Constant pressureTJ-27
Split Capstan Grips
Split capstan grips wrap the yarn around a curved surface, so tension decays around the wrap and the yarn is never crushed at a single clamping line — which is what a flat jaw would do.
SpecificationsTesting standard
ASTM D2256
Standard Test Method for Tensile Properties of Yarns by the Single-Strand Method
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ASTM D2256 measures the tensile properties of yarn by the single-strand method. One yarn is mounted between capstan grips over a defined gauge length, pretensioned, and pulled at a rate giving break in about twenty seconds. It reports breaking force, elongation, and tenacity — breaking force normalised by linear density rather than by area.
At a glance
- Test type
- Tensile — the specimen is pulled apart
- Published by
- ASTM
- Edition
- D2256/D2256M-21
- Runs on
- Series 7200 and Series 9000
What the test does
A single yarn is drawn from a conditioned package and mounted between capstan or bollard grips over a defined gauge length, commonly 250 mm. The yarn wraps around each capstan a specified number of turns, so it is held by friction distributed around a curve rather than crushed between flat faces. A defined pretension, calculated from the yarn's linear density, removes slack and crimp without stretching the specimen. The crosshead then separates at a rate chosen so the yarn breaks in about twenty seconds, recording force against grip separation. Because yarn is variable, the test is repeated across many specimens sampled from different parts of the package.
What it measures, and why it matters
The method reports breaking force, elongation at break, tenacity — breaking force normalised by linear density — and work of rupture, the area under the curve. Tenacity is what allows yarns of different counts to be compared, and it is the number a spinner is actually judged on. Elongation and work of rupture describe how the yarn survives processing: weaving and knitting apply repeated shock loads rather than a steady pull, and a strong yarn with little give will stop a loom more often than a slightly weaker one that stretches. The variability across specimens matters as much as the average, because it is the weakest place in a yarn that causes an end-break.
Specimen and grips
A yarn is a bundle of fibres held together by twist. Anything that crushes it or lets the twist run out changes what is being measured.
- Gauge length
- 250 mm commonlyOther lengths are used and must be reported — yarn strength falls as gauge length rises, because a longer specimen contains more chances of a weak place.
- Grips
- Capstan or bollard, wrapping the yarnThe wrap spreads the load and lets tension decay around the curve, so the yarn is not crushed at a single line.
- Pretension
- Defined, from the yarn's linear densityIt removes slack and crimp without stretching the yarn. Too little leaves a false toe; too much pre-strains the specimen.
- Linear density
- Measured, not assumedTenacity is breaking force divided by linear density, so an assumed count puts a systematic error into every tenacity figure.
- Conditioning
- 21 ± 1 °C and 65 ± 2 % RHThe textile standard atmosphere. Natural fibres in particular change strength markedly with moisture regain.
- Take specimens from different parts of the package
- Not one continuous lengthDakYarn varies along its length, and sampling one metre of it repeatedly measures that metre.
- Discard the first few metres from a package
- Before samplingDak
Breaking force alone does not compare two yarns unless they are the same count. Tenacity — force per unit linear density — is the comparable quantity, and it requires the linear density to have been measured on the yarn actually tested.
Test speed
- Rate
- Set to break the specimen in 20 ± 3 sA time to break rather than a fixed speed, because yarns range from a few per cent elongation to well over a hundred.
- Elongation
- From grip separation
- Number of breaks
- Enough for the required confidenceYarn is variable, so single-strand testing normally means tens of breaks rather than five.
- Reject slippage as well as jaw breaks
- Both invalidate the resultDakSlippage shows as an unusually high elongation with an ordinary breaking force.
Calculations
Breaking forceF
The maximum force recorded, in N or cN
Tenacity—
Tenacity = breaking force / linear density
- linear density
- mass per unit length, in tex or denier
Expressed in cN/tex or gf/den. This is the quantity that compares yarns of different counts, and it is why linear density has to be measured.
Elongation at breakε
ε = ΔL / L₀ × 100
- ΔL
- increase in grip separation, mm
- L₀
- gauge length, mm
Work of rupture—
Area under the force-elongation curve
Often the better predictor of how a yarn survives weaving or knitting, where shock loads matter more than a peak force.
How the test runs
- 01Condition the yarn package at 21 ± 1 °C and 65 ± 2 % RH.
- 02Discard the first few metres and sample from different parts of the package.
- 03Measure the linear density on the yarn being tested.
- 04Fit capstan or bollard grips and set the gauge length.
- 05Wrap the yarn around the capstan the specified number of turns.
- 06Apply the defined pretension.
- 07Set the rate to break in about 20 s.
- 08Pull to break, recording force against grip separation.
- 09Reject jaw breaks and any specimen showing slippage.
- 10Repeat for the required number of specimens.
- 11Compute breaking force, tenacity, elongation and work of rupture.
Grips and fixtures for this method
Prevents grip breaksTJ-12
Quick Release Bollard Grip
A quick release bollard grip gives the same wrapped hold with faster loading, which matters when a yarn programme means tens of breaks per sample.
