Self-tighteningTJ-10
Eccentric Roller Grips
Self-tightening rollers hold at near-zero load and grip harder as the piece stretches, which is what a rubber tab thinning through 500 % elongation needs.
SpecificationsTesting standard
ISO 37
Rubber, vulcanized or thermoplastic — Determination of tensile stress-strain properties
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ISO 37 is the tension test for vulcanised and thermoplastic rubber. A die-cut dumb-bell is pulled at a fixed rate until it ruptures, and the method reports tensile strength, elongation at break, and stress at a given elongation — the three numbers a compounder uses to confirm a batch reached its intended state of cure.
At a glance
- Test type
- Tensile — the specimen is pulled apart
- Published by
- ISO
- Edition
- ISO 37:2024
- Material
- Rubber, elastomers & foams
- Runs on
- Series 7200 and Series 9000
What the test does
A waisted strip of vulcanised or thermoplastic rubber — a dumb-bell — is held at its wide end tabs and pulled along its axis at a constant rate of traverse until it ruptures. Force and the separation of two reference marks inside the narrow test length are recorded throughout. The alternative ring test piece is not gripped at all: it runs on two pulleys that draw apart, one of them driven so the ring rotates as it stretches.
What it measures, and why it matters
The method reports tensile strength, elongation at break, and stress at a given elongation — the figures a compounder uses to release a batch, to confirm that a mix reached its intended state of cure, and to compare candidate compounds at the specification stage. The same three numbers measured before and after heat or fluid ageing quantify degradation, and a drop in elongation at break is the usual first evidence in a failure investigation. Rubber is strongly rate- and temperature-sensitive, so these values rank materials rather than predict service life.
Dumb-bell types and rings
Five dumb-bell types, distinguished by test length, plus a ring test piece for material a dumb-bell cannot be cut from. Which type was used changes the numbers, so it is reported with them.
- Type 1
- 25 mm test length, about 12 mm² sectionThe largest, and the one most comparable with ASTM Die C.
- Types 1A and 2
- 20 mm test length
- Types 3 and 4
- 10 mm test lengthType 4 presents only about 2 mm² of cross-section, for scarce or small samples.
- Type A ring
- 44.6 mm bore, two sections of about 16 mm² eachNot gripped at all — it runs on two pulleys, one driven so the ring rotates as it stretches.
- Cutting direction
- Fixed and recordedThe grain left by milling or calendering biases the result, so the orientation is part of the method rather than an operator's choice.
- Conditioning
- 23 ± 2 °C for at least 3 hWithout humidity control. At least 96 h with humidity control for latex-derived samples.
- Discard and replace
- Any rupture outside the test length
- Sharpen or replace dies
- On a scheduleDakRubber is notch-sensitive. A blunt die leaves an edge that seeds a tear, and the result looks like a compounding fault rather than a cutting one.
Rate of traverse
Fixed by test piece rather than calculated, which makes the frame requirement simple: it must hold three specific speeds steadily under almost no load.
- Types 1, 1A and 2
- 500 ± 50 mm/min
- Types 3 and 4
- 200 mm/min
- Type A ring
- 500 mm/min
- Type B ring
- 100 mm/min
- Ring pulley rotation
- 10 to 15 r/minOne pulley free-running with very low friction, the other driven.
- Hold the rate under no load
- The real requirementDakA rubber dumb-bell offers almost no resistance for the first part of its travel. A frame that only holds its rate under load will not hold it here.
Calculations
As in every rubber tension method, everything refers to the original cross-section and the original reference marks. The specimen thins enormously and none of that enters the equations.
Tensile strengthTS
TS = F / (w × t)
- F
- force at rupture, N
- w
- original width of the narrow portion, mm
- t
- original thickness, mm
For a ring, the load is carried by two cross-sections, so the area is doubled.
Elongation at breakEb
Eb = ((L − L₀) / L₀) × 100
- L
- separation of the reference marks at rupture, mm
- L₀
- original test length — 25 mm for type 1, 10 mm for type 4
Stress at a given elongationSe
Se = F_e / (w × t)
- F_e
- force recorded at the stated elongation, N
Commonly quoted at 100 % or 300 %. ASTM calls this quantity modulus; ISO does not, and the ISO name is the clearer one — it is a stress at a strain, not a slope.
