平板缺陷的板波散射與主動偵測研究

Abstract

本文探討以板波主動偵測平板圓孔及裂縫缺陷位置的技術及尺寸辨識的工作原理，並評估其檢測能力。利用黏貼於鋁合金平板之壓電陶瓷元件作為窄頻的板波致動器及 板波的感測器，並以小波轉換對於暫態板波訊號進行時頻域分析，由缺陷散射波波群的波程時間差與訊號分析鑑定缺陷位置及頻譜特徵。 含缺陷平板的板波訊號包括散射波訊號及壓電元件間直接波傳的板波訊號，在理想狀況下，含缺陷的板波訊號減去不含缺陷的板波訊號，可獲得自缺陷散射的板波訊號。本研究建立以簡單體法為基礎的尋優程式，反算平板缺陷的位置，所發展的缺陷定位目標函數具有唯一的全域極小值，可以保證缺陷定位計算的精確性，在實驗上已獲得良好的結果。散射波頻譜特徵與缺陷的種類、尺寸、波傳路徑、入射角度有關，裂縫尖端的繞射波訊號太小，裂縫尖端與壓電感測器之距離要足夠近，否則不易偵測裂縫尖端的繞射波。未來需發展散射波頻譜的相關數值分析，進一步比較實驗值與理論值，估算平板缺陷的尺寸。

This thesis experimentally investigates feasibility of detecting the locations and sizes of damages such as a circular hole and its edge crack in a plate using an active damage detection technology based on scattered plate waves. The experiments were carried out by an array of piezoelectric ceramic PZT discs surface mounted on both sides of the specimens as actuators and sensors to launch and detect the fundamental anti-symmetric plate waves. Damage positions are determined through time-of-flight of the envelop of wave group, which and the scattered spectra of plate waves from damages are characterized by time-frequency analysis. The time-frequency analysis on signals of plate wave was processed by continuous wavelet transform with a mother wavelet of Gaussian cosine pulse. Besides scattered plate waves from the damages, the signals for the damaged plate also include the signals of plate wave propagating direct from actuators to sensors. Under only ideal conditions, the signals of scattered plate waves can be evaluated from the difference between those signals for damaged plates and undamaged plates. The location of damage area is determined using the simplex algorithm, in which the objective function is a summation of the square of time-of-flight differences among scattered waves and those waves direct propagating between actuators and sensors. It guarantees seeking the exact location of damage due to existence of only one global minimum for the specimens having a single damage. A very good agreement between experimental results and predictions was achieved. The characteristics of scattered spectra are found to be dependent on damage types, sizes, paths of wave propagation, and angle of incidence. It results in a critical need to develop a numerical analysis of scattered spectra for various damages as references to compare them with experimental results in the near future. The present method can detect damages such as circular holes in a large plate. However, it is difficult to detect edge cracks extending from circular holes since the diffracted plate waves decay fast from the crack tips, which are far away from the actuators and sensors in these specimens.