小波轉換在有限尺寸平板音洩源定位研究之應用

Abstract

音洩實驗試片的尺寸通常較真實結構小，音洩傳受到試片邊緣多 次反射的影響，不易分辯音洩事件抵達各個探頭的時間，導致單洩源 定位計算的誤差。本論文以小波轉換分析受板緣反射影響之音洩波傳， 建立一個適用於各種尺寸板試片的音洩源平面定位方法。本研究以共 振式音洩探頭擷取脈衝雷射在薄鋁板上所產生的音洩模擬訊號，並在 探頭共振頻率附近之特定頻寬內進行訊號分析。小波轉換分析採用高斯 母小波函數，交板緣反射所產生的音洩訊號與直接抵達探頭的音洩訊 號分離，精確獲得音洩波抵達探頭的時間及群速度。適當地提高取樣 頻率，量測到的音洩波群速度會愈接近理論值，音洩定位計算也會愈 準確。

This thesis presents a wavelet-transform-baed method to determine the source position of acoustic emission (AE) in either small or large plate specimens. Contrast to the waveforms observed in field structures, the AE events are frequently composed of a number of edge reflections in most laboratory specimens. The multiple reflection results in difficulty to determine the arrival times of AE waves to receivers. The precision of differences of arrival times is of significance in source location. A resonant transducer is used in experiments to measure the AE signals induced by laser ultrasound on thin aluminum plate specimens. The AE waves directly propagating from sources to receivers are distinguished from those reflected from edges of the specimens using wavelet transform with respect to the Gaussian mother wavelet. The arrival times of AE waves and group velocities can be also determined accurately in a broadband frequency range. The higher sampling rate adopted in measurement, the calculations of AE waves becomes closer to the exact.