Adjustable spanTJ-124
Three Point Bend Fixture
A three-point bend fixture with the short span the method specifies. Span-to-thickness is the whole variable here — set it on the fixture, never assume it.
SpecificationsTesting standard
ISO 14130
Fibre-reinforced plastic composites — Determination of apparent interlaminar shear strength by short-beam method
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ISO 14130 determines the apparent interlaminar shear strength of fibre-reinforced plastic composites by loading a short, thick beam in three-point bending. The word apparent is doing real work: the standard states the method is not suitable for determining design parameters and is for screening or quality control.
At a glance
- Test type
- Shear
- Published by
- ISO
- Edition
- ISO 14130:1997
- Material
- Composites & sandwich structures
- Runs on
- Series 7200 and Series 9000
What the test does
A short, thick coupon is placed centrally on two supports and loaded at mid-span in three-point bending at a constant rate until it fails. The failure force, the specimen width and its thickness give the apparent interlaminar shear strength. The span and the roller radii are set to what the method specifies and are reported with the result, because the value depends on both. Every failed coupon is examined afterwards, since only a clean interlaminar crack makes the arithmetic mean anything.
What it measures, and why it matters
How well the plies of a laminate hold to one another — the property that governs delamination, which is how composite structures usually come apart. The short span is the whole design of the test: in three-point bending the peak bending stress rises with span while the shear stress does not, so a long beam breaks in tension on its underside, and shortening the span tips the balance until the interlaminar plane fails first. It is quick, cheap and discriminating, which is why it is the standard incoming-inspection test for laminates.
Short and thick, on purpose
The span-to-thickness ratio is deliberately small so the beam fails in shear between the plies rather than in bending.
- Geometry
- A short beam in three-point bendingA long beam fails in tension on its lower face. Shortening the span raises the shear stress relative to the bending stress until the interlaminar plane gives way first.
- Matrix
- Thermoset or thermoplasticBoth are in scope, provided interlaminar shear failure is actually obtained.
- Validity condition
- Interlaminar shear failure must occurIf the coupon fails in flexure or crushes under the loading nose, the number is not an interlaminar shear strength.
- Not for design
- Stated in the scopeIt is a screening and quality-control test. The word apparent is the standard's own warning.
- Corrigendum
- Technical Corrigendum 1:2003 applies
- Examine the failed coupon before recording the number
- DakA clean interlaminar crack, a tensile break on the underside and a crushed loading point look quite different and mean quite different things.
This is a quality-control number, not a design allowable. Quoting it as an interlaminar shear design value is the misuse the word apparent exists to prevent.
Test speed
- Rate
- A constant crosshead rate as specified
- Reported
- Apparent interlaminar shear strength
- Span and roller radii
- As the method specifies, and reportedThe result depends on both, so a value quoted without them cannot be reproduced.
- Reject any coupon that did not fail interlaminarly
- Dak
Calculations
Apparent interlaminar shear strengthτ
τ = 0,75 × F / (b × h)
- F
- failure force, N
- b
- specimen width, mm
- h
- specimen thickness, mm
The 0,75 is the parabolic shear distribution through a rectangular section — the peak shear stress at mid-thickness is 1,5 times the mean, and the mean here is F/2 over the area.
Why the span matters—
Shear stress is independent of span; bending stress rises with it
That asymmetry is the whole method. Shorten the span far enough and shear wins the race to failure.
How the test runs
- 01Cut coupons to the dimensions the method specifies for the laminate thickness.
- 02Condition them as required.
- 03Measure width and thickness on each coupon.
- 04Set the three-point fixture to the specified span and fit the specified roller radii.
- 05Record the span, because the result cannot be reproduced without it.
- 06Place the coupon centrally and square on the supports.
- 07Load at the specified constant rate until failure.
- 08Record the failure force.
- 09Examine the coupon: a clean interlaminar crack is valid, a tensile break or a crushed loading point is not.
- 10Calculate the apparent interlaminar shear strength.
- 11Report the value with the span, the roller radii and the observed failure mode.
