고등기술연구원

논문현황

Institute for Advanced Engineering

연구부서

논문현황

INSTITUTE FOR ADVANCED ENGINEERING

지능기계시스템센터 Acoustic emission characteristics of stress corrosion cracks in a type 304 stainless steel tube

· 저자

Woonggi Hwang, Seunggi Bae, Jaeseong Kim, Sungsik Kang, Nogwon Kwag, Boyoung Lee

Acoustic emission (AE) is one of the promising methods for detecting the formation of stress corrosion cracks (SCCs) in laboratory tests. This method has the advantage of online inspection. Some studies have been conducted to investigate the characteristics of AE parameters during SCC propagation. However, it is difficult to classify the distinct features of SCC behavior. Because the previous studies were performed on slow strain rate test or compact tension specimens, it is difficult to make certain correlations between AE signals and actual SCC behavior in real tube-type specimens. In this study, the specimen was a AISI 304 stainless steel tube widely applied in the nuclear industry, and an accelerated test was conducted at high temperature and pressure with a corrosive environmental condition. The study result indicated that intense AE signals were mainly detected in the elastic deformation region, and a good correlation was observed between AE activity and crack growth. By contrast, the behavior of accumulated counts was divided into four regions. According to the waveform analysis, a specific waveform pattern was observed during SCC development. It is suggested that AE can be used to detect and monitor SCC initiation and propagation in actual tubes.

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지능기계시스템센터 The Effect of Welding Residual Stress for Making Artificial Stress Corrosion Crack in the STS 304 Pipe

· 저자

Jae-Seong Kim, Bo-Young Lee, Woong-Gi Hwang and Sung-Sik Kang

The stress corrosion crack is one of the fracture phenomena for the major structure components in nuclear power plant. During the operation of a power plant, stress corrosion cracks are initiated and grown especially in dissimilar weldment of primary loop components. In particular, stress corrosion crack usually occurs when the following three factors exist at the same time: susceptible material, corrosive environment, and tensile stress (residual stress included). Thus, residual stress becomes a critical factor for stress corrosion crack when it is difficult to improve the material corrosivity of the components and their environment under operating conditions. In this study, stress corrosion cracks were artificially produced on STS 304 pipe itself by control of welding residual stress. We used the instrumented indentation technique and 3D FEM analysis (using ANSYS 12) to evaluate the residual stress values in the GTAW area. We used the custom-made device for fabricating the stress corrosion crack in the inner STS 304 pipe wall. As the result of both FEM analysis and experiment, the stress corrosion crack was quickly generated and could be reproduced, and it could be controlled by welding residual stress

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지능기계시스템센터 Characteristics of GMAW Narrow Gap Welding on the Armor Steel of Combat Vehicles

· 저자

Jae-Seong Kim and Hui-Jun Yi*

The primary purpose of this investigation was to study the characteristics of the armor steel weldment of combat vehicles by using GMAW narrow gap welding (NGW). The results showed that the mechanical properties and residual stress distribution of NGW weldment were improved, compared with conventional X-groove weldment. Additionally, ballistic tests according to MIL-HDBK-1941 were carried out to verify the ballistic ability of NGW weldment and the result showed that the NGW process was qualified for welding the armor steel of combat vehicle.

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지능기계시스템센터 The study for direct SCC fabrication in STS 304 pipe

· 저자

Sung-Sik Kang, Jae-Seong Kim & Bo-Young Lee

Stress corrosion cracking is one of the aging phenomena for the major structure components in nuclear power plant. During the operation of a power plant, stress corrosion cracks are initiated and grown especially in dissimilar weldment of primary loop components. Among the three factors (susceptible material, residual stress, and corrosive environment) which make the SCC, the residual stress becomes a critical factor for stress corrosion crack when it is difficult to improve the material of the components and their environment under operating conditions. In this study, stress corrosion cracks were artificially produced on STS (stainless steel) 304 pipe itself by control of welding residual stress. The instrumented indentation technique and 3D finite element method (FEM) analysis (using ANSYS 12) were used to evaluate the residual stress values in the GTAW area. As the result of both FEM analysis and experiment, the stress corrosion crack was quickly generated and could be reproduced, and controlled by welding residual stress. Also non-destructive evaluation signals by Acoustic Emission will be discussed for the initiation and growth of SCC.

