웹기반 크리프 물성 데이터베이스 개발

허용학,백운봉,이완규,박휘립,정인현 한국산업안전학회 2002 한국안전학회지 Vol.17 No.4

User-friendly-web-based database system for searching creep property data was developed. This system includes about 38000 creep data for 270 different materials including low carbon steel, stainless steel and alloy steel. Data on creep rupture, creep deformation, creep crack growth and creeping materials can be searched through this system. Retrieved data is displayed in numeric form and also presented in graphical form for visualizing the data. Furthermore, the creep rupture data is designed to be fitted to a regression equation of logarithmic stress using time-temperature parameter(TTP). The degree of the regression equation, orthogonal polynomials, was determined using analysis of variance.

철근 콘트리트 보의 손상 진전에 의해 발생하는 음향방출 신호 특성

박휘립,윤동진,정중채,김동진,허용학 한국비파괴검사학회 2003 한국비파괴검사학회지 Vol.23 No.2

콘크리트 구조물의 건전성 평가에 음향방출 기술을 적용하기 위한 기초연구로서, 철근 콘크리트 보 시험편을 대상으로 음향방출실험을 실시하여 다양한 크기의 하중이 가해진 철근 콘크리트 보에서 손상이 진행되는 동안의 음향방출 발생특성을 규명하였다. 철근 콘크리트 보의 파괴거동을 음향방출의 관점에서 단계별로 분석함으로써 콘크리트에서의 음향방출 신호원이 미세균열, 휨 균열, 사인장균열과 균열면간의 마찰 등으로부터 발생함을 추정할 수 있었다. 또한 철근 콘크리트 보 시험편을 이용한 반복재하실험과 파괴실험을 통하여 다양한 하중상태에서의 음향방출 신호를 취득하였다. 신호의 분석은 손상의 진전이 이루어질 때 발생하는 이상신호와 손상진전이 없을 때 발생하는 정상신호의 차이를 규명하는데 주안점을 두었는데, 특히 지금까지는 잡음신호로 간주되었던 균열면간의 마찰에 의한 신호의 발생특성에 관심을 집중하였다. 그 결과 음향방출 신호발생율(AE hit rate), 하중증가시의 음향방출 신호발생빈도(primary AE peak)와 하중감소시의 음향방출 신호발생빈도(secondary AE peak), 그리고 센서 부착위치에 따른 신호특성 등에서 정상신호와 이상신호간의 차이를 관찰할 수 있었다. 이러한 결과에 근거하는 실용적인 음향방출 적용을 위한 새로운 접근방법은 콘크리트 구조물의 손상과 성능저하를 평가하는데 유망한 새로운 방법으로 제시될 수 있다. As a preliminary study for applying the acoustic emission(AE) technique to assess the integrity of concrete structures, AE behavior of a reinforced concrete beam under cyclic loads of various loading stage was examined by laboratory experiments. By analyzing failure behavior of a reinforced concrete beam in the AE point of view, it was presumed that major sources of AE signals in concrete were micro-crack initiation, development of filexural and diagonal tension crack, and the friction between crack surfaces. In addition, cyclic loading tests and failure test were carried out to obtain the AE responses under various loading conditions. The analysis of the signal patterns was aimed at discussing the differences between the normal signal and the abnormal signal, which represent the safe condition and the condition of developing damage, respectively. In this study, especially, the behavior of friction signals from crack surfaces, which were usually treated as noises, was considered as a typical pattern of the normal signal. As a result, significant differences between the normal and abnormal signal patterns were observed in the such parameters as the AE hit rate, magnitude of the primary or secondary AE peak, and AE response according to the sensor location. Based on the preliminary results, this new approach for practical AE application may provide a promising method for estimating the level of damage and distress in concrete structures.

Effect of stress ratio on fatigue life of GFRP composites for WT blade

Yong-Hak Huh,Jae-Hyun Lee,Dong-Jin Kim,이영신 대한기계학회 2012 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.26 No.7

Fatigue life of GFRP (glass-fiber reinforced plastic) composites used in wind turbine rotor blades has been evaluated considering the glass fiber orientations. Three different laminate composites with the respective laminating orientation of 0°, ±45°, and 0°±45° were prepared using vacuum infusion method. Tensile properties and S-N curves for these composites were experimentally determined at room temperature. From the tensile tests, it was found that tensile properties were greatly dependent upon the fiber orientation and the tensile strength of unidirectional composite was the largest and bidirectional (45°) composite was the weakest among three composites. The fatigue properties were determined under constant amplitude load control at different stress ratios, R, of 0.5, 0.1 and -0.2. The properties also show the dependency of stress ratios and fiber orientation. The fatigue life diagrams of these three composite were relatively well presented with the double logarithmic S-N curve. The linear slopes of the respective S-N curves for three composite were not greatly different. The fatigue limits for the composites were evaluated and predicted with linear Goodman and Gerber diagrams.

