Extend the GursonTvergaardNeedleman model to account for other equivalent stresses

[hureje]

The Gurson-Tvergaard-Needleman yield criterion in the StandardElastoViscoPlasticity brick could be extended to account for other porous yield criteria available in the literature by allowing the user to choose the equivalent stress (von Mises, Hill, ...).

[thelfer]

What about the following syntax:

   inelastic_flow : "Plastic" {
      criterion : "GursonTvergaardNeedleman" {
         q1 : 1.5,
         q2 : 1.0,
         q3 : 2.2,
         fc : 0.01,
         fr : 0.1
         equivalent_stress: "Hosford"{
           a: 8
        }
      },
      isotropic_hardening : "Linear" {
         R0 : 150e6
      }
   },

[hureje]

Probably we should change the name of the criterion (as GursonTvergaardNeedleman refers to the use of von Mises stress in the literature). Something like GenericPorous ? (of GenericPorousGrowth as we should have some day GenericPorousCoalescence)

[thelfer]

GenericPorous seems a bit too much. What about GeneralisedGursonTvergaardNeedleman ?

[hureje]

Yes, that's great !