Testing standard
Standard Test Method for Determining Performance Strength of Geomembranes by the Wide Strip Tensile Method
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ASTM D4885 pulls a wide strip of geomembrane — much wider than its gauge length — so that the specimen cannot neck, and reports strength, elongation, three moduli and breaking toughness. It is a performance test intended as a design aid, not a specification test. The current edition is D4885-01, most recently reapproved as D4885-01(2023).
A wide strip of geomembrane — considerably wider than it is long in the gauge section — is clamped across its full width and pulled in tension until it fails. Strength and elongation are recorded, and the method also gives directions for calculating initial modulus, offset modulus, secant modulus and breaking toughness.
The two things that distinguish it from an ordinary tensile test are stated in the standard itself: the width of the specimens and the speed at which force is applied. Published work describes specimens 200 mm wide with a gauge length of 100 mm.
The word in the title is *performance*. A dumbbell tensile test of the kind ASTM D6693 specifies is a control and specification test; it produces numbers that identify the material. This method is intended as a design aid, to determine whether a geomembrane can withstand the stresses and strains imposed under design conditions.
The difference is necking. Most geosynthetics contract laterally when they are pulled — a narrow specimen pulls in at the edges and the stress concentrates. That contraction edge effect is an artefact of the specimen, not a property of the sheet, and a liner spread over a subgrade cannot neck because the material either side of any point holds it. Making the specimen much wider than the gauge length suppresses the contraction and produces a closer relationship to how the material actually behaves in service. The slower rate of applied strain does the same thing on the time axis.
That is why both figures exist for the same product and why they differ. A specification quoting D6693 values and a design quoting D4885 values are both correct and are describing different things.
The two things that make this method different are stated in the standard itself: the width of the specimen and the speed at which force is applied.
Narrowing the specimen to fit the available grips destroys the purpose of the method. The width is not a tolerance; it is the mechanism by which necking is suppressed, and a narrowed specimen reproduces exactly the artefact the method exists to remove.
Maximum force divided by specimen width
Reported per unit width, so specimens of different widths compare.
Three slopes taken from the load-elongation curve
Directions for all three are given by the method. Which one a design uses depends on the strain the liner is expected to see.
Energy absorbed by the specimen up to break
The wide-strip counterpart of tensile energy absorption: how much work the sheet takes before it splits.
Wide hydraulic faces clamping the full specimen width evenly. A 200 mm strip of heavy HDPE carries several times what a dumbbell does, and a specimen gripped only across its middle fails at the jaw corners rather than in the gauge section.
Specifications
Optical strain from marks on the strip. Where three moduli are reported, strain has to come from the specimen: on a wide compliant sheet the machine compliance and any grip slip are a large share of the crosshead movement.
SpecificationsForce is the constraint here in a way it is not for the dumbbell method. A 200 mm wide strip of a heavy HDPE liner carries several times what a dumbbell does, so a frame of 50 to 100 kN with grips wide enough to clamp the full specimen width is a realistic requirement. Force accuracy to ASTM E4 applies.
Grip width is what rules most laboratories out. The whole specimen width has to be clamped evenly; a specimen gripped only in the middle loads unevenly and fails at the grip corners. Wide vice-action or hydraulic jaws with a face that holds a slippery liner without cutting it are needed, and the clamping has to be uniform across the width.
Travel again is long, and where modulus values are reported the strain has to come from the specimen. Grip separation includes machine compliance and any slip, both of which are significant fractions of the movement on a wide, compliant specimen.
Narrowing the specimen to fit available grips destroys the whole point of the method and reintroduces the necking it was designed to suppress. Uneven clamping across a wide jaw makes the specimen fail at a corner. Taking modulus from crosshead travel gives values that are low and repeatable. And quoting a D4885 result against a D6693 specification, or the reverse, compares two deliberately different tests.
The same reasoning about specimen width appears in three places, on three different products.
| Geomembrane (D4885) | Geotextile (D4595) | Geotextile (ISO 10319) | |
|---|---|---|---|
| Specimen | Wide strip, wider than gauge length | 200 mm wide strip | 200 mm wide strip |
| Problem solved | Necking of a ductile sheet | Contraction of a fabric | Contraction of a fabric |
| Result | Performance strength per unit width | kN/m | kN/m |
| Intended as | A design aid | Index and design | Index and design |
| Grip requirement | Full-width even clamping | Full-width even clamping | Full-width even clamping |
All three fail the same way when the grips are too narrow: the specimen loads unevenly and fails at a grip corner. Grip width, not frame capacity, is what rules most laboratories out of wide strip work.
ASTM D4885 is the wide strip tensile method for geomembranes. A strip much wider than its gauge length is pulled to failure, and the method reports tensile strength and elongation together with initial, offset and secant modulus and breaking toughness. It is described by the standard as a performance test intended as a design aid. The current edition is D4885-01, most recently reapproved in 2023.
To stop it necking. A narrow tensile specimen contracts laterally as it is pulled, and the stress concentrates at that contraction — an artefact of the specimen shape rather than a property of the material. A geomembrane spread over a subgrade cannot contract that way, because the sheet either side of any point holds it. Making the specimen much wider than the gauge length suppresses the contraction and gives a closer relationship to how the liner actually behaves.
For the same reason the specimen is wider: to move the test closer to service. The standard names the width of the specimen and the speed of applied force as the two basic distinctions between this method and the other geomembrane tensile tests. A liner in the ground is strained over months by a settling subgrade, not over a minute by a crosshead, and a slower rate is a partial correction for that.
No, and treating it as one causes real confusion. D6693 is a control and specification test on a dumbbell — it identifies the material. D4885 is a performance test on a wide strip — it supports a design. They give different numbers on the same sheet, deliberately, and quoting a result from one against a specification written for the other is the commonest reporting error on geomembranes.
Grip width, not frame capacity. The full specimen width has to be clamped evenly, which needs wide vice-action or hydraulic jaws with a face that holds a slippery liner without cutting it. A specimen gripped only across its middle loads unevenly and fails at a jaw corner. Narrowing the specimen to fit smaller grips is not a workaround, because the width is the mechanism the method depends on.
Substantially more than the dumbbell method. A 200 mm wide strip of a heavy HDPE liner carries several times the load of a die-cut dumbbell, so a frame of 50 to 100 kN with full-width grips is a realistic requirement, with force accuracy to ASTM E4. Travel is long as well, because the sheet elongates a great deal before it breaks.
The one the design uses, and the report should say which. Initial modulus is the slope at the origin, offset modulus is taken from a defined offset for a curve with no straight portion, and secant modulus is the slope between the origin and a stated strain. A geomembrane curve rarely has a long linear region, so an unqualified modulus figure is ambiguous and the strain it was taken at matters as much as the number.
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 | Tens of kN — a 200 mm wide strip of heavy HDPE carries several times what a dumbbell does | 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 |
| Strain measurement | An extensometer of the class the method specifies | Certified to ISO 9513 Class 1 and ASTM E83 — non-contact video, clip-on and high-elongation |
| Gripping | Wide grips clamping the full specimen width evenly | Our self-tightening serrated wedge grips, with V-jaws for round specimens or vice-action grips, built to the specimen |
| Environment | Standard laboratory atmosphere for conditioning and test | 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.
