This blog post covers some of the common questions on axial extensometers. The guidance is based on ADMET equipment.
How is an axial extensometer related to my testing procedure?
When force is applied on a material, stress is produced and this stress causes the material to deform in the direction of the applied force. Stress vs. strain graphs are among the most common analyses on a tensile test report as they can be used to study multiple mechanical properties based on the specimen’s deformation profile and its reaction under applied load.
Strain (ε) represents the change in length. Strain measurements are obtained by dividing the change in length by the initial length of the specimen (ε = ∆L/L). Axial extensometers provide strain data by clipping onto the specimen with their knife edges and measuring the displacement in length as load is applied and the specimen is pulled.
ADMET’s standard clip-on extensometer is the EX-3542 extensometer (Class B-1). EX-3542 can be used for axial tensile, compression, and cyclic testing. It is designed for determining the strain of a wide range of materials including metals, plastics, composites and ceramics.
EX-3542 gauge length & travel range options
Selection of the gauge length and the travel range depends on the testing application and the specimen being tested. A variety of gauge lengths (both metric and US) and travel range options are offered with the EX-3542:
| Gauge Length | Travel Range | |
|---|---|---|
| Metric | US | |
| 10 mm 12 mm 12.5 mm 20 mm 25 mm 50 mm 100 mm |
0.5″ 0.64″ 1″ 1.4″ 2″ |
±5% ±10% +20%/-10% +25%/-10% +50%/-10% +100%/-10% |
ADMET’s EX-3542 axial extensometer
Using the same extensometer with specimens with different gauge lengths
Gauge length adapter kits are available to configure the same extensometer for multiple gauge lengths. Note that using an adapter kit will also change the % travel.
Can the EX-3542 be left on through specimen failure?
Yes.
High elongation specimens
Specimens such as elastomers and films elongate over the standard range of the EX-3542. We offer the EX-700CT and EX-1100CT long travel extensometers to accommodate such samples.
The travel range of EX-700CT is up to 700 mm, whereas the travel range of EX-1100CT is up to 1,100 mm. With these string-operating extensometers, gauge length is adjustable from 20 to 140 mm.
Available resolutions for both models include:
- Standard resolution: 0.025mm (0.001”)
- Hi-res: 0.005mm (0.0002”)
Testing applications in temperature controlled environments
We supply extensometers with custom temperature ranges to allow testing at low/high temperatures. Extensometers can be used in environmental chambers and left on through specimen failure.
Non-contacting extensometers
ADMET also offers non-contacting extensometers for applications where a clip-on is not practical. Some testing applications involve conditions where a traditional clip-on extensometer would cause low stress readings due to high contact stresses localized at the extensometer knife edges, be too heavy for the test sample, or not be able to stay on the sample through break.
Crosshead Displacement vs. Extensometry
Most universal test machine automation allows stress vs. strain calculations to be based on two options: either based on the movement of the crosshead or based on data from extensometry.
Depending on the user’s requirements, some test procedures can be set up to measure the change in the specimen length based on the movement of the crosshead, or the crosshead displacement data.
For example, when measuring modulus and yield of plastics, an extensometer is recommended as the linear region of plastics is very small and happens suddenly. Consequently, the crosshead displacement measures based on grip separation is not accurate enough.
The specimen geometry also determines the need for an extensometer. For example, dumbbell specimens do not have uniform widths so there will be errors when both the wide and narrow sections of the dumbbell shaped specimen elongate at different rates if crosshead displacement data is used for strain.
Examples of ASTM Test Methods
ASTM test methods where the use of an extensometer is recommended for strain data collection include:
ASTM A370 – Test Methods and Definitions for Mechanical Testing of Steel Products
ASTM D412 – Test Methods for Vulcanized Rubber and Thermoplastic Elastomers – Tension
ASTM D638 – Test Method for Tensile Properties of Plastics
ASTM D3039 – Test Method for Tensile Properties of Polymer Matrix Composite Materials
ASTM E8 – Test Methods for Tension Testing of Metallic Materials
ASTM E646 – Test Method for Tensile Strain-Hardening Exponents of Metallic Sheet Materials
Using an extensometer with the eP2 indicator or controller:
eP2 indicators and controllers are able to generate detailed tensile test reports. In order to accept input from extensometry, the strain channel of an eP2 needs to be activated. For existing eP2s without strain channels, we ask customers to send their eP2 box to ADMET to complete this basic modification.
eP2 back panel
Note: Different configurations may have different rear panels.
Using an extensometer with MTESTQuattro:
MTESTQuattro’s base configuration consists of load, crosshead position, axial strain, transverse strain and auxiliary input channels with options for three additional auxiliary input channels and servo control. No modifications are required for accommodating extensometry.
MTESTQuattro back panel
Note: Different configurations may have different rear panels.
How is compatibility ensured with ADMET extensometers and electronics?
ADMET extensometers come with connectors and smartducer chips to ensure compatibility with an existing controller. The smartducer identification chip allows the software to recognize the extensometer each time it is plugged back in, so you don’t have to worry about recalibration.
Conclusion
Please see our Extensometers and Transducers page for more information or contact ADMET sales for recommendations based on your testing application.
