Adjustable spanTJ-124
Three Point Bend Fixture
Procedure A, and the adjustable span this method needs — 32:1 on a 4 mm laminate is a 128 mm span, far longer than a plastics flexural test uses.
SpecificationsTesting standard
ASTM D7264
Standard Test Method for Flexural Properties of Polymer Matrix Composite Materials
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ASTM D7264 measures the flexural properties of polymer matrix composites. A laminate coupon is bent on a short span — three-point in Procedure A, four-point in Procedure B — and the method reports flexural stress at failure, flexural strain, and chord modulus between 0.001 and 0.003 strain. The span-to-thickness ratio is part of the result, not a setting.
At a glance
- Test type
- Flexure & bend — the specimen is bent
- Published by
- ASTM
- Edition
- D7264/D7264M-26
- Material
- Composites & sandwich structures
- Runs on
- Series 7200 and Series 9000
What the test does
A flat rectangular laminate coupon is laid across two hardened steel support cylinders — not gripped at all. The crosshead drives a loading nose down onto the middle of the beam (Procedure A, three-point) or two noses onto the quarter points (Procedure B, four-point) at a constant rate. Force and midspan deflection are recorded continuously until the beam fails.
What it measures, and why it matters
The method reports flexural stress at failure, flexural strain, and flexural chord modulus taken between 0.001 and 0.003 strain. Strength is used for incoming-lot acceptance of prepreg and cured panels, because a bend coupon catches resin cure, void content and fibre wetting faster than a tensile coupon. Modulus feeds panel stiffness calculations and laminate models. Flexure loads the outer plies hardest, so the figures rank processing quality and support failure investigations rather than serving directly as design allowables.
Coupon and span
The governing dimension here is the SPAN, not a gauge length. Change the ratio and you change the balance between bending and interlaminar shear, and therefore the number.
- Standard span-to-thickness
- 32 : 1128 mm span on a 4 mm laminate. 16:1, 40:1 and 60:1 are permitted where the material justifies them.
- Coupon length
- About 20 % longer than the spanRoughly 154 mm at the standard ratio, so it cannot roll off the supports.
- Standard width
- 13 mm
- Edges
- Machined square, free of saw delamination
- Report the ratio
- AlwaysIt is part of the result. A flexural strength quoted without its span ratio cannot be compared with anything.
- Conditioning
- Ambient, or to D5229 where moisture matters
- Check the failure mode
- Before recording a numberDakA coupon that fails by interlaminar shear, or under the loading nose, is not a valid flexural result — and the curve does not say so.
Test speed
- Crosshead rate
- 1.0 mm/minAn order of magnitude slower than D790 on a comparable coupon. Polymer matrices are rate-sensitive, so a faster run inflates both strength and modulus.
- Procedure A
- Three-point loading
- Procedure B
- Four-point loadingReport which was used — they do not give the same number.
Calculations
Flexural stress, three-pointσ
σ = 3 P L / (2 b h²)
- P
- force, N
- L
- support span, mm
- b
- width, mm
- h
- thickness, mm
Flexural chord modulusEf
Ef = Δσ / Δε between 0.001 and 0.003 strain
- Δσ
- change in flexural stress across the interval, MPa
- Δε
- corresponding change in flexural strain
A fixed interval, as in D3039 — 0.1 % to 0.3 % strain — rather than the steepest region of the curve.
How the test runs
- 01Cut coupons from the cured panel with edges machined square and free of saw damage.
- 02Measure width and thickness at the midspan.
- 03Choose the span ratio — 32:1 as standard — and set the span from the measured thickness.
- 04Cut or trim the coupon to about 20 % longer than the span.
- 05Fit the fixture for the procedure in use: one nose for A, two for B.
- 06Check the supports and noses are parallel and free to rotate.
- 07Place the coupon centred and confirm it does not rock.
- 08Run at 1.0 mm/min, recording force and midspan deflection.
- 09Continue to failure.
- 10Inspect the failure: reject anything that failed by interlaminar shear or under the nose.
- 11Take chord modulus between 0.001 and 0.003 strain.
- 12Report the span ratio and the procedure alongside every figure.
Watch the test
Grips and fixtures for this method
Uniform momentTJ-165
Four Point Bend Fixture
Procedure B, where the peak stress is spread across a length of the coupon rather than concentrated beneath a single nose.
SpecificationsWhat the report has to contain
- Reference to ASTM D7264 and whether Procedure A or B
- Material, lay-up, ply orientation and cure schedule
- Coupon dimensions and panel location
- SPAN-TO-THICKNESS RATIO and the span used
- Conditioning, including moisture conditioning where applied
- Crosshead rate
- Flexural stress at failure and flexural strain
- Flexural chord modulus with the strain interval
- Failure mode and location for every coupon
- Number of valid coupons and any rejected
What the machine must be capable of
Forces are modest. A 13 mm by 4 mm laminate on a 128 mm span typically peaks between a few hundred newtons and about 2 kN in three-point loading, and roughly twice that in four-point for the same outer-fibre stress, so a 5 kN frame is ample — but the load cell must resolve the early elastic region cleanly, because chord modulus is computed from data between 0.1 % and 0.3 % strain, near the bottom of the range. Force indication is verified to ASTM E4.