How the test runs
- 01Condition the sheet at 23 ± 2 °C for at least three hours.
- 02Cut dumb-bells with a sharp die, in one stroke, with the cutting direction relative to the grain recorded.
- 03Mark the reference lines inside the narrow test length on the unstrained piece.
- 04Measure thickness at three points in the narrow portion and take the median.
- 05Fit grips that hold at near-zero load, since a small prestress is applied to stop the piece bending.
- 06Mount the piece straight, without twist, and apply the prestress only.
- 07Set extension measurement on the test length — long-travel or non-contact, not a clip-on gauge.
- 08Run at the rate for that test piece type: 500 mm/min for types 1, 1A and 2.
- 09Record force at each stated elongation as it passes, and at rupture.
- 10Note where the piece ruptured, and replace anything that failed outside the test length.
- 11Repeat across the replicate count the material specification requires.
Watch the test
Grips and fixtures for this method
Standard 25 mm
Pneumatic Vice Action Grip
Constant air pressure across the jaw face, identical on every piece — the gentler option for soft compounds and the one that removes operator grip force as a variable.
SpecificationsWhat the report has to contain
- Reference to ISO 37 and the test piece type used
- Compound identification and cure conditions
- Cutting direction relative to the grain
- Median thickness and the cross-sectional area used
- Conditioning time and temperature
- Rate of traverse
- Tensile strength
- Elongation at break
- Stress at each stated elongation required by the specification
- Test temperature
- Number of test pieces and the statistic reported
What the machine must be capable of
Forces are small and the span between test types is wide. A type 1 dumb-bell usually breaks between roughly 50 N and 500 N, a type 4 miniature piece below 50 N, while a type A ring on a high-strength compound can approach 1 kN — so one load cell sized for the ring will resolve the miniature dumb-bell's early curve poorly, and interchangeable cells are the practical answer. Force indication must meet ISO 5893 Class 1.
Rate of traverse is fixed by test piece: 500 ± 50 mm/min for types 1, 1A and 2, 200 mm/min for types 3 and 4, 500 mm/min for type A rings and 100 mm/min for type B, so the frame must as a minimum provide 100, 200 and 500 mm/min and hold them steadily under almost no load.
Extension is measured on the test length, not from crosshead travel: ISO 5893 Class D for types 1, 1A and 2, Class E for types 3 and 4. Strains run from zero to rupture — commonly 100 % to 800 %, beyond 1000 % for some thermoplastic rubbers — which on a 25 mm test length is up to about 250 mm of travel, so a long-travel or non-contact device is required rather than a clip-on gauge. Grips must hold at near-zero load, since a small prestress is applied to stop the piece bending. Rings need the twin-pulley rig, one pulley free-running with very low friction and the other driven at 10–15 r/min. Tests away from 23 ± 2 °C need a thermostatically controlled chamber fitted to the frame.
What goes wrong in practice
Soft compounds creep out of the jaws; the trace stays smooth, so slippage reads as extra elongation and depresses the calculated stress at a given elongation. Grip pressure or too short an overall length puts the break in the shoulder radius, which understates strength. Above a few hundred per cent, reference marks thin and smear and a non-contact tracker can lose them near rupture, truncating the strain record. Ring results sit below dumb-bell results on the same compound, because the bore strains more than the outer wall.
ISO 37 or ASTM D412
The same materials, the same three properties, and dies that are close enough to be mistaken for one another.
| ISO 37 | ASTM D412 | |
|---|---|---|
| Test pieces | Five dumb-bell types plus rings | Six dies plus a cut ring |
| Largest test length | 25 mm, type 1 | 25 mm, Dies C and D |
| Speed | 500, 200 or 100 mm/min by type | 500 ± 50 mm/min |
| The stress-at-strain figure | Called stress at a given elongation | Called modulus |
| Force class | ISO 5893 Class 1 | ASTM E4 |
| Extension class | ISO 5893 Class D or E | Not classified the same way |
Close but not equal. The dies and speeds differ enough that results should not be treated as interchangeable without a correlation study — a compound can pass one specification and fail the other on the same material.
