ISO 14126
In-Plane Compressive Properties of Fibre-Reinforced Composites

Universal Grip ISO 14126 shear-loading compression fixture for composites

Downloads: ISO 14126 Shear-Loading Compression Fixture for Composites – Universal Grip Data Sheet (PDF)

Written by Chris Thomas, Universal Grip Company · Reviewed by Frank Bertini · Updated October 24, 2026

ISO 14126 is the international standard for the in-plane compressive properties of fibre-reinforced plastic composites: compressive strength, compressive modulus and failure strain measured parallel to the plane of lamination on flat coupons. Compression is the hardest basic property to measure on a laminate, because a thin, stiff strip will buckle long before it crushes unless the load is introduced through end tabs or a clamped length and the unsupported gauge section is kept short. The standard therefore defines two ways of introducing the load, Method 1 (shear loading through tabs in a wedge-type fixture) and Method 2 (combined end and shear loading), three specimen types, and a bending check with back-to-back strain measurement that every valid result must pass. It is the ISO twin of ASTM D3410 and ASTM D6641, and the compression partner of the ISO 527-4 and ISO 527-5 tensile methods.

This guide covers the current edition, ISO 14126:2023, and what changed from 1999 (Method 2 redefined as combined loading, a normative alignment annex, normative specimen preparation, a predicted tab-length annex and a digital image correlation annex); the two methods and which fixtures they imply; the Type A, B1 and B2 specimens with their dimensions, gauge lengths and tabs; the 1 mm/min speed, the 10 % bending limit and the 0.05 to 0.25 % modulus window; valid and invalid failure modes; the formulas with a worked example; the common errors; a numeric comparison with ASTM D3410 and D6641; and the shear-loading fixture, strain measurement and machines Universal Grip supplies. The compression testing page gives the lab overview and the ISO standards hub lists every ISO method we equip.

In this guide: What ISO 14126 Covers · Editions: 1999 to 2023 and What Changed · Method 1 (Shear Loading) vs Method 2 (Combined Loading) · Specimen Types A, B1 and B2 · Apparatus, Speed, Bending Check and Valid Failures · Step-by-Step Procedure · Formulas and a Worked Example · Common Errors and How Big They Are · ISO 14126 vs ASTM D3410 and ASTM D6641 · Equipment for ISO 14126: Fixture, Strain Measurement and Machines · FAQ

What ISO 14126 Covers

The standard applies to fibre-reinforced plastics with thermoset or thermoplastic matrices, tested as flat specimens machined from plates made to ISO 1268 or cut from finished and semi-finished products. It gives the compressive properties in directions parallel to the plane of lamination, which is what a designer needs for a laminate loaded in its own plane; through-thickness compression is a different test. Moulding compounds with fibres shorter than 7.5 mm are excluded and go to ISO 604, the plastics compression method, because their behaviour is governed by the matrix rather than by fibre microbuckling.

Compressive strength σcM: the maximum force divided by the original cross-section.
Compressive modulus Ec: the chord modulus between 0.05 % and 0.25 % strain, from the average of the strains measured on both faces.
Compressive failure strain εcM: the average of the two face strains at failure.
Subscripts: 11 for the fibre direction of a unidirectional laminate, 22 for transverse, following ISO 527-5.

The standard is explicit that different method and specimen combinations will not necessarily give the same result, which is why the report must state the method, the specimen type, the fixture and the tab configuration. A compressive strength quoted without them is not comparable to anything.

Editions: 1999 to 2023 and What Changed

The first edition, ISO 14126:1999 with Technical Corrigendum 1 of 2001, is the one most equipment pages still describe. The second edition, ISO 14126:2023, was published in October 2023 and changes the method structure in a way that matters for fixture selection:

Method 2 redefined as combined loading (end loading plus shear through tabs, the CLC-type fixture family). In 1999 Method 2 was end loading, with the ISO 8515 jig and the modified ASTM D695 fixture as examples. The 2023 principle clause states that end loading alone is not covered, so a pure end-loading jig no longer produces an ISO 14126 result.
Alignment of the specimen and load train is now a normative Annex A. Fixture alignment used to be implicit in the bending check; it is now a requirement in its own right.
Specimen preparation (Annex B) is normative, covering machining, tabbing and tolerances.
Annex F gives a predicted tab length so that the tab shear stress stays below the adhesive's limit, and Annex E gives the Euler buckling estimate used to set a minimum thickness for a given gauge length.
Annex G covers digital image correlation (DIC) as a strain measurement method, including strain on the narrow side faces, which brings optical systems such as our Vector 3D DIC extensometer formally into the standard.

