In the displacement formulation of the finite element method, the primary unknowns are the nodal displacements. They are automatically chosen by the computer so that the energy is minimized. Stresses are then obtained by performing numerical differentiation on the displacement field. This leads to poorer accuracy for the stresses. The stresses computed this way are also sensitive to the design of the local mesh in the area where they are computed: different meshes and element types give different results, sometimes widely varying. Moreover, the convergence of the pointwise stress values is far from monotonic. A more refined model will not necessarily improve the result. Apparent convergence of the stress values may be misleading.
In this paper we implement an extraction technique that computes the stress at a boundary point by performing a mathematical postprocessing of the finite element results. The stress is expressed as a weighted average of the displacements on the boundary. This extracted value is less sensitive to the local mesh and it converges much faster than the traditional point-wise stress. Comparative results are presented for a structure of moderate complexity.