Seongmin Lee

A critical evaluation of the stress-corrosion cracking mechanism in high-strength aluminum alloys

Attempts have been made to elucidate the mechanism of stress-corrosion cracking (SCC) in high-strength Al-Zn-Mg and Al-Li-Zr alloys exposed to aqueous environments by considering the temperature dependence of SCC susceptibility based upon the anodic dissolution and hydrogen embrittlement models. A quantitative correlation which involves the change of threshold stress intensity,KISCC, with temperature on the basis of anodic dissolution has been developed with the aid of linear elastic fracture mechanics. From the derived correlation, it is concluded that the threshold stress intensity decreases as the test temperature increases. This suggestion is inconsistent with that predicted on the basis of hydrogen embrittlement. It is experimentally observed from the Al-Zn-Mg and Al-Li-Zr alloys that the threshold stress intensity,K,ISCC, decreases and the crack propagation rate,da/dt, over the stress intensity increases with increasing test temperature. From considering the change in SCC susceptibility with temperature, it is suggested that a gradual transition in the mechanism for the stress-corrosion crack propagation occurs from anodic dissolution in stage I, where the crack propagation rate increases sharply with stress intensity, to hydrogen embrittlement in stage II, where the crack propagation rate is independent of stress intensity.

Effect of hydrogen on the impedance of passivating oxide films formed on aluminium in sodium nitrate solution

A.c. impedance has been determined at room temperature on hydrogen-free and hydrogen-charged passivating oxide films on aluminium in 0.05m Na2SO4 solution. Analysis of the impedance data enabled a clear distinction between the capacitance and inductance of the hydrogen-free oxide film and those of the hydrogen-charged oxide film to be made. The changes in capacitance and inductance by hydrogen-charging into the oxide films are discussed in relation to the compositional change from γ-Al2O3 to hydrated alumina.

Amortised Deep Parameter Optimisation of GPGPU Work Group Size for OpenCV

GPGPU (General Purpose computing on Graphics Processing Units) enables massive parallelism by taking advantage of the Single Instruction Multiple Data (SIMD) architecture of the large number of cores found on modern graphics cards. A parameter called local work group size controls how many work items are concurrently executed on a single compute unit. Though critical to the performance, there is no deterministic way to tune it, leaving developers to manual trial and error. This paper applies amortised optimisation to determine the best local work group size for GPGPU implementations of OpenCV template matching feature. The empirical evaluation shows that optimised local work group size can outperform the default value with large effect sizes.

Poster: MOBS: Multi-operator Observation-Based Slicing Using Lexical Approximation of Program Dependence

Observation-Based Slicing (ORBS) is a recently introduced program slicing technique based on direct observation of program semantics. Previous ORBS implementations slice a program by iteratively deleting adjacent lines of code. This paper introduces two new deletion operators based on lexical similarity. Furthermore, it presents a generalization of ORBS that can exploit multiple deletion operators: Multi-operator Observation-Based Slicing (MOBS). An empirical evaluation of MOBS using three real world Java projects finds that the use of lexical information, improves the efficiency of ORBS: MOBS can delete up to 87% of lines while taking only about 33% of the execution time with respect to the original ORBS.

Hyperheuristic Observation Based Slicing of Guava

Observation Based Slicing is a program slicing technique that depends purely on the observation of dynamic program behaviours. It iteratively applies a deletion operator to the source code, and accepts the deletion (i.e. slices the program) if the program is observed to behave in the same was as the original with respect to the slicing criterion. While the original observation based slicing only used a single deletion operator based on deletion window, the catalogue of applicable deletion operators grew recently with the addition of deletion operators based on lexical similarity. We apply a hyperheuristic approach to the problem of selecting the best deletion operator to each program line. Empirical evaluation using four slicing criteria from Guava shows that the Hyperheuristic Observation Based Slicing (HOBBES) can significantly improve the effeciency of observation based slicing.

Joint Design of Precoder and Backhaul Quantizer in Cooperative Cognitive Radio Networks

In cooperative cognitive radio networks (CRNs), multiple secondary base stations are managed by a central unit (CU) via finite-capacity backhaul links, which performs joint encoding and precoding of the messages intended for the secondary users (SUs). In this paper, the limitation of backhaul links is handled with the implementation of compression strategies, by which precoding is followed at the CU. In particular, focusing on underlay cooperative CRNs, we propose the joint optimal design of precoding and backhaul quantization with the aim of maximizing the sum throughput of the secondary network under limited backhaul capacity, transmit power, and interference power constraints. Furthermore, both perfect and imperfect channel state information (CSI) available at the CU are considered. In the imperfect-CSI case, the uncertainty of the channels is explicitly accounted for via a robust or worst-case optimization formulation. The proposed algorithmic solutions leverage rank relaxation and majorization-minimization programming, whose performances are evaluated via numerical results.

Integrated routing and fill for self-aligned double patterning (SADP) using grid-based design

Self-aligned double patterning (SADP) has been proposed as an alternative patterning solution for sub-10nm technology because of delay of advanced lithography beyond 193nm ArF. In conventional SADP, line and space style of dummy metal fills are inserted once main design is complete. A large buffer distance is required around the main design because no further verification of main design (in presence of fills) is performed. This causes irregular pattern density, which becomes a source of process variations. We propose integrated-fill, in which main design and dummy fill insertion are performed together. This requires a change in overall design flow, which we discuss. Integrated-fill is demonstrated in M2 layer of SADP process; M2 density increases by 15.7% with 2.3% reduction in standard deviation of density distribution; metal thickness variation is also reduced by 24%. More dummy fills cause increased coupling capacitance, which however is shown to be insignificant.

Effects of phosphorus segregated at grain boundaries on stress corrosion crack initiation and propagation of rotor steel in boiling 40 wt.% NaOH solution

Abstract The effects of phosphorus segregated at grain boundaries on intergranular corrosion have been investigated by an electrochemical method. The stress corrosion crack initiation and propagation of rotor steels was studied as a function of applied stress in boiling 40 wt.% NaOH solution at an applied passive potential of −530 mV SHE using the electrical potential method. Phosphorus segregation leads to intergranular corrosion susceptibility. Intergranular attack by anodic dissolution is responsible for the crack initiation, irrespective of the applied stress and phosphorus doping. The decrease in stress corrosion crack propagation time due to phosphorus segregated at grain boundaries is enhanced with increasing applied stress and is presumably related to crack branching. Fringe patterns on the intergranular facets of the fracture surface are found to be associated with martensite lath boundaries and are inferred to be caused by strain-induced dissolution.