TJ-144
Heavy Duty Hydraulic Grips
Heavy duty hydraulic wedge grips hold a constant clamping force as the coupon thins slightly under load, which mechanical wedges do not always manage at composite failure loads.
SpecificationsTesting standard
ISO 527-5
Plastics — Determination of tensile properties — Part 5: Test conditions for unidirectional fibre-reinforced plastic composites
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ISO 527-5 gives the tensile test conditions for unidirectional fibre-reinforced composites — the coupon types, the end tabbing, the rate and the strain measurement. Its practical burden is that a result only counts if the coupon failed in the gauge section, and getting it to do so is most of the work.
At a glance
- Test type
- Tensile — the specimen is pulled apart
- Published by
- ISO
- Edition
- ISO 527-5:2021
- Runs on
- Series 7200 and Series 9000
What the test does
A flat rectangular coupon with bonded end tabs is gripped in wedges and pulled at a constant crosshead rate, commonly 2 mm/min, while force and strain are recorded to failure. Two geometries are defined: Type A, nominally 250 mm by 15 mm by 1 mm, for testing along the fibres, and Type B, nominally 250 mm by 25 mm by 2 mm, for testing across them. Strain is measured with an extensometer or bonded gauges, and where Poisson's ratio is required it is measured in both the axial and transverse directions at once. Tensile strength, a chord modulus over the 0,0005 to 0,0025 strain window, strain at break and Poisson's ratio come out of the run.
What it measures, and why it matters
The stiffness and strength of a unidirectional laminate in a single, known fibre direction — which is the input every laminate analysis is built from. A structural composite part is a stack of plies at different angles, and its behaviour is predicted from the properties of one ply along and across the fibres. Those two sets of numbers come from Type A and Type B coupons respectively, and they differ by more than an order of magnitude, because one is carried by the fibres and the other by the matrix. Getting them wrong propagates into every prediction made from the laminate model, which is why the method is so specific about geometry and failure location.
Coupon types
Type A is the 0° coupon, Type B the 90°. They are different widths and thicknesses because they fail at wildly different loads.
- Type A — 0° unidirectional
- 250 mm × 15 mm × 1 mm nominalThin and narrow because the fibres carry everything and the load would otherwise be enormous.
- Type B — 90° unidirectional
- 250 mm × 25 mm × 2 mm nominalWider and thicker because it fails at a small fraction of the 0° load.
- End tabs
- Bonded, commonly 50 mm longRequired for Type A in practice. The tab spreads the grip load into shear instead of crushing the fibres.
- Gauge length
- Typically 50 mm between tabs for strain measurement
- Tab adhesive
- Tougher than the laminate matrixA brittle adhesive debonds before the coupon fails, and the test is over.
- Abrade and clean before bonding tabs
- Both surfacesDakTab debonding is the single most common reason a composite coupon is discarded, and it is nearly always a bonding problem rather than a material one.
A 0° coupon that failed at or under the tab has reported the tab, not the laminate. The failure mode code is part of the result and a report without it cannot be assessed.
Test speed
- Rate
- Commonly 2 mm/min
- Modulus strain range
- 0,0005 to 0,0025 strainThe same window as ISO 527-1, so moduli are comparable across the parts.
- Poisson's ratio
- From transverse and axial strain together
- Record the failure mode and location
- On every couponDakIt is what decides whether the number is admissible.
Calculations
Tensile strengthσM
σM = F / (b × h)
- F
- maximum force, N
- b
- width, mm
- h
- thickness, mm
On the gauge section dimensions, measured on the coupon, not nominal.
Tensile modulusEt
Et = (σ₂ − σ₁) / (ε₂ − ε₁)
- σ₁, σ₂
- stresses at strains ε₁ and ε₂
- ε₁, ε₂
- 0,0005 and 0,0025
A chord modulus over a fixed strain window rather than a tangent, so it does not depend on where anyone chose to draw a line.
Poisson's ratioν
ν = −εtransverse / εaxial
- εtransverse
- strain across the coupon
- εaxial
- strain along it
Requires strain measured in both directions simultaneously, which is why biaxial gauges are common on these coupons.
How the test runs
- 01Cut coupons to Type A or Type B geometry with the fibres correctly aligned.
- 02Abrade and clean both the coupon ends and the tab material.
- 03Bond end tabs with an adhesive tougher than the laminate matrix.
- 04Condition the coupons to the specified atmosphere.
- 05Measure width and thickness in the gauge section.
- 06Fit wedge grips and align the coupon so loading is axial.
- 07Fit an extensometer or bond strain gauges, in both directions if Poisson's ratio is wanted.
- 08Load at the specified constant crosshead rate.
- 09Record force and strain to failure.
- 10Calculate strength, chord modulus over 0,0005 to 0,0025 strain, and Poisson's ratio.
- 11Record the failure mode and location on every coupon, and discard any that failed in or at the tab.
