plasticity, three-dimensional deformation, static definability.

 Linear approximation of yield surface by three tangent planes Πk passing through the point Mc(σc1, σc2, σc3) is suggested for three-dimensional plastic statically indeterminate strain problem with smooth convex plasticity condition f(σ1, σ2, σ3) = 0 in the principal stress space (σ1, σ2, σ3). Point Mc is located near the point M of surface σbi = σi + k · ε · ∂f/∂σi |∇f| (ε - small parameter, k - typical strain). A statically indeterminate problem is approximated with small parameter ε by the system of three differential equations and three equations of plains.