The fracture problem for finite elastic body under static loading with unit crack or set of internal and surface cracks is considered. Using special orthogonal expansions, an approximation method to define the stress intensity factors is proposed. The orthogonal expansions in singular and regular parts are constructed in the norm associated with the potential energy. The proposed method allows to find the approximate values of the stress intensity factors for 2D and 3D problems. The asymptotic singular solutions near the tops of cracks, potential energy and Lagrangian for considered elasticity problem must be obtained in order to define the stress intensity factors.