A draft Method 3 (plain end loading, modulus only) was dropped before publication. If a supplier describes ISO 14126 Method 2 as end loading or sells a D695-type jig as an ISO 14126 fixture, they are working to the 1999 text.

Method 1 (Shear Loading) vs Method 2 (Combined Loading)

Both methods aim at the same thing: get the compressive load into a short, unsupported gauge section without buckling it and without crushing the specimen ends. They differ in how the load enters.

Method 1, shear loading: the specimen's tabbed ends are clamped between tapered wedges that seat in a housing; as the housing is pushed down the wedges close and the load passes into the specimen as shear through the tab adhesive and the tab faces. Fixtures of the IITRI type (the ASTM D3410 Method B fixture, with flat wedges and guide posts) and the older Celanese type (conical wedges, removed from D3410 in 2003) are Method 1 fixtures, as are aligned hydraulic grips on a stiff frame. The Universal Grip fixture described below is a Method 1 device.
Method 2, combined loading: the specimen is clamped between four blocks that apply a controlled clamping force (torqued screws), and the ends also bear on the fixture, so part of the load enters by end bearing and part by friction and shear through the clamped length. The ASTM D6641 combined loading compression (CLC) fixture is the archetype. Tabs are optional and the specimen is shorter and simpler to make.

Either specimen type may be used with either method, provided the result is valid: the standard accepts any fixture whose specimens fail in the gauge section with less than 10 % bending strain, measured between 10 % and 90 % of the maximum load. In practice Method 1 wedge fixtures are the choice for high-strength unidirectional carbon (Type A), where the tab shear capacity has to carry 30 kN or more into a 10 × 2 mm section, and Method 2 combined-loading fixtures are favoured for untabbed multidirectional laminates where end crushing is not the limiting failure. Our ASTM D3410 and ASTM D6641 guides describe each fixture family in detail.

Specimen Types A, B1 and B2

Table 1 of the standard gives three specimen types. Type A is the fixed-thickness, tabbed coupon for unidirectional and biaxial prepreg laminates tested in the fibre direction; Type B covers a range of thicknesses, tabbed or untabbed, in two sizes for multidirectional, mat and fabric reinforcements and coarser materials. Dimensions in millimetres:

Type A: overall length 110, width 10 ± 0.5, thickness 2 ± 0.2, gauge length (between tabs) 10 mm, end tabs mandatory, at least 50 mm long and about 1 mm thick, typically ±45° glass-epoxy or aluminium. For UD and biaxial prepreg with aligned continuous fibres or fibres longer than 7.5 mm.
Type B1: overall length 110, width 10 ± 0.5, thickness 2 to 10, gauge length 10 mm, tabs optional (50 mm long, 0.5 to 2 mm thick). For multidirectional laminates, mats and fabrics in the 10 mm width.
Type B2: overall length 125, width 25 ± 0.5, thickness 4 mm and above, gauge length 25 mm, tabs optional (50 mm long, 0.5 to 2 mm thick). The wider coupon for coarse fabrics and thick laminates, and the one that takes 10 mm strain gauges.

The gauge lengths are short by design: 10 mm on a 2 mm thick Type A coupon gives a slenderness that keeps a carbon UD laminate below its Euler buckling load up to around 1,500 MPa. Annex E lets you check that for your own material from the compressive modulus, the expected strength, the through-thickness shear modulus and the gauge length, and it is worth running the check before machining a thin Type B1 coupon at the 2 mm end of its range. Strain-gauge grids on Type A and B1 must be shorter than 3 mm because the gauge section is only 10 mm long; Type B2 accepts 10 mm grids. Specimens are prepared to the normative Annex B: machined from ISO 1268 plates, edges parallel and square, tabs bonded with a structural adhesive and their ends square to the axis so that the wedges clamp evenly.

