Yunok Kim

Development of limit load solutions for corroded gas pipelines

Abstract Pipelines have the highest capacity and are the safest and the least environmentally disruptive means for gas or oil transmission. Recently, failures due to corrosion defects have become of major concern in maintaining pipeline integrity. A number of solutions have been developed for the assessment of remaining strength of corroded pipelines. However, these solutions are known to be dependent on material properties and pipeline geometries. In this paper, a fitness-for-purpose (FFP) type limit load solution for corroded gas pipelines made of X65 steel is proposed. For this purpose, a series of burst tests with various types of machined pits are performed. Finite element simulations are carried out to derive an appropriate failure criterion. Then, further, extensive finite element analyses are performed to obtain the FFP type limit load solution for corroded X65 gas pipelines as a function of defect depth, length and pipeline geometry.

The Oxidation of TiB2 Particle-Reinforced TiAl Intermetallic Composites

The oxidation kinetics of TiAl alloys with and without (3, 5, 10 wt.%) TiB2 dispersoids were studied between 1073 and 1273 K in atmospheric air. The inert TiB2 dispersoids effectively increased the oxidation resistance of TiAl alloys. The higher the TiB2 dispersoids content, the more pronounced the effect. The oxide scale formed on TiAl–TiB2 composites was triple-layered, consisting mainly of an outer TiO2 layer, an intermediate Al2O3 layer, and an inner (TiO2+Al2O3) mixed layer. No B2O3 was observed within the oxide scale because of its high vapor pressure. A thin Ti3Al sublayer and discrete TiN particles were found at the oxide–substrate interface. During the oxidation of TiAl alloys with and without TiB2 dispersoids, titanium ions diffused outwardly to form the outer TiO2 layer, while oxygen ions transported inwardly to form the inner (TiO2+Al2O3) mixed layer. The increased oxidation resistance by the addition of TiB2 was attributed to the enhanced alumina-forming tendency and thin and dense scale formation.

Probing the Additional Capacity and Reaction Mechanism of the RuO2 Anode in Lithium Rechargeable Batteries.

The structural changes and electrochemical behavior of RuO2 are investigated by using in situ XRD, X-ray absorption spectroscopy, and electrochemical techniques to understand the electrochemical reaction mechanism of this metal oxide anode material. Intermediate phase-assisted transformation of RuO2 to LiRuO2 takes place at the start of discharge. Upon further lithiation, LiRuO2 formed by intercalation decomposes to nanosized Ru metal and Li2 O by a conversion reaction. A reversible capacity in addition to its theoretical capacity is observed on discharging below 0.5 V during which no redox activity involving Ru is observed. TEM, X-ray photoelectron spectroscopy, and the galvanostatic intermittent titration technique are used to probe this additional capacity. The results show that the additional capacity is a result of Li storage in the grain boundary between nanosized Ru metal and Li2 O. Findings of this study provide a better understanding of the quantitative share of capacity by a unique combination of intercalation, conversion, and interfacial Li storage in a RuO2 anode.

Lithium‐Ion Transport through a Tailored Disordered Phase on the LiNi0.5Mn1.5O4 Surface for High‐Power Cathode Materials

The phase control of spinel LiNi0.5 Mn1.5 O4 was achieved through surface treatment that led to an enhancement of its electrochemical properties. Li(+) diffusion inside spinel LiNi0.5 Mn1.5 O4 could be promoted by modifying the surface structure of LiNi0.5 Mn1.5 O4 through phosphidation into a disordered phase (Fd3m) that allows facile Li(+) transport. Phosphidated LiNi0.5 Mn1.5 O4 showed a significantly enhanced electrochemical performance, even at high rates exceeding 10 C, demonstrating that the improved kinetics (related to the amount of Mn(3+) ) can render LiNi0.5 Mn1.5 O4 competitive as a high-power cathode material for electric vehicles and hybrid electric vehicles.

Reduction of Electrical Hysteresis in Cyclically Bent Organic Field Effect Transistors by Incorporating Multistack Hybrid Gate Dielectrics

We have fabricated flexible organic field effect transistors (OFETs) on polyimide substrate with low hysteresis and low leakage current under repetitive bending. The insertion of an ultrathin atomic-layer-deposited Al 2 O 3 layer in between spin-coated poly-4-vinyl phenol organic layers in a multistack hybrid gate dielectric for OFETs significantly improved stability in the electrical hysteresis during cyclic bending. The observed hysteresis stability for cyclically bent multistack hybrid OFET devices was attributed to efficient blocking of charges injected from the gate electrode due to improved mechanical stability. Cyclically bent samples showed no cracking for thinner Al 2 O 3 layers in the multistack hybrid gate dielectrics.

