Structural Behavior of Grid-stiffened Tubes Under Axial, Bending, and Torsion Loads

Abstract

Advanced Grid-stiffened (AGS) structures are characterized by shells which are integrally stiffened by a series of ribs or stiffeners arranged in combinations of lateral, longitudinal, and angular patterns, all fabricated using composite materials. Such configurations possess excellent resistance to impact damage, delamination, and crack propagation while exhibiting high stiffness to weight ratio. Though the AGS structures are commonly used in fuselage structures, space vehicle shrouds, and wing structures, they are not used in rotor blade spar designs. The large size, high torsional stiffness requirements, large bending stresses along the blade length due to high lift and lower CF make it impractical to employ the conventional monocoque spar designs to the proposed heavy lift rotor blades. Considering the potential advantages of AGS structures, the feasibility of employing them to the heavy lift rotor blade spar designs has to be investigated thoroughly. As a preliminary study, the present investigation focuses on the structural behavior of closed cross-section cylindrical structures with helical grids under axial, torsion, and bending loads is investigated. The objective is to develop finite element and analytical models to predict the stiffness properties of the cylindrical grids and perform validation studies by conducting experiments. A general purpose finite element code is employed to create and analyze the grid and grid-stiffened models. Several grid specimens are fabricated and tested for deflections under various loading conditions and the results are compared with closed form analytical solutions and FEM. It is demonstrated that the results from FEM and experiments are in good agreement for the grid-only beams. It is also observed that the closed form analytical formulation has to be modified to incorporate out of plane bending of the grid segments, skin-stiffener interaction, and the curvature of the grid-stiffened structures to accurately predict the structural responses.