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Description
Sandwich laminates are often ramped down to a solid composite laminate at its end 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. This thesis investigates the effectiveness of using a functionally graded sandwich core material in the rampdown closure region of a sandwich panel to mitigate the large shear stresses induced by the stiffness and geometry changes, and the mismatch in facesheet and core stiffness properties. It further investigates methods to create functionally graded materials by densifying commercially available metallic honeycomb core materials. The thesis presents the finite element analysis of sandwich beam with end rampdown closure subjected to in-plane, transverse and bending loads. The effect of different functional grading on core-facesheet interface normal and shear stresses and the variation of bending stresses in the factsheet in the rampdown structures are presented. Investigations show that the exponential variation of core stiffness at the taper region provides the best solution. The main contribution of this thesis is the development and demonstration of a finite element model for rolling of honeycomb core to create edge wise graded properties. The commercially available Aluminum 5052-H39 foil Hexweb™ honeycomb core material is used as the initial material in the densification simulations. 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.
