Clip-On Extensometers

Written by Chris Thomas, Universal Grip Company · Reviewed by Frank Bertini · Updated October 17, 2026

A clip-on extensometer is the most direct way to measure strain in a tensile, compression or flexure test. Two knife edges clip onto the specimen a known gauge length apart, a strain-gauged flexure inside the body tracks how far they separate, and the machine records strain from the specimen itself rather than from crosshead travel. That is what makes modulus, offset yield, Poisson's ratio and any strain-based property valid: standards such as ASTM E8, ASTM D638 and ASTM D3039 require an extensometer of a stated accuracy class for those results, and crosshead displacement does not qualify. Universal Grip is a global distributor of Epsilon extensometers and strain gauges and configures clip-on extensometers for any universal testing machine, old or new, of any brand.

This page explains how a clip-on extensometer works, how to choose gauge length, travel and accuracy class for your tests, the axial, high-elongation and miniature models we supply, how they connect to Instron, MTS, Shimadzu, ZwickRoell, ADMET, United, Galdabini and other machines, how calibration is handled, and how to set one up so it does not slip, damage the specimen or get destroyed at fracture. For a comparison with video, long-travel and 3D DIC systems, see our guide to the types of extensometers and the extensometers hub.

In this guide: How a Clip-On Extensometer Works · How to Choose a Clip-On Extensometer · Our Clip-On Extensometer Models · Which Extensometer for Which Standard · Compatibility: Fitting an Extensometer to Your Machine · Calibration and Verification · Setting Up a Clip-On Extensometer · Common Mistakes · Related Products · FAQ

How a Clip-On Extensometer Works

The body of a clip-on extensometer is a pair of arms joined by a thin flexure. Each arm carries a hardened knife edge (or a rounded contact for soft materials) that presses against the specimen, held on by springs, elastic bands or a quick-attach clip. When the specimen stretches, the arms rotate apart and bend the flexure; strain gauges bonded to the flexure are wired in a Wheatstone bridge, so the bridge output is proportional to the change in gauge length. The testing machine's strain channel excites the bridge, reads the millivolt signal, and the software divides the displacement by the initial gauge length to give engineering strain.

Because the measurement comes from the gauge section itself, it ignores everything crosshead displacement includes by mistake: grip seating and slippage, machine and load-cell compliance, and stretching of the specimen shoulders and tabs. On a stiff material that difference is enormous. A steel specimen at 200 GPa reaches 0.2 % strain after only a few hundredths of a millimetre of gauge extension, while the crosshead may have moved ten times that; a modulus from crosshead data can be low by a factor of two or more. Extensometers are therefore mandatory wherever Young's modulus, Poisson's ratio, offset yield or a strain-based limit is reported.

Clip-on extensometers are classified by ASTM E83 (and ISO 9513) according to their fixed error and their error as a percentage of reading. Class B-1 and B-2 instruments are the common choices for modulus and yield; Class C suits elongation measurements at larger strains. The class is verified by calibration on a precision displacement fixture and is what a specification means when it says 'Class B-2 or better'.

How to Choose a Clip-On Extensometer

Four decisions cover almost every case, and they map directly onto the model lines below.

• Gauge length. Match the standard or the specimen: 50 mm (2 in) for the standard E8 round and sheet specimens and D3039 coupons, 25 mm (1 in) for E8 subsize, D638 Type I and most composite and plastic coupons, 10 to 12.5 mm for small rounds, wires, welds and miniature specimens. The gauge length must fit inside the reduced section with clearance from the fillets, and on an unreduced specimen must not exceed 80 % of the free length between the grips (E8).

• Travel (measuring range). The percentage of strain the extensometer can follow before it must be removed. Metals and rigid plastics need 10 to 50 % of gauge length; the modulus-only case needs only a few percent; elastomers, films and ductile plastics that stretch several hundred percent need a high-elongation design rather than a standard clip-on, or a long-travel or video extensometer.

• Accuracy class. Class B-2 (E83) for offset yield and modulus per E8, D638 and D3039; Class C is acceptable for elongation above 5 % strain. Ask for the class you need to be stated on the calibration certificate.

• Specimen and environment. Round or flat specimens (knife edges suit both; V-notched or chisel contacts for small rounds); specimen width and thickness within the clip range; temperature range if the test runs in an environmental chamber; and whether the extensometer will stay on to fracture (which needs a rugged, light body and a bracket that catches it) or be removed after yield.

Then there are the questions that decide whether a clip-on is the right type at all. If the specimen is delicate (thin film, foil, fibre), stretches more than the clip-on's travel, cannot carry the weight of an instrument, or is tested in volume where attaching and removing an extensometer per specimen costs more operator time than it is worth, a non-contact video extensometer or an automatic contacting extensometer is the better choice. For everything else, a clip-on is the simplest, most accurate and least expensive instrument for the job, and the clip-on purchasing and configuration guide walks through the options step by step.

