渦電流檢測是應用於非破壞性檢測的一項主要技術，惟國內的研究者參與渦電流檢測技術的開發並不多，而且在台灣商用的渦電流檢測儀器缺乏又昂貴。因此，為了往後的研究發展，首先我們根據渦電流理論設計開發了一套以電腦控制的脈衝式渦電流檢測儀器。這個脈衝式渦電流檢測儀器可用來檢測金屬中的裂縫和隱藏性腐蝕，也可使用於量測塗層特性與厚度，另外3D顯像功能可顯示待測物受損情形。

在非破壞性檢測的應用中，渦電流探頭的性能是值得重視的。我們探討當探頭線圈與待測體在檢測情形下相互作用時，渦電流探頭以頻域阻抗與時域電流所表現出的靈敏度。理論上，若探頭線圈的材料特性與形狀結構決定後，則測量非瑕疵材料與待測材料之間的線圈阻抗差或電流差即可由電磁理論的計算解得。我們設計了三組不同形狀的探頭來評估其靈敏度，以及二組實際的探頭用來驗證理論預測。我們透過掃頻式渦電流與脈衝式渦電流訊號有系統地研究這些渦電流探頭的靈敏度。

另一項主題是金屬裂縫檢測。本文中我們使用自行發展的脈衝式渦電流檢測儀器來檢測0.5 mm 到9 mm深的金屬表面裂縫，並且比較了使用不同裂縫深度、探頭形式和待測材料所量測的結果。這些實驗結果包含了對ＥＤＭ凹槽與疲勞性裂縫的檢測，並且發現脈衝式渦電流峰值訊號與裂縫深度有極為密切的關聯，很明顯地，脈衝式渦電流是一項可量化測量裂縫深度的方法。

使用脈衝式渦電流技術測定金屬基材上塗層的導電率與厚度是一項新的發展技術，其中金屬基材或塗層具導磁性。在這項主題中我們以銅基材上的鎳塗層(25 – 200 mm)、鎳基材上的銅塗層(25 – 200 mm)和鋼基材上的鋅塗層(50 – 400 mm)來說明這項技術。脈衝式渦電流檢測儀器紀錄所有電流差的測量，並且比對實際測量值與理論值來測定金屬上塗層的導電率與厚度，我們也進一步討論脈衝式渦電流與金屬塗層間的物理現象。

The eddy current is one of the main techniques that used frequently for nondestructive testing applications. Unfortunately, very few domestic researchers in Taiwan are involved in the study of the eddy-current techniques. Moreover, the commercial instruments for the eddy-current testing is not only deficient but also high-priced. In this research, we have designed and built a PC-based pulsed eddy-current (PEC) system. The PEC instrument has the ability to detect defects such as cracks and hidden corrosions in metals. Other applications of the PEC instrument include coating measurement and thickness inspection. In addition, the 3D image can be used to display the situation of defects in specimen.

The characterization of the eddy-current probes is an important issue for nondestructive testing applications. In the thesis, we study the sensitivity of eddy-current probes based on the frequency-domain impedance and the time-domain current when the coil is interacting with a specimen under testing. In theory, once the properties of the material and the geometry of the structure are determined, the electrical impedance change of the coil or the current difference can be calculated from the electromagnetic solutions. We designed three sets of different size probes to evaluate the sensitivity. Other two sets of practical probes were used to verify the theoretical predictions. The sensitivity of eddy current probes is systematically studied through the signals of the swept-frequency eddy current and the pulsed-eddy current techniques.

Another important subject is the crack sizing. In the thesis, the PC-based PEC testing system was also used for crack detection. For the cases studied, we show that the PEC technique can be used to inspect surface cracks with depth ranging from 0.5 mm to 9 mm. A comparison of measurement results for various combinations of crack depths, probes, and materials is presented. Experimental results including measurements from electrical-discharge-machined (EDM) notches and fatigue cracks are demonstrated and compared. We show that there is a strong relationship between the peak PEC signal amplitudes and crack depths. It is evident that the PEC technique is a potential method for quantitative determination of the depth of surface cracks.

A new measurement method has been developed using the pulsed eddy-current technique to determine the thickness or conductivity of metallic coatings on metal substrate for the case when either the coating or substrate is magnetic. We demonstrate this technique for nickel layers (25 – 200 mm) over copper substrates, copper layers (25 – 200 mm) over nickel substrates and zinc layers (50 – 400 mm) over magnetic steel substrates. All current difference measurements were recorded with the PEC instrument. The determination of coating thickness or conductivity of the metals is based on the comparison of the data taken with air-core coils and theoretical calculation using closed-form solutions. The physical phenomena of the PECs interactions with the coated magnetic metals are discussed.

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