ISO 14125 Flexural Testing of Fibre-Reinforced Composites

What the test does

A rectangular bar from the laminate is placed on two supports set to a span calculated from its 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, and force is recorded against deflection until the specimen fails. Flexural stress comes from the force, the span and the section; strain from the deflection, the thickness and the span; 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 many composite parts are actually loaded — panels, spars, leaf springs, blade shells — and because it loads the outer plies hardest while barely troubling the middle, it is unusually sensitive to how a laminate is stacked. That makes it a searching production control: a lay-up change, a cure problem or a resin-starved surface ply shows up here before it shows up in tension. It 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

The tabulated speed nearest an outer-fibre strain rate of 0,01 min⁻¹ (1 %/min)v = 0,01 L²/6h for three-point and 0,01 L²/4,7h for four-point, rounded to the nearest tabulated speed. A speed named in the material standard overrides the calculation.

Tabulated speeds

0,5 · 1 · 2 · 5 · 10 · 20 · 50 · 100 · 200 · 500 mm/minTolerance ±20 % up to 10 mm/min and ±10 % above it.

Worked through, three-point

Class I and II 2 mm/min (64 mm span, 4 mm thick); Class III 1 mm/min (40 mm, 2 mm); Class IV 5 mm/min (80 mm, 2 mm)2 mm/min for Class I is the standard's own worked example.

Worked through, four-point

2 mm/min (66 mm span), 2 mm/min (45 mm) and 5 mm/min (81 mm) on the preferred specimensDakSame rule, longer spans and the 4,7h denominator.

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.

What the machine must be capable of

Force to ISO 7500-1 Class 1 over a modest range, commonly a hundred newtons to a few kilonewtons, and a crosshead able to hold the rate the method derives from the specimen. The rule is a strain rate of 0,01 per minute — 1 % per minute — at the outer surface, so the speed is calculated as v = 0,01 × L² / 6h for three-point loading and 0,01 × L² / 4,7h for four-point, then set to whichever of the ten speeds the method tabulates comes nearest: 0,5, 1, 2, 5, 10, 20, 50, 100, 200 and 500 mm/min. Worked through on the preferred three-point specimens that gives 2 mm/min for Class I and Class II, on the 64 mm span and 4 mm thickness the method sets for them — the very figure the standard uses as its own example — 1 mm/min for Class III on a 40 mm span and 2 mm thickness, and 5 mm/min for Class IV on an 80 mm span and 2 mm thickness. Four-point spans are longer — 66, 45 and 81 mm — and the same arithmetic gives 2, 2 and 5 mm/min. A speed named in the standard for the material being tested takes precedence over the calculation. The fixture needs an adjustable span and the correct roller diameters. Where 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 read as deflection and all depressing the modulus.

Grips and fixtures for this method

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.

Specifications

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.

Specifications

How the test runs

1. 01Cut rectangular bars from the laminate and measure width and thickness.
2. 02Select the span from the specified span-to-thickness ratio.
3. 03Set the supports to that span and check the roller radii.
4. 04Condition to the specification.
5. 05Place the specimen centrally, span perpendicular to its length.
6. 06Set the rate to the tabulated speed nearest a 1 %/min outer-fibre strain rate — 2 mm/min on the preferred Class I and II specimens.
7. 07Load, recording force against deflection.
8. 08Stop at the first significant load drop.
9. 09Examine the specimen and classify the failure mode.
10. 10Reject any specimen that delaminated rather than failing in bending.
11. 11Compute stress with the expression matching the loading geometry.

Calculations

What 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 in mm/min and the outer-fibre strain rate it produced
• Flexural strength and modulus
• Failure mode for each specimen
• Specimens rejected, with the reason

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, and it looks 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.