Browsing by Subject "Numerical Simulation"

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    Polarization Analysis Based on Realistic Lithium Ion Battery Electrode Microstructure Using Numerical Simulation
    (Office of the Vice Chancellor for Research, 2015-04-17) Yan, Bo; Lim, Cheolwoong; Song, Zhibin; Zhu, Likun
    The performance of lithium ion battery (LIB) is limited by the inner polarization and it is important to understand the factors that affect the polarization. This study focuses on the polarization analysis based on realistic 3D electrode microstructures. A c++ software was developed to rebuild and mesh the microstructure of cathode and anode electrodes through Nano-CT and Micro-CT scanned images respectively. As a result, the LIB model was composed of electrolyte, cathode and anode active materials and current collectors. By employing 3D finite volume method (FVM), another c++ code was developed to simulate the discharge and charge processes by solving coupled model equations. The simulation revealed the distribution of physical and electrochemical variables such as concentration, voltage, current density, reaction rate, et al. In order to explore the correlation of local effects and electrode structural heterogeneity, the cathode electrode were divided equally into 8 sub-divisions, of which the porosity, tortuosity, specific surface area were calculated. We computed the polarizations in the sub-divisions due to different sub-processes, i.e., the activation of electrochemical reactions and charge transport of species. As shown in Fig. 1, the tortuosity is very irregular because of unevenly distributed cathode particle size and packing pattern with low porosity. There are no exact and direct relations among porosity, tortuosity and specific surface area. Fig. 2 shows that the polarizations are related to the porosity in sub-divisions. The knowledge from the study will help to figure out the mechanism of polarization and power loss in LIB, which could be useful to improve LIB design and manufacturing. Acknowledgments: This work was supported by US National Science Foundation under Grant No. 1335850. Fig. 1 Porosity and tortuosity in sub-divisions of a cathode electrode Fig. 2 Intercalation reaction polarization and ionic conduction polarization of sub-divisions at 120 sec during a 5 C charging process
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    Temperature-dependent impact properties of 3D printed 15-5 stainless steel
    (2018-05) Sagar, Sugrim; Zhang, Jing; Tovar, Andres; El-Mounayri, Hazim
    Since the conception of three dimensional (3D) printing circa 40 years ago, there has been the proliferation of several additive manufacturing (AM) technologies that enable its use in everyday applications such as aerospace, medicine, military, oil and gas and infrastructure. In order to improve its applicability and growth, 3D printed materials are subjected to the same or even higher levels of scrutiny for its mechanical behavior as its conventionally manufactured counterpart. One of the most important mechanical properties is toughness or the ability of a material to undergo large strain prior to fracture when loaded. The toughness of a material can be correlated to its impact energy or the increase in internal energy due to impact. In this study, the impact properties, including the toughness of 3D printed 15-5 stainless steel were investigated at low temperature (77 K), room temperature (298 K) and high temperature (723 K) using experimental and numerical modeling of the Charpy impact test. In addition, ballistic impact simulations were performed to determine the applicability of 3D printed 15-5 stainless steel in the defense industry. The 15-5 stainless steel specimens were printed (horizontal-build) using the direct metal laser sintering (DMLS) technique, cooled or heated to the specified temperature, then tested in accordance with the ASTM E23-2016b [1] standard. The Johnson-Cook (J-C) phenomenological material model and fracture parameters were used in the numerical modeling. The cross-sectional microstructures of surfaces and impact energies of the Charpy impact test were examined. For the ballistic impact simulations, a 3D printed 15-5 stainless steel typical plate was investigated at the same temperatures as the Charpy impact test. A typical missile using the J-C properties at room temperature (298 K) was assigned an initial velocity of 300 ms-1 for each plate temperature. The fracture surface investigation (microsurface analysis as well as visual inspection) and impact energy values of the Charpy impact test show that the 3D printed 15-5 stainless steel exhibited brittle behavior at low and room temperatures, but transitioned into a more ductile behavior at high temperature. At 77 K, 298 K and 723 K, the experimental Charpy impact test results were 0.00 J/cm2, 6.78±4.07 J/cm2 and 50.84±3.39 J/cm2 respectively; whereas the simulated impact energy were 1.05 J/cm2, 10.46 J/cm2 and 47.07 J/cm2 respectively. Hence, the impact energy for the experimental and numerical simulations were in good agreement; especially at higher temperatures. Consistent with the results from the Charpy impact test, the ballistic impact simulations show an increase in the impact energy, elastic plastic strain and deflection of the plate with an increase in temperature indicating brittle-to-ductile behavior. The high exit velocity at low and room temperature may not make the plate attractive in defense in its current configuration; however, at the high temperature, the exit velocity reduction was significant.