Differences in Young Modulus -> Tensile/Compressive tests

[daninazza]

Hello everybody. I've measured the Young Modulus of PDMS samples both in compression and in tension. From compressive tests I get a Modulus twice as big as the one I get from tensile tests.

Is it normal to get two different values for Young Modulus using tensile or compressive tests? Do you think this behaviour is due to differences in molecular interactions? (eg: you need more energy to bring molecules together than to separate them (!?))

Thanks!

 

[Replusz]

It could be due do elastic hysteresis.
(https://en.wikipedia.org/wiki/Hysteresis#Elastic_hysteresis)
Hope this helped.

 

[Chris Thomas]

The Young's modulus in both compression and tension tests should be theoretically the same but only at infinitesimal strain rates. As the strain rate increases, the effects of the viscoelasticity of the material also increase. If i were to guess, I would say that your tensile test is being performed at a much higher strain rate than the compression tests.

Since you are pulling your tensile test at higher strain rates, you will get effects from the viscoelasticity of your Polydimethylsiloxane. The loss from viscoelasticity will result in a reduced Young's modulus value. The compression test is run at a much slower rate of speed, so you will not see the viscoelastic effects as much. If you were to run the compression test at the same rate of speed as say the tensile test, you would compress the material so quickly that it would squish out of the compression platens and look highly viscous. A higher strain rate compression test would result in a closer comparison to the Young's modulus from the tensile test.

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