A quadrilateral macro-element, containing two-triangle elements, is developed by the hybrid-stress method with incompatible internal quadratic displacements. Stress approximation satisfies the energy compatibility condition. The element stiffness matrix is determined by the Hellinger-Reissner principle. Several test problems are used to compare displacement and stress solution accuracy of the proposed macro-element with published in the literature solutions.
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A hybrid stress method for eigenfrequencies analysis is developed using a plane rectangular hybrid element. Complex Trefftz functions which are solutions of elastostatic problem are used. By the complementary energy variational equation a relationship between the stress parameters and the nodal displacements is obtained. The Lagrange's variational equation for the dynamic case gives an expression for computation of eigenfrequencies.
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An application of hybrid stress method in studying the free vibrations of two-dimensional continuum in plane stress or plane strain problems by finite elements is investigated. The basis of the method is the assumed parametric stress field giving equilibrium of the internal forces inside the element area. The displacements on the element boundary are independent of the stress field and fully determined by the nodal displacements. The displacement field inside the element is independent of the assumed stress field and it is determined by parameters different than the assumed stress field parameters. Differentiating the assumed displacement field, a new stress field is obtained, which parameters are set to approach the assumed stress field by the least squares method.
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