Two laboratories test the same roll of netting and report 8 N and 250 kPa. Both figures are correct, both are quoted to buyers as “strength,” and they cannot be compared in any way. One describes how easily a crack runs once it has started. The other describes how much pressure the sheet takes before it fails at all. For an open mesh, only the second one behaves the way a buyer expects.
Two different questions
The confusion is structural, not a matter of units. The two tests answer different questions about failure:
- Tear strength — given a fabric that already has a cut in it, how much force is needed to propagate that cut?
- Bursting strength — given an intact fabric, how much distributed pressure can it take before it ruptures?
Tear testing is a propagation measurement. Bursting is an initiation measurement, and because the load is applied in every direction at once, it also captures propagation after initiation. That distinction is why the two numbers do not track each other across fabrics, and why choosing the wrong one for a material leads to specifications that do not predict field performance.
What a tear test measures
Tear methods come in several geometries, standardised under ISO 13937 and, in the United States, ASTM D2261 and D5587:
| Method | Geometry | Common for |
|---|---|---|
| ISO 13937-1 | Ballistic pendulum, single rip | Woven apparel fabrics |
| ISO 13937-2 | Single tongue (trouser shape) | Coated and industrial fabrics |
| ISO 13937-3 | Trapezoid | Fabrics that distort easily |
| ISO 13937-4 | Double tongue | Heavy and coated fabrics |
| ASTM D2261 | Single rip tongue | US apparel and home textiles |
| ASTM D5587 | Trapezoid | US general textiles |
Every one of these begins by cutting a deliberate slit or notch into the specimen. The instrument then measures the force required to keep that slit moving. The result is expressed in newtons or grams-force, and it is a measure of how much energy the fabric absorbs per unit of crack growth.
For a tightly woven fabric, that is a genuinely useful number. A tent flysheet or a coated tarpaulin tears from a small nick, and how fast that nick becomes a split is exactly what determines whether the product survives a windy night. Tear strength is the right metric for those materials.
What a bursting test measures
A bursting test clamps a circular specimen and applies increasing pressure to one face until it ruptures. No cut, no notch. The load radiates outward in all directions, which is closer to how a membrane actually carries pressure — a net taking wind load, a panel loaded by someone leaning on it, fabric pushed against a pole.
Two standard variants exist, and they are not interchangeable:
ISO 13938-1 — hydraulic, constant rate of extension. A rubber diaphragm expands against the fabric at a controlled rate until the specimen bursts. Pressure at rupture is recorded in kilopascals. This is the method most commonly used for netting, because it is repeatable and the diaphragm protects the specimen from the clamping edge.
ISO 13938-2 — pneumatic, constant pressure. Air pressure is applied and held or ramped according to a defined profile. The loading is more distributed and the specimen is not backed by a diaphragm, which changes how the fabric deforms before failure.
ISO 13938-1 versus 13938-2
Buyers frequently treat these as the same test with a different part number. They are not, and the difference is large enough to matter when you are comparing quotations.
| Aspect | ISO 13938-1 | ISO 13938-2 |
|---|---|---|
| Medium | Hydraulic, via a diaphragm | Compressed air, direct |
| Loading | Constant rate of extension | Controlled pressure profile |
| Specimen support | Diaphragm supports and deforms with the fabric | Fabric spans unsupported |
| Typical result | Higher rupture pressure | Lower rupture pressure for the same fabric |
| Where it is used | Netting, knitted fabrics, general textiles | Fabrics where hydraulic contact is undesirable |
The practical rule: a 250 kPa result from -1 and a 250 kPa result from -2 do not describe the same fabric. The -2 figure will generally come from a lighter or weaker construction. If you are comparing two suppliers and only one states the part number, you are comparing numbers that may not be measuring the same thing. Ask for the method before you ask for the value.
Reference specifications for insecticidal netting are written around hydraulic bursting, which is why the familiar ladder — 175 kPa at 50D, 250 kPa at 75D, 405 kPa at 100D — should be read as -1 results.
Why mesh is awkward to tear
Here is the specific reason tear testing fails as a durability predictor for netting, and it is geometric rather than statistical.
When a tear propagates through a continuous fabric, the crack must break yarn after yarn along a fairly well-defined path. In an open mesh, the crack reaches an aperture and has nothing to break. It stops, deflects, or jumps to a neighbouring yarn column. The result is a test that measures a mixture of yarn strength, structural geometry and luck, with very high scatter between specimens.
Three consequences follow:
- The values are low and compressed. With 80 percent of the area being hole, there is little material to resist propagation, and the differences between a good and a poor netting are small in absolute terms. The test loses discriminating power exactly where you need it.
- Repeatability is poor. High scatter means you need many specimens to get a confident mean, which most factory labs do not run.
- The failure mode is unrepresentative. Field failures in netting start at seams, corners and snag points, then grow under distributed load. That is a bursting scenario with a stress concentrator, not a single-crack propagation scenario.
This is not a theoretical preference. The specifications that the netting industry actually benchmarks against use bursting strength, and they require it for net and seams together. That last detail matters: a bursting test run across a seam captures the join, which is where products fail.
Reading the numbers
When a quotation arrives with a strength figure, work through these in order:
- Is it tear or bursting? Newtons or grams-force usually means tear. Kilopascals usually means bursting. If the unit is absent, the number is unusable.
- Which part number? For bursting, -1 or -2. For tear, which geometry. Different methods produce different numbers on identical fabric.
- Was the specimen flat goods or did it include a seam? For netting, the seam figure is the one that predicts returns.
- What was the conditioning? Standard atmosphere is 20°C and 65% relative humidity. Wet or unconditioned specimens read differently, and UV-aged specimens read very differently.
- Is it a minimum or a mean? A mean with no minimum allows a batch where half the rolls fall below the quoted figure. Specifications should state a minimum.
Get those five answers and the strength figure becomes a purchasing tool. Skip them and it becomes decoration — a number that looks technical and constrains nothing.
What this means when you buy
What to require, in the specification and in the test report:
- Specify bursting, not tear, for netting. ISO 13938-1 at the reference figure for your yarn weight, applied to net and seams. Tear results are appropriate for flysheets and tarpaulins, not for open mesh.
- Name the part number. “ISO 13938” alone is ambiguous. -1 and -2 give different values on the same fabric, and the gap is large enough to hide a downgrade.
- Require a minimum, not a mean. A mean of 250 kPa can be built from rolls at 210 and 290. A stated minimum with a sampling plan is what actually protects the shipment.
- Test the seam in the same run. A flat-goods figure that the join cannot match describes a fabric, not a product. Reference specifications already phrase it as “net and seams” — use that wording.
- Ask for the conditioning details. Standard atmosphere, and whether the sample was washed or UV-aged before testing. A strength figure on unwashed, unaged goods is a warehouse number.
One further note on cost: a factory that can produce a bursting report with the part number, the seam result and the conditioning stated is a factory that runs a real lab. That capability is worth a small premium, because it is the only way you find out about a downgrade before your customer does.