Testing standard
ISO 14126
Fibre-reinforced plastic composites — Determination of compressive properties in the in-plane direction
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ISO 14126 determines the in-plane compressive strength and modulus of a fibre-reinforced composite. The coupon is loaded in a fixture with a very short unsupported gauge length, because a thin composite coupon in compression is a column that will buckle long before it crushes. Back-to-back strain measurement is what proves which of the two actually happened.
At a glance
- Test type
- Compression — the specimen is squeezed
- Published by
- ISO
- Edition
- ISO 14126:1999
- Material
- Composites & sandwich structures
- Runs on
- Series 7200 and Series 9000
What the test does
A flat coupon, tabbed where the lay-up requires it, is assembled into a compression fixture that supports it right up to the edges of a very short unsupported gauge length. Load is introduced by shear through the tabs, by bearing directly on the coupon ends, or by a combination of the two, depending on the arrangement chosen. Strain gauges bonded back to back at the centre of the gauge section record both faces as the coupon is loaded at about 1 mm/min to failure. Compressive strength comes from the maximum force and the gauge area; modulus from the average of the two strain readings; and a percentage bending figure from their difference.
What it measures, and why it matters
The outputs are in-plane compressive strength and modulus, and a bending figure that decides whether the first two can be believed. Composites are markedly weaker in compression than in tension, so compression very often sizes a composite structure rather than tension does. Every laminate in bending has a compressive face; buckling and crippling calculations for panels and stiffeners need compressive modulus; and compression-after-impact work needs an undamaged compressive strength as its baseline. A design that used tensile allowables where compression governs would be unsafe by a wide margin.
Specimen and fixture
Compression testing of composites is a fight against buckling. Every dimension and every fixture detail exists to win it.
- Unsupported gauge
- Short — the fixture supports the coupon either side of itLong enough to be representative, short enough that buckling cannot precede crushing.
- Load introduction
- Shear through tabs, end loading, or bothThe standard recognises more than one route; which was used belongs in the report.
- Tabs
- Where the lay-up needs themHigh-strength unidirectional material is where end loading risks crushing the coupon ends.
- Faces
- Flat and parallel to close toleranceA coupon that sits crooked in the fixture bends from the first newton.
- Strain measurement
- Back to backNot redundancy — the difference between the faces is how bending is detected and quantified.
- Reject on the bending criterion, not on judgement
- Every specimenDakIt is the only thing separating a compressive strength from a buckling load.
A compressive result without a bending check is not a compressive strength. It is the load at which that coupon became unstable in that fixture, and it will be lower than the material's real capability by an amount nobody can estimate afterwards.
Test speed
- Crosshead speed
- 1 mm/min nominal
- Modulus
- Over a defined strain range, from the averaged strainsAveraging the two faces cancels bending in the modulus even where it invalidates the strength.
- Percentage bending
- Computed and checked against the limit
- Watch the two traces diverge
- It is the early warningDak
Calculations
σc = Fmax / A
- Fmax
- maximum force, N
- A
- gauge cross-sectional area, mm²
Ec = Δσ / Δε over the defined range
From the mean of the two back-to-back readings.
B = |ε₁ − ε₂| / |ε₁ + ε₂| × 200
- ε₁, ε₂
- strains on the two opposite faces
The validity check. Above the limit the coupon was bending, and the strength is not usable.
How the test runs
- 01Machine coupons keeping faces flat and parallel.
- 02Bond tabs where the lay-up and load route require them.
- 03Measure the gauge cross-section.
- 04Bond strain gauges back to back at the gauge centre.
- 05Condition to the specification.
- 06Assemble the coupon into the fixture, seating it evenly.
- 07Check alignment before loading.
- 08Load at 1 mm/min, recording force and both strain channels.
- 09Take the modulus from the averaged strains over the defined range.
- 10Compute percentage bending and check it against the limit.
- 11Classify the failure mode and reject anything failing outside the gauge.
What the report has to contain
- Reference to ISO 14126 and the edition
- Material, lay-up and cure schedule
- Coupon dimensions and gauge cross-section
- How load was introduced — shear, end loading or combined
- Tab details, or a note that none were used
- Conditioning and test temperature
- Crosshead speed
- Compressive strength and modulus per coupon
- Percentage bending per coupon
- Failure mode and location
- Coupons rejected, with the reason
What the machine must be capable of
Force measurement to ISO 7500-1 Class 1 at loads that can reach several tens of kilonewtons on a small section, a crosshead holding 1 mm/min, and at least two strain channels recorded simultaneously with force. Platens must be flat and parallel, since they load the fixture body and any tilt there passes straight into the coupon. Alignment dominates throughout: this specimen amplifies imperfection rather than correcting it, so a load path adequate for tensile work may not be adequate here. Conditioned or elevated-temperature testing requires the fixture to reach temperature with the coupon, because its thermal mass is large by comparison.
