## 3 Experimental Section  
In this section, the experimental platform and the experimental results are presented and the occurrence of the non-detection zone in square steel inspections is briefly described.  
As shown in Figure 4a, a 30# (30 mm \(\times\) 30 mm) square steel bar with a length of 6000 mm is used in this study; the length is sufficient to weaken the effect induced by signal trailing and facilitates the signal identification. These customized ultrasonic longitudinal-wave probes [21; 22] with a central frequency of 30 kHz are installed using the dual-probe echo method. The signals are acquired and displayed using an oscilloscope (Tektronix MD04104) and the waves are recorded and the data are captured using a wave recorder (HIOKI MR8875-30), as shown in Figure 4b.  
The Hamming pulse signal (10 cycles at the center frequency of \(f_{0}=30\left[\mathrm{kHz}\right]\)) is set by the signal source, which is loaded to the ultrasonic probe via a power amplifier. Transverse flaws with a width of 1 mm and a depth of 0-20 mm (\(D0\)) are created at a displacement \(L\) along the \(x\)-direction from the incident point. Because the thickness of the steel square bar is 30 mm and the length of the actual ultrasonic probe is 10 mm, a point (130.30) is selected as the measuring point. The excitation voltage amplitude is adjusted to acquire five reflected pulses and the waveforms of the reflected pulses are monitored in the time domain. Figure 4c shows the overall layout of the experimental platform.  
In a dual-probe echo long-distance inspection with the customized longitudinal-wave probe used in the experiment, the acquired reflected pulse wave of the transverse flaw occurs before reaching the first terminal; subsequently, the flaw can be located based on the modal wave velocity. However, both the experimental and the simulation results of the square steel inspection indicate that no reflected pulse is identified before the first terminal reflection pulse wave when the flaw is at a certain distance (\(L\in\) (5300 mm, 6000 mm) from the excitation point. The experimental result at \(L\) = 5500 mm is used as an example (Figure 5). In the conventional monitoring Interval I, the reflected pulse wave with the flaw cannot be identified. According to the experimental data, even when the depth of the flaw (\(D0\)) reaches approximately 20 mm (i.e., the flaw represents almost 75% of the 30-mm thickness), no echo information  
Figure 4: Establishment of the experimental platform (**a**) square steel bar with a length of 6000 mm; (**b**) picture of experimental instruments; (**c**) the diagram of the establishment of experimental platform.  
is detected in the conventional monitoring Interval I. In actual measurements, when the ultrasonic pulse excitation ends, there are attenuation oscillations in the signal, namely the trailing phenomenon of the pulse wave. Many factors affect signal trailing, such as the manufacturing technique of the probes, the capacitance shock effect of the excitation circuit, and the sound absorption effect of the substrate. The trailing phenomenon is common in long-distance inspections with low-frequency ultrasonic guided waves [23; 24]. In addition, the signal trailing compounds the difficulty of identifying the flaw-reflected pulse wave in the non-detection zone.  
At \(L=5500\) mm, the \(S0\) mode and \(A0\) mode flaw-reflected pulse waves are superposed by the first terminal-reflected pulse wave, various types of reflection pulse waves in the monitoring time interval II, and the pulse signal trailing. Therefore, in a conventional inspection, the non-detection zone can be easily determined in the actual experiment. A similar experimental phenomenon can also occur in the ultrasonic flaw detection of long rod-like structures and cannot be ignored for NDT.  
Figure 5: The reflected pulse waves with different values of \(D0\) at 5500 mm.