Extensometers are devices used in the measurement of length of objects. It has a high precision and used in stress-strain measurements and also in tensile strength tests. There are basically two major types of extensometers based on their principle of operation: They are the contact extensometers and the Non-contact extensometers. The contact extensometer is of various types of which the clip-on extensometer is the most common. The non-contact extensometers are the laser extensometer and the video extensometer. The Extensometer was invented by Charles Huston in 1879.
Laser extensometer is used in elongation measurements or strain measurements on certain materials or specimen. The working principle behind the Laser extensometer is that a laser beam is directed at the specimen and the resultant reflections from the surface of the specimen are then received by a Charge-Coupled device(CCD) whose information are then processed by very complex software program or algorithm.
Video extensometer work by capturing continuous images of the specimen using a digital camera that is attached to a computer. The pixel distance can then be measured in real time and then mapped accurately against a calibration value to give a direct strain measurement of the specimen. Resolution of even less than one micrometer can be achieved with a good calibration value and nice image processing data.
11 Types of Extensometers
There are 11 well known types of extensometers used in measurements both in the laboratory and in the industry.
These extensometers include:
1. Automatic contacting Extensometer
2. Average axial Clip-on Extensometer
3. Axial Clip-on Extensometer
4. Biaxial Clip-on Extensometer
5. Circumferential Extensometer
6. High Temperature Extensometer
7. Long Travel Extensometer
8. Non-contacting Video Extensometer
9. Transverse Clip-on Extensometer
11. Crack Opening Displacement (COD) Gauges
All Image(s) Credit: instron.co.uk
In mining, the extensometer is used in the measurement of displacement on highwalls. Geologists can then plot a graph of displacement against time to ascertain if there is an imminent fracture or failure on the walls. Complicated wall failures can be determined by the use of laser beams or radar to enable a 3-dimensional or 4-dimensional analysis.