When Should You Use an Extensometer?
- christopherthomasb
- Nov 10
- 3 min read
In materials testing, precision is everything. A universal testing machine (UTM) already measures strain by tracking the motion of its crosshead. However, when your goal is to determine accurate material properties—like elastic modulus, yield point, or elongation at break—a dedicated extensometer becomes essential.
In this article, we’ll explain when to use an extensometer, why it matters, and which type is best suited for your testing needs.
1. What Does a Universal Testing Machine Measure by Default?
Every universal testing machine measures load and displacement.
The load cell measures the force applied to the specimen.
The crosshead encoder tracks the machine’s movement to estimate strain.
However, the displacement measured at the crosshead includes more than just the specimen’s elongation — it also accounts for system compliance, grip slippage, and frame deformation.
That’s why the strain derived from crosshead movement is often accurate only to within about ±10%.
If your test standard or customer specification requires higher accuracy, that’s where an extensometer comes in.
2. Why Use an Extensometer?
An extensometer measures strain directly on the specimen, eliminating the effects of machine or grip compliance. This improves the precision of your stress–strain data, especially in the elastic region where small elongations make a big difference.
Using an extensometer can:✅ Increase strain measurement accuracy by up to 10%✅ Enable modulus and yield point calculations per ASTM and ISO standards✅ Improve data repeatability between tests✅ Meet the compliance requirements of standards like ASTM D638, E8, D412, and D3039.
3. When Is an Extensometer Required?
You should use an extensometer whenever:
You are measuring modulus of elasticity, yield strength, or proof stress.
The material is rigid or brittle, and small strain accuracy is critical (e.g., metals, composites, hard plastics).
You are testing according to ASTM or ISO standards that explicitly require one.
You need to compare results across multiple machines or labs with tight reproducibility.
For less demanding applications—such as quality control where only ultimate tensile strength is needed—crosshead strain measurements may be sufficient.
4. The Three Main Types of Extensometers
At Universal Grip Company, we provide three main types of extensometers to fit different materials and testing applications:
Contact-type devices that physically attach to the specimen.
Offer the highest accuracy in the elastic range.
Commonly used for metals, composites, and rigid plastics.
Easy to use and compatible with nearly any testing system.
Typical Use: ASTM E8, D638, D3039
Mechanically balanced devices mounted to the testing frame.
Designed for high-elongation materials such as rubber, elastomers, and films.
Track large displacements while maintaining low inertia.
Provide excellent accuracy for tests with long travel distances.
Typical Use: ASTM D412, D882
Use high-speed cameras and digital image correlation (DIC) to track deformation without touching the specimen.
Ideal for delicate, thin, or high-temperature materials.
Eliminate errors from slippage, alignment, or mass loading.
Now comparable in cost to long-travel mechanical extensometers.
Typical Use: ASTM D638, D3039, D1708
5. Choosing the Right Extensometer for Your Test
Application | Recommended Extensometer | Key Benefit |
Metals & Composites | Clip-On | Highest accuracy in elastic range |
Rubbers & Films | Long-Travel | Tracks large elongation |
Thin or Delicate Samples | Video | Non-contact measurement |
High-Temperature Testing | Video | Works in environmental chambers |
If you are using more than one clip-on extensometer per setup, upgrading to a video extensometer often makes sense — both for accuracy and cost-efficiency.
6. Integration with Universal Testing Machines
All extensometers from Universal Grip Company are designed to integrate seamlessly with most major UTM brands.
Each unit connects via a standard analog or digital input channel.
Long-travel extensometers can be mounted directly to the frame.
Video extensometers interface through dedicated software synchronized with machine control and data acquisition systems.
Our engineering team can help you choose the right setup for your testing application.
7. Summary
While your universal testing machine can measure strain on its own, an extensometer gives you that extra 10% of precision needed for professional, standards-compliant material testing. Whether you’re running basic quality control tests or advanced R&D programs, selecting the right extensometer ensures your data is both accurate and defensible.
Learn More or Request a Quote
Universal Grip Company offers a complete range of clip-on, long-travel, and video extensometers—along with grips, fixtures, and full testing solutions.
👉 Visit our Extensometers page to explore all models:🌐 www.universalgripco.com/extensometers
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