Extensometers for materials testing
Frequently Asked Questions
Can Epsilon extensometers be left on through specimen failure?
Yes. All Epsilon standard extensometers are designed to withstand remaining on the sample though specimen failure with most materials. On certain materials, such as high strength metals, knife edges will wear out faster if the extensometer is left on through failure. A spare set of knife edges is included.
Will the extensometer be compatible with my existing electronics?
All extensometers in this catalog can easily be shipped with the mating connector already installed, so you can plug right in to your existing electronics. We stock connectors for every common brand of test machine.
The majority of Epsilon’s extensometers are strain gaged transducers which may be connected to most brands of materials testing controls. If your controls have the signal conditioning module for a strain gaged extensometer, we can supply the unit with the necessary connector to plug in directly. If you are using a data acquisition board to acquire test data, the extensometers can almost always be interfaced properly. A few of our extensometers use capacitive sensing technology, and these include the required electronics.
If your controls are designed for LVDT type extensometers only, we can provide the electronics to work with our extensometers. Available electronics are covered here. This is also ideal for older test machines, which may not have any extensometer electronics, allowing the output to run a chart recorder or plotter, or interface to a data acquisition board.
The unique Shunt Calibration Module from Epsilon helps make calibrating the electronics for the extensometer quick and easy. This also allows you to send your extensometer back to Epsilon for periodic re-calibration. The VREF provides the same calibration function of the capacitive devices. Further details on shunt calibration or for the VREF.
What electronics are needed?
Most of our extensometers are strain gage based sensors. They use a full Wheatstone bridge design. Functionally they require the same signal conditioning electronics used for any strain gaged transducer (load cell, pressure sensors, etc.). If you do not already have the electronics, Epsilon can supply this.
How do I decide what measuring range I need?
The selection of extensometer measuring range can depend on several factors including resolution of the signal conditioner, desired dynamic performance, space constraints, desired calibration class, ergonomics of the extensometer, and hanging weight on the specimen. The general rule is that for optimum performance in the elastic region (i.e., Young’s modulus and yield strength measurements) a measuring range of 5% to 20% is typical. If there is a need to record strain at failure using the extensometer, the measuring range selected should provide sufficient travel including overage for outliers. Strain ranges of 20% to 50% are typical for many metallic materials, with more ductile materials requiring 50% to 100% strain range. Composites typically need no more than 10% to 20% strain range for measuring strain at failure.
Generally speaking, Epsilon’s extensometers will meet the higher levels of accuracy requirements in today’s standards, such as ASTM E83. You can thus be assured that reasonably accurate measurements at the low end of the range can still be made.
Why do I need an extensometer when I can get strain from crosshead displacement?
Measuring crosshead deflection during a test does not just measure strain in a defined region of a test sample. It also measures machine deflection, grip deflection and possible slippage and deflection of the part of the test sample outside the normal reduced section. Strain is defined as the change in length divided by the initial length (gauge length). There is no defined initial length without an extensometer and the change in length is not correctly measured due to the other deflections included in crosshead measuring range.
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