Jung-Hong Cho

Numerical analysis of secondary cooling and bulging in the continuous casting of slabs

Abstract In this paper, numerical analyses of the secondary cooling and the bulging of the strand are performed, as applied to the continuous casting of slabs. Solidification analyses of the strand under air-mist for each cooling zone are carried out by the one-dimensional finite difference method. The bulging deformation of cast slab is calculated with a two-dimensional elasto-plastic and creep finite element model. For efficient bulging analysis, the temperature distribution and shell thickness resulting from the solidification analysis are automatically transferred to input data for the bulging analysis. The solidification is analyzed more precisely with five sub-zones in the cooling zone and the temperature distribution is fully coupled in the bulging analysis. The adequacy of the model has been confirmed with experimental results. From this study the effects of the process variables such as casting speed, cooling condition, and roll pitch are examined. The results from the analyses are applicable to the design of the continuous casting process.

A study on the hot-deformation behavior and dynamic recrystallization of Al–5 wt.%Mg alloy

A numerical analysis was performed to predict flow curves and dynamic recrystallization behaviors of Al-5wt%Mg alloy on the basis of results of hot compression tests. The hot compression tests were carred out in the ranges of 350 ~ 500 and 5 ~ 3 /sec to obtain the Zener-Hollomon parameter Z. The modelling equation for flow stress was a function of strain, strain rate, temperature. The influence of these variables was quantifield using the Zener-Hollomon parameter. In the modelling equation, the effects of strain hardening and dynamic recrystallization were taken into consideration. Therefore, the modelling stress-strain curves of Al-5wt%Mg alloy were in good agreement with experimental results. Finally, the dynamic recrystallization kinetics were illustrated through the inspection of microstructure after deformation.

Three-dimensional finite element simulation of a spider hot forging process using a new remeshing scheme

Abstract A new method of three-dimensional remeshing is proposed for thermo-viscoplastic finite element analysis of complicated forging shapes. In the method, the deformed body to be remeshed is divided into a surface-adaptive layer and a core region. At the surface layer of hexahedral elements, the arrangement of nodal points is made by normal projection of boundary nodes in order to retard the mesh degeneracy, since the arrangement of boundary nodes can be achieved easily using the die surface patch information. After generating the mesh automatically at the surface-adaptive layer, the core region is meshed automatically by introducing a body-fitted mapping technique. In order to show the effectiveness of the method in a three-dimensional problem, forging of a spider has been simulated as a practical example. The complete simulation of spider forging has been carried out using the thermo-viscoplastic finite element method and the remeshing scheme. The deformed patterns of the computation are in good agreement with the results of experimens using the plasticine. It is thus shown that the proposed method can be applied successfully to complex geometries.

Measure of Effectiveness Analysis of Passive SONAR System for Detection

The optimal use of sonar systems for detection is a practical problem in a given ocean environment. In order to quantify the mission achievability in general, measure of effectiveness(MOE) is defined for specific missions. In this paper, using the specific MOE for detection, which is represented as cumulative detection probability(CDP), an integrated software package named as Optimal Acoustic Search Path Planning(OASPP) is developed. For a given ocean environment and sonar systems, the discrete observations for detection probability(PD) are used to calculate CDP incorporating sonar and environmental parameters. Also, counter-detection probability is considered for vulnerability analysis for a given scenario. Through modeling and simulation for a simple case for which an intuitive solution is known, the developed code is verified.

Optimal Acoustic Search Path Planning in Realistic Environments Based on Genetic Algorithm

The measure of effectiveness for detection of sonar system is directly related to the Cumulative Detection Probability (CDP) while its operation. In order to maximize CDP, there exists an optimal search path for given environment and sonar systems. Recently, with the advance of modeling and simulation methods, it has become possible to access the optimization problem more systematically. For this purpose, a program package named as OASPP (Optimal Acoustic Search Path Planning) was developed based on Genetic Algorithm (GA) and detection algorithm with environmental models. In this paper, the simulation results for a set of cases are presented and discussed, for which cases the intuitive solution are known.

Optimal Acoustic Search Path Planning Based on Genetic Algorithm in Continuous Path System

The design of efficient search path to maximize the Cumulative Detection Probability(CDP) is mainly dependent on experience and intuition when the searcher detect the target using SONAR in the ocean. Recently with the advance of modeling and simulation methods, it has been possible to access the optimization problem more systematically. In this paper, a method for the optimal search path calculation is developed based on the combination of the genetic algorithm with detection algorithm. We consider the continuous system for search path, space, and time, and use the movement direction of the SONAR as a gene of the genetic algorithm. The developed algorithm, OASPP(Optimal Acoustic Search Path Planning), is shown to be effective, via a simulation, in finding the optimal search path for the case where the intuitive solution exists.

An experimental study on the nonaxisymmetric square-die extrusion using model material test

Abstract In this paper, a design method is suggested to reduce the defects such as twisting and bending in the nonaxisymmetric square-die extrusion through a model material test. In the model material test, L-shaped and U-shaped plasticine bars are extruded by using a square-die. The effects of bearing-land width and flow guide of a die set are investigated, respectively. Firstly, from the products extruded by a square-die with uniform bearing-land width, a bearing-land width equation is derived to obtain an uniform exit velocity of the products and the bearing-land width is corrected. More improved products are obtained from the experiments using the corrected die. Secondly, flow guides are used in the square-die extrusion to obtain their influences on products. The flow guide retains the direction of material flow constantly.

Measure of Effectiveness Analysis for Tracking in SONAR System

Since the optimized use of sonar systems for target tracking is a practical problem for naval operations, the measure of mission achievability is needed for preparing efficient sonar-maneuver tactic. In order to quantify the mission achievability or Measure Of Effectiveness(MOE) for given sonar-maneuver tactics, we developed and tested a simulation algorithm. The proposed algorithm for tracking is based on Measure Of Performance(MOP) for localization and tracking system of sonar against target. Probability of Detection(PD) using steering beam patterns referenced to the aspect angle of sonar is presented to consider the tracking-performance of sonar. Also, the integrated software package, named as Optimal Acoustic Search Path Planning(OASPP) is used for generating sonar-maneuver patterns and vulnerability analysis for a given scenario. Through simulation of a simple case for which the intuitive solution is known, the proposed algorithm is verified.Keywords : Tracking, Measure Of Effectiveness(MOE), Localization, Probability of Detection(PD), Detection, Sonar-Maneuver Tactic

Measurement System for Performance Evaluation of Acoustic Materials in a Small Water Tank

Since the detection probability is critically dependent on the target strength (TS) in active sonar and on the radiated noise level (RNL) in passive sonar, the acoustic materials for echo reduction (ER) and transmission loss (TL) are widely used for the stealth of underwater targets. In this paper, a measurement system based on the small water tank, for the frequency range of greater than 30 kHz, is developed and verified using reference targets. In order to design the water tank and the geometry of test samples, a program is developed to calculate the arrival time of interfering signals due to the reflection from water tank walls and also due to the diffraction from the edge of the test samples. Considering all the interfering signals, an optimal experimental configuration for water tank and test samples is designed and used throughout the experiment. Next, the signal processing algorithms to estimate ER and TL are developed based on the measured propagation loss reflecting the geometric spreading characteristics of the transducer. Finally, a set of reference targets such as aluminium plate and perfectly reflecting plate are used in a small water tank to verify the developed measurement system.