Papers by Author: Young Chul Park

Abstract: This study examined the phase changes, nitride precipitation and variations in hardness of Fe-18Cr-1Mo-0.2Ti (436L) ferritic stainless steels after a nitrogen permeation heat treatment at temperatures ranging from 1050°C to 1150°C. The strong affinity between nitrogen and Ti/Cr enabled the permeation of nitrogen into the 436L ferritic stainless steels. The nitrogen content of the surface layers ranged from 0.40% to 0.87%, depending on the nitrogen permeation temperatures. The nitrogen-permeated surface layers changed into martensite plus retained austenite(RA) with rod type M2N and square type TiN precipitates. Ups and downs of hardness with increasing depth below the surface was observed, depending on the volume fraction of the RA, nitrogen content and quantity of precipitates. The maximum hardness of the nitrogen permeated surface layer was 700Hv.

Abstract: The welding process, a conventional production method of gate valves, has advantages such as light weight, but it also has disadvantages such high production price. However, the forging process, has economic merits and mass production capability. The main focus of this paper is the optimization of the preform in the forging process. This paper proposes an optimal design of the preform to improve the mechanical efficiency of the gate valve made by the forging method. The design of the preform is optimized by the use of the real response model to the Kriging model by computer simulation. Also, the optimized results were used to verify the response model. The verified response model confirmed the usefulness and reliability of the Kriging method in optimum structural design of preform achieved by finite element analysis and Kriging equations.

Abstract: Shape memory alloy (SMA) has been used to improve the tensile strength of composite materials. Because it produces compressive residual stress in the matrix by using its shape memory effect. In order to fabricate a shape memory alloy composite, TiNi alloy and Al2024, Al6061 were used as reinforcing material and matrix, respectively. In this study, TiNi/Al2024 and TiNi/Al6061 shape memory alloy composites were made by the hot press method. The fatigue limit of a shape memory alloy composite determined the volume ratio and reduction ratio. The probabilistic stresslife curve (P-S-N curve) about the shape memory alloy composite makes up using statistical method.

Abstract: In this study, elastic waves of ultrasonic and acoustic emission were used to evaluate the propagation characteristic of the wave in pipe, and study on mode conversion of the elastic wave due to the cracks in the pipe was also performed. An acoustic emission (AE) sensor was used to receive the propagated ultrasonic wave. AE technique has a merit that it can identify the received ultrasonic wave by the analysis of the AE parameters such as count, energy, frequency, duration time and amplitude. For transmitting and receiving of the wave, a wedge for universal angle was manufactured. The optimum angles for transmitting of ultrasonic wave and signal receiving at the attached AE sensor on the pipe were determined. Theoretical dispersion curve was compared with the results of the time-frequency analysis based on the wavelet transformation. The received modes showed a good agreement with theoretical one. The used ultrasonic sensor was 1MHz, and AE sensor was broadband (100kHz – 1200kHz). The artificial cracks were induced in the pipe to measure the propagation characteristics of the elastic wave for the cracks. AE parameters for the received signals were also varied with the crack types in the pipe. AE parameters of amplitude and duration time were more effective factors than the analysis of mode conversion for evaluation of the cracks in the pipe.

Abstract: DOE (design of experiments) was applied to the design of a knuckle as a part of a suspension system. Specifically, knuckle made of aluminum alloy was optimized considering the strength. On the other hand, design variables were set as shape variables. During structural optimization using DOE, an orthogonal array strategy was developed to determine the optimum design. The relevant discrete variables were treated as levels. Since the conventional orthogonal array did not consider the constraint, however, the characteristic function was defined to include the effect of constraint feasibility. The general DOE was expanded to include problems with constraints related to the new characteristic function.

Abstract: The optimization of gate valve was performed using Kriging based approximation model. The DACE modeling, known as the one of Kriging interpolation, is introduced to obtain the surrogate approximation model of the function. In addition, we describe the definition, the prediction function and the algorithm of Kriging method and examine the accuracy of Kriging by using validation method.

Abstract: This is a widespread requirement for low cost lightweight thermal imaging sensors for both military and civilian applications. The feasibility of micromechanical optical and infrared (IR) detection using microcantilevers is demonstrated. Microcantilevers provide a simple Structurefor developing single- and multi-element sensors for visible and infrared radiation that are smaller, more sensitive and lower in cost than quantum or thermal detectors. Microcantilevers coated with a heat absorbing layer undergo bending due to the differential stress originating from the bimetallic effect. This paper reports a micromachined silicon uncooled thermal imager intended for applications in automated process control.

Abstract: Tensile residual stress happen by difference of coefficients of thermal expansion between fiber and matrix is one of the serious problems in metal matrix composite (MMC). TiNi alloy fiber was used to solve the problem of the tensile residual stress as the reinforced material. TiNi alloy fiber improves the tensile strength of composite by occurring compressive residual stress using shape memory effect in the matrix. A hot press method was used to create the optimal condition for the fabrication of shape memory alloy (SMA) composite. The bonding effect between the matrix and the reinforcement within the SMA composite was strengthened by the cold rolling. The fabricated composite by these processes can be applied as a part of the aircraft, and this part is operated under severe flying condition such as low temperature and high pressure. In this study, an acoustic emission technique was used to quantify the microscopic damage behavior of cold rolled TiNi/Al6061 SMA composite at low temperature condition. The results showed that the tensile strength of the TiNi/Al6061 SMA composite increased with the TiNi reinforcement at low temperature condition, but the strength for the specimen subjected to the cold rolling decreased. AE parameters of AE counts, amplitude and energy were useful to evaluate the microscopic damage behavior of the composite.

Abstract: The current surface strengthening process of microalloyed unquenched and tempered steel components is usually induction or laser quenching treatment. Subsequent to heat treatment, these structural parts are subjected to grinding, during which impairment of hardened materials can be caused by thermo-mechanical influence of the grinding process. This paper studies a new method of surface heat treatment by making use of grinding heat and stress to create favorable microstructures and promote high wear and fatigue resistance. This work outlines the influence of grinding parameters on the superficial hardening effect of 48MnV microalloyed steel. It was found that the thickness and hardness of the treated surface layer could be up to 1.6mm and HV750 respectively. The beneficial microstructure of the layer was created by an enhanced martensite transformation. It is highly possible that the method can be used to incorporate grinding and surface hardening into a single grinding operation to develop a cost-effective production method.

Abstract: A smart material is used as spectacle frames and brassiere frames, and partly in medical supplies because of its shape memory effect. The smart composite can be used on the wing of an airplane instead of the existing aluminium to control crack propagation. In this study, the smart composite was fabricated by a hot press method. TiNi alloy as reinforcement and Al6061 as matrix were used, respectively. The mechanical properties of the smart composite under thermal shock cycles were evaluated. In addition, Acoustic Emission techniques were also used to clarify the damage behavior of the smart composite under thermal shock cycles nondestructively.