Behavior of Fatigue Crack around a Hole under Biaxial Loading: Effect of Biaxial Static Load

Huh, Yong Hak,Park, Philip,Kim, Dong Jin Trans Tech Publications, Ltd. 2005 Key Engineering Materials Vol.297 No.-

<P>Fatigue crack initiation and propagation behavior around a hole subjected to various biaxial fatigue loading modes has been investigated considering the effect of the biaxial static stress. Two different biaxial loading systems, i.e. cyclic tensile loading with static torsional load and cyclic torsional loading with static tensile load, were employed to thin-walled tubular specimens with a circular notch. Variation of the initiaiton and propagation directions of the crack around a hole under these loading ssytems was examined. The initiation directions were observed using replication technique and the crack propagation was measured by two crack gages mounted near the notch. It was found that the biaxial static stress superimposed on the cyclic tensile or torsional loading tests has no influence on the initiation and propagation directions of the cracks around a hole. The fatigue crack initiation and propagation direction under cyclic loading with biaxial static load can be very well explained by the maximum of tangential stress range, Dsqq,max. Furthermore, it was shown that the fatigue crack growth rates under biaixial faigue loading becomes higher with increment of biaxial static load.</P>

Effect of Biaxial Static Loads on Fatigue Crack Propagation Behavior under Cyclic Tensile and Torsional Loading

Huh, Yong Hak,Park, Philip,Kim, Dong Jin,Park, Jun Hyub Trans Tech Publications, Ltd. 2006 Key Engineering Materials Vol.321 No.-

<P>Fatigue crack propagation behavior under cyclic tensile or torsional loading with biaxial static loads has been investigated. Two different biaxial loading systems, i.e. cyclic tensile loading with static torsional load and cyclic torsional loading with static tensile load, were employed to thin-walled tubular specimens. The crack propagation was measured by two crack gages mounted near the notch and crack opening level was measured by unloading compliance method. The directions of the fatigue crack propagated under respective biaxial loading conditions were examined and the growth rates were evaluated by using several cyclic parameters, including equivalent stress intensity factor range, Keff, crack tip opening displacement range, CTD, minimum strain energy density factor range, Smin. Furthermore, the growth rates were evaluated by effective cyclic parameters considering crack closure. It was found that the biaxial static stress superimposed on the cyclic tensile or torsional loading tests has no influence on the propagation directions of the cracks. Furthermore, it was shown that the fatigue crack growth rates under biaixial faigue loading were well expressed by using the cyclic fatigue parameters, Keq,eff, CTDeff, Smin,eff considering crack closure effect.</P>

Response of impedance measured by polyvinylidene fluoride film sensors to damage propagation for wind turbine blade

Huh, Yong-Hak,Kim, Jongil,Hong, Seonggu SAGE Publications 2014 Journal of intelligent material systems and struct Vol.25 No.5

<P>To establish an effective operation and maintenance system for the wind turbine system, techniques for early detecting the damage, which may occur during its operation, may be required. In this study, responses of impedance, which can be a technique to early detect the damage, to the damage extension in the wind turbine blade were investigated through the full-scale structural test of the 100 kW wind blade. The impedance was measured with polyvinylidene fluoride film sensors with dimension of 25 × 20 mm<SUP>2</SUP> patched on the blade skin. The damages were initiated along the trailing and leading edges and extended with increasing static load level. Impedance signatures varied with increasing the damage size were analyzed in the frequency ranging from 1 to 300 MHz. It was found that the patterns of the impedance signatures are remarkably dependent upon the static load level and sensor location. The change in the signatures was quantitatively quantified by the damage metric. The scalar metric values were closely correlated to the damage extension size and the sensor locations to the extended damage.</P>

Measurement of Continuous Micro-Tensile Strain Using Micro-ESPI Technique

Huh, Yong Hak,Kim, Dong Iel,Kee, Chang Doo Trans Tech Publications, Ltd. 2005 Key Engineering Materials Vol.297 No.-

<P>To determine the micro-mechanical properties for micro/nano materials, it may be essential to measure the strain/deformation during micro-mechanical testing. Therefore, in this study, continuous measurement of in-plane tensile strain in micro-sized specimens of thin film materials was introduced using the micro-ESPI technique. TiN and Au thin films 1 and 0.47µm thick, respectively, were deposited on the silicon wafer and fabricated into the micro-sized tensile specimens by the electromachining process. The micro-tensile loading system and micro-ESPI system were developed to measure the tensile strain during micro-tensile loading. Micro-tensile stress-strain curves for these materials were determined using the algorithm for continuous strain measurement. Furthermore, the new algorithm for enhancing the sensitivity to measurement of in-plane tensile strain was suggested. Using the algorithm, micro-tensile strain data between interfringes were calculated. It is shown that the algorithm for enhancement of the sensitivity suggested in this study makes the sensitivity to the in-plane tensile strain increase.</P>

HARMONIZATION OF INTERNATIONAL WIND TURBINE STANDARD IN KOREA

Yong-Hak Huh,Seok-Jai Choi 한국신재생에너지학회 2013 AFORE Vol.2013 No.-

Back-Face Strain Compliance Calibration for the Four-Point Bend Specimen

Huh, Yong-Hak,Song, Ji-Ho The Korean Society of Mechanical Engineers 2000 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.14 No.3

Back-face strain compliance (BFS compliance) for the four-point bend specimen has been calibrated for various crack length ratios. Finite element technique was employed to simulate four-point loading and calculate back-face strain of the bend specimen. The numerically determined strain variation along the back face indicates that the sensitivity to gage placement increases with crack length and back-face strain at the gage length less than O.2W, where W is the width of the bend specimen, can be measured within 5% deviation of the maximum BFS. Non-dimensional back-face strain compliance, -E'BCW, was calibrated with FE analysis and experiment. The experimentally determined compliance indicates good agreement with the numerical compliance and can be expressed as a function of crack length ratio.

Back-face Strain Compliance Calibration ofr the Four-Point Bend Specimen

Yong Hak Huh,Ji Ho Song 대한기계학회 2000 KSME International Journal Vol.14 No.3