Testing standard
Nonwovens — Test methods — Part 18: Determination of tensile strength and elongation at break using the grab tensile test
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ISO 9073-18 determines the tensile strength and elongation at break of nonwovens by the grab method: only the centre of the specimen is clamped, over an area of 25,0 mm ± 0,5 mm square, and the surrounding fabric contributes to the strength recorded. The current edition is ISO 9073-18:2023, which replaced the withdrawn 2007 edition.
Only the middle of the specimen is clamped. A wide piece of nonwoven is gripped over a clamping area of 25,0 mm ± 0,5 mm square at the centre of its width, extended at a constant rate until it ruptures, and the maximum force is recorded together with the elongation at that point. The fabric either side of the jaws is unclamped but still attached, so it feeds load into the gripped section as the test runs. That is exactly what the method sets out to capture — its own definition calls it effective strength, the strength of the fibres in a specific width plus the additional strength contributed by the fibres beside them. Specimens are tested conditioned, wet, or both.
How a nonwoven behaves when a load is applied to part of it rather than to all of it, which is how these materials nearly always fail in service. A wipe is pulled at one corner, a hygiene product is stressed over a fastening, a medical drape catches on an instrument. A strip test loads every fibre in the section at once and answers a different question. Because the surrounding fabric contributes, the grab result is not a strip result scaled by width, and the two are not convertible in either direction — a specification names one method, and a report that does not say which was used cannot be compared with anything.
The clamped area is the defining dimension of a grab test, not the width of the piece that was cut.
A grab result belongs to the clamping geometry that produced it. Changing the jaw faces changes how much surrounding fabric helps, and therefore changes the number.
The maximum force applied to the specimen carried to rupture
Reported in newtons for the stated grab geometry. Nothing is divided by an area — a nonwoven's thickness compresses under the gauge, so a stress derived from it would depend mostly on how hard the gauge pressed.
The deformation in the direction of load, expressed as a percentage
The effective clamping points can be checked by clamping a specimen with carbon copy paper to leave a gripping pattern on the specimen or on the jaw faces.
Clamped fibres plus the contribution of the fibres beside them
The standard's own term is effective strength. It is not the strip strength scaled by width, and the two cannot be converted into each other.
Pneumatic vice action holds the clamping pressure constant from specimen to specimen, which is what a grab test needs — the result depends on how much surrounding fabric is allowed to help, and that changes with grip force. The faces have to be sized to give a 25 mm square clamped area, with one clamp at least 25 mm × 40 mm; jaw faces are made to suit.
SpecificationsMost nonwovens break between a few newtons and a few hundred, and the method wants the indicated maximum force within ±1 % anywhere in the range used. That is a cell chosen for the fabric, not for the frame.
SpecificationsA constant-rate-of-extension frame is the preferred machine, and it must reach 300 mm/min with an uncertainty of measurement of ±10 %. Force accuracy shall be class 1 of ISO 7500-1: the error of the indicated or recorded maximum force must not exceed ±1 % anywhere in the range used, and the error of indicated jaw separation must not exceed ±1 mm. A class 2 machine may be used, but that has to be stated in the test report. Where force is captured by data acquisition, the sampling frequency must be at least eight readings per second. Metrological confirmation of the machine follows ISO 10012. Jaw faces must be smooth, flat and parallel, holding the specimen without cutting it.
Comparing results from machines working on different principles, which the standard rules out — even the constant time-to-break convention of 20 s ± 3 s only narrows the gap rather than closing it. Running the method on a stretchy material it excludes. Reporting a grab figure against a strip specification. Letting the specimen slip, or accepting a break at the jaw edge. Using a class 2 machine and not saying so, which quietly removes the reader's ability to judge the number. And cutting the outer piece carelessly: a nonwoven has no yarns to redistribute load around a damaged edge, so a ragged cut is simply a notch.
| ISO 9073-18 (grab) | ISO 9073-3 (strip) | |
|---|---|---|
| Clamped | The centre only, 25 mm square | The full specimen width |
| Surrounding fabric | Contributes to the result | Contributes nothing — it is all clamped |
| Specimen width options | Set by the clamping area | 25 mm (Option A) or 50 mm (Option B) |
| Answers | How the web behaves when loaded over part of its width | How much load every fibre in the section carries |
| Comparable with the other | No | No |
Two parts of one series covering one property by different clamping. Neither converts into the other, and a specification names one. A report that does not say which method was used cannot be compared with anything.
