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I am trying to design and build a low cost tensile test machine for plastic specimens. Most machines I've seen measure strain data using an extensometer, either the clamp-on type or the noncontact type (using video). However, both options are expensive to implement, so I was wondering if it would be a good idea to measure crosshead movement vs. time to use it as elongation data, and the correlate it with cross-section area and force vs. time data to obtain a stress-strain curve. Would this be feasible, if extremely precise results are not required?

Since so many machines use extensometers, I assume there must be a good reason not to use crosshead movement as elongation data. If so, why are crosshead movement and specimen elongation different? And how significant is said difference? Would much would test results be affected if crosshead movement were used as elongation?

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It is usually not accurate or precise enough ; primarily because of movement between the jaws or what ever device hold the specimens and the specimen. On metal samples , even an extensometer can make a little irregularity in the beginning of the strain curve as it "seats-in".

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