散射路徑在平板裂縫偵測之分析研究

Analysis for the Path of Scattering Signal from Cracks in a Plate

時問反轉法理論（Time Reversal Method, TRM）已廣泛應用在結構非破壞檢測、水下聲學及醫學檢測上。其主要利用聲波可在非均勻介質中傳遞的特性，透過時間反轉法自適聚焦並還原至聲源點，將能量聚焦在裂縫上找出裂縫位置。但由於聲波在非均勻介質中傳遞仍會受幾何環境的影響，造成波形失焦及變形，以致聲東無法聚焦於聲源點，遠不到預期的效果，因此本研究中必須再透過陳列訊號處理技術來克服多徑效應及幾何的影響。本文主要藉由裂縫傳遞散射波（scattered wave）至接收陣列的到達時間不同，經時間反轉法並將訊號正規化處理後自接收陣列反發射回平板模型中，可藉著波與波的交疊形成一散射路徑聚焦回到裂縫，以偵測出裂縫實際位置。當中運用ANSYS有限元分析軟體，以模擬的方式驗證利用時反法所構成的散射路徑對平板裂縫位置偵測的可行性，並利用平面彎曲波在傳遞的過程中途經疲勞裂縫時所造成的散射波傅，透過時間反轉法找到裂縫位置。 The time reversal method has been widely used in non-destructive damage detection, underwater acoustic exploring, and medical monitoring. The proposed method utilizes the acoustic wave characteristics of having different propagating paths in an inhomogeneous media, such as a cracked plate. 8ased on this feature, the position of a crack in plate can be found by the time reversal method, which can selectively focuses the reemitting waves on the crack location to be detected. However, because of geometrical effects, which may result in the defocusing and distortion of the waveform and may cause the so-called multi-path problem on wave propagation, the array signal processing method is then applied to solve the multi-path problemIn this study, a method, consisting of the array signal processing method and the self-focusing method, is proposed to deal with the multi -path effects and to find the exact crack location in a plate. In the method, the delayed time caused by different wave propagating paths in a cracked plate, is utilized to reconstruct a scattered path pointing to the crack location. In the present investigation, the finite element analysis is used to simulate the surface wave propagation and to verify the validity of the proposed method in detecting a plate crack.