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지능기계시스템센터 A Study on the Cooling Performance of Newly Developed Slice Die in the Hot Press Forming Process

· 저자

Seung Hwan Lee, Jaewoong Park, Kiyoung Park, Dong Keun Kweon, Hyunwoo Lee, Daeho Yang, Hongrae Park and Jaeseung Kim*

The cooling performance of a slice die is studied numerically and experimentally. The slice die is designed to improve the cooling performance compared to that of a conventional die that is generally used in the Hot Press Forming (HPF) process by modifying the cooling channel layout and arrangement. In order to understand the physical phenomenon of the slice die cooling performance, the cooling performance of the conventional die is also simulated and their results are compared with the slice die results. From the results of the maximum temperature of the blank and die and the temperature distribution of the blank, the slice die has considerably improved cooling performance. To validate the simulation results, the slice die is prototyped and a blank is produced by the HPF process. Blank temperatures are measured by a thermal imaging camera at several holding times. The simulation and experimental results of the blank temperatures are compared and agree with the error rate of 3%. In order to verify the quality of the produced blank, ultimate tensile stress, yield stress, and elongation tests are conducted for specimens that are extracted from the blank and are compared with existing literature results.

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지능기계시스템센터 Development of two-dimensional piezoelectric laser scanner with large steering angle and fast response characteristics

· 저자

Ho-Sang Kim, Dae-Hee Lee, Deok Jae Hur and Dong-Chan Lee

We describe a two-dimensional piezoelectric laser scanner designed and tested to ob-tain the large steering angle of 1 degree and fast response characteristics with 200 Hz. To overcome the relatively small expansion capability of piezoelectric actuator, the displacement amplification mechanisms with two levers in series are employed to magnify the end tip of the lever which is connected to the 0.5-inch glass mirror. For fast response characteristics, the natural frequencies of the hinge mechanisms were calculated by using finite element analysis technique. In order to evaluate the perfor-mance of the proposed scanner, the hinge mechanism has been manufactured of titani-um alloy and by exciting the actuator, the natural frequencies have been estimated. Al-so, the actual machining test in the burning paper has been done by using high power laser and it is shown that the proposed laser scanner is capable of steering the laser beam in 1 degree and with frequency of 200 Hz successfully.

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지능기계시스템센터 Random Vibration Analysis of the Tip-tilt System in the GMT Fast Steering Secondary Mirror

· 저자

Kyoung-Don Lee, Young-Soo Kim, Ho-Sang Kim, Chan-Hee Lee, and Won Gi Lee

A random vibration analysis was accomplished on the tip-tilt system of the fast steering secondary mirror (FSM) for the Giant Magellan Telescope (GMT). As the FSM was to be mounted on the top end of the secondary truss and disturbed by the winds, dynamic effects of the FSM disturbances on the tip-tilt correction performance was studied. The coupled dynamic responses of the FSM segments were evaluated with a suggested tip-tilt correction modeling. Dynamic equations for the tip-tilt system were derived from the force and moment equilibrium on the segment mirror and the geometric compatibility conditions with four design parameters. Statically stationary responses for the tip-tilt actuations to correct the wind-induced disturbances were studied with two design parameters based on the spectral density function of the star image errors in the frequency domain. Frequency response functions and root mean square values of the dynamic responses and the residual star image errors were numerically calculated for the off-axis and on-axis segments of the FSM. A prototype of on-axis segment of the FSM was developed for tip-tilt actuation tests to confirm the ratio of tip-tilt force to tip-tilt angle calculated from the suggested dynamic equations of the tip-tilt system. Tip-tilt actuation tests were executed at 4, 8 and 12 Hz by measuring displacements of piezoelectric actuators and reaction forces acting on the axial supports. The derived ratios of rms tip-tilt force to rms tip-tilt angle from tests showed a good correlation with the numerical results. The suggested process of random vibration analysis on the tip-tilt system to correct the wind-induced disturbances of the FSM segments would be useful to advance the FSM design and upgrade the capability to achieve the least residual star image errors by understanding the details of dynamics.

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지능기계시스템센터 Diamond turning of large off-axis aspheric mirrors using a fast tool servo with on-machine measurement

· 저자

Ho-Sang Kim, Eui-Jung Kim and Byung-Suk Song

We describe a systematic approach of diamond turning which has been devised to fabricate large aspheric off-axis mirrors demanded for the construction of three-mirror-anastigmatic telescopes. To obtain sub-micrometer form accuracy, the machine axes errors are compensated for real time using a fast tool servo driven by a piezoelectric actuator along with an on-machine measurement scheme. Besides, a special chucking device is used to mount the mirror substrate on the spindle with a required large offset to the axis of rotation, which has been specially designed to minimize elastic deformation caused by centrifugal inertia and clamping force. Actual machining data are discussed in detail to prove that the proposed approach is capable of fabricating aluminum mirrors of 620 mm diameter to a form accuracy of 0.7 m in peak-to-valley error value. Key words: Diamond turning, large off-axis aspheric mirrors, real-time error compensation, fast tool servo, on-machine measurement, three-mirror-anastigmatic telescopes.

