Browsing by Author "Choi, Hyunhee"

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    Modeling of machining process of EB-PVD ceramic coatings using discrete element method
    (Elsevier, 2022-08) Zhang, Jian; Sagar, Sugrim; Dube, Tejsh; Yang, Xuehui; Choi, Hyunhee; Jung, Yeon-Gil; Koo, Dan Daehyun; Zhang, Jing; Mechanical and Energy Engineering, School of Engineering and Technology
    In this work, a new discrete element model (DEM) for simulating the machining process of thermal barrier coatings is presented. The effects of cutting processing parameters, including cutting depth and cutting speed, on the cutting force and chip morphology are studied. In the model, a columnar grain microstructure mimicking the electron-beam physical vapor deposition (EB-PVD) coating is used. The results show that, as the cutting depth increases, the cutting chip morphology changes from fine powder form (ductile mode) to large chuck pieces (brittle mode). The transition depth or the critical cutting depth is determined based on the Griffith fracture criterion. The transition is also illustrated using the numbers of broken bonds and cutting energy changes in the DEM model. In the ductile mode, the number of broken bonds is increased gradually. In contrast, at larger cutting depths, the brittle mode causes a step-wise increase. Moreover, the maximum cutting force is found correlated to the cutting depth, which agrees well with an analytical solution based on fracture mechanics principles. The period in the cutting force is consistent with the diameter of the column grain. Finally, the cutting speed has little effect on the cutting force and chip morphology due to no strain rate sensitivity.
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    Numerical Simulation of Impact Behavior of Ceramic Coatings Using Smoothed Particle Hydrodynamics Method
    (ASME, 2021-04) Zhang, Jian; Lu, Zhe; Sagar, Sugrim; Choi, Hyunhee; Jung, Yeon-Gil; Park, Heesung; Koo, Dan Daehyun; Zhang, Jing; Mechanical and Energy Engineering, School of Engineering and Technology
    In this work, the impact behavior of an alumina spherical particle on alumina coating is modeled using the smoothed particle hydrodynamics (SPH) method. The effects of impact angle (0 deg, 30 deg, and 60 deg) and velocity (100 m/s, 200 m/s, and 300 m/s) on the morphology changes of the impact pit and impacting particle, and their associated stress and energy are investigated. The results show that the combination of impact angle of 0 deg and velocity of 300 m/s produces the highest penetration depth and largest stress and deformation in the coating layer, while the combination of 100 m/s and 60 deg causes the minimum damage to the coating layer. This is because the penetration depth is determined by the vertical velocity component difference between the impacting particle and the coating layer, but irrelevant to the horizontal component. The total energy of the coating layer increases with the time, while the internal energy increases with the time after some peak values, which is due to energy transmission from the spherical particle to the coating layer and the stress shock waves. The energy transmission from impacting particle to coating layer increases with the increasing particle velocity and decreases with the increasing inclined angle. The simulated impact pit morphology is qualitatively similar to the experimental observation. This work demonstrates that the SPH method is useful to analyze the impact behavior of ceramic coatings.