Grips and fixtures for this method
Self-tighteningTJ-15
Universal Parallel Wedge Grips
Universal parallel wedge grips where the coupon width and failure load sit within their range.
SpecificationsWhat the report has to contain
- Reference to ISO 527-5 and the edition
- Material, fibre, matrix and fibre volume fraction
- Laminate lay-up and cure schedule
- Coupon type, and measured width and thickness
- Tab material, length and adhesive
- Conditioning atmosphere and time
- Test rate
- Tensile strength, modulus and strain at break
- Poisson's ratio where measured
- Failure mode and location for every coupon, with those excluded and why
What the machine must be capable of
Enough force for a 0° coupon — commonly thirty to a hundred kilonewtons — with wedge grips that clamp the tabbed ends squarely and alignment good enough that the load is genuinely axial, since any bending is concentrated at the tab ends where failure is already most likely. Strain measurement to Class 0,5 is required for modulus, and biaxial capability where Poisson's ratio is wanted. The frame needs the daylight for a 250 mm coupon plus grips, and enough stiffness that the sudden release when a 0° coupon fails does not disturb the load train.
What goes wrong in practice
Coupons failing in or at the tab, which reports the gripping arrangement rather than the laminate and makes the result inadmissible; this is why the failure mode and location are recorded on every specimen and why exclusions are stated. Brittle tab adhesive that peels before the coupon reaches its own limit. Misalignment, which biases every coupon the same way and is therefore invisible in the scatter. And quoting a modulus without naming the method, since ASTM D3039 uses a different strain window and produces a different figure from the same curve.
ISO 527-5 or ASTM D3039
| ISO 527-5 | ASTM D3039/D3039M | |
|---|---|---|
| Scope | Unidirectional composites specifically | Polymer matrix composites generally |
| Coupon | Type A 0°, Type B 90° | Width and thickness by lay-up |
| Modulus | Chord, 0,0005–0,0025 strain | Chord, 0,001–0,003 strain |
| Failure codes | Recorded | Three-character code required |
Close in intent, different in the strain window used for modulus — so moduli from the two are not directly interchangeable even on the same laminate. Specify which method was used whenever a modulus is quoted.
Questions we are asked about this test
What is ISO 527-5?
It is Part 5 of the ISO 527 tensile series, giving the test conditions for unidirectional fibre-reinforced composites. Parts 1 and 4 supply the general principles and the conditions for isotropic and orthotropic composites; Part 5 adds the coupon geometries, tabbing and strain measurement that unidirectional laminates specifically need.
Why are Type A and Type B different sizes?
Because they fail at completely different loads. A 0° coupon carries load on the fibres and can reach very high stresses, so it is made thin and narrow to keep the force within a sensible frame capacity. A 90° coupon carries load on the matrix across the fibres and fails at a small fraction of that, so it is made wider and thicker to give a measurable load and a coupon robust enough to handle.
Why do the coupons need end tabs?
Because a serrated grip face bites into the surface and crushes fibres exactly where the load is highest. A bonded tab converts that concentrated clamping into shear transferred gradually along the bond line, moving the peak stress away from the jaws and into the gauge section. Without tabs a 0° coupon fails at the grip almost every time, and that result reports the gripping arrangement rather than the laminate.
Why does the tab adhesive have to be tough?
Because if the adhesive is more brittle than the laminate matrix, the bond line fails first. The coupon then slips or the tab peels, the test ends before the material has been loaded to its own limit, and the coupon is wasted. Tab debonding is the most common reason a composite tensile coupon is discarded, and it is nearly always a surface preparation or adhesive selection problem rather than anything to do with the material under test.
Why is the modulus taken as a chord over a fixed strain window?
So that it does not depend on judgement. A tangent modulus depends on where the operator chose to draw the line, and on a composite whose curve has slight initial non-linearity from take-up that choice can move the answer by several per cent. Fixing the window at 0,0005 to 0,0025 strain makes every laboratory compute the same slope from the same two points.
Does the failure location really invalidate a result?
For a 0° coupon, yes. A failure in or immediately at the tab means the stress concentration from the grip caused it, so the recorded strength is a lower bound on the laminate rather than a measurement of it. The failure mode and location are recorded for every coupon precisely so that inadmissible results can be identified and excluded, with the exclusions stated in the report.
Can ISO 527-5 and ASTM D3039 results be compared?
Strength usually, modulus not directly. The two methods use different strain windows for the chord modulus — 0,0005 to 0,0025 here against 0,001 to 0,003 in D3039 — so on a laminate with any curvature the two produce different figures for the same material. Whenever a composite modulus is quoted, the method behind it should be quoted with it.
Running ISO 527-5 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 | Moderate to high — a 0° carbon coupon commonly needs 30 to 100 kN | 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 | Wedge grips with tabbed coupon ends; the tabs are part of the specimen, not the fixture | Our self-tightening serrated wedge grips, with V-jaws for round specimens, 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.