Jared N. Baucom

Energy harvesting concepts for small electric unmanned systems

In this study, we identify and survey energy harvesting technologies for small electrically powered unmanned systems designed for long-term (>1 day) time-on-station missions. An environmental energy harvesting scheme will provide long-term, energy additions to the on-board energy source. We have identified four technologies that cover a broad array of available energy sources: solar, kinetic (wind) flow, autophagous structure-power (both combustible and metal air-battery systems) and electromagnetic (EM) energy scavenging. We present existing conceptual designs, critical system components, performance, constraints and state-of-readiness for each technology. We have concluded that the solar and autophagous technologies are relatively matured for small-scale applications and are capable of moderate power output levels (>1 W). We have identified key components and possible multifunctionalities in each technology. The kinetic flow and EM energy scavenging technologies will require more in-depth study before they can be considered for implementation. We have also realized that all of the harvesting systems require design and integration of various electrical, mechanical and chemical components, which will require modeling and optimization using hybrid mechatronics-circuit simulation tools. This study provides a starting point for detailed investigation into the proposed technologies for unmanned system applications under current development.

Tiled Composite Laminates

This article describes our efforts to develop a meso-scale in-plane tiling technique for fiber-reinforced composite laminates. Such a technique expands the design space of the laminate, and the ability to tailor local laminate properties may provide a means, for example, to mitigate stress concentrations that arise in places such as along free edges of the laminate. Preliminary fabrication of tiled laminates produced material with an elastic modulus that is nearly equal to that of continuously reinforced laminates; however, the strength of these first specimens was significantly reduced. Finite element analyses were then performed to characterize the effects of features unique to tiled composite laminates, such as the existence of resin-rich tile-to-tile interfaces, and to explore the effects of relative arrangement of tiles through the thickness of the laminate. This led to a novel composite joint geometry as well as recommendations to minimize strength reduction. Strength retention of laminates fabricated using the new design guidelines was experimentally found to exceed 92% in comparison with the traditional analogs. Finally, we discuss the potential application of composite tiling for the suppression of free-edge delamination.

Multifunctional Applications of Thin Film Li Polymer Battery Cells

Commercial off-the-shelf (COTS) thin-film solid-polymer Li-ion battery cells appear to posses the requisite physical characteristics for dual use as both electrical energy-storage devices and structural members under a finite load. One realistic application could be small electric unmanned vehicles where the power requirements are in the range of 10 to 100 watts and the mechanical loads are relatively small. We tested the multifunctional feasibility of COTS battery cells by designing a specific mechanical testing protocol based on realistic use in unmanned vehicles. Our characterization protocol included randomized bending and shear testing and generation of energy-power relation (Ragone) plots of the COTS cells. The results indicate that multifunction applications of COTS Li polymer battery cells are feasible; however, battery packaging geometry and bonding are critical design issues.

EM Gun Bore Life Experiments at Naval Research Laboratory

The Naval Research Laboratory (NRL) performs basic and applied research on high power railguns as part of the US Navy EM Launcher program. The understanding of damage mechanisms as a function of armature and barrel materials, launch parameters, and bore geometry is of primary interest to the development of a viable high power railgun. Research is performed on a 6-m, 1.5-MJ railgun located at NRL. Barrel studies utilize in situ diagnostics coupled with detailed ex situ analysis of rail materials to provide clues to the conditions present during launch. Results are compared with coupled 3-D electromagnetic and mechanical finite element analysis models of railgun operation. Results of several experiments on rail wear will be discussed.

Autophagous structure-power systems

Novel autophagous (self-consuming) systems combining structure and power functionalities are under development for improved material utilization and performance enhancement in electric unmanned air vehicles (UAVs). Much of the mass of typical aircraft is devoted separately to the functions of structure and fuel-energy. Several methods are proposed to extract structure function from materials that can also serve as fuel for combustion or as a source of hydrogen. Combustion heat is converted to electrical energy by thermoelectric generation, and hydrogen gas is used in fuel cells to provide electrical energy. The development and implementation of these structure-fuels are discussed in the context of three specific designs of autophagous wing spars. The designs are analyzed with respect to mechanical performance and energy storage. Results indicate a high potential for these systems to provide enhanced performance in electric UAVs.

EM gun bore life experiments at the Naval Research Laboratory

The Naval Research Laboratory (NRL) performs basic and applied research on high power railguns as part of the US Navy EM Launcher program. The understanding of damage mechanisms as a function of armature and barrel materials, launch parameters, and bore geometry is of primary interest to the development of a viable high power railgun. Research is performed on a 6-m, 1.5 MJ railgun located at NRL. Barrel studies utilize in situ diagnostics coupled with detailed ex situ analysis of rail materials to provide clues to the conditions present during launch. Results are compared with coupled 3-D electromagnetic and mechanical Finite Element Analysis (FEA) models of railgun operation. Results of several experiments on rail wear will be discussed.

Electrospray Ionization of Polymers: Evaporation, Drop Fission, and Deposited Particle Morphology

The fundamental challenge of nanomanufacturing is to create, control, and place immense quantities of nanoscale objects controllably over large surface areas. Electrospray ionization (ESI) has the potential to address this challenge due to its simplicity, applicability to a broad range of materials, and intrinsic scalability. However, the interactions between electrospray parameters and final deposited morphology are not well understood. Experimental results are combined with physics-based models to explain how observed particle size distributions are caused in the spray by evaporation and Coulomb fission of drops with solute concentration gradients.

