# Review on Rail Damage Detection Technologies for High-Speed Trains
## 3 Non-Destructive Testing Methods for High-Speed Train Rails
### New Eddy Current Testing Methods  
At present, many foreign scholars use eddy current pulse thermal imaging technology to detect metal cracks, and their research results also prove the feasibility of eddy current pulse thermal imaging non-destructive testing technology for metal crack detection [47; 48]. [PERSON] summarized the defect detection techniques currently used for rolling contact fatigue cracks in railway rails and also stated that pulse eddy current thermal imaging non-destructive testing technology will be widely applied to the detection of surface and subsurface defects in rails [6]. [PERSON] et al. used pulsed eddy currents to detect surface and subsurface defects in steel rails and pointed out that processing time-domain and frequency  
Figure 8: Overall structure of terahertz time-domain spectroscopy system. domain signals can improve the efficiency of defect detection in the detection system [49]. [PERSON] conducted a study on the thermal response of surface cracks in magnetic materials and found that crack depth has an impact on the thermal response. Simulation analysis was conducted, and a formula for calculating crack depth was ultimately derived [50]. [PERSON] et al. found that eddy current pulse detection technology can identify two different types of defects at the same location and verified it using simulation analysis [51]. [PERSON] et al. proposed that boundary effects affect the detection efficiency of eddy current pulse detection and proposed using normalized input signals to improve detection efficiency. The results showed that this method can improve the detection efficiency of defect areas in specimens [49]. [PERSON] et al. used an eddy current pulse thermal imaging detection method to detect small fatigue cracks on metal surfaces and designed a non-destructive testing system [52]. With the development of technology, higher requirements have been put forward for the capacity of modern transportation, and at the same time, the load-bearing capacity of railway tracks has also increased. Under high-speed and high-load conditions, the stress on the railway track is also greater, and so is the damage to the railway track contact surface caused by the contact between the wheel and the rail. The main type of defect is the contact fatigue crack on the outer side of the railway track surface, also known as the rail head crack. In response to such defects in railway tracks, BAM [29] has collaborated with German [53] railways and other partners to develop a new eddy current measurement system that can perform real-time detection in the rail surface to determine the degree of rail damage and perform effective maintenance (Figure 9). The new eddy current testing car can achieve real-time recording, real-time data processing, and the analysis of surface defects on rails. It can simultaneously collect and analyze eddy current signals from eight probes in real time. Based on the real-time-collected rail surface defect data, it can select, convert, and provide real-time feedback on the strongest eddy current signal [54], including the depth, location, and remaining number of eddy current head inspections of rail surface defects, as well as the continuous availability of its data (Figure 10). [PERSON] et al. applied eddy current pulse thermal imaging non-destructive testing technology to defect detection in composite materials, and the experimental results proved that this testing technology provides a new idea for defect detection in non-conductive materials [55]. Eddy current pulse thermal imaging non-destructive testing technology is a relatively efficient and fast active non-destructive testing technology. Eddy current pulse thermal imaging non-destructive testing technology uses the principle of electromagnetic  
Figure 9: New eddy current detection system. induction to induce eddy currents on the surface of the test piece and then uses the principle of induction heating to achieve temperature differences between the defective and non-defective areas on the surface or subsurface of the test piece.