Adjustable spanTJ-124
Three Point Bend Fixture
A three-point bend fixture with adjustable span and the roller diameters the method specifies — span-to-depth ratio is set on the fixture, not assumed.
SpecificationsTesting standard
ISO 14125
Fibre-reinforced plastic composites — Determination of flexural properties
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ISO 14125 determines the flexural strength and modulus of a fibre-reinforced composite by bending a rectangular bar on two supports. The span-to-thickness ratio is the decision that matters: too short and the specimen fails in interlaminar shear instead of bending, and the number then describes the wrong property entirely.
At a glance
- Test type
- Flexure & bend — the specimen is bent
- Published by
- ISO
- Edition
- ISO 14125:1998
- Material
- Composites & sandwich structures
- Runs on
- Series 7200 and Series 9000
What the test does
A rectangular bar cut from the laminate is placed on two supports set to a span calculated from the specimen's thickness — commonly sixteen times it. A loading nose descends at a rate chosen to give a specified strain rate at the outer surface, either at mid-span for three-point loading or at two points for four-point. Force is recorded against deflection until the specimen fails. Flexural stress comes from the force, the span and the section; flexural strain from the deflection, the thickness and the span; and the modulus from the slope over a defined strain range.
What it measures, and why it matters
The outputs are flexural strength and flexural modulus. Bending is how a great many composite parts are actually loaded — panels, spars, leaf springs, skis, blade shells, enclosure walls — and because bending loads the outer plies hardest while barely troubling the middle, it is also unusually sensitive to how a laminate is stacked. That makes it a searching production control: a change in lay-up sequence, a cure problem or a resin-starved surface ply shows up here before it shows up in tension. The test is also quick and needs no tabs, gauges or grips, which is why it is so widely specified.
Specimen and span
Almost everything that goes wrong in composite flexure is a span problem. The ratio decides which failure you get.
- Specimen
- A rectangular bar cut from the laminate
- Span-to-thickness ratio
- Specified by class of material — commonly 16:1The single most important setting. Short spans shear; long spans bend.
- Loading nose and support radii
- SpecifiedA nose sharper than specified crushes the surface locally and the specimen fails under it.
- Three-point or four-point
- As the specification requiresFour-point puts a length under constant moment, so the failure finds the weakest section rather than the loaded one.
- Conditioning
- As the specification requires
- Check the failure mode before accepting a number
- Every specimenDakTensile failure on the lower face is a flexural result. Delamination in the mid-plane is a shear result wearing a flexural label.
Flexural strength is not a material property in the way tensile strength is. It depends on span, on roller radii and on the lay-up through the thickness, so it compares specimens tested identically and nothing else.
Test speed
- Rate
- Chosen to give the specified outer-fibre strain rateExpressed as a strain rate so specimens of different thickness and span are strained comparably.
- Modulus
- From a defined strain range on the curve
- Deflection source
- Crosshead travel, or a deflectometer where the modulus mattersCrosshead travel includes support indentation and frame compliance, which flatters nothing but the errors.
- Stop at the first load drop
- Not at the lastDakA delaminated specimen often carries load again afterwards; that second peak is not a flexural strength.
Calculations
Flexural stressσf
σf = 3 P L / (2 b h²)
- P
- force, N
- L
- support span, mm
- b
- width, mm
- h
- thickness, mm
For three-point loading. The four-point expression differs — using the three-point formula on four-point data is a common and invisible error.
Flexural strainεf
εf = 6 s h / L²
- s
- mid-span deflection, mm
Flexural modulusEf
Ef = Δσf / Δεf over the defined range
How the test runs
- 01Cut rectangular bars from the laminate and measure width and thickness.
- 02Select the span from the specified span-to-thickness ratio.
- 03Set the supports to that span and check the roller radii.
- 04Condition to the specification.
- 05Place the specimen centrally, span perpendicular to its length.
- 06Set the rate to give the specified outer-fibre strain rate.
- 07Load, recording force against deflection.
- 08Stop at the first significant load drop.
- 09Examine the specimen and classify the failure mode.
- 10Reject any specimen that delaminated rather than failing in bending.
- 11Compute stress with the expression matching the loading geometry.
Grips and fixtures for this method
Uniform momentTJ-165
Four Point Bend Fixture
Four-point loading where the specification calls for it, putting a length of the specimen under constant moment rather than concentrating it under one nose.
