Uniaxial Tension Test: Calculating Strain from Extension Data?

[KyleV]

I conducted a uniaxial tension test for a variety of materials but wasn't able to gather much useful axial strain data due to the extensometer continually slipping. I have axial strain data for the linear elastic region of the stress strain curve and I also have the extension of the crossbar of the machine. Since the materials of the machine are strained and thus stretch a certain amount, the extension data from the crossbar isn't very accurate for determining the strain in the specimen that's being tested. Using Hooke's law, is it possible to determine a modulus of elasticity of the materials in the testing machine, then use that value to calculate the strain of the specimen from the crossbar extension data?

Intuitively I think that it should work, but I'm having trouble verifying it.

Known Data:
σ
ε_axial
ε_extension

 

[Chris Thomas]

Hopefully I can help you understand what's going on with your tensile test.

So I am assuming you are testing dogbone style standard specimens. The Hooke's law conversion should work well if you are testing metals but will not work so well if you are testing rubber or highly elastic plastics. The reason for this is because the more elastic specimens will elongate outside of the gauge length area or the focus area of the test where the "neck" of the specimen is. They will elongate towards the base and since the base is thicker in material it will corrupt your results.

Regarding the extensometer slipping... You don't need to run the entire test with the extensometer on the sample. You can remove the extensometer halfway through the test. If you are able to recognize about when the extensometer will slip in the test (maybe 20 seconds in) you can remove the extensometer and you should have gathered enough data to calculate a modulus.

It also sounds like you may need a new extensometer to go with your universal testing machine and grips. If you have any questions related to tensile testing, visit us at www.universalgripco.com and we should be able to help you out.

- Chris