A universal testing machine tells you the force on a specimen very precisely. What it does not tell you directly is how much the gauge section of that specimen has stretched. For that you need an extensometer. This guide explains what an extensometer measures, how the three common types differ, and what to consider when you add one to a tensile testing setup.
What an extensometer measures, and why crosshead travel is not enough
Strain is the change in length of a defined gauge length divided by that original gauge length. During a tensile test, the crosshead of the machine moves, but that movement includes much more than the stretch of the gauge section. It also includes slip and seating in the grips, deformation of the shoulders of the specimen, elastic deflection of the load frame, the load cell and the grip bodies, and any play in the load train.
For a rough elongation-at-break figure, crosshead displacement may be acceptable for some product tests. But wherever the result depends on the early, elastic part of the curve, crosshead readings are unreliable. That includes the modulus of elasticity, the 0.2% proof stress (Rp0.2) of materials without a sharp yield point, and strain-rate control in the elastic range. An extensometer is attached to, or optically tracks, the gauge section itself, so it measures only the deformation that matters.
Mechanical extensometers
A mechanical extensometer uses knife edges clamped to the specimen and a mechanical linkage that drives a dial gauge or similar indicator. It is a simple, robust instrument with no electronics, and it is still widely used in teaching laboratories and in labs that run analog machines.
- Strengths: straightforward to understand, no software dependency, good for demonstrating the principle of strain measurement.
- Limitations: readings are taken manually, so plotting a continuous stress-strain curve is slow; the operator usually removes it before fracture to protect it.
Akuracy offers a mechanical extensometer for this kind of work.
Electronic (clip-on) extensometers
An electronic extensometer also attaches to the specimen with knife edges, but the displacement is converted into an electrical signal, typically by strain-gauged arms or a displacement transducer. The signal goes to the controller or the testing software alongside the load signal, so the software can plot stress against strain in real time and calculate proof stress and modulus automatically.
- Strengths: continuous data, automatic calculation, compatible with computerized and servo-controlled machines.
- Limitations: knife edges can slip on very smooth or hard surfaces, and a clip-on unit may need to be removed before fracture of energetic specimens such as high-strength steel to avoid damage, unless the model is designed to stay on.
See the electronic extensometer for computerized UTM setups.
Non-contact video extensometers
A video extensometer uses a camera and image processing to track marks or features on the specimen surface. Because nothing touches the specimen, there is no knife-edge loading, no risk of the instrument damaging soft materials, and no need to remove anything before fracture.
- Strengths: suitable for soft, thin or delicate specimens such as films, rubbers and fibres; can follow very large extensions; measures through to break.
- Limitations: needs good lighting and contrast, a clear line of sight and marking of the specimen; set-up takes some care.
Akuracy lists a non-contact video extensometer with up to 800 mm extension, which suits materials that stretch far beyond the range of a clip-on unit.
Comparison at a glance
| Feature | Mechanical | Electronic clip-on | Non-contact video |
|---|---|---|---|
| Contact with specimen | Yes | Yes | No |
| Continuous data to software | No | Yes | Yes |
| Usually kept on to fracture | No | Depends on model | Yes |
| Typical use | Teaching, analog machines | Metals, routine QC | Soft or high-elongation materials |
Gauge length and specimen fit
The extensometer gauge length must match the gauge length specified for the specimen by your test standard or product specification. For proportional metal specimens, the original gauge length is related to the cross-sectional area of the specimen, so a lab testing several bar sizes may need more than one gauge length setting. Check also the maximum and minimum specimen diameter or thickness the knife edges can clamp, and the travel of the instrument compared with the elongation you expect.
Using an extensometer for proof stress and modulus
Metallic tensile testing in India is usually carried out to IS 1608 (Part 1), which is harmonised with ISO 6892-1. For materials without a distinct yield point, the standard defines proof strength at a specified plastic extension, most commonly 0.2%. Determining it means drawing a line parallel to the elastic part of the curve, offset by 0.2% strain, which is only meaningful if the strain axis comes from an extensometer.
The extensometer itself should be verified. ISO 9513 (and its Indian equivalent) sets out calibration procedures and accuracy classes for extensometers used in uniaxial testing. Test standards state which class is needed for which property, so check that your instrument and its calibration certificate meet the class your standard requires.
Choosing one for your lab
- List the materials and standards you test, and the properties you must report.
- Note specimen shapes, sizes and gauge lengths.
- Estimate the maximum elongation you need to follow.
- Confirm your machine and software can take the extensometer signal; older analog machines can often be upgraded.
- Plan for calibration of the extensometer as part of your lab's routine.
You can browse the full range in the extensometer collection.
Frequently asked questions
Do I need an extensometer to report 0.2% proof stress?
In practice, yes. Proof stress is defined from strain in the gauge length, and crosshead travel includes grip slip and frame deflection, so it does not give a reliable offset line.
Can a video extensometer be used on metals?
Yes, provided the specimen is marked or has enough surface contrast for the camera to track. It is especially useful when you want strain data through to fracture.
Can I add an extensometer to an existing UTM?
Often, yes. A computerized or retrofitted machine with a spare input channel and suitable software can usually accept an electronic extensometer. Share your machine details with us to check.
Request a quote
Akuracy (Ratnakar Enterprises) can advise on a configuration for your materials, standards and test volume. Share your specimen details and the standard you test to, and our team will reply with a suitable option. Request a quote online or message us on WhatsApp at +91 94038 91579.