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지능기계시스템센터 Feed-forward control of fast tool servo for real-time correction of spindle error in diamond turning of flat surfaces

· 저자

Ho-Sang Kim and Eui-Jung Kim

We describe a fast tool servo designed and tested to obtain sub-micrometer form accuracy in diamond turning of flat surfaces. The thermal growth spindle error is compensated for real time using a fast tool servo driven by a piezoelectric actuator along with a capacitive displacement sensor. To overcome the inherent non-linearity of the piezoelectric actuator, Proportional Integral (PI) feedback control with a notch filter is implemented. Besides, feed-forward control based on a simple feed-forward predictor is added to achieve better tracking performance. Actual machining data are discussed in detail to prove that the proposed fast tool servo is capable of fabricating flat aluminum specimens of 100 mm in diameter to a form accuracy of 0.10 m in peak-to-valley error value. Key words: Fast tool servo, diamond turning, real-time error compensation, feed-forward control, piezoelectric actuator.

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지능기계시스템센터 Fabrication of free-form surfaces using a long-stroke fast tool servo and corrective figuring with on-machine measurement

· 저자

Ho-Sang Kim, Kwang-Il Lee, Kyoung-Min Lee, Young-Bong Bang

Fabrication of free-form surfaces that are frequently demanded for the construction of optical imaging systems is described. To obtain a tool motion with large amplitude and high bandwidth, a novel long-stroke fast tool servo is proposed and installed on the Z-axis of a diamond turning machine as an additional synchronized axis. In addition, a special on-machine measurement device is used to measure the optical parameters of the machined surface and to compensate for the residual form errors that are commonly produced in the diamond turning process. Actual machining test results show that the proposed procedures are capable of generating the copper free-form mirrors of 50 mm diameter to a form accuracy of 0.15 m in peak-to-valley value error. Key words: Long-stroke fast tool servo, diamond turning, free-form surface, on-machine measurement, corrective figuring.

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지능기계시스템센터 Effects of Hospital Ward Curtains on Ventilation in a Four-bed Hospital Ward

· 저자

Jung-Hun Noh, Jungsuk Lee, Kwang-Chul Noh, Young-Won Kim, Se-Jin Yook

Various highly infective pathogens that are present in hospital wards can spread through the air and infect patients. To remove pathogens effectively in order to reduce infections, the efficient ventilation of hospital wards is critical. This study used age of air data to analyze the ventilation efficiency within hospital wards. The validity of the numerical analysis method was verified by comparison with the age of air data obtained from an experimental test on a four-bed hospital ward. The verified numerical analysis method was used to establish the airflow within a hospital ward in relation to the use of ventilation systems, air cleaners, and individual patient curtains. The efficiency of ventilation according to each factor was compared using age of air data. Considerable differences in the age of the air were identified, depending on the location of the patient with respect to both the air cleaner and the inlet/outlet duct of the ventilation system. Furthermore, individual curtains were found to interfere with air circulation, reducing ventilation efficiency and increasing the age of the air at the location of each patient.

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지능기계시스템센터 Numerical Modeling and Performance Prediction of COS Hydrolysis Reactor in an Integrated Gasification Fuel Cell in terms of Thermo-Chemical Transport Phenomena

· 저자

Jung-Hun Noh, Dong-Shin Ko, Seung-Jong Lee and Deog-Jae Hur

During the recent decades, global warming by greenhouse gas evolution has attracted worldwide attention and ever increasing strict regulations thereon have become institutionalized as international policies. In the process, more environment-friendly power generation technologies have been developed utilizing fossil fuels with a view to timely commercialization. As one such “clean coal” technology, an Integrated Gasification Fuel Cell system is a promising power generation means where a carbonyl sulfide (COS) hydrolysis reactor is installed downstream of coal syngas to remove acidic gas constituents such as H2S and COS. The most significant design parameters affecting performance of the COS hydrolysis reactor were selected to be gas hourly space velocity (GHSV), reaction temperature, and length ratio, and numerical modeling was performed considering heat and fluid flow transfer as well as chemical reaction kinetics. Effect of the selected design parameters on the variation of conversion rate and reactant gas mixture concentration were comprehensively investigated to predict performance of the COS hydrolysis reactor. Stochastic modeling of reactor performance was finally performed using Monte Carlo simulation and linear regression fitting.

