ASTM D751 Testing of Coated Fabrics

What the test does

For breaking strength — the most commonly cited part of the standard — a specimen of coated fabric is clamped in flat-faced grips and pulled to break, in either a grab geometry with 25 mm jaws on a wider specimen, or a strip geometry with the full width clamped. The grab procedure runs at 300 ± 10 mm/min over a 75 mm gauge. Warp and fill directions are tested separately. The standard also contains procedures for tearing strength, adhesion of the coating to the substrate, hydrostatic resistance and other properties, each with its own specimen and apparatus.

What it measures, and why it matters

Coated fabrics are composite materials: a textile substrate carries the load while a polymer coating provides weatherproofing, abrasion resistance, air retention or chemical protection. The breaking strength procedures characterise what the substrate can carry, which decides whether a tarpaulin survives being tensioned, an awning holds up in wind, an inflatable structure retains its shape, or a protective fabric resists a snag. Because the coating and the substrate can fail independently, the standard's other procedures — particularly coating adhesion — often matter as much as strength, since a fabric whose coating delaminates has failed even with its substrate intact.

Specimen and procedure choice

This is a family of methods rather than one, and the first decision is which procedure the specification actually calls for.

Grab procedure

25 mm jaws on a wider specimenQuick, and the most commonly specified for breaking strength.

Strip procedure

Full width clampedMore representative of the material's intrinsic strength; gives lower numbers than grab.

Directions

Warp and fill tested separately

Coated side

Recorded, and consistent in the gripsAn asymmetric coating grips differently on each side, and results drift if the orientation is not kept.

Conditioning

Textile standard atmospherePracticeThe retrievable ASTM record prints no atmosphere figures, so none are quoted here.

Grip faces

Chosen for the coatingPracticeThe recurring problem in coated fabric testing — smooth faces slip, aggressive faces cut.

Check for coating damage at the jaw line

After each breakDakA grip that has cut through the coating into the substrate has notched the specimen, and the break is not valid.

D751 contains many procedures and a bare reference to the standard is ambiguous. A certificate must name the procedure — grab or strip, and which of the tear, adhesion or burst methods — or it cannot be reproduced.

Test speed

Grab breaking strength

300 ± 10 mm/min

Gauge length

75 mm for the grab procedure

Elongation

From grip separation

Watch for the coating breaking before the substrate

A two-stage curveDakOn some laminates the coating cracks first, giving a shoulder in the trace, and the substrate then carries on. Which one the specification means by breaking strength is worth settling before the test.

What the machine must be capable of

Force measurement across a wide range — from a few hundred newtons for light coated cloth to well over ten kilonewtons for heavy industrial laminates — and a crosshead holding 300 mm/min for the grab procedure. At that speed over a 75 mm gauge, tests finish in seconds, so the data rate must be high enough to capture the peak and any two-stage behaviour. Grips must be wide enough for the specimen and must clamp evenly at a constant pressure, which favours pneumatic actuation. The retrievable ASTM record for D751 names no accuracy class and no machine specification, so none is quoted here. The laboratory needs the textile standard atmosphere.

Grips and fixtures for this method

Standard 25 mm

Pneumatic Vice Action Grip

Pneumatic vice action grips hold a coated fabric at a constant even pressure, which is the practical answer to a material that slips under smooth faces and is cut by aggressive ones.

Specifications

Two face setsTJ-26

100mm Wide Vice Action Grips

A 100 mm wide vice grip suits the strip procedure on heavier industrial coated fabrics, where the full specimen width has to be clamped.

Specifications

How the test runs

1. 01Identify which D751 procedure the specification requires.
2. 02Cut specimens in warp and fill directions to the dimensions that procedure specifies.
3. 03Note which face is coated and keep the orientation consistent.
4. 04Condition in the textile standard atmosphere.
5. 05Select grip faces suited to the coating.
6. 06Set the gauge length for the chosen procedure.
7. 07Mount the specimen square, clamping evenly.
8. 08Pull at the specified rate to break.
9. 09Inspect the jaw line for coating damage.
10. 10Reject specimens that slipped or were cut by the grips.
11. 11Report warp and fill separately, naming the procedure used.

Calculations

What the report has to contain

• Reference to ASTM D751, the edition, and the specific procedure used
• Material identification — coating polymer, substrate construction and mass
• Which face was coated, and the orientation tested
• Specimen dimensions and gauge length
• Direction for each result set
• Grip face type
• Conditioning atmosphere and duration
• Rate of extension
• Breaking strength and elongation for each specimen
• Number rejected for slippage or grip damage
• Mean, standard deviation and coefficient of variation

What goes wrong in practice

The most damaging error is a certificate that cites only ASTM D751 without naming the procedure, which leaves the result unreproducible — grab and strip give different numbers, and the tear, adhesion and burst methods measure different properties entirely. In the laboratory, grip problems dominate: slippage on the coating and cutting through it are opposite failures with the same cure, which is face selection rather than clamping pressure. Inconsistent specimen orientation on asymmetrically coated fabrics adds avoidable scatter. And a two-stage curve, where the coating fails before the substrate, needs a decision about which peak is being reported.