5. Plasticity

This chapter describes the input syntax of plasticity models. Plasticity is specified as yielding or hardening. See Chapter 17 for background theory.

• For concrete, the Mohr-Coulomb plasticity model is often used for the compressive regime. Generally, the compressive strength f_c and the friction angle $\phi$ ( $\approx$ 30° ) are known. If Drucker-Prager plasticity is used to model the failure surface in plane stress, the friction angle $\phi$ is approximately 10° otherwise the biaxial strength is overestimated. See §17.1.4.2 for an elaborated example. The cohesion then follows from c = f_c(1 - sin$\phi$)/2 cos$\phi$ . For the tensile regime, a tension cut-off with Smeared cracking can be added [§6.1.1]. A special plasticity model for concrete is also available [§5.1.3].

• Nonassociated plasticity gives an asymmetric tangential stiffness matrix. Low to moderate degrees of asymmetry |$\phi$ - $\psi$| < $\approx$ 20° , are best solved with DIANA's nonsymmetric solver. For high degrees of asymmetry the Constant or Linear Stiffness iteration methods should be applied [Vol. Analysis Procedures]. For plane stress, the value of $\psi$ is not essential, take $\psi$ = $\phi$ for simplicity.

    (syntax)

YIELD

yldcri is the name of the yield criterion to be used.

The description of the yield surface and hardening depends on the criterion, as outlined in the following subsections.