為確保結構體安全性防止意外發生，一個可靠、非破壞性的缺陷檢測技術非常重要。利用非破壞檢測技術檢驗結構有無缺陷存在之方法有很多種，其中直流電位降(direct current-potential drop; dc-pd)技術即為優良之非破壞缺陷檢測方法之一。其原理係利用輸入導電結構直流電，產生穩定電位場後，藉由檢知電位場的分佈變化以估測缺陷的存在。因其不需要製作試片或昂貴電子裝備與技術，方法簡單，且可以在導電結構上進行，值得吾人有系統深入探討。
本論文首先以電位場理論為基礎，建立直流電位降有限單元分析模型。為剔除可能影響缺陷檢測之相關參數，如電流大小、輸入方式與位置及結構形狀等，本論文定義了「電位缺陷影響因子 」(即含缺陷結構與完整結構電位場之比值)以作為結構缺陷檢測之依據，藉由繪製電位缺陷影響因子等位線，可將結構之缺陷清晰顯現。為探討檢測多裂縫、缺陷之可行性，本論文亦將Chen等(1994)所發展之有限單元互疊法( finite element alternating method)延伸應用於含多裂縫平板電位場之計算。為印證本論文建立之直流電位降缺陷檢出程序及判定準則，本論文並針對多種單一或多數個不同缺陷如圓孔、方形孔、開槽及裂縫等案例進行探討。有限單元/有限單元互疊法數值分析模擬檢測結果並與實驗相驗證，獲得良好之成果，可成功的將結構缺陷之形狀、位置、分佈及數目檢測出來。

由於直流電位降技術係採取以探針直接與結構接觸的點量測方式，故在使用上有其限制。此外，當電流通入導電結構後，電阻會產生焦耳熱，亦常會造成結構組件之熱破壞。為克服上述之限制與熱破壞，如何利用紅外線進行非接觸式溫度量測，並建立電溫 (electro-thermal; ET) 測技術，亦為本論文研究重點。

與直流電位降技術類同，為剔除如電流大小、輸入位置與方式，及結構幾何形狀等參數影響，本論文亦提出了「溫度缺陷影響因子 」觀念(即含缺陷結構與完整結構昇溫場比值)以作為電溫測技術檢測結構缺陷之依據，藉由溫度缺陷影響因子等位線之繪製，而將結構缺陷檢測出來。為驗證本論文開發之電溫測技術缺陷檢測性能，除了建立電溫場有限單元分析模型外，並應用紅外線熱像儀(Infrared thermography)進行溫度場量測。檢測案例包括薄四方扁平(thin quad flat package; TQFP)電子封裝爆米花(pop-corn)及脫層破壞以及金屬結構裂縫等。前者結果並與x光檢驗結果互相比較，顯示本論文所建立之電溫技術確具潛在工程實用價值。

本論文所發展缺陷判定準則(即 及 )其值與導電結構的材質等參數有關，若要推廣運用於工業檢驗，須有系統的材料歸類、缺陷影響因子分析並列出判定標準表以供參考使用。另上述之直流電位降與電溫測技術檢測案例，除了薄四方扁平電子封裝外，均僅侷限於穿透缺陷或裂縫，唯實際工程結構或因雜質、異物、氧化、腐蝕等常產生部份貫穿缺陷或裂縫，或因結構微奈米化，檢測之解析度要求更高，凡此均值得吾人未來進一步探究。

To ensure the safety of structure and prevent it from catastrophic failure, it is highly desirable to seek a reliable non-destructive inspection (NDI) for detecting the defects in structure. There are numerous NDI methods for detecting the cracks or defects, among them direct current potential drop (dc-pd) technique is useful and has been proved effective. By applying direct current to the electrical conductive structure and investigating the variation of potential distribution disturbed by the discontinuity of the structure, the defects in the structure can be identified. Because of its robust and direct application on the electrical conductive structure, test can be done without the making of specimen or using expensive electronic equipment and techniques, and it also can be used in real time monitoring of engineering structure. Thus it is worth systematically exploring further this technique.
This paper has firstly established the finite element analysis model for the electrical potential drop based on the electrical field theory. Then a factor, called “defect influence factor ”, has also been devised to identify the defects in an attempt to filter out the interferences caused by the parameters such as the quantity of current, input types and places of current supply, and the geometry of structure except for the defects. The factor is the ratio of the electrical potential drop on a defective structure to that of an identical one without defects. By depicting the contours of the defect influence factor , the defects of the structure can be clearly shown. For dealing with the multiple defects or cracks detection, the work has extended the finite element alternating method developed by Chen et al. (1994) originally for heat conduction analysis of multiple cracks to dc-pd to calculate the electrical potential distribution. Several samples with single or multiple defects (such as circular holes, square holes, and slots) and cracks were undergone the experiment. With the set of defect detection procedure and defect identification criteria, the shape, size, number and location of defects have been accurately identified by depicting the contours of the defect influence factor. Good agreement between the computed results of finite element method/ finite element alternating method and experimental data shows the merits of this technique.

Although dc-pd technique has advantages, there are disadvantageous constraints in it. It will cause measuring difficulties due to point-to-point contact measuring using two probes. In addition, the internal resistance of structure results in joules heating which may in turn make damage to the structure parts after the currents flow through the electrical conductive structure for a while. In order to overcome these measuring constraints and thermal damages, finding to use infrared to implement non-contact thermal measurement, and establish electro-thermal (ET) technique for defect detection is also an important part of this work.

Quite similar to the considerations taken in dc-pd technique, a factor called “temperature defect influence factor ” is devised for the detection of structural defects. The factor , defined as the ratio of temperature rise of a defected structure to that of a perfect one, has the effect of filtering out the possible interferences on the detection coming from parameters such as the quantity of current, input types and places of current and the geometry of structure except for defects. By depicting the contours of the temperature defect influence factor, the defects or cracks in the structure can be clearly identified. To verify the effectiveness of the ET technique newly developed in this work, in addition to the establishment of ET finite element analysis model, an infrared thermography is used to measure temperature distribution. Samples used for detection test include two thin quad flat packages (TQFP) with pop corn and debonding defects and one stainless plate with crack. The results from the former experiments are then compared with x-ray data. The comparison shows positive usefulness of the ET technique developed in this work.

The defect identification criteria ( and ) of this work are variable according to the structural material. When extension this present technique to apply in the industrial product inspection, a vast calculation of the or and classified the variation with different material for establishing a table of crack test specification will be necessary. The detection of defects with dc-pd and ET technique all deals with through-wall defects or cracks, except that in TQFP specimen. But in reality, the structural defects are present in partial through-wall cracks caused by delamination, impurity, and oxidation and corrosion. Also taken into consideration is the coming of micro, nano level structures design of products. These all require high resolution of defect detection which lies ahead for future work.

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