Browsing by Author "Nguyen, Hoang"
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Item Analysis of ductile damage with sigmoidal hardening(2022-04-14) Nguyen, HoangHexagonal Closed-Packed (HCP) materials are widely used in engineering applications – from transportation to biomedical devices. Magnesium alloys are a class of HCP materials that are promising candidates because they are cheaper, more light-weight, and biodegradable, compared to titanium, and aluminum. However, their damage mechanisms are poorly understood, because of its multi-stage stress-strain response, often referred to as sigmoidal (or S-shaped) hardening. This makes it difficult to obtain robust engineering projections regarding their durability and damage tolerance. The goal of this research is to investigate the coupled effects of the material characteristics and specimen geometry on the macroscale damage behavior of magnesium alloys. Two plasticity models considered are Power-law hardening and Sigmoidal hardening. Also, damage modeled using is the GTN model. Four specimen geometries modeled using finite elements in ABAQUS/STANDARD (Rights Reserved) to account for increasing levels of stress triaxiality (T).Item Degradation of PET Microplastics by Controlled Microbial & Photocatalytic, MoO3, Exposure(2020-09-29) Harris, Kristen E.; Holt, ZacharyPolyethylene Terephthalate (PET) plastic is a material widely used to manufacture common household items like clothing and water bottles. Unfortunately, millions of tons of microplastic PET waste matriculate into our environment annually, resulting from dumped wastewater and litter. Microplastic PET fibers are extremely difficult to hydrolyze and can release chemicals, creating a potential for toxicity in seafood that has unknown consequences on human health. This research is a comparative study of the chemical and biological degradation of PET microplastics by photocatalytic and microbial means. The chemical degradation process was performed with the use of the photocatalyst, MoO3, under visible light irradiation for sixty hours with PET in water. The biodegradation process was initiated by an incubation period of soil media with microplastic discs analyzed for degradation every eight weeks. The photocatalytic degradation of PET was analyzed with atomic force microscopy (AFM) and by measuring the changes in the contact angle of a water droplet on its surface to evaluate its hydrophilic nature. The biodegradation process is ongoing but has not yet shown progress in breaking the fundamental groups of the PET polymer. Our results from the photocatalytic degradation show an increase in the hydrophilic nature of the PET surface and average surface roughness, suggesting that chemical degradation is an effective method for PET microplastic waste removal. Furthermore, there are limitations to photocatalytic degradation of PET in wastewater treatment and more research needs to be done to establish parameters for exposure time when using MoO3 as a photocatalyst in water.Item Detection and quantification of Pb2+ in Drinking Water Using Dark Field Smartphone Microscope(2017) Nguyen, Hoang; Shih, Wei-ChuanWater is the essence of life, but easy to become contaminated. Lead (Pb) is the most dangerous source of water contamination. There is a significant unmet need for Pb2+ sensing device that is portable, affordable, and ideally, available to the general population. Such a device will enable on-demand Pb2+ detection in virtually any environmental setting and empower individuals to examine the safety of drinking water whenever and wherever needed.Item Fabrication of Multi-point Side-Firing Optical Fiber by Laser Micro- ablation(Optics Letters, 1/16/2018) Nguyen, Hoang; Arnob, Md Masud Parvez; Becker, Aaron T.; Wolfe, John C.; Hogan, Matthew K.; Horner, Philip J.; Shih, Wei-ChuanA multipoint, side-firing design enables an optical fiber to output light at multiple desired locations along the fiber body. This provides advantages over traditional end-to-end fibers, especially in applications requiring fiber bundles such as brain stimulation or remote sensing. This Letter demonstrates that continuous wave (CW) laser micro-ablation can controllably create conical-shaped cavities, or side windows, for outputting light. The dimensions of these cavities determine the amount of firing light and their firing angle. Experimental data show that a single side window on a 730 μm fiber can deliver more than 8% of the input light. This can be increased to more than 19% on a 65 μm fiber with side windows created using femtosecond laser ablation and chemical etching. Fine control of light distribution along an optical fiber is critical for various biomedical applications such as light-activated drug-release and optogenetics studies.Item Multiscale Analysis of Ductile Damage In Material With Sigmoidal Hardening(2022-05-11) Nguyen, HoangHexagonal Closed-Packed (HCP) materials are widely used in engineering applications - from transportation to biomedical devices. Magnesium alloys are a class of HCP materials that are promising candidates because they are cheaper, more light-weight, and biodegradable, compared to titanium and aluminum. Additionally, magnesium currently expands its application to automotive and battery. However, their damage mechanism are poorly understood, because of its multi-stage stress-strain response, which often referred to as sigmoidal (or S-shaped) hardening. Therefore, the goal of this thesis will be computationally examining the material response of the sigmoidal hardening on two perspectives: macroscopic and microscopic. For macroscopic re- sponse, the higher the stress triaxitality T, or the sharper of the notch region, the postpeak nominal axial stress σeq will be higher and void volume fracture f will exponentially grows at a lower level of nominal axial strain. This can be conclude higher triaxiality will have lower strain to failure, regardless of the difference in the porosity propagate differently between power-law hardening and sigmoidal hardening. For microscopic response, stress triaxiality T related directly to the geometry shape of the void and the growth rate of the void volume. While the spherical void volume does not propagate at a low triaxiality stress, the oblate void volume yield at every triaxility stress level.Item Spatiotemporal Temperature and Concentration Measurements in a Monolith Reactor(2015-12) Nguyen, Hoang; Luss, Dan; Harold, Michael P.; Jacobson, Allan J.; Balakotaiah, Vemuri; Brankovic, Stanko R.Honeycomb reactor is often used in pollution abatement applications. A typical commercial honeycomb reactor is made of a ceramic material, which is deposited with the high surface area washcoat carrier such as Al2O3. The polluted gases enter the reactor channels and diffuse through this porous structure to the catalytic sites where they are converted to less harmful gases. Stringent environmental regulations have driven the markets toward increasing engine efficiencies and better after-treatment strategies. Achieving high hydrocarbon conversion requires comprehensive information about the thermal and concentration fields inside the monolith reactor. The concentration fields can be studied by a Spatially Resolved Capillary Inlet Mass Spectrometer technique (Spaci-MS). On the other hand, the temperature field is often measured by positioning thermocouples at various locations inside the monolith channel, which can be intrusive to the reactor and not suitable for fast transient processes. To overcome this limitation, I use a new optoelectronic technique called coherent optical frequency domain reflectometry (c-OFDR). This technique enables instant temperature measurement along a fiber with high spatial and temporal resolutions. The objectives of my works are: i) illustrate the capability of the c-OFDR method to measure transient temperature inside the monolith channel, ii) employed c-OFDR method to analyze the veracity of the concentration measurement by the Spaci-MS probe, iii) develop the experimental setup combining the c-OFDR and Spaci-MS techniques to study the dynamic behaviors of the Pt/CeO2/Al2O3 catalyst system under periodic lean-rich operation.