The fixture this method needs
What the report has to contain
- Reference to ISO 14130 and the edition, and whether Cor 1:2003 applied
- Material, fibre, matrix and fibre volume fraction
- Laminate lay-up and cure schedule
- Measured width and thickness
- Span and roller radii used
- Conditioning and test rate
- Failure force and apparent interlaminar shear strength
- Observed failure mode for every coupon
- Number of coupons rejected and why
- A statement that the value is not a design parameter
What the machine must be capable of
Modest force — a short-beam coupon commonly fails between a few hundred newtons and a few kilonewtons — with a three-point fixture whose span is settable to the specified value and whose rollers are the specified radii. The span is the parameter that decides the answer, so it is set deliberately and recorded rather than inherited from whatever the fixture was last used for. Nothing else is demanding: no extensometer, no strain measurement, and a rate that only has to be constant.
What goes wrong in practice
Quoting the result as a design allowable. The standard states in its own scope that the method is not suitable for determining design parameters and is for screening or quality control, and the word *apparent* in its title is the same warning in one word — the stress state under a loading nose on a short beam is not pure shear. After that: not examining the failure mode, so flexural and bearing failures average in silently; and omitting the span from the report, which makes the value unreproducible, because two laboratories at different span-to-thickness ratios will get different numbers and both will be right about what they measured. A quieter one is inheriting whatever span the fixture was last set to, which produces a result that is internally consistent and comparable with nothing.
ISO 14130 or ASTM D2344
| ISO 14130 | ASTM D2344/D2344M | |
|---|---|---|
| Principle | Short-beam three-point bending | Short-beam three-point bending |
| Reports | Apparent interlaminar shear strength | Short-beam strength |
| Design use | Explicitly excluded | Explicitly cautioned |
| Failure mode | Must be interlaminar to be valid | Must be interlaminar to be valid |
The same test in two families, and both are careful about the same thing: the number is comparative. Keep to one within a data set, since specimen dimensions and span ratios differ.
Questions we are asked about this test
What is ISO 14130?
It is the ISO short-beam method for the apparent interlaminar shear strength of fibre-reinforced plastic composites. A short, thick coupon is loaded in three-point bending until it fails between the plies, and the failure force gives a shear strength. It suits thermoset and thermoplastic matrices alike, provided interlaminar shear failure is actually obtained.
Why is the beam short?
Because span decides which failure wins. In three-point bending the peak bending stress rises with span while the shear stress does not, so a long beam breaks in tension on its underside. Shortening the span far enough tips the balance until the interlaminar plane — the weakest thing in a laminate — gives way first. The short span is not a convenience, it is the method.
What does apparent mean here?
That the number describes the coupon rather than the material. The stress state under a loading nose on a short beam is not pure shear: there is through-thickness compression, local crushing and a stress concentration, all mixed into one failure force. The standard is explicit that the result is not suitable for determining design parameters and is for screening materials or quality control.
When is a result invalid?
When the coupon did not fail interlaminarly. If it broke in tension on the lower face, that is a flexural result. If it crushed under the loading nose, that is a bearing result. Either way the arithmetic still produces a number, and that number is not an interlaminar shear strength. Examining every failed coupon is what separates the two.
Where does the 0,75 in the formula come from?
From the shape of the shear stress distribution through a rectangular section, which is parabolic with a peak at mid-thickness equal to one and a half times the mean. Combined with the fact that each support carries half the applied force, that gives three quarters of F over the cross-sectional area. It is geometry, not a fudge factor.
Why must the span be reported?
Because the result depends on it. Two laboratories testing the same laminate at different span-to-thickness ratios will get different numbers, and neither is wrong — they measured under different conditions. Reporting the span and the roller radii is what makes a value reproducible rather than merely plausible.
Can it be compared with ASTM D2344?
Cautiously. Both use short-beam three-point bending and both warn against design use, but specimen dimensions and span ratios differ between them, so results should not be pooled. Cite the standard alongside the figure and keep a data set to one method.
Running ISO 14130 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 — a short-beam coupon commonly fails between a few hundred newtons and a few kilonewtons | 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 | A three-point bend fixture at the short span the method specifies, with rollers of the specified 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.