臺灣博碩士論文加值系統

超音波在非破壞性檢測中有著非常重要的用途，因為超音波能夠穿透大部分的物體，故在瑕疵檢測上有著廣泛的應用。由於超音波在使用上受到許多因素的干擾，如幾何介面、晶粒介面、波式轉換等雜訊及餘音等，故若直接以所接收到之超音波信號來做判讀的依據，困難度相當高，甚至導致誤判。對於此情況，利用信號處理與小波轉換等技術來消除雜訊、以偵測並凸顯瑕疵信號。因為探頭音束在遠場中的發散現象，以致造成瑕疵在二維及三維影像的變形，利用合成孔徑聚焦技術(SAFT)能夠以類似多感測器合成的方法減少瑕疵影像的變形，讓超音波所偵測到的瑕疵信號更為清晰，並以此來偵測出瑕疵位置。對於瑕疵尺寸的評估，是利用各量測位置與瑕疵間距離上的不同作為依據，再以比例轉換的方法來分辨瑕疵的大小。此外我們也對單一圓孔，平行雙圓孔，裂縫等瑕疵類型做檢測，模擬結果顯示我們的方法可得到良好的成效。

Ultrasound has many important applications in nondestructive testing. Since ultrasonic signal can penetrate many materials, it is widely used for detection of flaws. The evaluations of welding are the main purpose of this paper. The use of ultrasound is limited by several interferences, such as geometric inter-face, coarse grain, mode conversion and ringing. It will be practically difficult to perform evaluation if the originally received ultrasonic signal is used directly as false judgment may be obtained. This paper will use the methods of signal processing and wavelet transform to reject noisy signal and detect the position and relative information of flaws. Owing to effect that the sound beam will diverge in far field, the flaw information will be distorted in 2-D or 3-D ultrasonic images. Synthetic aperture focusing technique (SAFT) uses the methods similar to sensor fusion to reduce the distortion and increase the accuracy for evaluation. The appraisal of the size of the flaw utilizes the distance between the flaw and transducer. The size of flaw can then be measured according to a proportionally transfer function. We also performed measurement on various types of side-drilled holes and cracks. Simulations show that our method provides very effective results.

第一章 緒論…………………………………………………1
1.1 研究動機………………………………………………1
1.2 研究背景………………………………………………2
1.3 研究目的………………………………………………3
1.4 論文架構………………………………………………4
第二章 超音波信號與小波轉換……………………………5
2.1 超音波檢測原理………………………………………5
2.2 小波轉換………………………………………………10
2.3 應用小波轉換抑制信號雜訊…………………………14
第三章 SAFT於瑕疵檢測與大小評估方法…………………19
3.1 SAFT法則………………………………………………19
3.2應用SAFT於超音波影像重建……………………………23
3.3 瑕疵大小評估之比例轉換法…………………………25
第四章 實驗結果與分析……………………………………31
4.1 實驗設備與檢測方式……………………………………31
4.2 圓孔瑕疵位置偵測………………………………………32
4.3 單一圓孔瑕疵大小檢測…………………………………37
4.4 平行雙圓孔瑕疵檢測……………………………………46
4.5裂縫瑕疵長度檢測………………………………………57
第五章 結論與未來研究方向………………………………61
5.1 結論………………………………………………………61
5.2 未來研究方向……………………………………………62
參考文獻……………………………………………………64

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