What is an Extensometer?
An Extensometer is a type of strain gauge that is used in conjunction with a Universal Testing Machine in order to measure elongation at high accuracies. Extensometers come in different shapes and sizes depending on the type of sample being measured. Extensometers can also be used to measure in the lateral direction in order to observe and quantify changes to the material in the X reference plane. Poisson’s Ratio is a specific calculation that combines XY strain gauge extensometer measurements in order to characterize the necking effect . Poisson’s calculation is commonly used in Metallurgy to characterize different types of commercial grade steel, especially cold rolled steel sheets.
Clip-on extensometers are the most common and lowest cost type, however these measurement instruments have a tendency to fall from the test apparatus and break. Clip-on extensometers are commonly used for tensile tests involving metal and are removed before a sample break occurs. There are two main specifications for a clip-on extensometer. The first is the distance between the two reference points which are two equidistant points inside the neck area of the sample - usually 1” or 2” apart. The second specification is the amount of elongation to be measured. Brittle metals will have elongation of less than 10%, where as a more ductile metal like Aluminum may have up to 50% elongation.
Balanced Extensometers are used for materials such as elastomers, plastics, and textiles that could have elongations above 50% or 100%. These types of extensometers are supported by the machine and hang off to the side. When a sample is loaded into the grips, the long travel extensometer will swivel into place and attach onto the sample. The extensometer will stay attached throughout the test including during the sample break. Universal Grip offers the Micron extensometer which is used in conjunction with the Galdabini Graphworks 6 testing software. The Micron extensometer is capable of measuring elongations of over 1000% which are usually seen when testing thermoplastic elastomers.
Other Types of Strain Measurement
Extensometers are used exclusively for tensile testing. Similar devices are used for compression and bend tests, however they are slightly different. Compression tests, such as those involving concrete cylinders, will use a type of strain gauge called a Compressometer. Bend tests will use a deflectometer which is able to accurately measure the deflection of the bend testing specimen. These types of contact measurement methods are much more precise than simply using the crosshead position data from the materials testing system. The extensometer is in direct contact with the sample and therefore gives a much more accurate representation of what is happening during the test procedure.
Measuring Elongation in an Environmental Chamber
If the extensometer is used in an environmental chamber, special precautions must be taken in order to account for the temperature. Specially designed high temperature extensometers that can withstand over 1000 degrees Celsius are available from Universal Grip. The high temperature extensometers use ceramic components in order to withstand the harsh conditions. If you have more questions regarding these types of strain gauge measurement devices, please contact us.