- 學位論文
差動式渦電流探頭於缺陷偵測之最佳激發頻率研究
Optimal Excitation Frequency for Defect Detection with Differential Eddy-Current Probe
摘要
渦電流檢測法在工業上的應用極廣，但是由於一般的渦電流機台均操作在較高的磁場激發頻率且受到材料在此時較低的肌膚深度的限制，通常只能對表面及近表面的缺陷作檢測。而本研究使用差動式線圈探頭分別以梯度計及磁量計的方式作比較，在良導體鋁層板的情況之下；其操作頻率最低可以到10 Hz且可檢測的裂縫缺陷深度達12.5 mm。而訊雜比的表現在使用作梯度計的檢測方式時，於60 Hz的環境磁場之下仍然有不錯的效果。並且不論是以梯度計或是磁量計的形式對缺陷作深度判斷的結果，其深度對感應訊號延遲相位角之間的線性變化斜率仍幾乎相同。 另外由於探頭尺寸的關係，差動式探頭在小面積圓孔缺陷輪廓檢測的效果並不佳，僅能就缺陷位置作定位的功能。但由其感應訊號的分析結果仍然可以發現其在缺陷深度判斷上的應用與裂縫缺陷的作法有極為接近的關係，且不受到探頭激磁或檢磁形式的影響。不過由於缺陷外形的差異，在訊號的分析方法上則與裂縫缺陷時略有不同。 而差動式探頭在兩種不同形式的檢測應用時，以本研究所使用之良導體鋁層板的試驗結果；其不同深度之下的最有效激發頻率範圍略有差異，但仍大約在80 Hz至1000 Hz之間，對於以相位延遲角的變化作缺陷深度判斷的能力仍然足夠。
並列摘要
The eddy-current non-destructive evaluation (EC NDE) is a powerful method of flaw detection in industrial applications. The conventional EC NDE instrument is very sensitive to the surface-breaking crack. However, they are not suitable for the detection of deep-lying cracks in good conductors since the skin depth is small at the typically high excitation frequencies from 0.1 MHz to 10 MHz. In this work, we design and fabricate the differential eddy-current probe consisting of differential and absolute induction coils. The eddy-current probe can be operated either with the differential coil as the gradiometer and the absolute coil as the exciter, or the absolute coil as the magnetometer and the differential coil as the exciter. For detecting the crack in layered aluminum sheets, the lowest excitation frequency of the probe is 10 Hz, and the detectable depth of the crack is as deep as 12.5 mm. Notably, the power-line magnetic interference is reduced with the induction coil gradiometer. The signal-to-noise ratio of the gradiometer probe is better that of the magnetometer probe at 60 Hz. The optimal excitation frequency at varies flaw depth is slightly different between the gradiometer and the magnetometer. The range of the optimal frequency is from 80 Hz to 1000 Hz for the layered aluminum sheets with the crack depth from 12.5 mm to 0.5 mm. The signal-to-noise at varies excitation frequencies shows that both the gradiometer and magnetometer probes are suitable for flaw depth evaluation. The linear relationship between phase lag angle and flaw depth for the cylindrical flaw is almost the same as that for the crack-like flaw by using either the gradiometer probe or the magnetometer probe. Moreover, the slope of the linear relation between the phase lag angle and the flaw depth varies with the skin depth in the same way for crack-like flaws and cylindrical flaws. However, both probes are not sensitive to the profile of small cylindrical flaws due to the large probe-size. Nevertheless, the eddy-current magnetic field map of the cylindrical flaw is generally different from that of the crack-like flaw. This implies that further information about the shape of flaw may be extracted from the eddy-current mapping measured with the proposed eddy-current probe.