Axial/Torsional Extensometers – Model 3550

Designed for simultaneously measuring axial and torsional deflections on specimens tested in axial/torsional machines.

38 EXTENSOMETER MODELS are available in many thousands of variations. Our focus on helping customers has led to the development of models to cover nearly any test method.

Operation of these, and all of Epsilon’s extensometers, are


using in-house calibration systems that are calibrated with Epsilon’s laser interferometer system.


to the materials testing community, you know we will be there to stand behind our products when your critical application requires it.

The Model 3550 extensometer is most often used on round specimens tested in bi-axial test machines capable of simultaneous axial and torsional loading. The extensometer is often customized for particular applications. All units are capable of bi-directional displacement, so they may be used for cyclic testing under fully reversed loading conditions. The standard sized model is self-supporting on the specimen, and works on specimens from 9.5 to 25.4 mm (0.375 to 1.000 inches) diameter. The conical point contacts included with the extensometer are made from tungsten carbide. Their unique design directly measures the surface shear strain angle, which allows operation on a wide range of specimen diameters without changing calibration.

All models are designed specifically to minimize crosstalk between axes and to provide high accuracy measurements. All feature the dual flexure design common to other Epsilon extensometers.

Available options cover temperature ranges from -270 °C to 200 °C (the standard temperature range is -40 °C to 100 °C). Both inch and metric versions are available. A wide range of gauge length and measuring ranges are available. Because these transducers are often used for specialized tests, contact Epsilon with your specific test needs.

The Model 3550 extensometers are strain gaged devices, making them compatible with any electronics designed for strain gaged transducers. Most often they are connected to a test machine controller. The signal conditioning electronics for the extensometer is typically included with the test machine controller or may often be added. In this case the extensometer is shipped with the proper connector and wiring to plug directly into the electronics. For systems lacking the required electronics, Epsilon can provide a variety of solutions, allowing the extensometer output to be connected to data acquisition boards, chart recorders or other equipment.

Download Data Sheet


  • Full bridge, 350 ohm strain gaged design for compatibility with nearly any test system.
  • Self-supporting on the specimen.
  • Cyclic testing may be performed at up to 2 Hz.
  • Includes the Epsilon Shunt Calibration System for on-site electrical calibration.
  • Rugged, dual flexure design for strength and improved performance – much stronger than single flexure designs.
  • Includes high quality foam lined case.

Download Data Sheet


Excitation: 5 to 10 VDC recommended, 12 VDC or VAC max.

Output: 2 to 4 mV/V, nominal, depending on model

Accuracy: Standard configurations meet ASTM E83 class B-1 and ISO 9513 class 0,5 requirements for accuracy in the axial direction. A test certificate is included.

Axial: ≤0.15% of full scale measuring range, typical
Torsional: ≤0.20% of full scale measuring range, typical

Temperature Range: Standard (-ST) is -40 °C to +100 °C (-40 °F to 210°F)

Cable: Integral, ultra-flexible cable, 2.5 m (8 ft) standard

Operating Force: <30 g typical

Crosstalk: Less than 0.5%



Download Data Sheet

Download Data Sheet

Epsilon’s axial/torsional extensometer was designed to directly measure the shear strain, γ, shown in the figure. This design allows the correct determination of the shear strain without having to know the radius of the specimen being tested, R1. The shear strain is related to the angle of twist (a commonly referenced parameter in torsion testing), φ, by the following relationship:



For extensometers that measure φ directly instead of γ, corrections need to be made for every different specimen diameter tested and for axial strain to correctly determine the shear strain. The following two examples show the variation of the angle of twist versus a change in specimen diameter relative to the constant resulting shear strain:

Product Videos

Setup instructions for Model 3550 axial-torsional extensometers.

Ordering Information

Model Number: 3550025M005002ST

Model Number:

Axial Gauge Length: 25 mm; Axial Strain: ±5%; Torsional Shear Strain Angle: ±2°

Model Number: 3550025M005003ST

Model Number:

Axial Gauge Length: 25 mm; Axial Strain: ±5%; Torsional Shear Strain Angle: ±3°

Model Number: 3550025M010002ST
Model Number:

Axial Gauge Length: 25 mm; Axial Strain: ±10%; Torsional Shear Strain Angle: ±2°

Model Number: 35500100010002ST
Model Number:

Axial Gauge Length: 1 inch; Axial Strain: ±10%; Torsional Shear Strain Angle: ±2°

-ST temperature range is -40 °C to 100 °C (-40 °F to 210 °F), but -HT1, -HT2, -LT, and -LHT options are also available. This unit is built to order, contact Epsilon for a unit specific to your testing requirements.


If your unit is in need of service, calibration or repair, initiate the RMA process with us and we will be happy to assist.

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Our 2022 Extensometer Catalog, Version 110 is now available! Not finding something you’re looking for? Ask us about a custom quote.