Questions we are asked about this test
What is ISO 37?
It is the international standard for the tensile stress-strain properties of vulcanised and thermoplastic rubber. A die-cut dumb-bell is pulled at a fixed rate until it ruptures, and the method reports tensile strength, elongation at break, and stress at stated elongations. A ring test piece run on two pulleys is the alternative where a dumb-bell cannot be cut.
What is the difference between ISO 37 and ASTM D412?
They cover the same materials and the same properties, but the dies and speeds differ, so results are close rather than equal and should not be treated as interchangeable without a correlation study. There is also a naming difference worth knowing: what ASTM calls modulus, ISO calls stress at a given elongation — the ISO name being the more accurate, since it is a stress at a strain rather than a slope.
Which dumb-bell type should I use?
Type 1, with its 25 mm test length and roughly 12 mm² cross-section, unless there is a reason not to — it is the largest and the most comparable with ASTM Die C. Types 3 and 4 exist for scarce material, but type 4 presents only about 2 mm² of section, so it breaks at very low force and scatters more. The type used has to be reported, because the numbers are not interchangeable between types.
What speed does ISO 37 use?
It depends on the test piece. Types 1, 1A and 2 run at 500 ± 50 mm/min, types 3 and 4 at 200 mm/min, type A rings at 500 mm/min and type B rings at 100 mm/min. The frame must hold those speeds steadily under almost no load, which is a harder requirement than it sounds — a rubber dumb-bell offers very little resistance through the first part of its travel.
What extensometer does ISO 37 need?
One with very long travel, or a non-contact device. Elongation commonly runs from 100 to 800 % and beyond 1000 % for some thermoplastic rubbers, which on a 25 mm test length is up to about 250 mm of extension. A clip-on gauge cannot follow that. ISO 5893 Class D applies for types 1, 1A and 2, and Class E for the smaller types.
What load cell does ISO 37 need?
It depends on the test piece, and the span is wide enough that one cell will not serve well across all of them. A type 1 dumb-bell usually breaks between about 50 N and 500 N, a type 4 below 50 N, and a type A ring on a high-strength compound can approach 1 kN. A cell sized for the ring resolves the miniature dumb-bell's early curve poorly, so interchangeable cells are the practical answer.
Why does the cutting direction matter?
Because milling and calendering leave a grain in the sheet, and a piece cut along it behaves differently from one cut across it. The standard therefore fixes the direction and requires it to be recorded rather than leaving it to the operator. It is a common source of scatter between laboratories testing the same compound.
Why must elongation be measured on the test length?
Because crosshead travel also contains the machine's own stretch and any creep of the tab through the grips, and both push elongation up while pushing the stress-at-strain figures down. On a 25 mm test length stretching several hundred per cent, a small error near the grips becomes a large error in the result.
Running ISO 37 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 | A type 1 dumbbell presents about 12 mm² and usually breaks between roughly 50 N and 500 N; a type A ring carries two cross-sections of 16 mm² each and can approach 1 kN on a high-strength compound, while a type 4 miniature dumbbell of only 2 mm² may break under 50 N and needs a small load cell to read its early curve. | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | ISO 5893 Class 1 (force) | ISO 7500-1 Class 0.5, verified to ASTM E4, DIN 51221 and BS 1610 |
| Strain measurement | An extensometer to ISO 5893 Class D (dumbbell types 1, 1A, 2); ISO 5893 Class E (types 3, 4), gauge length 25 (type 1); 20 (types 1A and 2); 10 (types 3 and 4); n/a for rings | Certified to ISO 9513 Class 1 and ASTM E83 — non-contact video, clip-on and high-elongation |
| Gripping | Self-tightening or pneumatic grips for dumbbells; a twin-pulley ring rig with one free-running and one driven pulley for type A and type B rings. | Our vice-action grips or self-tightening eccentric roller grips, built to the specimen |
| Environment | Ambient standard laboratory temperature, 23 ± 2 °C; samples conditioned without humidity control (latex-derived samples excepted); a thermostatically controlled chamber fitted to the frame for any non-ambient 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.