1. The problem statement, all variables and given/known data I have a Mohr-Coulomb plasticity model with isotropic hardening on the cohesion c(k). The angle of internal friction is constant. k=sqrt((2/3)*(de)'Q*de), where de is the time derivative of the plastic strain. Q is diag[1,1,1,0.5,0.5,0.5]. It is a triaxial test assuming associated plasticity and a constant confining pressure (sigma2=sigma3=constant) 2. Relevant equations I know that for associated plasticity the plastic potential function has the same shape as the yield function. 3. The attempt at a solution I have tried the following: sigma(ij)=Dijkl(dE(kl)dekl)=Dijkl(dE(kl)-dlambda*(df/dsigma(kl)), where f is the yield function and dE is the time derivative of the axial strain. dlambda=(1/h)*(df/dsigma(kl)*Dijkl*dE I have no idea how to continue to fill in the equation or the elastic matrix. Please help me.