# Uniaxial compressive test using Voronoi elements

Hello. I conducted an UCS test simulation using small voronoi blocks. The reason i am using voronoi is because i want to find out the normal stiffness and shear stiffness of the voronoi contacts by doing calibration. I am also using the voronoi to see the deformations and slippage in the UCS simulation I want to fit the stress-strain curve obtained from the simulation with the experimental data we got from UCS test. But the main problem is I am unable to get a shear plane failure. It mostly looks like the picture I have sent below. And all the stress-strain curves I get from the the simulation have too much fluctuations as shown in the pictures. Can someone tell me how do I solve this problem. Thank you. I am using yvelocity as the boundary condition on top and bottom.

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It looks like you have friction on the joint between the platen and the voronoi blocks. For a true UCS test, you probably want friction = 0 on this joint. It could also be that the loading rate is too fast, or it could be a problem with the way you are measuring stress and strain. Please send your model to technical support if you are still having problems.

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Yes the problem is solved. Thank you very much.

Is there any specific reason to use platens above and below rather than applying stress (or velocity) directly on the material ?

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The issue is that the platens can affect the results. This is the same for the physical laboratory test.

If you use a stress at the end, this is the equivalent to a soft test machine and the sample will explode because the stress does not back off as the sample fails. This is not the way most laboratory tests are performed.

Most laboratory tests are conducted using a constant strain rate. This would be the equivalent to a constant vertical velocity at one or both ends of the sample.

If you also prescribe zero x velocity at the ends, this is equivalent to a rough platen which does not allow expansion at the end.

If you do not restrict the x velocity at the ends, it is the equivalent to a frictionless platen (the ideal platen, but difficult to achieve in the lab).

If you model the platen as well, the platen itself should have a constant velocity boundary applied to it. Then you can apply a friction between the platen and the sample. This more closely models the lab test. However, I do not know what friction value to use.

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Hello,

I would like to ask if you have used any specific command to visualize the fracture in your model.