Our Clip-On Extensometer Models

Extensometers are assembled to order to the customer's exact gauge length, travel and connector specification. The three families below cover nearly every tensile testing application; if you do not see what you need, Universal Grip will find the most suitable configuration.

3500 Series – Axial Extensometer (model 3542)

Designed for seamless integration with any universal testing machine, the 3542 basic axial extensometer is the go-to instrument for precise, reliable strain measurement in axial tensile testing, and the most popular extensometer we supply. It is the standard choice for ASTM E8 metals (0.2 % offset yield, modulus), ASTM D638 plastics (modulus, yield), ASTM D3039 composite coupons and the compression modulus specimens of ASTM D695, and it is compatible with any UTM that has a strain input channel. Gauge lengths and travel ranges are configured to order; ask for the data sheet for the full option list.

3800 Series – High-Elongation Extensometer

The 3800 high-elongation extensometer uses a string and a potentiometer instead of a strain-gauged flexure, so it can follow long-elongation specimens such as elastomers, plastics and rubber. Travel lengths go up to 20 inches, or about 500 mm, which covers ASTM D412 dumbbells and ductile plastics to rupture. For even longer travel, or where no contact with the specimen is wanted, see our balanced long-travel extensometers and video extensometers.

3400 Series – Miniature Extensometer

The 3400 miniature extensometer is a lightweight, low-friction instrument for small specimens and short gauge lengths: wires, small rounds, welds, thin strip and the subsize specimens of E8 and D638. Its low mass and friction make it the ideal choice for dynamic and cyclic testing, where a heavier extensometer would ring or slip under reversing load.

Which Extensometer for Which Standard

• ASTM E8/E8M – Class B-2 for offset and EUL yield; Class B-2 below 5 % strain, C from 5 to 50 %, D above 50 % for extensometer-based elongation; 50 mm gauge on standard specimens, 25 mm on subsize. Axial 3500 series.

• ASTM D638 – modulus requires an extensometer accurate to at least ±1 % of strain (Class B-2 is the usual choice); 25 or 50 mm gauge depending on type; stays on to yield, removed before break for brittle plastics or kept on with a catch bracket. Axial 3500 series; 3800 high-elongation for ductile types to rupture.

• ASTM D3039 – chord modulus between 1,000 and 3,000 µε and Poisson's ratio; 25 or 50 mm axial gauge, transverse extensometer or gauges for ν. Axial 3500 series (a biaxial or transverse unit for Poisson's ratio).

• ASTM D695 – compressive modulus on the 50.8 mm slenderness specimen needs a Class B-2 compressometer or an extensometer mounted on the specimen. Axial 3500 series.

• ASTM D412 and other elastomer methods – elongation to several hundred percent; 3800 high-elongation, long-travel or video extensometer.

• ASTM D790 – flexural modulus is measured with a deflectometer at mid-span rather than a clip-on; see the bend fixture page.

• ASTM A370, A615, E345, B557 – product standards that adopt E8's extensometer rules; same models, with the gauge length of the product specimen.

• ISO 6892-1, ISO 527-2, ISO 527-4 – the ISO equivalents, with ISO 9513 classes (Class 1 for yield, 0.5 for modulus) in place of E83.

Compatibility: Fitting an Extensometer to Your Machine

Universal Grip works directly with each customer to specify the right extensometer configuration for their machine. Universal testing machines built within the last 15 years are straightforward: most, including Instron, MTS, ADMET, Shimadzu, United and ZwickRoell frames, have a data-acquisition card and a strain channel ready to accept a strain-gauge extensometer, and the most common connector is a 25-pin D-sub plug from Amphenol. If your machine has a strain channel, we supply the extensometer wired to the correct connector and cable assembly for that controller, with the bridge excitation and sensitivity matched so the channel reads correctly after calibration.

Older machines, and frames without a spare strain channel, can be fitted with a signal conditioner that converts the bridge output to a ±10 V analogue signal or a digital interface the software can read; tell us the controller model and we will specify it. The Galdabini universal testing machines we supply are delivered with the extensometer channel configured, and the Mark-10 testers accept extensometers through their analogue input on the higher-capacity models. Adapters are not involved for extensometers; the mounting is on the specimen, and only the electrical interface changes from machine to machine.

Calibration and Verification

Universal Grip's clip-on extensometers are easy to calibrate and set up. Universal Grip does not provide certified calibration services; the instruments may be calibrated by any professional third-party calibrator to ASTM E83 or ISO 9513, and the class is stated on the certificate. For laboratories and calibration houses that verify extensometers in-house, Universal Grip also offers digital electronic extensometer calibrators: precision displacement fixtures that move the knife edges a known distance so the output can be checked at the points E83 requires. Verify annually, after any drop or knife-edge replacement, and whenever a machine's strain channel is changed; see our calibration page for the services we coordinate.

Setting Up a Clip-On Extensometer

• Mark or measure the gauge length. Set the extensometer to its gauge length with the setting pin or stop, attach it, then remove the pin before the test starts; a forgotten pin destroys the flexure.

