Stability of the biaxial tension of an elementary cube problem is considered. Material properties of this cube are described by the potential. The potential possesses convexity and concavity. A monotone incremental strain is applied through two linear elastic bars. Bifurcation sets and a separatrix for this gradient mechanical system are obtained. The loss-of-stability moment of the deformation process is determined. A numerical method that permits to construct the separatrix approximately without a nonlinear equilibrium equation is presented.
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