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Strain gauged flexures are compact and lightweight making them easy to integrate into a, compact, clip-on extensometer design. They have a very high inherent resolution; the limiting factor is the noise level or digital resolution of the conditioning electronics. The accuracy of clip-on extensometers using strain-gauged flexures will usually be sufficient to meet ASTM E83 grade B1/B2 or ISO 9513 grade 0.5 (i.e. accuracy is better than 0.5% of reading). It should be noted that parameters such as modulus are usually measured at small strains (e.g. 0.1% to 0.3% for an ASTM D3039 Tensile test) and that the extensometer used should be calibrated in this region.

Operation at temperatures above about 200°C requires either a strain gauged extensometer with some form of cooling (e.g. water or air) or the use of a transducer capable of operating at the test temperature e.g., a high-temperature encapsulated strain gauge or capacitive type.

Clip-on extensometers can be used when testing in environments other than air e.g. in high humidity air, in fluids or gases. In tmchese situations care should be taken to ensure that all the materials used in the construction of the extensometer are compatible with the particular environment in use.

In addition to single axial clip-on extensometers there are a number of other clip-on extensometers types used for composites testing.

The averaging axial clip-on extensometer uses a pair of axial extensometers positioned across the width or thickness of the test specimen. As noted earlier the measurement of average axial strain eliminates strain measurement errors arising from the presence of bending in the test specimen caused by alignment errors in the load string.

Measurement of shear strains and Poisson’s Ratio requires the simultaneous measurement of axial and transverse strain. Transverse clip-on extensometers are similar in design to axial types, but they measure the change in width of the test specimen. A gauge length setting mechanism is usually not required; the gauge length (in this case a Gauge Width) is determined by the dimensions of the specimen. Biaxial extensometers, which integrate both axial and transverse extensometers in to a single unit, are available and these units are more convenient to attach than two separate extensometers.

Extensometers v Strain Gauges Some of the important characteristics of strain gauges and clip-on extensometers are summarized in the table below:

  Extensometer  Strain Gauge 

Ease of application 

Simple  Complex  

Recurring Cost  Low  High 

Strain Measurement 

Average(over gauge length) 

Local (over grid) 

Compatibility with compression fixtures 

Limited  Good (small size) 

Transverse sensitivity 

Low  May be significant 

Conclusion Both strain gauges and extensometers have a role is composites testing. Generally extensometers are preferred because of their ease of use and consequently lower costs; however there are many test situations where strain gauges are still required. Examples of tests where strain gauges are required include; compression tests where the short specimen gauge sections and/or the available space inside the fixture prevent the use of an extensometer and V-Notch shear tests where the strain is concentrated in a small region.

References Mechanical testing of advanced fiber composites Woodhead Publishing ISBN: 1 85573 312 9

Handbook of Experimental Solid Mechanics Springer ISBN: 978-0-387-26883-5

ASTM E83 – 10a Standard Practice for Verification and Classification of Extensometer Systems

ISO 9513:2012 Metallic materials – Calibration of extensometer systems used in uniaxial testing

ASTM D6641-09: Standard Test Method for Compressive Properties of Polymer Matrix Composite Materials Using a Combined Loading Compression (CLC) Test Fixture