SpecificationsWhat the report has to contain
- Reference to ISO 14125 and the edition
- Material, lay-up and cure schedule
- Specimen dimensions
- Support span and span-to-thickness ratio
- Three-point or four-point loading
- Loading nose and support radii
- Conditioning and test temperature
- Rate and the outer-fibre strain rate it produced
- Flexural strength and modulus
- Failure mode for each specimen
- Specimens rejected, with the reason
What the machine must be capable of
Force measurement to ISO 7500-1 Class 1 across a modest range, commonly a hundred newtons to a few kilonewtons, and a crosshead able to hold the rate that produces the specified outer-fibre strain rate — which changes with span and thickness, so it is calculated per specimen rather than fixed. The bend fixture must offer an adjustable span and the correct roller diameters. Where flexural modulus is required rather than strength alone, a deflectometer reading mid-span deflection directly is worth having: crosshead travel includes the specimen sinking into the supports, the fixture flexing and the frame's compliance, all of which are read as deflection and depress the modulus.
What goes wrong in practice
Nearly everything traces back to the span. A ratio too short delaminates the specimen and produces an interlaminar shear result labelled as a flexural strength — not a slightly wrong number but a measurement of another property entirely, and it looks perfectly reasonable on the certificate. The second recurring error is taking the peak force from the wrong place: a delaminated composite often carries load again as the separated plies bear together, giving a second peak higher than the first, and only the first significant drop is the material failing. Using the three-point stress expression on four-point data is a third, and it is invisible in the result.
ISO 14125 or ASTM D7264
| ISO 14125 | ASTM D7264 | |
|---|---|---|
| Geometry | Three-point and four-point | Three-point (Procedure A) and four-point (Procedure B) |
| Span ratio | Specified by material class | 16:1 default, adjustable |
| Rate basis | Outer-fibre strain rate | Outer-fibre strain rate |
| Used by | International and European specifications | North American specifications |
Close in intent and in the quantities reported. Span, roller radii and loading geometry all change a flexural number, so results transfer only when those match — the method actually run belongs on the certificate.
Questions we are asked about this test
What is ISO 14125?
It is the ISO method for the flexural properties of fibre-reinforced plastic composites. A rectangular bar is supported on two rollers and loaded either at mid-span by a single nose or at two points, and the force-deflection record gives flexural strength and flexural modulus.
Why does the span-to-thickness ratio matter so much?
Because it decides which failure you get. On a short span the dominant stress between the plies is shear, and the specimen delaminates in its mid-plane before the outer fibres reach their limit — so the number you record is an interlaminar shear failure wearing a flexural label. A longer span lets bending dominate. Getting this wrong does not produce a slightly wrong flexural strength; it produces a measurement of a different property.
Is flexural strength a real material property?
Not in the way tensile strength is. It depends on the span, on the roller radii and on how the lay-up is arranged through the thickness, because bending loads the outer plies hardest and barely troubles the middle. It is a genuinely useful comparative and quality-control figure, and it is not a value to feed into a design calculation as though it were intrinsic.
Why is four-point loading sometimes specified?
Because it puts a length of the specimen under constant bending moment rather than concentrating the maximum under a single nose. The failure then occurs at the weakest section within that length rather than wherever the nose happened to be, which is more representative of the material and avoids the local crushing a single nose can cause on a soft laminate.
Why should deflection come from a deflectometer rather than the crosshead?
Because crosshead travel includes the specimen sinking into the supports, the fixture flexing and the frame's own compliance. For a strength figure that hardly matters, since only the peak force is used. For a modulus it matters a great deal — every bit of that extra travel is read as specimen deflection and the modulus comes out low.
What do I do if the specimen delaminates?
Reject it and check the span. A mid-plane delamination is an interlaminar shear failure, which means the span was too short for this laminate. Lengthening the span to the specified ratio usually moves the failure into bending. If it does not, the material genuinely has poor interlaminar strength — which is itself a finding, and one that ASTM D2344 short-beam strength is designed to quantify.
Why stop at the first load drop?
Because a composite that has delaminated will often carry load again as the separated plies bear on one another, producing a second peak higher than the first. That second peak is not a flexural strength — it is a broken specimen being pressed together. The first significant drop is where the material actually failed.
Running ISO 14125 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 | Low to moderate — commonly 100 N to 5 kN depending on lay-up and span | 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 |
| Gripping | Three-point or four-point bend fixture with specified loading nose and support radii | Our bend fixtures, 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.