ISO 14130 Short-Beam Interlaminar Shear Testing of Composites

What the test does

A short, thick coupon is placed centrally on two supports and loaded at mid-span in three-point bending at 1 mm/min (0.04 in./min) 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 and discriminating, which is why it is the usual 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

1 mm/min (0.04 in./min), held constantThe speed is part of the designation the report cites — a run at 1 mm/min is reported as ISO 14130/1.

Span

Five times the specimen thickness — 10 mm on the standard 2 mm coupon

Roller radii

Loading member 5 mm ± 0,2 mm; supports 2 mm ± 0,2 mm

Reported

Apparent interlaminar shear strength

Span and roller radii

Set to the method and reported with the resultThe result depends on both, so a value quoted without them cannot be reproduced.

Reject any coupon that did not fail interlaminarly

Dak

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 crosshead holding 1 mm/min and a three-point fixture whose span is settable and whose rollers are the specified radii: 5 mm ± 0,2 mm on the loading member and 2 mm ± 0,2 mm on the supports. The span is the parameter that decides the answer, and the method fixes it at five times the specimen thickness, which on the standard 2 mm × 10 mm × 20 mm coupon is a 10 mm span. It is set deliberately and recorded rather than inherited from whatever the fixture was last used for. The speed travels with the result: the standard has the test report cite the method and the rate together, so a run at 1 mm/min is reported as ISO 14130/1. The machine follows ISO 5893, with force indicated to better than ±1 % of full scale. Nothing else is demanding — no extensometer and no strain measurement.

The fixture this method needs

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.

Specifications

How the test runs

1. 01Cut coupons to the dimensions the method specifies for the laminate thickness.
2. 02Condition them as required.
3. 03Measure width and thickness on each coupon.
4. 04Set the three-point fixture to the specified span and fit the specified roller radii.
5. 05Record the span, because the result cannot be reproduced without it.
6. 06Place the coupon centrally and square on the supports.
7. 07Load at 1 mm/min until failure.
8. 08Record the failure force.
9. 09Examine the coupon: a clean interlaminar crack is valid, a tensile break or a crushed loading point is not.
10. 10Calculate the apparent interlaminar shear strength.
11. 11Report the value with the span, the roller radii and the observed failure mode.

Calculations

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 goes wrong in practice

Quoting the result as a design allowable. The scope says plainly that the method is not for design parameters and is for screening or quality control, and the word *apparent* in the 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 gives a result that is internally consistent and comparable with nothing.