## 3 Numerical Simulation Study on Mechanical Properties of Corroded Pipe
### Numerical Simulation Results Analysis  
Analysis of Numerical Simulation Results of Pipe Failure Mode  
The maximum principal stress diagram and plastic strain cloud diagram of four groups of pipe mechanics are shown in Figures 7 and 8, respectively. The maximum principal stress of the A1 group, B1 group, B2 group and B3 group are shown in Figure 7a-d. It can be seen that, because the load on the pipe is symmetrical, the maximum stress of the pipe is symmetrically distributed. The tension zone and compression zone of the four groups of pipes are roughly the same, among which the upper crown and the inner side of the lower bottom plate are the tension zone, and the maximum tensile stress appears in the middle of the pipe crown and the bottom plate, which is consistent with the damage of the pipe in the upper section. The bottom and the middle of the top span are the dangerous sections under the TEB test conditions. At the same time, the outside of the pipe side is the tension area, and the compression area appears on the top of the pipe, the outside of the pipe bottom and the inner wall of the pipe side. Comparing the maximum principal stress diagrams of pipes with different corrosion depths, it is found that, with the increase of corrosion depth, the maximum tensile stress inside the pipe increases and the maximum compressive stress decreases, while the tensile resistance of concrete materials is extremely weak. It can be inferred that the corrosion depth of the pipe has an effect on the ultimate bearing capacity of the pipe. As the corrosion depth of the pipe increases, the ultimate bearing capacity of the pipe decreases and the pipe is more dangerous. This conclusion is consistent with the experimental results. Figure 8: _Cont._  
Figure 7: Distribution of maximum principal stress of pipe with different corrosion depths: (**a**) A1, pipe; (**b**) B1 pipe; (**c**) B2 pipe; (**d**) B3 pipe. As shown in Figure 8a-d, the position and area of the plastic strain area of the pipe with different corrosion depths is almost unchanged, and the maximum plastic strain area is in the middle of the top span of the pipe. With the increase of corrosion depth, the development of the plastic strain area is from the top of the pipe to the side of the pipe, from the middle of the span to both sides. It can be concluded that, when the corrosion depth of the pipe is less than the thickness of the protective layer, the change of the corrosion depth has little effect on the plastic deformation area of the pipe, and the maximum plastic deformation is in the middle of the pipe top span. Based on the above rules, the numerical simulation results are in good agreement with the experimental results. According to the simulation results, the reaction force and the vertical displacement of the simulated pipe loading reference point are derived, and the load-displacement curve is obtained, as shown in Figure 9. The curve trend of pipe simulation with different corrosion depths is the same: the load-displacement curve of the pipe is a straight line in the early stage of loading, and the load and displacement of the pipe are in direct proportion. This section corresponds to the uncracked stage and crack development stage of the pipe. In the early part of this stage, the pipe did not crack and showed elastic deformation. With the increase of displacement, the concrete in the tensile zone developed fine cracks, the pipe structure was not damaged, and the pipe was in good working condition. Then the bearing capacity of the pipe section continues to increase with the increase of the displacement, and stops growing when the ultimate bearing capacity of the pipe section is reached. The slope of the load-displacement curve decreases with the increase of the displacement, and the growth rate of the bearing capacity of the pipe slows down. Corresponding to the pipe failure stage in the test, the pipe crack gradually develops from the tensile zone to the compressive zone at this stage.