A Split Hopkinson Pressure Bar Apparatus for High Strain Rate Testing of Interpenetrating Phase Composites (IPC): Measurements and Modeling
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Novel materials with enhanced mechanical performance and multifunctionality are of interest to automotive, aerospace and marine industries alike. Designing materials with multiple thermo-mechanical attributes while satisfying lightweight constraint is rather challenging. This challenge becomes even greater when materials are to withstand elevated rates of loading. In this thesis, (i) a 3D Interpenetrating Phase Composite (IPC) made of Syntactic Foam (SF) in an open-cell aluminum scaffold is proposed for high-strain rate applications and (ii) a split-Hopkinson pressure bar apparatus is developed for characterizing low-impedance materials such as the SF and IPC materials at high-strain rates. The SF used for making IPC is prepared by dispersing hollow glass microballoons into an epoxy matrix. The resulting SF is a two phase composite on a microscopic scale and a lightweight homogeneous, isotropic material on a macro scale. The infusion of SF in its uncured state into the aluminum scaffold results in an IPC with a continuous 3D interpenetrating network throughout the material volume.A split Hopkinson pressure bar has been developed and calibrated for carrying out high-strain rate stress-strain response measurements on low impedance materials. Dynamic compression characteristics, including strength and energy absorption features of IPC made of four different volume fractions of microballoons in SF are measured using this apparatus and compared. Measurements on pure SF counterparts are also carried out for evaluation relative to IPC. The results show that in general, the IPC foams outperform the SF samples in terms of compressive strength and energy absorption per unit volume. The underlying deformation processes are evaluated optically using real-time high-speed photography as well as post-mortem analysis of deformed samples using scanning electron microscopy. The former includes the development of a novel grid-based method for measuring surface displacements and strains. An idealized elasto-plastic unit cell finite element (FE) model based on explicit dynamics is proposed for studying IPC foams. The 3D geometry of the aluminum component in the cubic unit cell is modeled as a tetrakaidecahedron, a 14 sided polyhedron, also called the Kelvin cell. The SF constituent of the IPC is modeled as an occupant of the rest of the unit cell. The computational model incorporates infinite elements to represent the far-field regions surrounding the unit cell. The infinite elements help simulate stress wave dynamics of a sample undergoing deformation in a split Hopkinson pressure bar. The computations are successfully compared with measurements for all the four cases. The contour plots of von-Mises stress, equivalent plastic strain and displacements in the loading direction are analyzed at different strain levels. In addition, the kinetic and strain energy histories absorbed by SF and aluminum ligaments in the IPC are quantified.