Local buckling modelling of isogrid and anisogrid lattice cylindrical shells with hexagonal cells

The analytical modelling of the local buckling failure mode for composite anisogrid lattice cylindrical shells with the typical system of hexagonal cells is here discussed. The aim is to complement the set of constraint equations that are associated with the preliminary design phase of such structures under axial compressive loads, and to prospectively improve the final solutions.The basic constraint equations for anisogrids without the skin are focused on the global buckling of the shell, the local buckling of helical ribs, and the material failure of these ribs. However, the local buckling of helical ribs is normally based on a simplified and qualitative approach. Conversely, the developed modelling, which is based on the Ritz method, accounts for the helical angle of the periodic cell, the stiffness of intersecting hoop and helical ribs, and the positive effect of the prebuckling tensile force in hoop ribs. This model has been verified with the aid of finite-element analysis, demonstrating a noteworthy accuracy.Thanks to the comparison with the parallel study on anisogrid shells with triangular cells, the effect of hoop rib positioning on the local buckling strength (i.e., hexagonal or triangular system of cells) is finally assessed.