Apparatus, Speed, Bending Check and Valid Failures

Machine: ISO 7500-1 class 1 (force within 1 %), with a load train aligned to the normative Annex A; the compression platen or fixture coupling must be stiff and square, and a spherical seat, if used, must be locked once aligned.
Strain measurement: on both faces (back-to-back), by strain gauges, mechanical or optical extensometers or DIC, with an error no more than 1 %. Strain on the narrow side faces is permitted with DIC (Annex G).
Micrometer: 0.01 mm resolution with about 6 mm faces, width and thickness measured in the gauge section.
Speed: crosshead at 1 ± 0.5 mm/min, constant to failure.
Number of specimens: at least 5 per direction and condition, invalid ones replaced; more if the mean is needed to a tighter confidence.
Conditioning: per the material specification, otherwise ISO 291 at 23 °C and 50 % RH.
Bending check: with εa and εb the strains on the two faces, the test is valid only if |εb − εa| ≤ 0.1 |εb + εa|, that is 10 % bending, checked over the range 10 % to 90 % of the maximum force. Specimens that exceed it are discarded and replaced, and the fixture alignment is reviewed.
Valid failure: within or at the end of the gauge length. Failures inside the tabs or grips (tab debonding, wedge-face crushing, end brooming in a Method 2 fixture) are invalid and the specimen is replaced; for Method 2 the specimen ends must not fail.

The bending check is the heart of the method. A 10 mm gauge section with 5 % bending at 1,000 MPa mean stress has one face at 1,050 MPa and the other at 950, which is survivable; at 20 % bending the faces are at 1,200 and 800 MPa and the specimen fails early on the compressed face, under-reporting strength by 10 to 20 %. Record both face strains for every specimen, not just the first; a fixture that was aligned on Monday drifts as the wedges wear.

Step-by-Step Procedure

Machine and tab the coupons to Annex B; measure width and thickness in the gauge section with the 0.01 mm micrometer and record the tab configuration.
Bond or mount back-to-back strain gauges (grid under 3 mm on A and B1) or set up extensometers or DIC on both faces.
Condition to ISO 291 or the material specification.
Mount the specimen in the fixture with the gauge section centred: for Method 1, seat the tabs in the wedge pairs on the bench, insert the wedge pairs into the housing and bring the guide posts together; for Method 2, torque the clamping screws to the fixture's specified value, square the ends to the end plates.
Verify alignment per Annex A (the first few specimens of a batch are run with the bending check watched live).
Load at 1 ± 0.5 mm/min to failure, recording force and both face strains continuously.
Check bending between 10 % and 90 % of the maximum force; check that the failure is in or at the end of the gauge section; discard and replace invalid specimens.
Calculate σcM, Ec and εcM; report mean, standard deviation and coefficient of variation for at least 5 valid specimens, with the method, specimen type, fixture, tabs, dimensions, conditioning, speed, bending percentage and failure modes.

Formulas and a Worked Example

Compressive stress: σc = F / (b × h). Compressive strength: σcM = Fmax / (b × h). Strain: εc = the mean of the two face strains at each force. Compressive modulus: Ec = (σ″ − σ′) / (0.0025 − 0.0005), the chord between 0.05 % and 0.25 % strain. Percent bending: (εa − εb) / (εa + εb) × 100.