Crosshead rate is 1.0 mm/min [0.05 in./min] for both procedures, an order of magnitude slower than D790 on a comparable coupon; polymer matrices are rate-sensitive, so a faster run inflates both strength and modulus.
Midspan deflection must be read at the centre of the support span. A deflectometer working upright against the underside of the beam gives the true sag; crosshead travel is permitted but includes fixture and frame compliance. No public source consulted states a required ASTM E83 class for the deflectometer.
The fixture is a long-beam bend rig with hardened cylindrical supports and loading noses of parallel axis, commonly 5 mm radius, fixed, rotatable or rolling. Procedure B uses quarter-point loading — the load span exactly half the support span, and unlike D6272 third-point loading is not permitted. Non-parallel or undersized noses crush the surface plies and end the test early. Elevated- or low-temperature work needs an environmental chamber around the span.
What goes wrong in practice
Interlaminar shear failure is the standard trap: too short a span and the beam fails by plies sliding, giving a low number that is not a flexural strength. Loading-nose crushing does the same locally on soft-matrix or thin-faced laminates. Delamination initiating from a sawn edge drops strength with no visible cause. At long spans, large deflections make the simple beam equations non-conservative, so strain and stress are overstated unless the run is kept within the method's limits.
D7264, D790 and D6272
| ASTM D7264 | ASTM D790 | ASTM D6272 | |
|---|---|---|---|
| Subject | Polymer matrix composites | Plastics and insulation | Plastics and insulation |
| Loading | Three- or four-point | Three-point | Four-point |
| Standard span ratio | 32 : 1 | 16 : 1 | 16 : 1 |
| Rate | 1.0 mm/min fixed | Strain-rate derived | Strain-rate derived |
| Modulus | Chord, 0.001–0.003 strain | Initial tangent | Initial tangent |
The longer span is the point. At 16:1 a stiff laminate fails in interlaminar shear rather than in the tension face, and the recorded number is not a flexural strength at all — which is exactly the failure D790 warns about for reinforced materials.
Questions we are asked about this test
What is ASTM D7264?
It is the ASTM method for flexural properties of polymer matrix composites. A laminate coupon is bent on a short span, in three-point loading under Procedure A or four-point under Procedure B, and the method reports flexural stress at failure, flexural strain and flexural chord modulus.
Why does D7264 use a 32:1 span when D790 uses 16:1?
Because a stiff laminate on a short span fails by interlaminar shear rather than in the tension face, and a shear failure is not a flexural strength. Lengthening the span reduces the shear stress relative to the bending stress, so the coupon fails the way the method intends. That is also why the ratio has to be reported with the result.
What is the difference between Procedure A and Procedure B?
The loading geometry. Procedure A uses a single nose at midspan; Procedure B uses two noses at the quarter points, spreading peak stress across a length of the coupon so failure finds the weakest section within it. They do not give the same number and the procedure used must be stated.
Why is D7264 run so much slower than D790?
1.0 mm/min against roughly 1.4 or 13.7 mm/min. Polymer matrices are rate-sensitive, and a faster run inflates both strength and modulus. The slower fixed rate makes results comparable between laboratories without each having to derive a speed from the specimen.
How do I know a flexural result is valid?
By looking at the coupon, not at the curve. A specimen that failed by interlaminar shear between the plies, or crushed under the loading nose, has not produced a flexural strength — and the force-deflection trace can look entirely normal in both cases. Failure mode is recorded for every coupon for this reason.
Are flexural figures design allowables for composites?
No. Flexure loads the outer plies hardest, so the numbers rank processing quality — cure, void content, fibre wetting — and support failure investigations. They are excellent at catching a bad panel quickly, which is why they are used for incoming-lot acceptance, but design allowables come from tension, compression and shear coupons.
Running ASTM D7264 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 | A standard 13 mm wide, 4 mm thick laminate on a 128 mm span typically peaks between a few hundred newtons and about 2 kN in three-point loading, with four-point (quarter-point) loading needing roughly twice that for the same outer-fibre stress. | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | ASTM E4 | Verified to ASTM E4, and to ISO 7500-1 Class 0.5 |
| Gripping | Long-beam three- or four-point bend rig at 32:1 span-to-thickness; quarter-point loading for Procedure B | Our bend fixtures, built to the specimen |
| Environment | Ambient laboratory conditions as standard; the method is routinely run inside a chamber at the intended service temperature, and moisture conditioning is referenced to Practice D5229/D5229M | 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.