Non-contacting extensometers

Most non contacting type extensometers are stand-alone units and can be used with other manufacturers tensometer equipment if specified at purchase. 

Often high-extension and low-extension models are offered to give the degree of accuracy required. Horizontal versions are also available. 

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Automation frequently aids accuracy and/or reproducibility by being more consistent than humans. Non-contact extensometers ensure no unwanted stresses on the test piece and any automatic extensometer will be less subjective than a technician with a ruler. Digital thermometers, load cell balances and many other apparatus introductions have made measurements easier and less prone to operator error. 

The first commercial non-contact extensometers were optical extensometers which used either visible or infrared light to illuminate targets on the test piece. The essential difference between the optical and contact types is in the method of following the extension as illustrated in Figure 8.12, thereafter they both use some form of transducer to measure the movement which largely dictates the range and sensitivity. 

The principle and use of an optical non-contact extensometer available commercially has been described in some detail77. Two photoelectric sensing devices automatically follow, by means of a servo mechanism, contrastingly coloured gauge marks on the test piece. The separation of the auto followers... 

Figure 9.14 Schematic view of an infrared non-contacting extensometer.

For reading the temperature as an in-process control a non-contact laser infrared digital thermometer is used. The consistency could be measured using two glass plates as a simple extensometer. 

The mechanical performance of all samples was evaluated by tensile testing with an Instron 5567 machine. As the samples used were too weak to support a conventional mechanical extensometer, an Instron Advanced Video Extensometer model 2663-821 that allows for accurate non-contact strain measurements was used. The tensile testing was performed according to the ASTM standard D882 with a nominal gauge length of 50 mm, a crosshead speed of 6 mm/min, at a temperature of 21.5 °C, and a humidity of 50%. As per the standard, the results of at least five specimens were averaged for each sample. They are summarized in Table 11.2. 

Figure 2-10. (a) Diagram illustrating an extensometer (strain gauge) attached to the test specimen. (Courtesy of Instron Corporation.) (b) Non-contact optical extensometer (Courtesy Zwick USA). 

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