Specimen: Type A carbon-epoxy UD, measured width 10.02 mm, thickness 2.04 mm; area = 10.02 × 2.04 = 20.44 mm².
Modulus points: at 0.05 % mean strain the force is 1,370 N → σ′ = 1,370 / 20.44 = 67.0 MPa; at 0.25 % the force is 6,850 N → σ″ = 335.1 MPa. Ec = (335.1 − 67.0) / 0.0020 = 268.1 / 0.0020 = 134,000 MPa (134 GPa).
Bending at the modulus check: face strains 0.00258 and 0.00242 at the upper point → (0.00258 − 0.00242) / (0.00258 + 0.00242) = 0.00016 / 0.00500 = 3.2 %, inside the 10 % limit.
Strength: Fmax = 27,300 N → σcM = 27,300 / 20.44 = 1,336 MPa; failure in the gauge section with fibre kinking across the width: valid.
Failure strain: face strains at Fmax 0.0104 and 0.0097 → εcM = 1.0 % (mean 0.01005); bending at failure (0.0007 / 0.0201) = 3.5 %.
Capacity check: 27.3 kN on a 100 kN fixture is 27 % of range; a Type B2 glass coupon of 25 × 5 mm (125 mm²) at 600 MPa needs 75 kN; a Type B2 carbon coupon of 25 × 10 mm (250 mm²) at 600 MPa needs 150 kN, which is why the 100 kN standard fixture has a higher-capacity option.

Note the modulus window: ISO uses 0.0005 to 0.0025 strain, the ASTM methods use 0.001 to 0.003 (1,000 to 3,000 µε). On a linear carbon laminate the two agree within a percent; on a laminate that stiffens or softens early they do not, and the window must be stated with the value.

Common Errors and How Big They Are

Single-face strain. Without back-to-back measurement the bending check cannot be made and the modulus is biased by whatever bending exists: 10 % bending puts a single gauge 10 % high or low.
Tab failure counted as strength. A tab that debonds at 20 kN on a coupon that would have reached 27 kN reports a strength 26 % low. Tab failures are invalid; check the adhesive and the predicted tab length of Annex F.
End-loading jig sold as Method 2. Since 2023, end loading alone is not an ISO 14126 method; results from a D695-type jig cannot be reported to the standard.
Too long a gauge for a thin coupon. A 2 mm Type B1 coupon with a 25 mm unsupported length buckles elastically at a fraction of its strength; Annex E gives the minimum thickness, and the 10 mm gauge exists for a reason.
Wrong speed. 1 ± 0.5 mm/min is a narrow band; 5 mm/min raises apparent strength of a rate-sensitive matrix by a few percent and is outside the standard.
Unlocked spherical seat. A free seat lets the fixture tilt as the specimen starts to kink; bending shoots up at the moment it matters. Align, then lock.
Serrated wedge faces on thin tabs. Deep serrations can cut through 1 mm tabs and start a failure at the tab end; smooth or lightly surfaced faces with correct wedge angle carry the load by friction over the whole tab.
Comparing across methods. A Method 1 Type A result and a Method 2 Type B2 result on the same laminate can differ by 10 % or more; the standard warns that combinations are not interchangeable.

ISO 14126 vs ASTM D3410 and ASTM D6641

The three standards share the physics and the 10 % bending rule but differ in the numbers. Our ASTM D3410 and ASTM D6641 guides give the ASTM figures in full; the comparison here uses the same values.

Method mapping: ISO Method 1 (shear loading) ≈ ASTM D3410 (IITRI wedge fixture); ISO Method 2 (combined loading) ≈ ASTM D6641 (CLC four-block fixture). The modified D695 end-loading fixture is neither.
Specimens: ISO Type A 110 × 10 × 2 mm, gauge 10 mm, tabs mandatory 50 × 1 mm; D3410 140 to 155 mm long, 10 mm (0°) or 25 mm wide, gauge 10 to 25 mm (12.7 typical), tabs 65 × 1.5 mm; D6641 140 × 13 mm, gauge 13 mm, about 2 to 2.5 mm thick, Procedure A untabbed or Procedure B tabbed (about 1.6 mm tabs) for laminates with more than 50 % 0° plies.
Speed: ISO 1 ± 0.5 mm/min; D3410 1.5 mm/min (0.05 in/min); D6641 1.3 mm/min.
Clamping: ISO Method 2 and D6641 set a clamping force; D6641 specifies eight screws torqued to 2.5 to 3.0 N·m.
Modulus window: ISO 0.05 to 0.25 % strain; ASTM 0.1 to 0.3 % (1,000 to 3,000 µε).
Bending check: ISO 10 % over 10 to 90 % of Fmax; D6641 10 % at 2,000 µε for modulus and at failure; D3410 10 %.
Specimens per test: at least 5 in all three.
Extras: ISO has a normative alignment annex, a predicted tab-length annex and a DIC annex; ASTM D3410 has a three-part failure code and the ASTM methods give precision statements.

