## 1 Introduction  
Currently, ultrasonic guided wave inspection is an effective long-distance nondestructive inspection technique [1; 2; 3]. Low-frequency ultrasonic guided waves are widely used for long-distance ultrasonic guided wave inspection [4]. The ultrasonic pulse width is wider at low frequency than at high frequency. Due to the influence of the pulse dispersion for long-distance transmissions, the reflected signal pulse is wider than in short-distance transmission [5]. The superposition state occurs easily during actual waveform measurements when the flaw is close to the terminal. In addition, the signal trailing interference aggravates the superposition state and the flaw reflected pulse is more difficult to distinguish. In the low-frequency ultrasonic guided wave flaw inspection experiments for a steel square bar as described in this study, we find that the reflected pulse wave of the flaw 'disappears' before the first terminal reflected pulse wave (in the conventional monitoring interval) when the flaw is near the reflection terminal; this results in a loss of the flaw's location information, which is referred to as the non-detection zone in this paper. Therefore, the conventional ultrasonic echo method is invalid under this condition. The literature does not provide clear details on low-frequency ultrasonic guided wave excitation pulse conditions and no studies have focused on providing a solution for the problem of the non-detection zone mentioned in this study. However, the non-detection zone cannot be ignored in studies on nondestructive testing (NDT). In this study, we designed experiments and simulations for the inspection of a square steel bar with the focus on the above-mentioned problem.  
Research on ultrasonic reflected pulse waves should be concerned with solving the problem of the non-detection zone to facilitate the actual detection of flaws. To determine the ultrasonic propagation characteristics caused by different types of flaws or structural damages, the reflected pulse wave is used to analyze the reflection and transmission. In reference [5] the flaws of different opening angles were analyzed and in reference [6], the first reflected pulses in a 20-45 Hz range for four sizes and three orientations of transverse rail head flaws were investigated. Moreover, the reflected pulse wave was used to locate the flaws. For example, the flaws in a steel rail flange were detected in reference [7] and the orientation and position of small flaws in pipes were detected in reference [8]. In previous studies, researchers have provided considerable advances in long-distance non-destructive testing (NDT). However, in most studies, the flaw-reflected pulse before the first terminal reflected pulse wave was used to locate and identify the flaw, a method we refer to a \"monitoring Interval I\" hereafter. Different from the methods used in existing studies, the analysis time domain of the reflected pulse waves is expanded to the \"monitoring Interval II\" in the following sections. In order to resolve the non-detection zone problem in long-distance low-frequency ultrasonic guided wave detection, a feasible solution to locate the flaw is proposed in this study.  
To describe the non-detection zone clearly, the cause of the existence of the non-detection zone is clarified by using a simulation with an equivalent model of low-frequency ultrasonic guided waves for the inspection of a square steel bar in Section 2.The experimental platform and the results are described in Section 3. The characteristics of the non-detection zone are analyzed and discussed. Based on the characteristics of the reflected pulse waveform in the non-detection zone, a solution is proposed. Finally, the experimental verification is described in Section 4.