must not damage the specimen. That is also important for any pretreatment before bonding strain gages.

Note 1 to entry: Due to the thin nature of the specimen any mechanical surface interaction may influence the test result stronger than with specimens with larger thickness.

If optical strain measurement systems are used the system must be able to detect and correct out of plane deformations. Alternatively, a full 3D system encapsulating all deformations can be used.

Recommendation is 8 mm/min.

Note 1 to entry: When following the recommended speeds of testing strain rate is comparable in both methods

Recommendation is 2 mm/min.

Preferably record the force and the corresponding values of the increase of the gauge length and the cross head displacement over time. This requires three data channels for data acquisition. If only two channels are available, record the force signal and the extensometer signal. It is preferable to use an automatic recording system.

The recording of data shall conform to ISO 527-1:2019, 9.7.

Calculate all stress values, defined in 3.6, using Formula (1):

(1)

where

is the stress value in question, expressed in megapascals (MPa);

is the measured force concerned, expressed in newtons (N);

is the initial mean cross-sectional area of the specimen, expressed in square millimetres (mm²) as described in 8.4.

The property calculation shall conform to ISO 527-1:2019, Clause 10, except that the definitions given in Clause 3 apply and strain values shall be reported to three significant figures.

For materials and/or test conditions for which a homogeneous strain distribution is prevalent in the parallel section of the test specimen, i.e. for strains prior and up to a yield point, calculate all strain values, defined in 3.7, using Formula (2):

(2)

where

is the strain value in question, expressed as a dimensionless ratio, or as a percentage;

is the gauge length of the test specimen, expressed in millimetres (mm);

is the increase of the specimen length between the gauge marks, expressed in millimetres (mm).

The determination of strain values using an extensometer averages strains over the gauge length. This is correct and useful, as long as the deformation of the test specimen within the gauge length is homogeneous. If the material starts necking, the strain distribution becomes inhomogeneous and strains determined with an extensometer are strongly influenced by the position and size of the neck zone.

Calculate the tensile modulus, defined in 3.8, using one of the following alternatives [see Formula (3) to Formula (4)].

(3)

where

is the tensile modulus, expressed in megapascals (MPa);

is the stress, expressed in megapascals (MPa), measured at the strain value ε1 = 0,000 5 (0,05 %);

is the stress, expressed in megapascals (MPa), measured at the strain value ε2 = 0,002 5 (0,25 %).

With computer-aided equipment, the determination of the tensile modulus, E_t, using two distinct stress/ strain points can be replaced by a linear regression procedure applied on the part of the curve between these mentioned points.

(4)

where is the slope of a least-squares regression line fit to the part of the stress/strain curve in the strain interval 0,000 5 ≤ ε ≤ 0,002 5, expressed in megapascals (MPa).

Calculate the arithmetic means of the test results and, if required, the standard deviations and the 95 % confidence intervals of the mean values in accordance with the procedure given in ISO 2602.

Calculate the stresses and the tensile modulus to three significant figures. Calculate the strains to five significant figures if strain is expressed as a dimensionless ratio, or to 3 significant figures if strain is expressed as a percentage ratio.

See Annex A.

The test report shall include the following information:

a) a reference to this document, including the type of specimen and the test speed, written in the following format:
tensile test ISO XXX / type of specimen /test speed

b) all the data necessary for identification of the material tested, including type (matrix and fibre), source, manufacturer's code number and history, where these are known;

c) fibre content and fibre geometry (e.g. unidirectional)

d) type of gripping device(method A or method B1 or B2), the gripping distance L;

e) method of preparing the test specimens (slitting or cutting or as produced),

f) number of the tested specimen;

g) standard environment (e.g. temperature and humidity) for conditioning and for testing, plus any special conditioning treatment, if required by the relevant standard for the material or product concerned;

h) accuracy grading of the test machine and extensometer (see ISO 7500-1, ISO 9513 and ISO 527-1 5.1.5);

i) type of elongation or strain indicator, and the gauge length L₀;

j) testing speeds;

k) for each test specimen width, thickness, cross-section and test results of the properties defined in Clause 3;

l) standard deviation, and/or coefficient of variation, and/or confidence limits of the mean, if required;

m) statement as to whether any test specimens have been rejected and replaced, and, if so, the reasons, and reasons for testing non-conforming specimens;

n) date of measurement.

1. 
   
   Precision Data
2. 
   
   Specimen Preparation

This standard applies to preimpregnated fibre-reinforced materials, which usually are produced in a continuous manner and are provided on large spools with a significant higher width and length than the specimen dimension. As the thickness remains unchanged for monolayer testing, only a cutting of the specimen out of the rolls in length and width has to be carried out. If the material is directly produced in specimen width, only the length has to be cut.

That can include cutting using knives with the help of gauges, ripping of the tapes between the fibres or the use of stationary equipment such as bench mounted cutting shears.

For the preparation of specimen for method A, the long specimen length of typically 800 - 900 mm might not be suitable for stationary equipment. For method B, the use of bench mounted cutting shears has been found to be convenient.

The fibre orientation shall be controlled and documented if the fibres are not oriented straight and precisely in parallel to the length direction of the specimen.

To cut the specimens in length sharp scissors can be used.

It is also recommended to note the cutting method in the testing protocol.

References

[1]

[2]

[3]