Displacement oscillation in plane quadratic isoparametric elements in orthotropic situations

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Abstract
Long thin test specimens of orthotropic material are modelledby quadratic isoparametric finite elements in plane strain. Displacement oscillations occur down the long free edge of themodel with both reduced (2 × 2) and full (3 × 3) integration, themagnitude of the oscillations being affected by element aspectratio and the elastic constants of the material. The rate ofdiffusion of stress as predicted by the classical shear‐lag theory, being dependent on the material constants, is reflectedin the displacement oscillations near the load in 3 × 3integration. In 2 × 2 integration, however, this effect is swampedby the build‐up of oscillations away from the restraints in alowenergy deformation mode which becomes dominant as the elementaspect ration is increased. These latter voscillations arepredicted by the illconditioning of the structural stiffnessmatrix measured by a solution diagonal decay factor.

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