SpecificationsWhat the report has to contain
- Reference to ASTM D2256 and the edition
- Yarn identification — fibre, construction and twist
- Measured linear density
- Gauge length used
- Grip type and number of capstan wraps
- Pretension applied
- Conditioning atmosphere and duration
- Rate used and the time to break achieved
- Breaking force, tenacity, elongation and work of rupture
- Number of specimens, mean, standard deviation and coefficient of variation
- Number rejected for jaw breaks or slippage
What the machine must be capable of
Very small forces measured accurately. Fine spun yarns break below a newton, and a load cell sized for fabric or film will report them as noise, so the cell has to be matched to the yarn count. The crosshead must offer a range of rates, since achieving a twenty-second break across yarns ranging from a few per cent elongation to well over a hundred cannot be done at one speed. Capstan or bollard grips are effectively mandatory. Because elongation is taken from grip separation, machine compliance matters at these low forces, and the laboratory must hold the textile standard atmosphere.
What goes wrong in practice
Jaw breaks and slippage account for most rejected specimens, and they have opposite signatures — a jaw break gives a low force, while slippage gives a normal force with an inflated elongation, because the slip is counted as extension. Both are cured by capstan wraps and correct pretension rather than by heavier clamping. Testing in the wrong atmosphere is a systematic error that a plastics-oriented laboratory makes easily and rarely notices. Sampling one continuous length instead of across the package understates variability, which is precisely the information the single-strand method exists to provide. And an assumed rather than measured linear density puts a quiet bias into every tenacity figure.
ASTM D2256 or ASTM D2524 skein testing
| Single strand (D2256) | Skein | |
|---|---|---|
| What is broken | One yarn | A skein of many wraps |
| Result | Individual breaking force and variability | An average force over many strands |
| Shows variability | Yes, directly | No — it is averaged away |
| Speed | Slower, many breaks needed | Quick |
Skein testing is faster but averages the yarn's variability into a single number. Since it is very often the weakest place in a yarn that stops a loom, the distribution the single-strand method reveals is usually the more useful information.
Questions we are asked about this test
What is ASTM D2256?
It is the ASTM method for the tensile properties of yarns by the single-strand method. A single yarn is mounted between capstan grips over a defined gauge length, pretensioned, and pulled at a rate that breaks it in about twenty seconds. It reports breaking force, elongation at break, tenacity and work of rupture.
Why capstan grips instead of flat jaws?
Because a yarn clamped between flat faces is crushed along a single line, and the fibres break there rather than in the gauge length. A capstan wraps the yarn around a curved surface, so the tension decays gradually around the wrap and the load is transferred over a long contact rather than at one point. Without that, almost every specimen would produce a jaw break.
What is tenacity and why not just quote breaking force?
Tenacity is breaking force divided by linear density, expressed in cN/tex or gf/den. Breaking force alone only compares yarns of the same count — a thicker yarn is stronger simply because there is more of it. Tenacity removes that and compares the fibre and the spinning. It is why linear density has to be measured on the yarn actually tested rather than taken from the label.
Why does the gauge length affect the result?
Because a longer specimen contains more opportunities for a weak place, and a yarn breaks at its weakest point. Increasing the gauge length therefore lowers the average breaking force — a real statistical effect rather than an artefact. This is why the gauge length is always reported, and why results from different gauge lengths cannot be compared directly.
Why is the textile atmosphere 21 °C and 65 % RH rather than 23 and 50?
Because textiles have long been tested in that atmosphere, and natural fibres in particular change strength and elongation markedly with moisture regain. Cotton gains strength as it takes up moisture, while some regenerated fibres lose it. Testing in the textile standard atmosphere keeps results comparable across the industry, and it is one of the commonest mistakes for a laboratory that mostly tests plastics.
How many specimens do I need?
Considerably more than for a moulded plastic. Yarn is variable along its length and between packages, so single-strand testing normally means tens of breaks rather than five, and the coefficient of variation is usually reported alongside the mean. That variability is itself useful information — a yarn with a good average and a wide spread will still break on a loom.
How do I tell slippage from a genuine result?
By the elongation. A specimen that slipped through the capstan shows an unusually high elongation with an otherwise ordinary breaking force, because the slip is counted as extension. If a set shows a few points with markedly higher elongation and no corresponding strength increase, slippage is the likely cause — add a wrap to the capstan and check the pretension.
Running ASTM D2256 on the Series 7200 and Series 9000
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 | Very low — from under 1 N on fine spun yarn to a few hundred newtons on industrial filament | 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 over the working range | Verified to ASTM E4, and to ISO 7500-1 Class 0.5 |
| Gripping | Capstan or bollard grips that wrap the yarn, so it is held by friction over a curve rather than crushed in a jaw | Our split capstan grips, built to the specimen |
| Environment | 21 ± 1 °C and 65 ± 2 % RH — the textile standard atmosphere, not the 23/50 used elsewhere | 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.