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Can I Trust Strain Data from Crosshead Position or must I Use an Extensometer?
This is a commonly asked question.
To begin, ASTM and ISO industry test standards for materials such as plastics, rubber, steel and composites tend to require or strongly recommend an extensometer to measure strain values when the test data is used to validate critical engineered designs.
To the user, the implications of providing an axial extensometer to a new test machine purchase add approximately $3000 higher cost for the extensometer and controller strain channel.
To contrast, the crosshead movement or stroke of a universal test machine or electromechanical actuator is measured by the digital position encoder that is included as part of the mechanical assemble and can typically measure with submicron resolution possible. Crosshead position movement is not the same as mechanical sample based strain, but in certain applications the crosshead movement measured by the encoder can be even more accurate than an extensometer, at least when the load train is unloaded or loads are small.
An extensometer measures true sample strain independent of sample loading. Higher or lower load values do not influence the strain measurement of an extensometer which is in contrast with use of the encoder that is subject to system compliance or loading considerations.
If compliance of your specimen is 100 times more than the compliance of the load frame and the load string components, you can assume that the reported crosshead movement is equivalent to the deflection experienced by the specimen – as a general guideline.
Another way to approach the challenge of capturing inexpensive sample position data is to measure system compliance prior to the test by installing a stiff sample and applying a tensile load. Measure the resulting deflection measurement. When you test with a real test specimen, remove the deflection values from the results.
Compliance refers to the stiffness of the test frame, related load train components, and test sample under loading. Note that when a load is applied to a load train component such as crosshead, it deflects. It acts as a very stiff spring and although the deflection is small, it is measurable. Compliance if the opposite of stiffness.
Load train compliance is usually unknown because of the large combination of different test fixtures, grips and machines that do not have published compliance data.
Next time consider the cost of adding an extensometer to a purchase decision may be an easier decision, assuming you have a clear test sample (geometry, dimensions and material properties) in mind.
