Axial & transverse
Clip-On Extensometers
A clip-on extensometer is the usual subject of this calibration; its class is established over the range of extension it will actually be used across.
SpecificationsTesting standard
ISO 9513
Metallic materials — Calibration of extensometer systems used in uniaxial testing
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ISO 9513 specifies the static calibration of extensometer systems used in uniaxial testing, covering axial and diametral systems and both contacting and non-contacting types. It assigns a class — 0,2, 0,5, 1 or 2 — where the class number is the maximum permissible relative error of gauge length in per cent.
At a glance
- Test type
- Calibration & verification
- Published by
- ISO
- Edition
- ISO 9513:2012
- Material
- Metals, alloys & welds
- Runs on
- Series 7200 and Series 9000
What the test does
The extensometer system is mounted in a calibration apparatus in its working configuration and known displacements are applied across the range of extension it is used over. Indicated extension is recorded against true extension at each point, the series is repeated as the standard requires, and three quantities are determined: the relative error of gauge length, the resolution, and the bias error. All three are checked against the limits for the intended class, and the system is then assigned a class — 0,2, 0,5, 1 or 2 — together with the range of extension over which that class holds.
What it measures, and why it matters
Whether the strain the machine reports is the strain in the specimen. The scheme mirrors ISO 7500-1 exactly: the class number is the maximum permissible relative error in per cent, so Class 0,5 permits half a per cent. Which class is needed depends entirely on what is being measured. A modulus divides a stress interval by a deliberately small strain interval — often a window only a couple of thousandths wide — so a percentage error in that strain passes into the modulus undiminished, and tight classes are essential. Elongation at break, measured over tens or hundreds of per cent, tolerates far more.
The classes
The same scheme as ISO 7500-1 for force: the class number is the permitted relative error in per cent.
- Class 0,2
- ±0,2 % maximum permissible relative error of gauge length
- Class 0,5
- ±0,5 %The usual requirement where a modulus is to be determined.
- Class 1
- ±1 %
- Class 2
- ±2 %Adequate for elongation at break, where the extensions are large.
- Not accuracy alone
- Resolution and bias error each carry their own limit per classThe standard's table gives relative gauge length error, resolution and bias error together.
- Systems, not transducers
- Contacting and non-contacting, axial and diametralThe 2012 edition renamed the subject from extensometers to extensometer systems. A video system is calibrated with its optics and software, as used.
Like a force class, an extensometer class holds over a stated range of extension. A system classified at small extensions has not been classified at large ones, and long-travel work is exactly where that bites.
Test speed
- Application
- Static — displacements applied and held
- Reported
- Class, and the range of extension it applies over
- Scope
- Axial and diametral, contacting and non-contacting
- Calibrate the system as it is used
- Same optics, same software, same settingsDakRecalibrating a video system after changing lens or field of view is not pedantry — the field of view is part of the gauge.
Calculations
Relative error of gauge length—
(indicated extension − true extension) / true extension × 100
A percentage of the extension, so the absolute tolerance shrinks as the extension falls. This is why modulus work — measured over a very small strain window — demands the tightest classes.
Class—
The class number is the maximum permissible relative error of gauge length, in per cent
Class 0,5 permits ±0,5 %. Resolution and bias error each have their own limit for the same class, and all must be met.
Why modulus needs a tight classEt
Et = Δσ / Δε
- Δε
- a strain interval that may be as small as 0,002
The modulus divides by a very small strain, so a percentage error in that strain passes straight into the modulus undiminished.
How the test runs
- 01Identify the extensometer system, its gauge length and the range of extension it is used over.
- 02Calibrate it as a system — same optics, software and settings as in service.
- 03Mount it in the calibration apparatus in its working configuration.
- 04Apply known displacements across the intended range.
- 05Record indicated against true extension at each point.
- 06Repeat as the standard requires.
- 07Calculate the relative error of gauge length at each point.
- 08Determine the resolution and the bias error.
- 09Check all three against the limits for the intended class.
- 10Assign the class together with the range of extension it covers.
- 11State the gauge length on the certificate — the class belongs to that gauge length.
Grips and fixtures for this method
Non-contact
Advanced Video Extensometer
Non-contacting systems are explicitly in scope, and are calibrated as a system — optics, software and all — not as a transducer.
Specifications
800 mm travel
High Elongation Extensometer
Long-travel systems are classified over their own range, and a class held at small extensions does not carry to large ones.
