## 4 Results  
Figure 9 shows a typical pattern of the output signal measured in the secondary coil in the case of damaged external tendon. As the AC is delivered to the primary coil, the output signal of the secondary coil has a form similar to the input signal delivered to the primary coil with the same frequency and is symmetric with reference to the horizontal axis. In the case where the DC is delivered to the primary coil instead of an AC and the moving speed of the device is irregular, undesired disturbance would occur in the induced voltage on the secondary coil. In this research, a sufficiently stable magnetic flux was generated by means of AC that inverted the direction of the power source as much as its frequency even though the operator was moving the device without specific speed control. Since the device was moved slowly from the left end to the right end of the external tendon, a very large number of positive and negative peaks of the secondary coil output signal was recorded according to the distance travelled during the total time of motion. The measured secondary coil output takes thus the form of the compressed sinusoidal wave along the horizontal axis shown in Figure 9. Only the positive peaks were picked from the measured signal for the convenience of the analysis. Figure 8: Equipment used to deliver AC to the primary coil of the device and measure the output of the secondary coil. Figure 7: View of external PSC tendon installed for measurement test of sectional loss by the Total Flux Leakage (TFL) method. The peak curves of the voltage induced in the secondary coil in accordance with the change in length of one-strand damage of the external tendon are presented in Figure 10. The lowest point in the curves corresponds the center of the damage length. The centerline of the damage length is shifted slightly to the right due to the fixed left end of the damage as shown in Figure 4. It is observed that this phenomenon is reflected on the measured peak curves. In the same manner, the peak curves of the voltage induced in the secondary coil in accordance with the change in length of two- and three-strand-damage of the external tendon are presented in Figures 11 and 12, respectively. In addition, the secondary voltage reduction ratios of the one to three-strands damage with respect to the damage length are presented in Figure 13. The damage detectable level in Figure 13 was visually suggested as the peak curve for the 1-strand damage with 0 cm damage length in Figure 10 is difficult to be distinguished and can be hidden if the signal to noise ratio decreases. The decrement ratio in Figure 13 is the voltage change due to the damage to the voltage value just before the decrement. Figure 10: Secondary coil output peak induced by 1-strand damage according to damage length. Figure 9: Induced voltage and its peaks in the secondary coil. The location of the sectional loss can be estimated simply and intuitively. Moreover, the change in the damage length is also well reflected in Figures 10-12. The reduction of the magnitude of the voltage induced in the secondary coil caused by the magnetic flux leaked from the damaged section that is not permeating the secondary coil shown in Figure 3 can be clearly distinguished in the graphs. In addition, it appears that deeper and longer damage introduces stronger magnetic flux leakage as well as increases the magnitude of the decrement of the voltage induced in the secondary coil as shown in Figure 13. The ratio of decrement in induced voltage of the secondary coil for all cases except for the 1-strand damage with a length of 0 cm case were above the designated damage detectable level. However, as such the case is neglectable as the external tendon is always in tension and the damage length would not be zero when cut. Figure 11: Secondary coil output peak induced by 2-strand damage according to damage length. Figure 8:0 Secondary coil output peak induced by 3-strand damage according to damage length.