Equipment for ISO 14126: Fixture, Strain Measurement and Machines

Universal Grip ISO 14126 shear-loading compression fixture (part ref. UGC-ISO14126-CP). From the data sheet: a wedge-grip compression fixture for in-plane compressive testing of fibre-reinforced composites. Tabbed specimens are clamped in tapered wedge pairs that seat in a guided housing; linear guide posts keep the upper and lower blocks aligned, introducing load by shear through the tabs to give a short, unsupported gauge length. Key features: tapered wedge grips that introduce load by shear through the specimen end tabs; guide posts with linear bearings that keep the upper and lower halves aligned; removable wedge pairs for bench loading of specimens; a short unsupported gauge length to prevent buckling; and adapters to suit the load frame (clevis or threaded). Specifications: in-plane compression per ISO 14126, shear loading; compression introduced via wedge-gripped tabs; tabbed flat composite strips per ISO 14126; load capacity 100 kN standard, higher capacities on request; hardened steel wedges and housing; serrated or surfaced wedge faces; twin guide posts with linear bearings; clevis pin or threaded adapter to suit the customer frame. Applications listed: unidirectional and multidirectional laminates, aerospace composite qualification, carbon and glass prepreg systems, design allowables and material screening. When ordering, specify specimen width, thickness and tab length, required capacity, wedge surface and machine connection.

Two honest notes on the sheet. Its title reads 'compression platens', but the device it describes is a wedge-grip, guide-post fixture of the IITRI type, and that is what you receive: it is a Method 1 (shear loading) fixture and is the right choice for Type A and tabbed Type B coupons. It also describes Method 2 as 'combined/end loading'; under ISO 14126:2023 Method 2 is combined loading, and for that family we supply the ASTM D6641 combined-loading fixture, which runs ISO Method 2 on untabbed or tabbed Type B coupons. For thin tabs we recommend the surfaced wedge faces over the serrated option.

Capacity: 100 kN covers every Type A and Type B1 coupon (a 10 × 2 mm carbon UD at 1,500 MPa is 30 kN; a 10 × 10 mm Type B1 at 600 MPa is 60 kN) and glass Type B2 up to about 5 mm thick; carbon Type B2 at 25 × 10 mm and 600 MPa needs about 150 kN, so specify the higher-capacity option.
Machines: the 100 kN benchtop UTM suits Type A and B1 work; the 200 kN floor-standing UTM covers the heavy Type B2 coupons and the 150 kN fixture option with margin. Both accept the fixture on a clevis or threaded adapter and have the stiffness the alignment annex assumes.
Strain on both faces: bonded strain gauges with grids under 3 mm on Type A/B1; or non-contact measurement with a video extensometer on each face, or our Vector 3D DIC extensometer, which gives full-field strain on the gauge section and the side faces as Annex G allows and reports the bending check directly. The extensometers page compares the options.
Fixtures for related tests: compression fixtures and the compression testing overview; the tensile twins ISO 527-4 and ISO 527-5; flexure to ISO 14125 and interlaminar shear to ISO 14130 for a full laminate characterisation.
Environment: our environmental chambers take the fixture for hot/wet compression; keep the guide posts and bearings rated for the chamber temperature.

ISO 14126 FAQ

What is ISO 14126?

ISO 14126 is the ISO standard for the in-plane compressive strength, modulus and failure strain of fibre-reinforced plastic composites. Flat tabbed or untabbed coupons are loaded in compression at 1 mm/min through a shear-loading (Method 1) or combined-loading (Method 2) fixture, with strain measured on both faces and a 10 % bending limit. The current edition is ISO 14126:2023.

