Two nonlinear models of brittle fracture for solids. The paper considers isotropic and orthotropic nonlinear constitutive relations for brittle materials in the case of plane stresses. Numerical algorithms based on the finite element method are developed. The resulting material models are incorporated in the PIONER software. The correctness of crack path determination is examined by solving a test problem of crack propagation. The isotropic model gives mesh-dependent results, whereas the orthotropic model provides an adequate solution. It is shown that the solutions obtained by using the isotropic model are close to those obtained by eliminating failed elements