Testing standard
Standard Test Method for Grab Breaking Load and Elongation of Geotextiles
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ASTM D4632 measures the grab breaking load and elongation of a geotextile. Jaw faces of at least 25.4 × 50.8 mm grip the centre of a 100 mm wide specimen and pull at 300 mm/min. Because the fabric either side of the jaws helps carry load, the result is higher than a wide-width test — which is why it is an index property for acceptance rather than a design value.
A specimen 100 mm wide is clamped between jaws whose faces are at least 25.4 × 50.8 mm, centred on the specimen so that roughly 37 mm of material extends beyond the jaw on each side. The gauge length is 75 mm. The crosshead separates at 300 ± 10 mm/min and the specimen is pulled to failure, with force recorded against grip separation. Machine and cross-machine directions are tested and reported separately. The fabric outside the jaws is not clamped but is not idle either — it assists the gripped yarns through the fabric structure, which is the defining feature of the grab geometry.
The result is grab breaking load in newtons, together with elongation at maximum load. It is an index property: quick, repeatable, and widely written into geotextile specifications for acceptance and quality control. On a construction site it answers the practical question of whether the roll delivered matches the roll specified, and it does so in minutes with a small specimen. What it is not is a design value. Because the surrounding fabric contributes an amount that depends on construction, the grab load cannot be expressed per unit width or used to size a reinforcement application — that requires wide-width testing.
The whole method rests on one geometric relationship: a narrow jaw gripping a wide specimen. Change either dimension and you have changed the test.
A grab breaking load is an INDEX property. It cannot be converted to a strength per unit width, and it must not be used as a design value — wide-width tensile testing to ISO 10319 or ASTM D4595 is what design uses.
The maximum force recorded, in N
Reported as a force for the defined jaw size on a 100 mm specimen. It is not divided by any width — doing so would imply a strength per unit width the test does not measure.
ε = ΔL / L₀ × 100
A vice action grip of this size covers the 25.4 mm across-specimen dimension the method calls for. Check the face against the standard before a certification run: D4632 asks for at least 25.4 × 50.8 mm, so the jaw must be rectangular rather than square, and the across-specimen dimension is part of the test definition rather than a matter of convenience.
Specifications
Pneumatic actuation gives the same across-specimen jaw dimension with a constant clamping force, which matters on a compressible nonwoven that a screw grip would keep bedding into.
SpecificationsForce measurement to ASTM E4 over a range from a few hundred newtons for light nonwovens to several kilonewtons for heavy woven geotextiles, and a crosshead holding 300 mm/min. That speed, combined with a 75 mm gauge, means tests are over in seconds, so the data acquisition rate must be high enough to catch the peak of a stepped trace rather than sampling past it. Grips must present a face of at least 25.4 × 50.8 mm — a rectangle rather than a square, and the across-specimen dimension is part of the method's definition rather than a convenience — and clamp evenly on a compressible nonwoven, which favours pneumatic actuation over screw tightening.
The most consequential error happens after the test, when a grab breaking load is divided by a width to produce an apparent strength per unit width and then used for design. There is no valid conversion, and the resulting figure overstates what the material can be relied on for. In the laboratory, poor jaw centring is the commonest source of scatter and is cured by marking the specimen. Slippage on slick nonwovens and jaw-edge failures both invalidate specimens. And a data acquisition rate too low for a 300 mm/min test can miss the true peak on a stepped woven trace.
| ASTM D4632 grab | ASTM D4595 / ISO 10319 wide-width | |
|---|---|---|
| Specimen | 100 mm wide, jaw faces at least 25.4 × 50.8 mm | 200 mm wide, fully clamped |
| Result | Breaking load, in N | Strength per unit width, in kN/m |
| Lateral contraction | Restrained by surrounding fabric | Minimal — the wide specimen restrains itself |
| Use | Index property, site acceptance, QC | Design value for reinforcement |
Grab results cannot be converted to wide-width values. The grab test deliberately allows the surrounding fabric to assist, which is useful as a quick index but wrong as a design basis. Reinforcement design uses wide-width strength, without exception.
It is the ASTM grab tensile test for geotextiles. Jaw faces of at least 25.4 × 50.8 mm grip the centre of a 100 mm wide specimen over a 75 mm gauge length, and the specimen is pulled at 300 mm/min to failure. It reports grab breaking load in newtons and elongation at maximum load.
Deliberately, so that the fabric either side of the jaw can assist the clamped yarns through the fabric structure. This is meant to represent the way a geotextile is loaded in the ground, where the material around any loaded region helps carry load, rather than the idealised case of a strip loaded alone. It is also what makes the grab value higher than a strip or wide-width value on the same material.
No, and it is one of the more common errors in geotextile specification. The grab test allows surrounding fabric to contribute an amount that depends on the fabric's construction and lateral stiffness, so there is no fixed relationship to divide out. Any strength per unit width derived from a grab result is arithmetic without meaning.
Whenever the number will be used for design. Reinforcement applications — retaining structures, basal reinforcement, steepened slopes — are designed on wide-width tensile strength from ASTM D4595 or ISO 10319, which uses a 200 mm specimen clamped across its full width and reports kN/m. Grab is an index property for acceptance and quality control, and it is used because it is quick, not because it is more representative.
Because the test is designed for throughput. Most textile methods set a time to break so that materials of very different extensibility are strained comparably, but D4632 is a site-acceptance and quality-control test where consistency and speed matter more than that refinement. A fixed rate makes every result directly comparable and lets a laboratory run large numbers of specimens quickly.
Not necessarily — on woven geotextiles it is characteristic of the grab geometry. Yarns at the jaw edge take the highest load and break first, load transfers outward to the next yarns, and the pattern repeats. The peak of that trace is still the grab breaking load. A smooth curve is more typical of nonwovens, where load is shared through a bonded structure rather than by discrete yarns.
Yes, and it drifts more than people expect across a set of ten specimens. An off-centre grab gives more assisting fabric on one side than the other, which loads the specimen asymmetrically and lowers the result. Marking the jaw position on each specimen before clamping takes seconds and removes a source of scatter that otherwise looks like material variability.
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 | Moderate — commonly 0.3 to 5 kN depending on geotextile mass and construction | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | ASTM E4 | Verified to ASTM E4, and to ISO 7500-1 Class 0.5 |
| Gripping | Grab jaws: 25 mm square clamping faces gripping the centre of a 100 mm wide specimen | Wedge, vice-action, pneumatic and hydraulic grips, built to the specimen |
| Environment | 21 ± 1 °C and 65 ± 2 % RH, the textile standard 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.