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지능기계시스템센터 Natural convection flow around heated disk in cubical enclosure

· 저자

Dong-Bin Kwak, Jung-Hun Noh and Se-Jin Yook

In this study, a numerical analysis of the natural convection characteristics around a circular disk in a cubical enclosure was carried out. The disk and the enclosure were used to simulate a wafer placed in a front-opening unified pod. The disk position, size, and temperature were considered as the main variables, and the effects of each variable on the isotherm, streamline, and local Nusselt number were examined. The results confirmed that the disk position and size in an enclosure had greater effects on the natural convection flow pattern than the disk temperature. Because air flowed in from the edge of the disk, the local Nusselt number was the highest at the edge of the disk. It was also found that a large disk size generated a plume and formed a new thermal boundary layer.

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지능기계시스템센터 Optimization of the radial heat sink with a concentric cylinder and triangular fins installed on a circular base

· 저자

Dong-Bin Kwak, Hoon-Pyo Kwak, Jung-Hun Noh and Se-Jin Yook

In this study, a radial heat sink with a concentric cylinder and triangular fins installed on a circular base, that is, a heat sink representing a general shape of the heat sinks employed for most of the commercial LED light bulbs, was optimized in terms of mass and thermal resistance. A numerical study was performed by considering natural convection and radiation heat transfer, and was validated through experiment. The effects of various geometric parameters, that is, radius of heat sink base, number of fins, fin height, and radius of hollow cylinder, on thermal resistance and convectional heat transfer coefficient were investigated. Multiobjective optimization was conducted by using the weighted sum method and the genetic algorithm. As a result, Pareto fronts were obtained, and the thermal resistance and mass of the radial heat sink with triangular fins were reduced compared to those of the reference radial heat sink.

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지능기계시스템센터 Effects of wall thickness and material property on inverse heat conduction analysis of a hollow cylindrical tube

· 저자

Jung-Hun Noh, Dong-Bin Kwak and Se-Jin Yook

In this study, we examined the effects of a hollow cylindrical tube’s thickness and material properties on estimated time delay and waveform distortion in a one-dimensional inverse heat transfer analysis model using the thermal resistance method and an input estimation algorithm. Results indicated a persistent time delay for various heat flux amounts applied to different tube thicknesses. As the tube thickness increased, the numerically determined temperature data also experienced a time delay, which affected the inverse heat transfer response curve. Results also indicated that the transient heat flux waveform estimated for different material properties showed higher levels of distortion for materials having relatively low thermal conductivity. These materials also exhibited greater time delays. To address these issues, we applied a Fourier number (a dimensionless number representing the tube’s thickness and material properties) and proposed an equation to calculate sharpness, which can subsequently be used to predict probable time delays and heat flux waveform distortion. In conclusion, a correction is required when a low Fourier number is used in inverse heat transfer analysis.

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지능기계시스템센터 A study on improving bone conduction speaker performance by electromagnetic prediction and performance distribution by statistical analysis method

· 저자

Dong Shin Ko, Deog Jae Hur, Tae Won Park and Jai Hyuk Lee

The present paper is focused on the stochastic characteristics of the electromagnetic force, one of the performance parameters for the bone conduction speaker which is one of the devices in the “smart glass”. The design parameters were taken as significant, affecting the electromagnetic force. Characteristic analysis of significant parameters was considered by using the factorial design method. Significant factor of main effect was selected via fractional factorial design method. Main effect and interaction of selected factor were analyzed applying the full factorial design method. The independency of the selected parameter and their significant interaction were examined by using the F-test method. Linear and non-linear characteristics for the selected parameters and performance were examined by creation of the median point within the analysis results for significance analysis. Therefore, prediction model derived non-linear regression model from the central composite design of response surface method. For probability distribution of the electromagnetic characteristics, related prediction model and Monte Carlo simulation method were applied. Electromagnetic performance prediction result showed 98.5 % level and improved maximum 99.8 % reliability level under 3σ level of dimensional management. In view of this, such stochastic design approach could improve design efficiency via verification of individual design parameters’ effect on the performance levels, thereby proving design reliability based on the object levels.

