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Is it possible to model nonlinear elasticity in ANSYS with SOLID 18x elements?

Dear all iMechanicians Smile,

Since several days I am trying to simulate a granular material very similar to sand. The uniaxial compression test performed to the material shows a nonlinear elasticity behavior during the unloading curve.

The goal is to fully reproduce the behavior of the material with a nonlinear elastic model when the material stress-state is inside the yield surface and the plastic behavior with a multilinear isotropic hardening law. The yield surface would be given by the Drucker-Prager-Cap criterion.

I know that the legacy element SOLID45 can reproduce multilinear elasticity, which would be very appropiate for our case. Unfortunately, it only supports the linear Drucker-Prager yield model and not the DP-Cap, necessary for accounting the compaction of the material.

I tried to switch to hyperelasticity to solve the nonlinear elasticity issue with the so-called "new technology elements" (SOLID18x). For this I define a Uniaxial COmpression Test Data in the Engineering Data of Workbench and then I apply a Response Function to define the hyperelastic properties. Additionally,I define the DP-Cap yield criterion and the flow rule with a user command . But when I launch the simulation the results shows only elastic behavior and no plasticity at all: ANSYS seems to ignore the DP-Cap criterion. If I replace the hyperelasticity with a classical linear elastic young modulus, ANSYS calculates the corresponding plastic behavior according to the DP-Cap yield criterion, to my surprise.

My question is then: is it not possible at all to represent nonlinear elastic behavior of a granular material while reproducing also plasticity by means of DP-Cap?

Thanks in advance for your kind help! 

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