The frequency and the phase of the
observed images were 50 Hz and 0 radian,and 1 kHz and p/4 radians,where by the eddy current patterns were maximized,and
50 Hz and p/2 radians,and 1 kHz and 3p/4 radians,where by the
eddy current patterns were minimized,respectively.The tangential magnetic field strengths of both sides of the weld areas were stronger than that of the weld area at the frequency of 50 Hz,the patterns of which were the same as that of the square sample. Additionally,these patterns were observed not only in the single-layer sample,but also by scanning the double-layer sample,and it was possible to detect the weld area in side the double-layer tube. The eddy current patterns,which were observed in the case of the square plate sample,were not observed at the frequency of 50 Hz.This was because the measured areas were around the center of the sample,and the eddy current patterns were weak at the low frequency.