It is Part 18 of the ISO 9073 series of nonwoven test methods, covering tensile strength and elongation at break by the grab tensile test — only the centre of the specimen is clamped, over an area of 25 mm square. The current edition is ISO 9073-18:2023, the second, published in November 2023 and replacing the withdrawn 2007 first edition.
The title, for one thing: the 2007 edition was headed Textiles — Test methods for nonwovens — Part 18: Determination of breaking strength and elongation of nonwoven materials using the grab tensile test, and the current one reads Nonwovens — Test methods — Part 18: Determination of tensile strength and elongation at break using the grab tensile test. The scope was clarified, new terms were added, and separate clauses were introduced for sampling, conditioning, specimen preparation, apparatus verification, expression of results, and precision and bias.
In a strip test the full width of the specimen is clamped, so every fibre in the section carries load. In a grab test only the centre is clamped, and the unclamped fabric on either side remains attached and feeds load into the gripped section as the test runs. The standard calls the result effective strength: the strength of the fibres in a specific width plus the additional strength contributed by the fibres beside them. That is a real property of the material in use, and it is not the strip strength scaled by width.
Because the mechanism the grab test relies on breaks down. The contribution of the surrounding fabric depends on it transferring load into the clamped section; a material with a high percentage of stretch simply extends instead, the load path becomes indeterminate, and the number stops describing anything repeatable. The standard states plainly that the test method is not applicable to those materials.
Class 1 of ISO 7500-1 under conditions of use: the error of the indicated or recorded maximum force must not exceed ±1 % at any point in the range used, and the error of indicated jaw separation must not exceed ±1 mm. A class 2 machine may be used, but the standard requires that to be stated in the test report. Where force is recorded through data acquisition boards and software, the sampling frequency must be at least eight per second.
By agreement, and with a caveat. The method specifies constant rate of extension machines and names CRE as preferred. Constant rate of loading instruments are covered for information by reference to ISO 2062:2009 Annex A, in recognition that they remain in use. Results from machines operating on different principles are not comparable — even holding a constant time to break of 20 s ± 3 s, which is the established way of producing comparable data, the standard warns the results can still differ significantly.
Because a large share of nonwovens do their work wet — wipes, hygiene products, medical drapes, filtration media — and many lose most of their strength when saturated. Reporting only the conditioned figure would describe a property the product does not have at the moment it matters. The method specifies grade 3 water to ISO 3696 with a non-ionic wetting agent, and blotting paper for handling the wet specimens.
A great deal. A nonwoven web is laid down on a moving belt and the process orients the fibres along the direction of travel, so the machine direction is commonly several times stronger than the cross direction. Both are ordinarily tested and reported separately. An average would describe neither and would let a product satisfy a specification while being weak across the web, which is the direction most tearing failures start in.
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 — most nonwovens break between a few newtons and a few hundred newtons, so the cell is chosen for the fabric rather than for the frame | 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 under conditions of use; a Class 2 machine may be used if that is stated in the test report | ISO 7500-1 Class 0.5 — a class tighter than the method asks |
| Gripping | Grab clamps giving a fabric clamping area of 25,0 mm ± 0,5 mm square | Wedge, vice-action, pneumatic and hydraulic grips, built to the specimen |
| Environment | Conditioned in the ISO 139 atmosphere; a wet state is an explicit alternative, using grade 3 water to ISO 3696 with a non-ionic wetting agent | 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.
