## 5 Comparison of Conventional Testing Method and TRM  
The dispersive behaviors of different modes are represented by dispersion curves like the ones shown in Figure 4. Mode \(L(0,1)\), the only one which can be generated and received by the testing system in the frequency range between 0-200 kHz, is preferred as a candidate for damage detection because it is unnoticeably dispersed and apparently identified without disturbance from other modes. The point of minimum dispersion in mode \(L(0,2)\) at 700 kHz is at the point of maximum group velocity, which leads to the wave packet of mode \(L(0,2)\) easily separated from others to make accurate interpretation. For the foregoing reasons, modes \(L(0,1)\) and \(L(0,2)\) are generated at the central frequencies of 150 kHz and 700 kHz respectively under an excitation of 8 V, and then the normalized amplitudes of their defect echoes are applied to evaluate the damages as the conventional testing method in Ref. [5; 6; 27]. The results towards different depths are shown in Figure 10.  
Regarding the results of mode \(L(0,1)\) shown in Figure 10a, the amplitudes corresponding to the aggravating damages rise with immensely different rates. Distinct increases occur at depths of 3 and 5 mm, but subtle changes occur at other points. It is reasonably deduced that only when the damage accumulates to some extent can feedback be given by mode \(L(0,1)\), but the sensitivity is not sufficiently high to identify tiny variations for its excessively large wavelength at 150 kHz. With regard to the results of mode \(L(0,2)\) shown in Figure 10b, the amplitudes can prop  
Figure 10: Damage assessment. (**a**) Tendency of normalized amplitudes of defect echoes of mode \(L(0,1)\) versus damage size; (**b**) Tendency of normalized amplitudes of defect echoes of mode \(L(0,2)\) versus damage size. Figure 9: Damage assessment: tendency of normalized amplitudes of reconstructed input signal versus damage size. increase within the depth of 2 mm. When the depth goes up to 3 mm, the amplitudes turn around to decrease unexpectedly, and then bounce up at 5 mm. The wavelength of mode \(L(0,2)\) at 700 kHz is much shorter; other than that, the NAPFD which is defined to describe the spatial distribution and transmitting direction of the wave energy has a significant surface distribution with respect to mode \(L(0,2)\) at 700 kHz, as illustrated in Figure 11a. Both the characteristics demonstrate that mode \(L(0,2)\) is sensitive to superficial damage. However, with the continuous development of pitting, the interaction of the guided waves and the defect becomes increasingly complex, therefore mode \(L(0,2)\) is no longer the only component of the defect echoes. Other modes, which should have been suppressed, are aroused as well, and share the energy with mode \(L(0,2)\). On the other hand, Figure 11a shows that the NAPFD of mode \(L(0,2)\) has a negligible component at the core area of the wire, which means that the energy increment contributing to the defect echo is bound to be minimal. As can be seen from Figure 12, when the damage depths are within 2 mm, the defect echoes of mode \(L(0,2)\) are evident while the other modes are suppressed well. Due to the increasingly remarkable multimode nature of guided waves at 3 mm depth, the amplitudes of defect echoes can not keep growing any more. When pitting reaches 4 mm, the defect echo of mode \(L(0,2)\) is hardly distinguished from the others, for it is no longer a dominant signal. To sum up, the amplitude of defect echo of mode \(L(0,2)\) at 700 kHz can not be utilized to indicate the entire historical development of corrosion damage. Refer to the damage qualification using TRM presented in Section 4.3, mode \(L(0,3)\) is simultaneously generated with a large amplitude approximately equal to mode \(L(0,2)\) to indicate different damages universally. Notably, the NAPFD of mode \(L(0,3)\), which concentrates in the core area of a wire, is strongly complementary with that of mode \(L(0,2)\) at 790 kHz, as illustrated in Figure 11b.