## 6 Conclusions  
This paper proposes a sweeping Eddy Current Pulse Thermal-Tomography system (ECPTT) and verifies the feasibility of sweeping frequency heating for detecting multi-layer defects. This study compares the detection effect of five coils and selects the center trench plane coil as performing the best. In this paper, skewness is used to detect and locate defects. Furthermore, TSR values are used to quantify the temperature rise of defects, enhancing the linear relationship between different sets of data and facilitating comparative analyses. The distinctions of types of multi-layer defects are based on the variation in TSR values with frequency.  
The differentiation can be summarized as follows: the TSR value of shallow defects increases significantly with increasing excitation frequency, while deeper defects show no significant change. Single-layer defects display no mutation in the TSR value with varying excitation frequency, while double-layer defects and triple-layer defects show a mutation in the TSR value when the excitation frequency increases to a certain value. In this experiment, the double-layer defect has a mutation in maximum temperature at an excitation frequency of 16 kHz, while the triple-layer defect has a mutation in maximum temperature at an excitation frequency of 25 kHz.  
Most previous studies have used a single excitation frequency to heat the specimen for detection. However, the heating depth of a single frequency is limited by the skin effect, impacting the detection and reconstruction of defects. This paper uses multiple excitation frequencies to heat the defects, overcoming the heating depth limitation and facilitating the acquisition of defect information at various depths, particularly for identifying multi-layer defects. The number of layers and depths of rolling contact fatigue (RCF) multi-layer defects can be distinguished through the transient thermal response at scanning frequencies.  
Future work will focus on the tomographic reconstruction of artificial RCF multi-layer defects and the application of the proposed system to the detection and reconstruction of natural multi-layer RCF defects.  
The authors confirm their contributions to the paper as follows: conceptualization and supervision: X.C.; methodology: S.Z. and H.Z.; design and implementation: Y.Z. ([PERSON]) and H.Z.; experiment: H.Z., S.Z., Y.L. and Y.Z. ([PERSON]); software, data curation and result analysis: H.Z. and Y.Z. ([PERSON]); writing--draft manuscript preparation: H.Z., S.Z., Y.L. and Y.Z. ([PERSON]); writing--review and editing: X.C. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Natural Science Foundation of China, grant number 6203312, and the Natural Science Foundation of Sichaun Province of China, grant number 2024 NSFSC1484.  
Not applicable.  
Data Availability Statement: Data are contained within the article.  
We would like to thank all of the individuals who participated in this study and in particular [PERSON] for their consultation and recommendations regarding the codification process of the present study.  
The authors declare no conflicts of interest.