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Sandwich laminates are often ramped down to a solid composite laminate at the ends to enable the use of mechanical fasteners to connect them to support structures. The rampdown regions are often the site of fatigue damage initiation and delamination failure. Functionally grading sandwich core material in the rampdown closure region of a sandwich panel can mitigate the large shear stresses induced by stiffness and geometry changes, and the mismatch in facesheet and core stiffness properties. However, in practice, producing such functional grading has remained a challenge. This paper investigates methods to create functionally graded materials by densifying commercially available metallic honeycomb core materials. It describes the development and demonstration of a finite element model for rolling of honeycomb core to create edge wise graded properties. Finite element simulation of honeycomb core crushing (including effects of large nonlinear deformations, plasticity, and contact between cell walls) was performed using ABAQUS™ for uniform in-plane crushing (as a validation case) and for edge rolling induced crushing (to create functionally graded cores). The results indicate that a multistage rolling operation with careful selection of roller diameter and crushing depth for each pass can achieve desired densification gradient in the honeycomb core.

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