A spinning mill reports 16.5 RKM. The weaving customer measures 14.8 on the same lot and raises a claim. Both labs have calibrated load cells and both quote ISO 2062.
Single-end yarn strength is not a property you read off a machine. It is the result of a defined test, and small differences in that definition — gauge length, speed, how breaks at the jaw are treated — move the figure by more than the difference between two good yarn lots.
ISO 2062 is four methods
ISO 2062 defines single-end breaking force and elongation using a constant rate of extension (CRE) tester. It contains four methods:
| Method | Operation | Specimen |
|---|---|---|
| A | Manual | Taken directly from conditioned packages |
| B | Automatic | Taken directly from conditioned packages |
| C | Manual | Relaxed test skeins, after conditioning |
| D | Manual | Specimens tested after wetting |
A report that says "ISO 2062" without the method letter leaves out how the specimen was prepared. Method C, with relaxed skeins, can give different elongation from yarn taken straight off a tightly wound package. Method D is a wet strength and must never be compared with a conditioned result.
The standard does not cover glass, elastomeric, aramid, HMPE/UHMPE, ceramic and carbon yarns, or polyolefin tape. Those materials have their own methods.
Gauge length and speed
ISO 2062 normally uses a 500 mm gauge length with a constant rate of extension of 100 % of the gauge length per minute — 500 mm/min. A 250 mm gauge length is also used, with the speed scaled accordingly.
Gauge length matters more than most people expect. A yarn breaks at its weakest point, and a longer specimen is more likely to contain a weak place. A shorter gauge length therefore gives a higher average breaking force. If the spinner tests at 250 mm and the customer at 500 mm, the spinner's figure will look better even if both labs are perfect.
ASTM D2256 takes a different approach to rate. Its primary requirement is an average time-to-break of 20 ± 3 seconds, with the machine speed adjusted to reach it. Alternate speeds are provided — for example 300 mm/min with a 250 mm gauge length — but these may be used only by agreement or when a material specification requires them.
The practical consequence: an ISO 2062 result and an ASTM D2256 result are not automatically comparable, even on identical yarn. Agree the standard, the gauge length and the speed in the contract, not after the dispute.
Jaw breaks
A specimen that breaks at or inside the jaw has not shown its true strength — the jaw has damaged it. On slippery filament yarns the opposite happens: the yarn slips in the jaw, and the elongation reading is inflated.
- Record jaw breaks separately. Do not quietly delete them, and do not average them in without noting it; follow the treatment set out in the standard.
- If more than an occasional specimen breaks at the jaw, look at the jaw faces and the clamping pressure before blaming the yarn.
- Pneumatic jaws give more consistent clamping than manual screw jaws, which is one reason automatic testers usually show lower scatter.
Pretension
Before the test starts, the yarn must be straightened under a small, defined pretension. Too little, and the first part of the elongation curve is just slack being taken up. Too much, and the specimen is pre-strained before the clock starts.
Use the pretension specified in the method for the yarn's linear density, and remember that this means you need the yarn count first. A wrong count gives a wrong pretension, and the same wrong count is then used again to calculate tenacity.
Units: N, cN/tex and RKM
Breaking force on its own says little, because a thicker yarn is naturally stronger. Tenacity divides force by linear density.
- cN/tex is the SI-based unit used in ISO practice.
- RKM (Reißkilometer) is the length in kilometres at which a yarn would break under its own weight. Numerically it equals breaking force in gram-force per tex.
- 1 RKM ≈ 0.981 cN/tex.
Mixing up RKM and cN/tex creates an apparent 2 % difference out of nothing. State the unit on the report.
Number of tests and scatter
Single-end tests scatter widely by nature, because they sample individual weak places. Report the coefficient of variation alongside the mean. A mean taken from too few breaks is not comparable with a mean from the number of tests the method requires. Draw specimens from several packages, not from one cone.
Conditioning
Strength and elongation depend on moisture. Cotton gains strength when moist; viscose loses it. Condition packages to the standard atmosphere of ISO 139 for long enough for the moisture to reach the inside of the package — not just its surface. A cone brought from a warm warehouse and tested an hour later is not conditioned.
Before you report
- ISO 2062 method A, B, C or D — or ASTM D2256?
- Gauge length and speed (or time-to-break) as specified?
- Jaw breaks recorded and treated as the standard requires?
- Pretension based on the correct linear density?
- Unit stated: cN/tex or RKM?
- Enough breaks from enough packages, with CV reported?
- Packages conditioned through, not just on the surface?
Our yarn strength testers include a 12-station automatic-feed model with pneumatic grips and results in gf, cN and RKM, as well as a portable unit for yarn and splice strength. They are built in our own workshop in Istanbul.