What is the difference between Method 1 and Method 2?

Method 1 introduces the load by shear through end tabs clamped in tapered wedges (IITRI/Celanese-type fixtures or aligned hydraulic grips). Method 2, redefined in 2023, is combined loading: end bearing plus shear through a clamped length, as in the ASTM D6641 CLC fixture. Pure end loading (a D695-type jig) is no longer an ISO 14126 method.

What are the ISO 14126 specimen dimensions?

Type A: 110 × 10 × 2 mm, 10 mm gauge length, tabs mandatory (at least 50 mm long, about 1 mm thick). Type B1: 110 × 10 mm, 2 to 10 mm thick, 10 mm gauge, tabs optional. Type B2: 125 × 25 mm, at least 4 mm thick, 25 mm gauge, tabs optional (50 mm long, 0.5 to 2 mm thick).

What test speed does ISO 14126 specify?

1 ± 0.5 mm/min crosshead speed, constant to failure. ASTM D3410 uses 1.5 mm/min and ASTM D6641 1.3 mm/min.

How is the compressive modulus calculated?

As a chord modulus between 0.05 % and 0.25 % strain: Ec = (σ at 0.25 % − σ at 0.05 %) / 0.0020, using the average of the strains measured on both faces. The ASTM methods use 0.1 to 0.3 % (1,000 to 3,000 µε).

What is the bending limit in ISO 14126?

10 %: |εb − εa| must not exceed 0.1 |εb + εa|, checked between 10 % and 90 % of the maximum force. Specimens that exceed it are discarded and replaced, and the fixture alignment is checked against the normative Annex A.

Which failure modes are valid?

Failures within or at the end of the gauge section (fibre kinking, through-thickness shear, transverse splitting across the gauge). Tab debonding, failures inside the wedges or grips and end brooming in a Method 2 fixture are invalid and the specimen is replaced.

How many specimens are needed?

At least 5 valid specimens per direction and condition; invalid ones are replaced. The report gives the mean, standard deviation and coefficient of variation.

What changed in ISO 14126:2023?

Method 2 was redefined as combined loading (end loading alone is excluded), specimen and load-train alignment became a normative annex, specimen preparation became normative, and new annexes cover Euler buckling, predicted tab length and digital image correlation. A draft Method 3 was dropped.

How does ISO 14126 compare with ASTM D3410 and D6641?

ISO Method 1 maps to D3410 (shear loading, IITRI fixture, 140 to 155 mm coupons, 1.5 mm/min) and Method 2 to D6641 (combined loading, 140 × 13 mm, 1.3 mm/min, screws at 2.5 to 3.0 N·m). ISO uses a 10 mm gauge on Type A versus 10 to 25 mm (D3410) and 13 mm (D6641), a 0.05 to 0.25 % modulus window versus 0.1 to 0.3 %, and 1 mm/min. All three limit bending to 10 % and need at least 5 specimens.

What fixture capacity do I need?

A Type A carbon UD coupon (10 × 2 mm) at 1,500 MPa needs 30 kN; a Type B1 at 10 × 10 mm and 600 MPa needs 60 kN; a Type B2 carbon coupon at 25 × 10 mm and 600 MPa needs 150 kN. The Universal Grip shear-loading fixture is 100 kN standard with higher capacities on request.

Can I use DIC or a video extensometer instead of strain gauges?

Yes. ISO 14126:2023 Annex G covers digital image correlation and permits strain on the narrow side faces; any method with an error of 1 % or less is acceptable provided strain is measured on both faces so the bending check can be made. Strain-gauge grids on Type A and B1 must be shorter than 3 mm.

Need a shear-loading compression fixture, combined-loading fixture, strain measurement or a complete composites compression system for ISO 14126? Contact Universal Grip with your specimen type, laminate and expected strength and we will quote the right setup.

Literature

Pricing and Availability

ISO 14126 In-Plane Compressive Properties of Fibre-Reinforced Composites