  A new yield/damage function is proposed for modelling the inelastic behaviour
of a broad class of pressure-sensitive, frictional, ductile and
brittle-cohesive materials. The yield function allows the possibility of
describing a transition between the shape of a yield surface typical of a class
of materials to that typical of another class of materals. This is a
fundamental key to model the behaviour of materials which become cohesive
during hardening (so that the shape of the yield surface evolves from that
typical of a granular material to that typical of a dense material), or which
decrease cohesion due to damage accumulation. The proposed yield function is
shown to agree with a variety of experimental data relative to soil, concrete,
rock, metallic and composite powders, metallic foams, porous metals, and
polymers. The yield function represents a single, convex and smooth surface in
stress space approaching as limit situations well-known criteria and the
extreme limits of convexity in the deviatoric plane. The yield function is
therefore a generalization of several criteria, including von Mises,
Drucker-Prager, Tresca, modified Tresca, Coulomb-Mohr, modified Cam-clay, and
--concerning the deviatoric section-- Rankine and Ottosen. Convexity of the
function is proved by developing two general propositions relating convexity of
the yield surface to convexity of the corresponding function. These
propositions are general and therefore may be employed to generate other convex
yield functions.
