Interlaminar stress recovery for three-dimensional finite elements

An accurate evaluation of interlaminar stresses in multilayer composite laminates is crucial for the correct prediction of the onset of delamination. In general, three dimensional finite element models are required for acceptable accuracy, especially near free edges and stress concentrations. Interlaminar stresses are continuous both across and along layer interfaces. Nonetheless, the continuity of interlaminar stresses is difficult to enforce in interpolated elements. Nodal values of the stresses are usually retrieved using extrapolation techniques from super-convergent points, if known, or Gauss points inside the element. Stress fields within an element can be deduced using either constitutive relations or variationally consistent procedures. In either case, spurious oscillations in stress fields may be encountered leading to a reduced accuracy of the recovered stresses at nodes. In this paper, an efficient interlaminar stress recovery procedure for three-dimensional finite element formulations is presented. The proposed procedure does not rely on extrapolation techniques from super-convergent or integration points. Interlaminar stress values are retrieved directly at nodes and stress continuity at the inter-element boundary is automatically satisfied. Several benchmark problems were analysed. Comparisons with finite element software and available solutions in the literature confirmed the accuracy of the procedure. Accurate interlaminar stresses were obtained using coarser meshes compared to customary recovery procedures.