Hello everyone,
I am trying to use strain softening Mohr-Coulomb criteria in concrete and I am trying to understand how the softening of cohesion and hardening of friction angle can be determined (I refer to the curves implemented through tables in 3DEC) experimentally? From what I understood I could run multiple triaxial tests at different confinement stresses and get values from the post-peak behavior by doing a strain-controlled loading. I guess the other way could be doing a sensitivity analysis until the model matches the experimental data. But I would like to know what kind of approach you guys have used to get these kind of parameters? I'm attaching pictures of the curves I am talking about. Thanks for the help.
Great question—this is exactly the kind of calibration 3DEC was built for. Below is a practical workflow used in geomechanics labs and in DEM/FEM calibration to obtain the strain-softening cohesion c(εps)c(\varepsilon^{ps})c(εps) and strain-hardening (then softening) friction angle φ(εps)\varphi(\varepsilon^{ps})φ(εps) for the Mohr–Coulomb model in 3DEC.
Do drained triaxial compression on (at least) 4 confining pressures, e.g. σ3={0,5,10,20}\sigma_3=\{0,5,10,20\}σ3={0,5,10,20} MPa, on similar material (same mix, curing, density). Record, vs axial strain:
- Deviator stress q=σ1−σ3q=\sigma_1-\sigma_3q=σ1−σ3
- Mean stress p=(σ1+2σ3)/3p=(\sigma_1+2\sigma_3)/3p=(σ1+2σ3)/3
- Axial and radial strains εa,εr\varepsilon_a, \varepsilon_rεa,εr
- Volumetric strain εv=εa+2εr\varepsilon_v=\varepsilon_a+2\varepsilon_rεv=εa+2εr
Between a few load steps, perform small unload–reload cycles (or know E,νE,\nuE,ν) so you can split total strains into elastic and plastic.
2º. Convert to the “internal” strain measure used by 3DEC.
3º. Get ccc and φ\varphiφ at each plastic-strain level.
4º. Calibrate dilation consistently.
5º. Regularize for mesh/zone size.
6º. Implement & verify in 3DEC.
Minimal example (shape only; numbers illustrative):
https://docs.google.com/spreadsheets/d/1XbEOMdEwc9KtO32jEJ4_SkfLt70rzt7qovZs0xGUzm4/edit?usp=sharing
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