# Performance of CRTS-II Ballastless Track-Bridge Structural System Rebars under Fatigue Loading Test
## 3 The Result of the Experiment
### Rebar Strain Analysis under Static Load
According to the experimental results analysis, it can be seen that under 3 million fatigue loading cycles, the strain amplitude of the steel rebar decreases gradually and the slope of the load-strain curve of the steel rebar under a static load increases gradually, which indicates enhancement of the steel rebar mechanical properties. This is the fatigue strengthening stage. In this paper, the reasons for fatigue strengthening of the steel rebar are analyzed based on the whole structural system and the single steel rebar. Figure 7: Slope change curve (a) L/2 section; (b) L/4 section. The strain amplitude of the steel rebar under the first stage fatigue loading stage is 26.08 \(\mu\)\(\varepsilon\), corresponding to a stress amplitude of 5.22 MPa, which is relatively low. According to the strain compatibility, the stress level of concrete is also very low. The ballastless track-bridge structural system is a composite structural system composed of various materials in which concrete has the largest specific gravity. The concrete structure is a discrete structure with micro-cracks and voids in the concrete before fatigue loading. Under the action of lower stress fatigue loading, the original micro-cracks and voids in the concrete in the structural system are compressed and closed by repeated micro-disturbances. The internal structure of material shows a tendency of \"optimization\", so the mechanical properties of the steel rebar are enhanced. On the one hand, the reason why low stress fatigue can improve the fatigue resistance of the steel rebar material itself is due to the unique fatigue strengthening phenomenon of the metal material itself. [PERSON] [30] explained the mechanism of the fatigue hardening of metals from the microscopic structure and shows that a certain degree of pre-strain can positively affect the strengthening of lattices in metals. The reason is that pre-strain can restructure the lattices, making them more uniform in arrangement and volume; this is the key to fatigue hardening. On the other hand, the overall slip and interfacial dislocation of the lattice structure under pre-strain contribute to the consumption of incompatible strain energy. [PERSON] [31] showed that the fatigue strength and fatigue life of steel plate and 20 Mncr5 alloy structural steel with better toughness are improved by suitable low-amplitude fatigue loading; [PERSON] [32] studied the fatigue damage of steel members under two-stage variable amplitude loading and showed that the load interaction had an enhanced effect on their fatigue life. The first low-stress amplitude fatigue loading stage in this test can be assimilated to apply pre-strain to the steel rebar, and the rebars can be ideally considered to be in the elasto-plastic loading stage below the fatigue limit, which will not cause serious internal  
Figure 8: Slope ratio curve. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Section**} & \multirow{2}{*}{**Initial Ratio**} & \multicolumn{2}{c}{**Stage 1**} & \multicolumn{2}{c}{**Stage 2**} \\ \cline{3-6}  & & **Slope Ratio** & **Degradation Rate (\%)** & **Slope Ratio** & **Degradation Rate (\%)** \\ \hline L/2 & 1.00 & 1.04 & \(-\)4.44 & 1.00 & 4.01 \\ L/4 & 1.00 & 1.05 & \(-\)5.02 & 1.00 & 4.33 \\ \hline \multirow{2}{*}{**Section**} & \multicolumn{2}{c}{**Stage 3**} & \multicolumn{2}{c}{**Stage 4**} & \multicolumn{2}{c}{**Total Degradation**} \\ \cline{2-6}  & **Slope Ratio** & **Degradation Rate (\%)** & **Slope Ratio** & **Degradation Rate (\%)** & **Rate (\%)** \\ \hline L/2 & 0.96 & 3.98 & 0.95 & 1.47 & 5.15 \\ L/4 & 0.97 & 3.93 & 0.95 & 1.40 & 4.81 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Slope evolution. cracks and defects in the steel bars. At the same time, the development can appropriately restructure the steel lattice, the lattice arrangement is more uniform, and the volume is more uniform, so the internal strength of the steel is increased and the mechanical properties of the steel are improved.