Stiffness analysis of closed cross-section composite grid-stiffened cylinders

A new closed-form analytical model was developed to predict the stiffness properties of closed cross-section composite grid-stiffened (CGS) cylindrical structures. The analysis is applicable to structure with various stiffening configurations with combinations of helical, circumferential, and longitudinal stiffeners. Curved grid-stiffened structures, in general, have their stiffeners oriented in space exhibiting coupling between torsion, out-of-plane bending, in-plane bending, and axial deformation modes. The out-of-plane bending stiffness of the stiffeners was included in the present analysis by modeling them as curved composite beams made of uniaxial fibers. Smeared stiffness approach was employed to derive the overall cross-section stiffness characteristics. Analyses were performed to predict the torsional and bending stiffness of CGS structures with various stiffening configurations. The analytical results were compared against those predicted using finite element analysis. Results were generated for a range of helical stiffener angles from 15 to 75 deg with respect to the structural axis. The effect of stiffener cross-section geometry on the stiffness characteristics was also studied by varying the depth of the stiffeners. Stiffness analyses were performed on three different stiffener configurations: (1) helical, circumferential, and longitudinal stiffeners (2) helical and longitudinal stiffeners, (3) helical and circumferential stiffeners. The analytical model predicted the bending stiffness coefficients within engineering accuracy at lower helical angles. Maximum error observed was within 9% compared to FEM predictions. The error was observed to increase to nearly 18% for higher stiffener angles. Torsion stiffness predictions showed considerable deviations in the range of stiffener angles considered. The errors in torsion stiffness predictions ranged from 19% at lower stiffener angles to 8% at higher stiffener angles. The discrepancies observed between analytical and FEA predictions could be attributed to the assumptions employed to derive the stiffness matrices of the CGS structure. The eccentricity of the stiffeners causes extension-bending coupling, which adversely affected the overall grid-stiffened structure stiffness properties.