Lithium-excess olivine electrode for lithium rechargeable batteries

Lithium iron phosphate (LFP) has attracted tremendous attention as an electrode material for next-generation lithium-rechargeable battery systems due to the use of low-cost iron and its electrochemical stability. While the lithium diffusion in LFP, the essential property in battery operation, is relatively fast due to the one-dimensional tunnel present in the olivine crystal, the tunnel is inherently vulnerable to the presence of FeLi anti-site defects (Fe ions in Li ion sites), if any, that block the lithium diffusion and lead to inferior performance. Herein, we demonstrate that the kinetic issue arising from the FeLi defects in LFP can be completely eliminated in lithium-excess olivine LFP. The presence of an excess amount of lithium in the Fe ion sites (LiFe) energetically destabilizes the FeLi-related defects, resulting in reducing the amount of Fe defects in the tunnel. Moreover, we observe that the spinodal decomposition barrier is notably reduced in lithium-excess olivine LFP. The presence of LiFe and the absence of FeLi in lithium-excess olivine LFP additionally induce faster kinetics, resulting in an enhanced rate capability and a significantly reduced memory effect. The lithium-excess concept in the electrode crystal brings up unexpected properties for the pristine crystal and offers a novel and interesting approach to enhance the diffusivity and open up additional diffusion paths in solid-state ionic conductors.

Development of expert system for nuclear piping integrity

The objective of this paper is to develop a nuclear piping integrity expert system (NPIES) for nuclear piping integrity. This paper describes the structure and the development strategy of the NPIES system. The NPIES system consists of five parts; user interface, database, knowledge base, expert and integrity evaluation parts. The user interface part is developed to connect the user and the NPIES system effectively. In the database part, nuclear piping material properties are stored; the unknown material properties are stored in the knowledge base part. Various rules for inferring material properties are stored in the knowledge base part. The most appropriate evaluation method for a given input condition is recommended in the expert part. Finally, the integrity evaluation part is developed for the evaluation of piping integrity effectively.

A parametric study on pressure–temperature limit curve using 3-D finite element analyses

Abstract In order to operate a reactor pressure vessel (RPV) safely, it is necessary to keep the pressure–temperature ( P – T ) limit during the heatup and cooldown process. While the ASME Code provides the P – T limit curve for safe operation, this limit curve has been prepared under conservative assumptions. In this paper, the effects of conservative assumptions involved in the P – T limit curve specified in the ASME Code Sec. XI were investigated. Three different parameters, the crack depth, the cladding thickness and the cooling rate, were reviewed based on 3-D finite element analyses. Also, the constraint effect on P – T limit curve generation was investigated based on J – T approach. It was shown that the crack depth and constraint effect change the safety region in P – T limit curve dramatically, and thus it is recommended to prepare a more precise P – T limit curve based on finite element analysis to obtain P – T limit for safe operation of a RPV.

Development of elastic-plastic integrity evaluation system for pressure vessel and pipings

Abstract A practically useful system for elastic-plastic fracture mechanics analysis has been developed. The developed system is comprised of the J- integral/Tearing modulus (J/T) approach and the deformation plasticity failure assessment diagram (DPFAD) approach. The program contains analysis routines for five types of fracture mechanics specimens and five types of flaws in cylindrical geometries. A double and triple interpolation schemes were adopted to interpolate J values from the EPRI developed EPFM handbooks and a material property database was also developed. Several case studies were performed to evaluate the accuracy and the usefulness of the code. It was found that the J/T approach and the DPFAD approach yielded similar results. However, the DPFAD approach is more convenient for quick assessment of cracked structures while the J/T approach is more useful in evaluating the full history of the fracture process.

Effect of cladding on the stress intensity factors in the reactor pressure vessel

Abstract In general, reactor pressure vessels (RPV) are cladded with stainless steel to prevent corrosion and radiation embrittlement. The ASME Sec. XI specifies that a subclad crack which may be found during the in-service inspection must be considered as a semi-elliptical surface crack when the thickness of cladding is less than 40% of the crack depth. In order to refine the fracture assessment procedures for such subclad cracks, three-dimensional finite element analyses were applied for various subclad cracks embedded in the base metal. A total of 18 crack geometries were analyzed, and the results were compared with those for idealized semi-elliptical surface cracks for two different loading conditions, i.e. internal pressure and pressurized thermal shock. The resulting stress intensity factors for subclad cracks were 50–70% less than those for idealized surface cracks. It has been proven that the condition specified on the ASME Sec. XI is overly conservative for subclad cracks which are assumed to be surface cracks.