What goes wrong in practice
The central failure is accepting a buckling load as a compressive strength. It is convincing — a smooth curve, a clean maximum, nothing in the force trace to suggest anything is wrong — and only the two strain channels reveal it, which is exactly why the bending check is mandatory. Beyond that, the recurring problems are coupons machined without adequate flatness, tabs of uneven thickness that tilt the specimen in the fixture, and failures at the tab ends being reported rather than rejected. A subtler error is taking modulus from a single gauge, which builds any bending straight into the number instead of cancelling it.
ISO 14126, ASTM D6641 or ASTM D3410
| ISO 14126 | ASTM D6641 | ASTM D3410 | |
|---|---|---|---|
| Load route | Shear, end, or combined | Combined loading | Shear only |
| Tabs | Where needed | Often unnecessary | Usually required |
| Fixture | Several recognised | CLC fixture | Shear-loading fixture |
| Bending check | Mandatory | Mandatory | Mandatory |
All three demand back-to-back strain and reject on a bending criterion, because all three are fighting the same problem. Where a specification names one, run that one — the fixtures introduce load differently and the results are not pooled.
Questions we are asked about this test
What is ISO 14126?
It is the ISO method for the in-plane compressive properties of fibre-reinforced plastic composites. A coupon with a short unsupported gauge length is loaded in a fixture — by shear through its tabs, by end loading, or by a combination — while back-to-back strain gauges record both faces.
Why is composite compression so much harder than tension?
Because tension is self-correcting and compression is not. A coupon in tension straightens itself as it loads; a coupon in compression amplifies any initial crookedness, so a small imperfection in flatness, tab thickness or fixture alignment grows into bending. That is why compression methods specify fixtures in such detail, demand strain measurement on both faces, and reject results on a bending criterion — none of which a tensile method needs.
Why must the gauge length be so short?
Because a thin composite coupon is essentially a column, and a column buckles long before it reaches its material compressive strength. Shortening the unsupported length raises the buckling load above the compressive strength so the coupon crushes instead. Everything else in the fixture exists to support the coupon right up to the edges of that short gauge.
What is percentage bending and why does it decide validity?
It is the difference between the strains on the two faces expressed against their average, and it is the only thing distinguishing a real compressive strength from a buckling load. A coupon that is bending gives a smooth curve and a clean maximum that looks entirely convincing — the force trace cannot tell you it is wrong. Only the two strain channels can, which is why the check is mandatory rather than advisory.
Do I need tabs?
It depends on the lay-up and the load route. Tabs spread the grip or fixture load so the coupon is not damaged where it is held, and high-strength unidirectional laminates usually need them because end loading risks crushing the coupon ends. Softer and lower-strength lay-ups often test satisfactorily untabbed. The practical test is where failures occur: repeated failures at the tab ends mean the tabbing needs attention.
Why average the two strain readings for modulus?
Because averaging cancels the bending component. If one face is in more compression than the other, the mean of the two is the axial strain the coupon would have seen with no bending at all. That is why a modulus can survive a degree of bending that would invalidate a strength — but only if it is taken from the average rather than from one gauge.
How does it relate to the ASTM methods?
Closely. ASTM D6641 uses a combined loading fixture and often needs no tabs; ASTM D3410 introduces load purely by shear through wedge grips. ISO 14126 recognises more than one route and is the international equivalent. All three demand back-to-back strain and reject on bending, because all three are fighting the same instability. Where a specification names one, run that one — the results are not pooled.
Running ISO 14126 on the Series 7200 and Series 9000
Dak verifies against whichever standard the method names, and where a class applies our frames sit a class tighter than it asks.
| The method asks for | Dak supplies | |
|---|---|---|
| Capacity | High for the section — commonly 20 to 60 kN on carbon laminates | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | ISO 7500-1 Class 1 over the working range | ISO 7500-1 Class 0.5 — a class tighter than the method asks |
| Strain measurement | An extensometer of the class the method specifies | Certified to ISO 9513 Class 1 and ASTM E83 — non-contact video, clip-on and high-elongation |
| Gripping | A compression fixture introducing load by shear through the tabs, by end loading, or by a combination, with a short unsupported gauge | Our compression anvils, built to the specimen |
| Environment | 23 ± 2 °C standard laboratory atmosphere | 3009 series chambers, −150 °C to +400 °C — temperature only |
This page describes the method as practised. The governing text is the current edition from the issuing body. Tell us what you are testing and we will answer with the machine, the fixture and a quotation.