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지능기계시스템센터 A study of optimization for vibration performance and electromagnetic force of the bone conduction speaker actuator asymmetry

· 저자

Jai Hyuk Lee, Dong Shin Ko, Sang Youp Kwon, Jae Yong Kim and Deog Jae Hur

This study derives an optimal design of electromagnetic force, verifies the analysis of the optimum, and runs the vibration test to consider various properties of the optimal design in order to optimize the performance of the bone conduction speakers used in Smart Glasses, one of smart wearable devices. For performance factors that affect the electromagnetism of actuator in a bone conduction speaker which holds a magnetic structure, the diaphragm height, yoke pole height, magnet height, magnet and plate width, and coil turn number were selected. To analyze the properties of the performance factors, responsive factors needed to be classified first using fractional factorial design and full factorial design was used for influence analysis. The F-test was done as the means to conduct the valence test to show the independence and reciprocal action for selected performance factors, and it concluded that three independence factors were valid. Based on the valid performance factors, a regression equation to predict its performance was deducted and using the equation, an optimal design to maximize the electromagnetic force performance per component. To verify the characteristics of the optimal model, the finite element method (FEM) was used for analysis. Through electromagnetic analysis, magnetic flux density was obtained, and the particular information along with current and coil length contributed to deriving 0.052 N of electromagnetic force. After completing the frequency response analysis based on the electromagnetic force, it resulted in the 0.0772 mm of displacement at 590.12 Hz of resonant frequency. A sample model was fabricated, followed by vibration testing, after optimal design and analytical verification. Hence, it is verified that the optimal design method and the credibility of the analysis of this study is deemed very high. Furthermore, utilizing this mechanism would inspect the effect of the design parameter performance and increase the credibility and effic…

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지능기계시스템센터 A study of optimization for vibration performance and electromagnetic force of the bone conduction speaker actuator asymmetry

· 저자

Jai Hyuk Lee, Dong Shin Ko, Sang Youp Kwon, Jae Yong Kim and Deog Jae Hur

This study derives an optimal design of electromagnetic force, verifies the analysis of the optimum, and runs the vibration test to consider various properties of the optimal design in order to optimize the performance of the bone conduction speakers used in Smart Glasses, one of smart wearable devices. For performance factors that affect the electromagnetism of actuator in a bone conduction speaker which holds a magnetic structure, the diaphragm height, yoke pole height, magnet height, magnet and plate width, and coil turn number were selected. To analyze the properties of the performance factors, responsive factors needed to be classified first using fractional factorial design and full factorial design was used for influence analysis. The F-test was done as the means to conduct the valence test to show the independence and reciprocal action for selected performance factors, and it concluded that three independence factors were valid. Based on the valid performance factors, a regression equation to predict its performance was deducted and using the equation, an optimal design to maximize the electromagnetic force performance per component. To verify the characteristics of the optimal model, the finite element method (FEM) was used for analysis. Through electromagnetic analysis, magnetic flux density was obtained, and the particular information along with current and coil length contributed to deriving 0.052 N of electromagnetic force. After completing the frequency response analysis based on the electromagnetic force, it resulted in the 0.0772 mm of displacement at 590.12 Hz of resonant frequency. A sample model was fabricated, followed by vibration testing, after optimal design and analytical verification. Hence, it is verified that the optimal design method and the credibility of the analysis of this study is deemed very high. Furthermore, utilizing this mechanism would inspect the effect of the design parameter performance and increase the credibility and effic…

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지능기계시스템센터 Evaluation of stress distribution with wind speed in a greenhouse structure

· 저자

Deog-jae Hur, Jung-Hun Noh, Hyun ju Lee and Hyoung woon Song

In this paper, stress distribution for a structurally stable greenhouse is considered in the present paper with subsequent investigation into the detailed stress distribution contour with the variation of self-weight and wind pressure level designation method under wind velocity of less than 30 m/sec. For reliable analysis, wind pressure coefficients of a single greenhouse unit were modeled and compared with experiment with correlation coefficient greater than 0.99. Wind load level was designated twofold: direct mapping of fluid dynamic analysis and conversion of modeled results into wind pressure coefficients (CP). Finally, design criteria of EN1991-1-4 and NEN3859 were applied in terms of their wind pressure coefficients for comparison. CP of CFD result was low in the most of the modeled area but was high only in the first roof wind facing and the last lee facing areas. Besides, structural analysis results were similar in terms of stress distribution as per EN and direct mapping while NEN revealed higher level of stress for the last roof area. The maximum stress levels are arranged in decreasing order of mapping, EN, and NEN, generating 8% error observed between the EN and mapping results under 30 m/sec of wind velocity. On the other hand, effect of dead weight on the stress distribution was investigated via variation of high stress position with wind velocity, confirming shift of such position from the center to the forward head wind direction. The sensitivity of stress for wind velocity was less than 0.8% and negligible at wind velocity greater than 20 m/sec, thus eliminating self-weight effect.

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