# A Quantitative Detection Method for Surface Cracks on Slab Track Based on Infrared Thermography
## 5 Concluding Remarks  
In view of the actual condition of HSR maintenance within the relevant time window, infrared thermography can be an effective approach to detecting the surface cracks in track slabs. The aim of this paper was to solve the key problem of the quantitative detection of surface cracks based on the infrared thermogram of a slab track. The main contribution of this paper is the development of a thermographic processing method which can extract detailed information on surface cracks in slab tracks. It consists of two steps: feature extraction and quantitative detection. In the process of crack-feature extraction, this paper proposes a novel infrared-image-enhancement algorithm based on the NSCT multi-scale product-thresholding method and a phase\(-\)congruency-based crack-edge-detection method. In the quantitative detection process, the lengths, widths, and areas of surface cracks are obtained based on the calculation of pixel numbers in the crack areas. Finally, the proposed method was verified by a laboratory test, in which a scale track model was used. The main conclusions are as follows:  
1. The proposed infrared-image enhancement algorithm can effectively solve the problem of fuzzy edge details and low contrast in the infrared thermograms of slab tracks and strengthen the crack-edge details in the image;
2. The crack area can be located from the infrared thermogram via the edge-detection algorithm and morphological processing algorithm; the morphological processing method can be used to effectively remove the isolated noise and false edge areas in the image;
3. The scale model of the track slab designed in this study can reflect the surface-temperature field of the original slab-track structure based on the results of the FE simulation of the slab-temperature field;
4. As verified by the laboratory test, the quantitative accuracy of the detection of crack width by the proposed algorithm can be higher than 90%, whilst the accuracy of the algorithm's length and area detection is 85%, which essentially meets the requirements of current track maintenance for HSRs.  
In summary, the proposed method can be used to improve the detection of surface cracks in HSR slab tracks based on thermal imaging, and its quantitative detection ability was verified. However, the actual operational condition of HSRs are much more complex than those in the laboratory. The combined effect of ambient temperature, rainwater, waterproof materials on the slab surface, and other external factors probably influence the accuracy with which surface cracks are detected. Future research may focus on how to further improve the effectiveness of IRT-based detection for slab tracks under the actual condition of HSRs. Furthermore, to improve the application of the proposed method in real HSR slab tracks, a track-inspection trolley equipped with an IR camera should be developed. However the parameters of the trolley, including the distance between the camera and the slab surface, the velocity of the trolley, and the frame rate, need further study to meet the requirements of crack inspection.  
Conceptualization, X.-Y.Y. and Y.-Y.L.; methodology, X.-Y.Y., Y.-Y.L. and Z.-W.L.; modeling, X.-Y.Y. and X.-Z.L.; validation, Z.-W.L. and X.-Z.L.; formal analysis, X.-Y.Y. and Y.-Y.L.; investigation, Z.-W.L. and X.-Z.L.; resources, Y.-Y.L. and X.-Z.L.; data curation, X.-Y.Y. and Z.-W.L.; writing\(-\)original draft preparation, X.-Y.Y. and X.-Z.L.; writing\(-\)review and editing, Y.-Y.L. and Z.-W.L.; visualization, X.-Z.L.; supervision, Y.-Y.L. and Z.-W.L.; project administration, Y.-Y.L.; funding acquisition, Y.-Y.L. and Z.-W.L. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China (grant nos. 52178430 and 52208441), Innovation Initiative Program of Shanghai (grant no. 20 dz1203104), and Natural Science Foundation of Top Talent of SZTU (grant no. GDRC202128).