  The constitutive modelling of granular, porous and quasi-brittle materials is
based on yield (or damage) functions, which may exhibit features (for instance,
lack of convexity, or branches where the values go to infinity, or false
elastic domains) preventing the use of efficient return-mapping integration
schemes. This problem is solved by proposing a general construction strategy to
define an implicitly defined convex yield function starting from any convex
yield surface. Based on this implicit definition of the yield function, a
return-mapping integration scheme is implemented and tested for elastic-plastic
(or -damaging) rate equations. The scheme is general and, although it
introduces a numerical cost when compared to situations where the scheme is not
needed, is demonstrated to perform correctly and accurately.