Mitigation of Free-Edge Effects by Meso-Scale Structuring

We are developing a new class of fiber-reinforced polymer composite materials to facilitate the embedment of multifunctional features and devices in material systems and to manage interlaminar stresses at the external free edges and internal free surfaces of holes and cut-outs in composite laminates. The idea is centered on the introduction of one or more additional dimensions of design space by a tessellation of individual laminae into sets of discrete tiles, each possessing the same levels of design freedom normally associated with an entire lamina (material constituents, fiber orientation, and so on). In this work, we have focused on the development of tiling schemes that will allow blending of disparate laminates (lay-ups), where a lay-up suitable for suppressing interlaminar stresses could be substituted at necessary locations in place of another lay-up that may be more suitable for the global structural loads. This technique results in the inclusion of possibly detrimental matrix-rich tile-to-tile interface pockets in the plane of each lamina. Mechanical testing has shown that uniaxially reinforced tiled composites maintain stiffness levels near those of their traditional continuously reinforced counterparts, yet with a potential degradation of strength. We have used the finite element method to investigate the effects of resin-rich pocket size, the use of supporting continuous layers, tile size, and tile overlapping schemes (interface stacking geometry) on the distribution of stress and transfer of load around interfaces in uniaxially reinforced tiled composites. This was done with the aim to identify parameters controlling overall strength. We discovered that alignment of the resin-rich pockets through the thickness exacerbates stress-concentration and that outer continuous layers on the composite may help in better load transfer and more efficient material utilization. Failure analyses of the finite element results using three-dimensional Hashin-Rotem failure criteria [1] have shown the concept to be effective in the suppression of free-edge delamination in traditional quasi-isotropic and angle-ply laminates under tensile loading. Although each meso-scale structured solution must be tailored to the exact structural geometry and anticipated loads, the technique shows promise to have broad application

Microstructural and Mechanical Characterization of Carbon Nanofiber Reinforced Composites

Carbon nanofibers, such as single walled carbon nanotubes (SWNT), multiwalled carbon nanotubes (MWNT) and vapor-grown carbon nanofibers (VGCF or VGCNF) are routinely compounded with polymers to create thermally and electrically conductive polymer nanocomposites. Our group is interested in combining the conduction with structural functionality by reinforcing a high-performance thermotropic liquid crystal polymer (LCP) matrix with vapor-grown carbon nanofibers and single walled carbon nanotubes. High strength and stiffness can be achieved in LCPs through the alignment of molecular domains during high-shear mixing and extrusion. Further strength and stiffness enhancements are potentially possible if the carbon nanofibers could also be aligned, perhaps, with the assistance of the aligned domains of the LCP matrix. However, the geometrical structure of VGCF is quite different and the diameter is one to two orders of magnitude larger than that of SWNT. Therefore, the processing conditions and the interactions between the LCP domains and the nanofibers are expected to lead to different dispersion and alignment characteristics of VGCF and SWNT within the LCP matrix. In this work, twin-screw and Maxwell-type mixer-extruders were used to produce neat LCP filaments and LCP-nanofiber composite filaments with various concentrations of VGCF and SWNT. The dispersion and orientation of the VGCF and SWNT reinforcements were determined by X-ray diffraction and electron microscopy. The filaments were loaded in quasi-static uniaxial tension until fracture to determine the tensile modulus, strength and strain-to-failure. The mechanical properties showed a strong dependence on the filament diameter, nanofiber concentration and processing parameters. A significant increase in mechanical performance was observed with decreasing filament diameter irrespective of the carbon nanofiber concentration. Fracture surfaces examined under electron microscopy revealed hierarchical features at multiple length scales. At the macroscopic scale, a skin-core configuration was observed in the filament cross-section with the skin possessing a greater degree of LCP molecular alignment and nanofiber alignment than the core. The mechanical and electrical properties of the LCP, LCP-VGCF and LCP-SWNT nanocomposite filaments will be described and related to processing parameters, the type of carbon nanofibers (VGCF or SWNT) and the resulting composite microstructure.Copyright

A Preliminary Study on the Mechanical Performance of Tiled Polymer Composites

We are developing and exploring the concept of in-plane tiling of composite laminates, called MOSAIC, to mitigate or control delamination at free edges due to interlaminar stresses. Initial mechanical testing has shown that MOSAIC composites with uniaxial graphite-fiber reinforced tiles retain the stiffness levels of traditional uniaxially reinforced composites but with reduced strength. The reduction in strength is attributed to the formation of resin-rich pockets between adjacent tiles. In this study, we have performed detailed finite element analyses to identify the critical design parameters that affect the mechanical performance of uniaxially reinforced MOSAIC composites. We have found that using continuous laminae on the outer surfaces significantly increases the overall loadcarrying capacity. Increasing aspect ratio of the pocket and decreasing material property differences between resin and tiles also cause better load transfer between tiles but may not necessarily improve overall strength due to increasing stress concentration. The tiling scheme and density of pocket placement influence the interaction of local stress concentrations. Consequently, a novel composite joint is proposed and found to have superior performance.Copyright