Compression Response and Modeling of Interpenetrating Phase Composites and Foam-Filled Honeycombs

Abstract

Although multiphase materials with discrete, dispersed and/or embedded phases in a matrix have been evolving over the years, there are limitations in terms of the degree of concentration of the secondary phase that can be dispersed into the primary phase. Nature has addressed this by adopting a 3D interpenetrating network of phases as evident in skeletal tissues and some tree trunk microstructures. This observation has inspired a relatively new category of materials called Interpenetrating Phase Composites (IPC). Thus in an IPC constituent phases are interconnected three-dimensionally and topologically throughout the microstructure. Consequently, each phase of an IPC contributes its property to the overall macro scale characteristics while adding mechanical constraint synergistically. In this thesis, the feasibility of processing a lightweight interpenetrating phase composite (IPC) made of aluminum and syntactic polymer foams is demonstrated.

A syntactic foam-filled aluminum honeycomb composite is also examined as a 2D variant of the IPC. Pressureless infiltration of uncured syntactic epoxy foam into an open-cell aluminum preform or a honeycomb structure is used for producing the composite systems. The compression characteristics of these novel materials relative to syntactic foams are studied. Different varieties of IPC foam and foam-filled honeycombs are prepared by varying the volume fraction of microballoons in the syntactic epoxy foam while keeping the volume fraction of the metallic network the same. Two variations of IPC foam are produced by using the aluminum preform in ‘as-received’ condition and after coating it with silane to increase adhesion between the metallic network and polymer foam. Uniaxial compression tests are then carried out on syntactic foam and foam-filled composites. The IPC foam and foam-filled honeycomb samples show enhancement in elastic modulus, yield stress and plateau stress when compared to the corresponding syntactic foam samples. Silane coated IPC foam samples in particular show significant improvements in these properties. The silane treated IPC foam consistently shows about 50% higher energy absorption relative to the corresponding syntactic foam. The maximum increase in the energy absorption for syntactic foam-filled honeycomb composite is found to be approximately 48%. A unit-cell based 3D elastic-plastic finite element model is developed to predict the stress-strain response of the IPC foam. A space filling Kelvin cell (tetrakaidecahedron) is used to represent the microstructure of the IPC. In case of foam-filled honeycombs, 2D elastic-plastic analyses on 8 x 8 array of cells are carried out. Measurements are used to validate compression behavior of both IPC and foam-filled honeycomb models up to 40% strain. The measured elastic moduli of the syntactic foam and foam-filled composites are also compared with a few existing micromechanics models.