Cylindrical sheet forming analysis of hexagonal cell honeycomb core using 3D finite element analysis

Abstract

Honeycomb structures have become one of the most popular alternatives for substituting conventional solid materials in the aviation industry due to their excellent high stiffness and low weight capabilities. Honeycomb sandwich panels, formed of a cellular core glued in between two face sheets, are typical of aircraft skins and other high curvature parts. The honeycomb cores are bent into the desired radius of curvature through expensive forming processes that consist of a trial and error approach to achieve an undamaged formed core panel. Despite their popularity, efforts in the open literature that fully characterize and understand the complex mechanical behavior of honeycomb core are insufficient. This research work describes the creation of a fully descriptive honeycomb core model that replicates the complex loading experienced during the forming process of honeycomb cores. The results will help reduce the time and money spent on the trial and error process used to establish the formability of cores. This research describes the creation of a complete 3D numerical model of a fiberglass/phenolic hexagonal honeycomb core for studying in-plane and flexural responses of the structure. The core geometry is captured in detail to be representative of the actual characteristics of the honeycomb studied, including the cell wall curvature, double walls, and full adhesive fillet. A building block approach is followed to create the final multi-cell model using the finite element package LSDYNA. Uniaxial in-plane numerical analyses are conducted over a representative volume (RV) and a 10x10 cell model to then compare to the experimental data available. The results can capture the non-linear orthotropic behavior of the structure as well as the typical failure modes seen experimentally (ribbon fracture and adhesive debonding). Flexural numerical simulations that imitate the forming process are conducted by bending a 30x40 cell core model over a female-male cylindrical tool. Three radii (50, 75, and 100 in.) and two orientations of the core with respect to the tooling were investigated to determine the forming limits, established based on the failure of the structure. Results conclude that the core fails catastrophically when formed over tooling of 50 in. and smaller radii. Failure exists, even though not as extraneously, when the core is formed using the 75 in. radius tool. No damage occurs on the core panel when formed over tooling of 100 in. and larger radii. Failure occurs due to high shear strains caused by a state of planar biaxial tension/compression when transitioning from anti-clastic bending to cylindrical forming over a high curvature tool.