difference between the normal magnetic field strengths Bn,measured at each point,and B 0,measured at the left side of the lines. In the cross-section diagram of the 1 kHz type,the in tensity of the normal magnetic field strengths of the double-layer sample was more than twice the strength of the single layer,which was the same as the case of the tangential magnetic field,and that was due to the eddy current flows being generated not only in the inner layer,but also the outer layer.In the cross-section diagram of the 50 Hz type,the peak amplitude of the normal magnetic field strength of the double-layer sample was slightly weaker than that
of the single-layer sample at the weld area,which was affected by the outer layer.The DB curve showed no apparent dependence on the frequency,and these phenomena indicate the effect of the
permeability change that is shown and discussed in Fig. 5. The peak amplitude of DB of the double-layer sample was slightly weaker than that of the single-layer sample,which was affected
by the outer layer.