SpecificationsWhat the report has to contain
- Reference to ISO 9513 and the edition
- Extensometer system identification, type and gauge length
- Whether contacting or non-contacting, axial or diametral
- Configuration as calibrated, including optics and software settings for a video system
- Identification and traceability of the calibration apparatus
- Ambient temperature
- Indicated and true extension at every point
- Relative error of gauge length, resolution and bias error
- The class assigned and the range of extension it applies over
- Date of calibration and the interval to the next
What the machine must be capable of
Nothing, in the sense that no force is applied — the calibration apparatus imposes displacement and the frame is not involved. What the *system* must be capable of is resolution fine enough that the class is not resolution-limited, and repeatable mounting, since a clip-on gauge that seats differently each time will fail on bias error rather than on accuracy. Long-travel and non-contacting systems are explicitly in scope and are classified over their own ranges.
What goes wrong in practice
Assuming a class holds at every extension, when it holds only over the stated range — a trap in long-travel work on elastomers, films and textiles, where service extensions can far exceed anything calibrated. Changing a video system's lens or field of view and carrying the old certificate forward. Using a Class 1 system for modulus work that needed Class 0,5. And holding an immaculate force certificate alongside an uncalibrated extensometer, which still yields a wrong modulus with nothing in the result to say which half was responsible.
ISO 9513 or ASTM E83
| ISO 9513 | ASTM E83 | |
|---|---|---|
| Family | ISO | ASTM |
| Result | Class 0,2 / 0,5 / 1 / 2 | Its own classification scheme |
| Scope | Axial and diametral, contacting and non-contacting | Extensometer systems and COD gauges |
| Paired with | ISO 6892-1, ISO 527-1 | ASTM E8/E8M, ASTM D638 |
Both classify the extension-measuring chain rather than the transducer alone. The class names do not map across, so cite the standard alongside the class or the figure cannot be interpreted.
Questions we are asked about this test
What is ISO 9513?
It is the ISO standard for the static calibration of extensometer systems used in uniaxial testing. It covers axial and diametral systems, and both contacting and non-contacting types, and assigns a class — 0,2, 0,5, 1 or 2. The current edition is ISO 9513:2012, the third, with a technical corrigendum issued in 2013.
What does Class 0,5 mean?
That the maximum permissible relative error of gauge length is ±0,5 %. As with ISO 7500-1 for force, the class number is the tolerance in per cent, which makes the two schemes read the same way. And as with force, accuracy alone is not enough — the standard's table sets limits on resolution and bias error for each class too, and a system must satisfy all of them to hold the class.
Why does a modulus need a tighter class than an elongation?
Because of what it divides by. A modulus is a stress interval over a strain interval, and that strain interval is deliberately small — often a window a couple of thousandths wide. A percentage error in a very small strain passes straight into the modulus at full size. Elongation at break, by contrast, is measured over tens or hundreds of per cent, where the same percentage error is far less consequential.
Why did the title change from extensometers to extensometer systems?
Because what gets calibrated is the whole chain, not the transducer. For a video or laser system that means the optics, the software and the settings as configured, not a sensor in isolation. It follows that changing a lens or a field of view changes the gauge, and the calibration should be repeated — a point that is easy to overlook precisely because nothing physical has been unplugged.
Does the class apply at every extension?
No. Like a force class, an extensometer class holds over a stated range of extension, and a system classified at small extensions has not been classified at large ones. This bites hardest in long-travel work — elastomers, films, textiles — where the extensions in service can be far beyond anything the calibration covered. The range belongs on the certificate next to the class.
Does the gauge length matter to the certificate?
Yes, and it should be stated on it. The relative error is an error of gauge length, so the class is tied to the gauge length at which it was established. Using the same instrument at a different gauge length is not automatically covered, which is worth checking before assuming a certificate applies to a new test set-up.
How does this relate to the force calibration?
They are separate certificates for the same machine and both are needed. ISO 7500-1 covers the force-measuring system; this covers the extension-measuring system. A modulus depends on both, so a machine with an immaculate force certificate and an uncalibrated extensometer will still produce a wrong modulus — and nothing in the result will indicate which half was at fault.
Running ISO 9513 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 | None — the extensometer is displaced against a reference, not loaded | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | A calibration apparatus of a resolution appropriate to the class sought | ISO 7500-1 Class 0.5, verified to ASTM E4, DIN 51221 and BS 1610 |
| Gripping | A calibration apparatus that imposes known displacements on the extensometer | Wedge, vice-action, pneumatic and hydraulic grips, built to the specimen |
| Environment | 23 ± 2 °C standard laboratory atmosphere | 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.