  The proposed yield criterion depends upon two material constants and is
proven to be smooth and convex under a simple condition. These properties
induce a mathematical robustness that allows a further use in a damage
mechanics model. The analytical gradient and hessian are given. The obtained
yield surface is relevant to Kupfer's biaxial testings on concrete. The
identification procedure, with respect to the classical uniaxial tension and
compression testings, is detailed.
