Yong-Hak Huh

Anisotropic fatigue behavior of rolled Mg-3Al-1Zn alloy

The anisotropy in the fatigue behavior of rolled Mg–3Al–1Zn alloy between the rolling direction and normal direction to the rolling plane was investigated. The {10-12} twinning–detwinning characteristics were found to play key roles in the anisotropic fatigue deformation behavior by inducing a change in the predominant plastic deformation mechanism, which controlled the flow stress and finally influenced the fatigue resistance by generating mean stress. Energy-based approach was successfully used to describe anisotropic fatigue life behavior.

Damage assessment of wind turbine blade under static loading test using acoustic emission

Acoustic emission is known as a powerful nondestructive tool to detect any further growth of preexisting cracks or to characterize failure mechanisms. Recently, this kind of technique, which is an in situ monitoring of integrity of materials or structures, becomes increasingly popular for monitoring the conditions of large structures such as a wind turbine blade. Therefore, it is required to find a symptom of damage progress before catastrophic failure through a continuous monitoring. In this study, acoustic emission technology was applied to assess the damage in the wind turbine blade during step-by-step static load test. In this static loading test, we have used a full-scale blade of 100 kW in capacity, and an attempt was made to apply a new source location method using a new algorithm with energy contour mapping concept. We also measured the deflection of blade tip by linear variable differential transformer (LVDT) and the strain of inner shear web in order to analyze the correlation between stress condition and damage identification. The results show that the acoustic emission activities give a good agreement with the stress distribution and damage location in the blade, especially in bonding edges around 1000–1500 mm far from the root. Finally, the applicability of new source location method was confirmed by comparison of the result of source location and experimental damage location.

A tip deflection calculation method for a wind turbine blade using temperature compensated FBG sensors

The tip deflections of wind turbine blades should be monitored continuously to prevent catastrophic failures of wind turbine power plants caused by blades hitting the tower. In this paper, a calculation method for wind turbine blade tip deflection is proposed using a finite difference method based on arbitrary beam bending and moment theory using measured strains. The blade strains were measured using fiber optic Bragg grating sensors. In order to confirm this method, a 100 kW composite wind turbine blade was manufactured with epoxy molded fiber optic Bragg grating (FBG) sensors installed in the shear web of the blade. A number of these sensors, normal FBG probes, were fabricated to only measure strains and the other sensors, temperature compensated FBG probes, were prepared to also measure strain and temperature. Because the output signals of FBG sensors are dependent on strains as well as temperatures, the sensor output signals should be compensated by the temperatures to obtain accurate strains. These FBG sensors were attached on the lower and upper parts of the web at one meter intervals throughout the entire length of the blade. To evaluate the measurement accuracy of the FBG sensors, conventional electrical strain gauges were also bonded onto the surface of the web beside each FBG sensor. By performing a static load test of the blade, the calculated tip deflection of the blade was well determined within an average error of 2.25%. (Some figures may appear in colour only in the online journal)

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

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 mm2 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.

Source Location on Full-Scale Wind Turbine Blade Using Acoustic Emission Energy Based Signal Mapping Method

Acoustic emission(AE) has emerged as a powerful nondestructive tool to detect any further growth or expansion of preexisting defects or to characterize failure mechanisms. Recently, this kind of technique, that is an in-situ monitoring of inside damages of materials or structures, becomes increasingly popular for monitoring the integrity of large structures like a huge wind turbine blade. In this study, the activities of AE signals generated from external artificial sources was evaluated and located by new developed signal mapping source location method and this test is conducted by 750 kW full-scale blade. And a new source location method was applied to assess the damage in the wind turbine blade during step-by-step static load test. In this static loading test, we have used a full scale blade of 100 kW in capacity. The results show that the acoustic emission activities give a good agreement with the stress distribution and damage location in the blade. Finally, the applicability of the new source location method was confirmed by comparison of the result of source location and experimental damage location.

Damage Monitoring for Wind Turbine Blade using Impedance Technique

Impedance based monitoring technique was investigated to evaluate the damage occurring in wind turbine blade. In this study, PVDF film piezo sensors were patched on the 10 kW wind turbine blade, and impedance was measured over the frequency range of 1~200 MHz under fatigue loading. With applying fatigue loads on the blade, change in maximum deflection of the blade and local strain values could be obtained from the strain gages attached on the blade, and difference of the impedance signatures was also observed. From these data, it could be found that local damage or geometrical change in the blade structure happened. To quantitatively compare the impedance signature patterns, a statistical algorithm, scalar damage metric M was used. It was calculated from the impedance signatures considering fatigue loads and location of the sensors. The metric values were compared to correlate the metrics with damage in the blade.

RESEARCH ON SPACE ENVIRONMENTAL EFFECT OF ORGANIC COMPOSITE MATERIALS FOR THERMAL MANAGEMENT OF SATELLITES USING MC-50 CYCLOTRON

유기재료(organic material)는 위성이나 우주비행체의 열을 조절하고 우주환경에 직접 노출되는 것을 차단하기 위하여 가장 흔하게 사용되는 재료 중 하나이다. 본 논문에서는 지상설비를 이용하여 모사한 우주환경에서 유기재료의 물성변화를 관찰하였다. 대표적인 위성용 유기 열조절 재료 중 하나인 2mil ITO(Indium Tin Oxide) coated aluminized KAPTON을 실험 대상으로 선택하였다. 양성자에 의한 단일 우주환경효과를 실험하기 위하여, 한국 원자력의학원의 MC-50 싸이클로트론(cyclotron)을 이용하여 양성자를 조사(irradiation)하였으며, 조사조건은 지구궤도 주변 최고의 양성자 발생기록인 1972년 8월의 최고치 상황을 적용하였다. 조사에너지는 평균 관측에너지인 30MeV으로 고정하였으며, 등가 조사량은 우주노출 시간 1년, 3년, 5년 및 10년을 기준으로 설정하였다. 분석과정은 인장강도를 측정하여 정량적 물성저하를 확인하였고, 전계방출 전자주사현미경 등으로 결정성변화와 노출표면의 손상을 분석하였다. 【The organic material is one of the most popular material for the satellites and the spacecrafts in order to perform the thermal management, and to protect direct exposure from the space environment. The present paper observes material property changes of organic material under the space environment by using ground facilities. One of the representative organic thermal management material of satellites, 2 mil ITO(Indium Tin Oxide) coated aluminized KAPTON was selected for experiments. In order to investigate the single parametric effect of protons in space environment, MC-50 cyclotron system in KIRAMS(Korea Institute of Radiological and Medical Science) was utilized for the ion beam irradiation of protons and ion beam dose was set to the Very Large August 1972 EVENT model, the highest protons occurrence near the earth orbit in history. The energy of ion beam is fixed to 30MeV(mesa electron volt), observed average energy, and the equivalent irradiance time conditions were set to 1-year, 3-year, 5-year and 10-year exposure in space. The procedure of analyses includes the measurement of the ultimate tensile strength for the assessment of quantitative degradation in material properties, and the imaging analyses of crystalline transformation and damages on the exposed surface by FE-SEM(Field Emission Scanning Electron Spectroscopy) etc.】