超音波檢測金屬缺陷之提升方法

Abstract

非破壞性檢測技術 (Nondestructive testing, NDT) 被用來評估組件壽命，也幫助工廠提高產品質量，已廣泛應用於運輸、航空、管道等行業中。超音波陣列成像能用來探測受測物體內部之缺陷，因其具有靈活及快速使用等特性，已成為非破壞性檢測的主要方法，其中又以總聚焦法 (Total focusing method, TFM) 為常用方法之一。然而此方法之成像結果仍受探測方向及缺陷取向所影響，當探測方向與缺陷法向量夾角大於45度時，探頭無法接收到來自缺陷輪廓的回聲訊號，會有無法探測的限制。此外，當探測方向與缺陷法向量夾角小於45度時，聲波可能會在探頭與缺陷間來回反射，會導致多重反射假影 (Reverberation artifact) 產生，而這些現象都會導致錯誤判斷。在本論文中，我們探討上述現象的發生條件，並利用前人提出之多角度全圖聚焦法 (multi-mode TFM) 配合時域窗遮罩 (Time-domain window mask) 以濾除來自背板的強烈反射訊號，使探測方向與缺陷法向量夾角大於45度的缺陷仍可以被探測。另外，本論文提出一種創新的方法，利用符號同調因子 (Sign coherence factor)，來判別由於聲波在探頭與缺陷間來回反射所造成的多重反射假影，以更簡單地辨識出缺陷與多重反射假影。為了使上述兩種成像方法能夠即時地應用在超音波系統上，本文在可程式的超音波影像系統上利用 NVIDIA 的統一計算架構 (Compute unified device architecture, CUDA) 實踐軟體波束形成，其實際成像速率可分別達到每秒17張及每秒24張，已證明可達到即時成像。此外，本論文使用模擬仿體及實際金屬仿體實驗取得的超音波信號來驗證上述方法的可行性。

Nondestructive testing is performed to evaluate component life and quality. It has been widely used in transportation, aviation, and other industries. Ultrasound array imaging has the ability to detect defects under the surface of the object. Because of its flexibility and usability, it has become one of the main methods of nondestructive testing. Among all the array imaging methods, the total focusing method has been widely used. However, the performance of this method is limited by the detection direction and defect orientation. When the angle between the detection direction and the normal vector of the defect is greater than 45 degrees, the ultrasound probe cannot receive the echo signal from the defect contour. In addition, when the angle between the detection direction and the normal vector of the defect is less than 45 degrees, sound waves may be reflected back and forth between the probe and the defect, which may cause reverberation artifacts, and these phenomena lead to misjudgment. In this study, we propose the use of the multi-angle total focusing method, which had been reported in the literature with the time-domain window to resolve the first issue. For the second issue, we propose a new method that uses the sign coherence factor to distinguish reverberation artifacts. To implement the above two methods in real-time, we perform software beamforming on a programmable ultrasound imaging system. With the current hardware, the imaging frame rate can reach 17 frames per second and 24 frames per second, respectively. Both simulations and metal phantom experiments were done to verify the feasibility of the above methods.