• Centre it on the reduced section, square to the specimen axis, with the knife edges seated firmly but not gouging. On soft plastics use rounded or rubber contacts; on round metal specimens use V-notched contacts.

• Balance the spring force to the specimen. Too little and the knife edges slip under load (a flat spot or step in the curve); too much and the edges indent thin specimens or start a fracture at the contact.

• Support the cable so its weight does not pull the extensometer sideways, and route it clear of the grips.

• Zero the strain channel after attachment, with the specimen under the small preload used to seat the grips.

• Decide the removal point. Clip-ons rated for small travel come off after yield; those that stay to fracture need a catch bracket or lanyard so the recoil does not throw the instrument. The software should switch to crosshead displacement at removal without a step in the curve.

• Check the first specimen's modulus against a known value for the material; a low modulus almost always means slippage or a wrong gauge length entered in the software.

Common Mistakes

• Reporting modulus or offset yield from crosshead displacement; E8, D638 and D3039 do not allow it.

• A gauge length that spans the fillet or sits within the grip free-length limit, so part of the gauge is outside the reduced section.

• Wrong gauge length entered in the software after changing an extensometer.

• Extensometer class not stated on the calibration certificate, so the lab cannot show compliance.

• Leaving the gauge-length setting pin in place.

• Knife edges worn or chipped, giving slip at low load.

• Using a standard clip-on on an elastomer or film and exceeding its travel at break.

• Mismatched excitation or sensitivity between the extensometer and the strain channel, so the reading is scaled wrongly until recalibrated.

Related Products

• Video extensometers: non-contact strain to fracture, multiple gauge lengths on one specimen, no attachment time.

• Long-travel extensometers: counterbalanced contact extensometers for elongations up to 700 mm.

• Vector 3D extensometer: DIC full-field strain, biaxial and Poisson's ratio.

• Gauge-length marking tool: repeatable marks for video extensometers and for elongation after fracture.

• Environmental chambers: with extensometer ports and high- or low-temperature extensometer options.

• Guides: when to use an extensometer, video vs clip-on, clip-on purchasing guide and the extensometers hub.

Clip-On Extensometer FAQ

What is a clip-on extensometer?

A strain-measuring instrument that clips onto a test specimen with two knife edges a known gauge length apart and measures their separation with a strain-gauged flexure, so the testing machine records strain directly from the specimen rather than from crosshead travel.

Why can't I use crosshead displacement instead of an extensometer?

Crosshead displacement includes grip seating, slippage, machine compliance and stretching of the specimen shoulders, so modulus and yield values from it are wrong, often by a factor of two on stiff materials. ASTM E8, D638 and D3039 require an extensometer of a stated class for those results.

What gauge length should I choose?

The one the standard or specimen calls for: 50 mm (2 in) for standard ASTM E8 and D3039 specimens, 25 mm (1 in) for E8 subsize and ASTM D638 Type I, and 10 to 12.5 mm for small rounds, wires and welds. The gauge length must fit inside the reduced section clear of the fillets.

What is extensometer Class B-2?

An ASTM E83 accuracy class with a fixed error of not more than 0.0002 mm/mm or 0.5 % of reading, whichever is greater. It is the class usually required for offset yield and modulus in ASTM E8, D638 and D3039; Class C is acceptable for elongation above 5 % strain.

Will a clip-on extensometer work with my Instron, MTS or Shimadzu machine?

Yes. Any machine with a strain input channel can use one; most frames built in the last 15 years have a data-acquisition card and strain channel, and the usual connector is a 25-pin D-sub. Universal Grip wires the extensometer to the correct connector and cable for your controller.

Which model do I need for rubber or plastics with high elongation?

The 3800 high-elongation extensometer, which uses a string and potentiometer and travels up to about 500 mm (20 in). For longer travel or delicate specimens, a long-travel or video extensometer is the better choice.

Which extensometer suits small specimens or cyclic testing?

The 3400 miniature extensometer: lightweight, low friction and made for short gauge lengths, which makes it suitable for dynamic and fatigue testing where a heavier instrument would slip or ring.

Does Universal Grip calibrate extensometers?

Universal Grip does not provide certified calibration services. Our extensometers can be calibrated by any professional third-party calibrator to ASTM E83 or ISO 9513, and we supply digital extensometer calibrators for labs and calibration houses that verify in-house.

Can a clip-on extensometer stay on the specimen until it breaks?

Only if its travel covers the strain at fracture and it is a rugged, light design with a catch bracket; otherwise it is removed after yield and the software continues on crosshead displacement. For strain to fracture on ductile materials a video extensometer is usually simpler.

How do I know the extensometer is reading correctly?

Run a specimen of a material with a known modulus and compare; a low result almost always means knife-edge slippage, a wrong gauge length entered in the software, or a mismatched strain-channel setting. Verify annually against E83 with a calibrator.

Not sure which extensometer, gauge length or connector your machine needs? Contact Universal Grip with the test standard, specimen size and your machine's make and model, and we will configure the right clip-on extensometer and cable.