相位陣列壓電探頭之超音波場分析

Abstract

本文以有限元素法分析直線陣列壓電探頭的暫態超音波波傳，探討同時發射、各別接收之相位陣列壓電探頭在汽機葉片裂紋偵測上的應用，並分析不同檢測參數的影響，提供陣列探頭設計及檢測規劃的參考。超音波B掃描影像僅能呈現反射波抵達接收點的波程時間，無法辨識反射體的空間位置，必須仰賴檢測人員進一步判讀。汽機葉片根槽的結構複雜，幾何反射訊號常會高過裂縫尖端的繞射訊號，以致於無法檢測出根槽裂紋的發生。本研究應用角頻譜合成孔徑成像技術，重建各個反射或繞射訊號源的空間影像，克服葉片根槽結構幾何反射的干擾，改進超音波檢測葉片根槽裂紋位置、走向的能力。

This thesis presents an investigation of the acoustic field induced by linear array piezoelectric transducers using transient finite element analysis. Ultrasonic inspection of defects near roots of turbine blades was numerically studied by assuming that the ultrasound were transmitted simultaneously and received individually by the phase array transducers. The influences of various inspection parameters are also discussed and used as a reference for design of an array transducer and planning of an inspection. Conventional B-scan can only provide time-of-flight information of the reflected/diffracted signals from internal discontinuities in specimens. The spatial relations among those discontinuities can not be directly interpreted from B-scan images. The interpretation results in a need of well-experienced inspectors. However, the echoes reflected from complex dovetails of the blades usually have greater amplitudes than those diffracted signals from the crack tips. The inspectors were often confused with signals of geometric reflection. This research applied an angular-spectrum-based synthetic aperture imaging technique to reconstruct images of the defects and geometric boundaries of the dovetails. The present study provides a promising method to improve the capability of ultrasonic inspection for the defects and their orientations in turbine blade dovetails.