Browsing by Author "Ryu, Jong"

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    Design of compliant mechanism lattice structures for impact energy absorption
    (2017-12) Najmon, Joel Christian; Tovar, Andres; Ryu, Jong; Nematollahi, Khosrow
    Lattice structures have seen increasing use in several industries including automotive, aerospace, and construction. Lattice structures are lightweight and can achieve a wide range of mechanical behaviors through their inherent cellular design. Moreover, the unit cells of lattice structures can easily be meshed and conformed to a wide variety of volumes. Compliant mechanism make suitable micro-structures for units cells in lattice structures that are designed for impact energy absorption. The flexibility of compliant mechanisms allows for energy dissipation via straining of the members and also mitigates the effects of impact direction uncertainties. Density-based topology optimization methods can be used to synthesize compliant mechanisms. To aid with this task, a proposed optimization tool, coded in MATLAB, is created. The program is built on a modular structure and allows for the easy addition of new algorithms and objective functions beyond what is developed in this study. An adjacent investigation is also performed to determine the dependencies and trends of mechanical and geometric advantages of compliant mechanisms. The implications of such are discussed. The result of this study is a compliant mechanism lattice structure for impact energy absorption. The performance of this structure is analyzed through the application of it in a football helmet. Two types of unit cell compliant mechanisms are synthesized and assembled into three liner configurations. Helmet liners are further developed through a series of ballistic impact analysis simulations to determine the best lattice structure configuration and mechanism rubber hardness. The final liner is compared with a traditional expanded polypropylene foam liner to appraise the protection capabilities of the proposed lattice structure.
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    High extinction ratio mid-infrared polarizer based on sulfuric polymer
    (2018-08) Berndt, Aaron James; Ryu, Jong; Tovar, Andres; Gautam, Vemuri; Cheng, Rui
    Transparent polymers with low-loss and high refractive index are critical components of integrated optical devices including filters, lenses, and polarizers. Current conventional mid infrared (MIR) polarizers are fabricated from inorganic semiconductor materials and are intrinsically expensive, brittle, and difficult to manufacture. This represents a significant challenge in developing a surface mountable low-cost component. Herein, an alternative sulfur polymer-based material will be used to create MIR polarizers reducing cost and simplifying fabrication. Sulfur polymer ellipsometry data indicates low loss material with a refractive index of 1.64 across the MIR spectrum. Transmission data of thin film samples also support the fact that sulfur polymer is transparent in the MIR with an even transmission through the range. Sulfur polymer was created by the mixing of molten sulfur with 1-3, diisopropenyl- benzene (DIB) and allowing the chains of sulfur rings to break open and cross link with the DIB. To form the polarizer, sulfur polymer solution was spin-coated onto a silicon (Si) wafer and imprinted with a polydimethylsiloxane (PDMS) linear grid stamp. After imprinting, a thin layer of gold was deposited onto the surface of the grating, completing the bilayer structure polarizer. To measure the performance of the polarizer, transverse magnetic (TM) and trans- verse electric (TE) transmission data was collected with an Fourier-transform infrared spectroscope (FTIR). The linear polarizer should allow transmission of the TM light while blocking all TE light. The extinction ratio (dimensionless) is used to com- pare the two polarization states, defined as the TM transmission divided by the TE transmission. With high refractive index and transmission in MIR sulfur polymer is ideal for fabrication of optical components and can be applied as a substitute for conventional brittle inorganic materials. Sulfur polymer based polarizers showed experimental FTIR TM transmission over 95 percent. Combined with low FTIR TE transmission the physical samples displayed extinction ratios of over 600 in MIR. High polarization performance is attributed to the surface plasmon effect along the grating surface as well as the Fabry–P´erot cavity conditions between the multiple films. These assumptions are reinforced by the correlation between the simulation data and experimental results.