foam-extend4.1-coherent-io/applications/solvers/solidMechanics/elasticPlasticNonLinULSolidFoam/calculateDEpsilonDSigma.H

DEpsilon = symm(gradDU) + 0.5*symm(gradDU & gradDU.T());

DSigma = 2*mu*(DEpsilon - DEpsilonP) + I*(lambda*tr(DEpsilon));