# Evaluation of Bol Corrosion Degree Based on Non-Destructive Testing and Neural Network
## 6 Conclusions  
This study investigated the recognition of accelerated electrochemical corrosion defects in anchor bolts using convolutional neural networks through testing on anchor specimens with different corrosion periods. The conclusions are as follows:  
(1) Guided waves propagate in multiple modes within the anchor bolts, and the energy of different modes propagates at different speeds. This allows guided waves to be influenced by different modes simultaneously. In the study of the 20 mm diameter bolt, using a 30 KHz signal excitation can ensure that only one modal \(L(0,1)\) of longitudinal wave exists, which facilitates the analysis of the signal.  
(2) The relationship between the ultrasonic guided wave waveform and the degree of corrosion in anchor bolts at different corrosion levels was studied. The propagation speed and reflection intensity of ultrasonic guided waves in the corroded region vary. The strength (i.e., high amplitude) or weakness (low amplitude) of the guided wave reflection signal is influenced by many factors, such as the shape, size, and roughness of the object surface. The corrosion degree of anchor bolt cannot be quantified solely based on the amplitude of the guided wave.  
(3) A corrosion evaluation method for anchor bolts based on non-destructive testing and deep learning was proposed. This method utilizes a multi-scale convolutional neural network model to analyze the ultrasonic guided wave signals of corroded anchor bolts and quickly and accurately assess the degree of corrosion. On a dataset containing 608 waveform groups, the accuracy rate for identifying the corrosion degree of anchor bolts reached 99.4%, outperforming other comparative methods.  
However, this study still has certain limitations. When analyzing the reflected ultrasonic guided waves, the uncertainty of the actual corrosion form should be taken into account. The signals of guided waves may be influenced by various factors, such as the diameter of the steel bar and the integrity of the anchoring agent. Further research is needed to incorporate the uncertainty of other influencing factors. Additionally, the corrosion form and degree in rock masses may differ from those in electrochemical corrosion, requiring further investigation.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Method** & **Accuracy** & **MSE** & **Precision Ratio** & **Recall Ratio** & **F1 Score** \\ \hline SqueezeNet & 92.3 & 3.293 & 0.934 & 0.913 & 0.919 \\ TI-CNN & 97.0 & 0.694 & 0.970 & 0.970 & 0.970 \\ CNN-LSTM & 96.1 & 0.568 & 0.960 & 0.961 & 0.960 \\ CNN-BiLSTM-SE & 98.6 & 0.262 & 0.989 & 0.988 & 0.988 \\ MS-CNN & 99.4 & 0.026 & 0.994 & 0.993 & 0.993 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Predicted results of corrosion level types under different methods.  
**Author Contributions:** Conceptualization, G.H., S.L. and Z.T.; methodology, G.H., S.L. and X.S.; Software, S.L. and B.D.; validation and formal analysis, G.H., S.L. and B.D.; Writing--original draft preparation, G.H. and S.L.; writing--review and editing, G.H., Z.T. and X.S.; funding acquisition, G.H., Z.T. and X.S. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This work is supported part by State Key Laboratory for GeoMechanics and Deep Underground Engineering, China University of Mining Technology under [Grant No.SKLGDUEK2222], Key independent research project of Hebei Provincial Collaborative Innovation Center of Transportation Power Grid Intelligent Integration Technology and Equipment under [Grant No.TPGIITE12], Hebei Provincial Science and Technology Plan Funded Project [Grant No.22375413D].  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Data available in a publicly accessible repository. The original data presented in the study are openly available in [ResearchGate] at [10.13140/RG.2.2.11268.95365].  
**Conflicts of Interest:** The authors declare no conflicts of interest.