The primary results in a finite element analysis are grid point displacements and rotations. Element results are derived from those results. Results of a finite element analysis are post-processed using a graphical tool.
The grid point results calculated by OptiStruct in a linear static analysis are the grid point displacements and grid point rotations (if available). From those results (depending on the output option), the stresses, strains, and strain energy densities are calculated.
The eigenvector is the primary result in a normal modes analysis. It is normalized with respect to the mass matrix (i.e. the left and right hand multiplication of the mass matrix with an eigenvector yields one). Strain energy densities are calculated depending on the output option.
Displacements and eigenvectors are plotted as a deformed structure, or as a contour on the undeformed structure. Some post-processors, such as Altair HyperMesh and Altair HyperView, also allow the animation of the displacements.
Stresses and strains are usually plotted on the deformed or undeformed structure using colors for the different values. Contour and assign plots are usually available. Since the stress results of OptiStruct are element values, the use of assign plots is strongly recommended. The interpolation of the contour plot might give a wrong impression of the true stress results. Vector plots can help in the evaluation of the load path in a structure.
The stresses are secondary results. Stresses near notches and other sharp corners, point loads and boundary conditions, and rigid elements are often unreliable due to the singularities in these points. This is not a trait unique to OptiStruct, but is inherent in the finite element method itself. A mesh refinement in such places can improve the stress prediction. A theoretically infinite stress cannot be predicted by finite elements.
Stresses are calculated at the centroid of the element. The stresses of interest are usually found on the surface of a structure. It is common practice to use a skin of thin membrane elements in 3-D modeling, or rod elements in 2-D modeling, to evaluate the stresses on element surfaces or edges, respectively. This method also has the advantage of much faster post-processing of solid models because only the membrane skin needs to be plotted.
The definition of the output options can be found in the I/O Options Section.