4.1.4 Guided wave testing (GWT)

GWT techniques have been widely investigated over the past decades because of the potential for long-range interrogation and detecting vertical-transverse defects under shelling and weld defects [62, 63]. They are ideal in SHM applications that can benefit from built-in transduction, moderately large inspection ranges and high sensitivity to small flaws. In rail applications, since ultrasonic guided wave can propagate through the discontinuous defects on rail surface, the screening effect of lateral cracks distributed underneath produced by surface detachment can be minimised.

Rose et al. [63] developed a GWT inspection system with non-contact aircoupled and EMATs to transmit and receive guided waves for the detection of transverse defects under shelling. Wilcox et al. [64] developed a GWT system with a dry-coupled piezoelectric transducer array to detect smooth transverse-vertical defects and alumino-thermic welds, but this system requires interruption of the operation of trains. Lanza et al. [54] developed a GWT system using a pulsed laser

Figure 4. Rail cracks detection using EMAT [59].

to generate ultrasonic guided waves and air-coupled transducers to sense the guided waves for the detection of vertical cracks hidden below horizontal cracks. Park et al. [65] proposed a built-in active sensing system consisting of two piezoelectric patches in conjunction with both impedance and guided wave propagation methods for rail defect detection. Marine Technology Association of South Africa and Council of Scientific and Industrial Research [66] jointly developed solar power GWT detection system (Figure 5). The coverage of the single system for rail defect detection is up to 2 km. Imperial College London and Guided Ultrasonics Ltd. cooperatively developed a G-shaped scanning ultrasonic rail track detection device, which can inspect vertically distributed defects and alumino-thermic weld joint [67], as shown in Figure 6. It can effectively inspect 18-mm-deep defects under rail crossing nose.

### 4.1.5 Acoustic emission testing (AET)

Different from common ultrasonic inspection, acoustic emission (AE) is instantaneous elastic waves by quick release of localised energy in solid materials under external applied loads. AE events can be captured by the piezoelectric sensors, generated elastic waves along all directions. Many sensors can be utilised to document arrival time of the signals and the variation of frequency during the crack initiation process. Hence, the nature of cracks can be determined. Through experimental study, AE has been proven a feasible solution in defecting rail detection, especially in rotating machinery [68]. A simplified analytical model, which separates defects caused by AE activities from background noise, was proposed by Thakkar et al. [48]. They also investigated the physical interaction between AE and axial load, speed, as well as traction through experiment. It is found that AE signal can be used for analysing the defects on the surface of the rail under normal operating speed.

The application of AET on rail defect detection is rarely reported. Previous research [69] shows that the benefit of AET may be limited due to imperfection of materials, which can produce different nature of signal source. Besides, the installation of the sensor may also affect the AE signal generation. It may also be affected by wheel and track defect for any misalignment [48]. Another concern of AET is signal processing for those AE waves that have similar amplitude with that of background noise produced by the rolling wheel [18]. Advanced data learning and

Figure 5. Monitoring system on freight line in South Africa [66].

Contemporary Inspection and Monitoring for High-Speed Rail System DOI: http://dx.doi.org/10.5772/intechopen.81159

Figure 6. G-type scan guided wave in inspection device in UK [67].
