Surface Crack Detection by A.C. Potential Method

采用交流电位下降法开发了裂纹检测系统。在该系统中，应用了交流的趋肤效应。利用该系统进行了表面裂纹测量，得出了以下主要结论：1. 开发了一种新的交流电位裂纹检测系统，该系统有效地利用了交流的集肤效应。该系统由函数发生器、高频功率放大器、锁相放大器、FFT分析仪和微机组成。系统的最大电流和频率分别为5A和20khz。经验证，该系统具有设计性能。2. 表面裂纹测量采用交流电位系统进行。使用的是频率在50赫兹到10千赫之间的交流正弦波。对于较小的裂纹，频率较高的交流电位增加较多，而对于较大的裂纹，频率较低的交流电位增加较多。将交流的集肤效应成功地应用于所开发的电势系统中，有效地检测了表面小裂纹。3. 为了实现高效测量，采用了功率谱范围在50hz ~ 5khz之间的矩形交流波。利用FFT分析仪对检测到的矩形裂纹产生的交流电位进行功率谱分析。表面裂纹越小，高频电位越高；表面裂纹越大，低频电位越高。矩形波交流电位法与正弦波法相比，测试时间缩短了50倍。4. 将矩形交流电势法应用于内部缺陷检测。当缺陷靠近表面时，高频交流电位增加较多，而当缺陷远离表面时，低频交流电位增加较多。

The crack detection system was developed using a.c. potential drop method. In this system, the skin effect of a.c. was applied. The surface crack measurements were carried out using the system and the main conclusions obtained are as follows. 1. The new a.c. potential crack detection system was developed, where the skin effect of a.c. was effectively applied. The system was composed of function generator, high frequency power amplifier, lockin amplifier, FFT analyzer and micro computer. The maximum current and frequency of the system are respectively 5A and 20 KHz. The system was confirmed to have a designed performance. 2. Surface crack measurements were carried out using the a.c. potential system. Used was an a.c. sinusoidal wave with frequencies between 50 Hz to 10 KHz. For the smaller crack, the a.c. potential with higher frequencies was more increased while for the larger crack, that with lower frequencies was specifically increased. The skin effect of a.c. was successfully used in the potential system developed to detect the small surface crack effectively. 3. For the efficient measurement, the rectangular a.c. wave which has the power spectrum range between 50 Hz and 5 KHz was used. The detected rectangular a.c. potential raised by cracks was analyzed by FFT analyzer into the power spectrum. The potential of higher frequencies was more increased by the smaller surface crack while that of lower frequencies was more increased by the larger surface crack. The a.c. potential method using the rectangular wave shortened the testing time 50 times in comparison with the sinusoidal method. 4. The a.c. potential method suing the rectangular a.c. was applied to the inside defect detection. The a.c. potential with higher frequencies was more increased if the defect was near the surface but that with lower frequencies was more increased when the defect was apart from the surface.