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I am trying to apply a compression force to samples using air. I do not want to directly apply my loading since my samples are irregular. I do not wish to process them with a flattening step. I hope to use fluid and pressure to get a single compression force with even distribution across the surface. In the left example I have a spacer that has a negligible poisson's ratio so i will neglect expansion orthogonal to the applied pressure. In the right hand example I would have pressure on the side correct?

Based on the cartoon below I am hopping to have the pressurization of the air transfer to the samples. Assume the vessel is air tight with the exception of the flow restriction to control pressure levels.

Can I make this assumption?

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    $\begingroup$ Correct, in the right hand picture you will get uniform hydrostatic pressure (stress) in the sample, not uniaxial. In the left hand picture, presumably you have a thin flexible membrane or something to separate the water from the samples and the spacers? Also if the base of samples is "irregular" you will obviously not get uniform stress if they are resting on the container at a few "random" points. $\endgroup$ – alephzero Jul 2 '19 at 19:09
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Both water and air are fluid. And they transfer the pressure equally every where. On the right hand case if your samples are submerged in water they feel the pressure everywhere.

In material testing labs when they need to test a sample for its compressive strength they prepare the sample with capping it smooth with a special paste which is chemically neutral and and solidifies to a smooth level finish.

Then they can test it under uniaxial pressure.

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