18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS SOURCE LOCATION METHOD FOR GFRP WIND TURBINE BLADE USING ACOUSTIC EMISSION SIGNAL MAPPING B. Han 1 , D. Yoon 1 * 1 Center for Safety Measurement, Korea Research Institute of Standards and Science, 1 Doryong-dong, Yuseong-gu, Daejeon, South Korea * djyoon@kriss.re.kr Keywords : Wind blade, Acoustic emission, Nondestructive testing, Source location case of composite materials or heterogeneous 1 Introduction structures consisted of each different material, it is Recent social issues related to energy crisis and very hard to find the location of damages exactly. environmental pollution caused a concern for the This study describes a new concept for identification renewable energy sources. One of the promising of damage sources in heterogeneous composite future energy, wind energy is taking its place as an materials and discusses how they can be verified alternative energy and the market of wind turbine both to laboratory blade certification testing and to system is growing steadily. Moreover wind turbine actual full scale wind turbine blade. Finally we blades are getting large in size due to the demands of suggest a new algorithm for source location of high power wind turbine application [1]. As wind damages and verified its usefulness in field turbine blades increase in size, there is an increasing application. need to monitor the health of the structures [2, 3]. And also they use composite materials in 2 Acoustic emission signal mapping method for manufacturing blades for ensuring weight vs. damage location strength ratio, appropriate nondestructive testing method for evaluating the integrity of composite 2.1 Damage index map material structures is required. There were many Usually the limitation of traditional AE source studies for the applying acoustic emission technique location method strongly showed the dependence for to composite blades in the Europe. But these wave speed in the corresponding material of tested research theme was mainly focused on evaluation of structures, especially in the inhomogeneous material structural integrity during static and fatigue test [4], or heterogeneous structures. Therefore new method and it was used as a tool for detection of damage to be considered should be less affected by the wave occurrence [5]. speed in these kinds of composite blades. Also it Recently, there have been advances in developing will be better to install minimum number of sensors damage localization method in composite materials on the structures to be covered. In order to satisfy using a structural neural system [6], and in these conditions, we developed a new source monitoring of acoustic emission from real wind location algorithm using damage index based turbine blades undergoing static and fatigue testing database map. [7]. However these methods usually used a number Database map is consisted of each intensity value of sensors in their system, since there exists a acquired from pre-set data point on the blade surface practical problem for real application as large blades. before installing the blade onto the tower and nacelle. It is clearly important to detect the location and Considering several kinds of damages, each different severity of any damages which occurs during the arbitrary input source was used to get initial static test in order to be able to improve blade design database map. Each value of database was calculated and also to monitor such areas during the fatigue test. from power spectrum density of the signal after Conventional source location technique has used measuring AE events. That is, the measurement of typically arrival time difference between elastic signal energy changes in the composite materials is waves generated from each separated sensor better than time arrival method in its reproducibility mounted on the structure surface. However, in the point of view.
Fig. 3 shows the result of interpolation process. Interpolation process of signal map will minimize source location error by increasing signal map resolution. 2.2 Procedure of source location Now, the database of signal map acquired from the Fig. 1 Test specimen : a part of 750 kW blade tested blade will be used to calculate the location of unknown damage sources. Basic principle of this method is to find a solution by comparing certain value between each four sensor output and pre- acquired database of the interested blade. In these procedures, some numerical process such as iteration method, adding tolerance, error correction can be applied to optimize source location exactly shown as Fig. 4. It means that this numerical process helps Fig. 2 Sensors and test point on the specimen source location area smaller by increasing measured damage index value based on tolerance concept. That is, the energy changes of AE signal is less And also, this procedure covers the problems of affected than the measurement of arrival time unexpected foreign input sources which are different difference in location damage source of composite from initial input source. materials. Fig. 1 shows the specimen for algorithm verification test and Fig. 2 shows the position of sensor and data point to be tested. Four sensors are used to get one signal map of database of blade area interested. In case of the Fig. 2, total four sensors and 144 test points was used to calculated intensity and constructs a signal map of blade to be tested. From the several steps of signal processing, signal map was completed for each sensor. The signal map (a) 2 % tolerance indicates a pattern of signal attenuation according to the position of sensor and input source. (b) 5 % tolerance (c) 8% tolerance Fig 4. Damage location result with increasing Fig. 3 Enhanced resolution of the signal map by tolerance applying interpolation process
PAPER TITLE 3 Experiments for verification 3.1 Experiments for the laboratory specimen Verification test for new suggested location In order to verify the performance of new suggested algorithm was carried out in the laboratory specimen localization method, traditional acoustic emission test and in the full scale blade test. The specimen of source location method was tried together. Same laboratory consisted of 1,500 mm in length, 1,000 four AE sensors were used and wave speed was mm in width and is a part of actual full scale blade. directly obtained from same blade. Measurement of In the full scale test of blade which is a 25 m in wave speed in the blade was done for three different length, 750 kW capacity, the pressure side of the materials such as GFRP, PVC and GFRP-PVC area. blade was used as test area. Therefore three kinds of different wave speed were used to calculate reasonable source location. The database of signal map was measured from 16 points in row, 9 points in column, 100 mm interval, that is, total 144 points. Also we composed the 12 signal maps, which are each different input source such as spring impact, pencil lead break, equo-tip test. Fig. 5 shows the signal map of each different channel for the same impact source. On the other hand, Fig. 6 shows the signal map of same channel for the each different impact source. (1) CH 1 (2) CH 2 (1) Pencil break (2) Spring impact (3) CH 3 (3) Equo-tip D type (4) CH 4 Fig. 6 Signal map by each different source input Fig. 5 Signal map by each different channel 3
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