## 3 Analyzing Experimental Phenomena and Results  
### Load-Deflection Curve  
The load-deflection curves were plotted by taking the average deflection values of 1/4 span and 1/2 span for each group of two specimens, as shown in Figure 6, while the deflection specific data are shown in Table 4. The three working stages of the load-deflection curve of the steel-ECC composite structure are obvious from the curves: the elastic stage, crack extension stage, and yielding stage. The whole composite structure is in the elastic working stage from the beginning of loading the ECC slab before the cracks appear when the steel strain is not large and the growth of both the ECC surface and the steel strain is relatively stable. With the further increase in the load, the reinforcement in the ECC plate starts to enter the yielding stage, and the curve takes the first turn, the deflection curve is out of linear growth, and the whole curve tends to flatten out, at which time it enters the crack expansion stage. As the load continued to increase, the reinforcement reached the yielding point and the deflection curve began to turn into the yielding stage for the second time, the cracks in the ECC plate expanded rapidly toward the bottom of the beam, the beam deflection increased rapidly again, and the load-bearing capacity remained stable. After the test unloading, the whole specimen showed an obvious rebound phenomenon, and the ECC layer did not flake off, indicating that the ECC has high ductility, and the steel plate did not yield during the whole loading period. Finally, a large area of cracks appeared in the flexural-only section of the lower ECC layer, and the load-bearing capacity was lost, indicating damage to the member.  
The steel-ECC composite structure can maintain good tensile strength even after being subjected to negative moment stress. This is partly due to the material properties of the ECC material itself, which has high tensile strength, and partly due to its ability to maintain the ultimate tensile strength in its high strain state after reaching the ultimate tensile strength.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Symbol** & **1/2 Span Maximum Deflection/mm** & **1/4 Span Maximum Deflection/mm** \\ \hline SE-50-150-1 & 125.62 & 83.56 \\ SE-50-150-2 & 124.52 & 81.43 \\ SE-50-200-1 & 111.96 & 76.13 \\ SE-50-200-2 & 112.54 & 74.62 \\ SE-75-150-1 & 96.21 & 66.46 \\ SE-75-150-2 & 95.49 & 66.24 \\ SE-75-200-1 & 88.34 & 62.07 \\ SE-75-200-2 & 90.54 & 61.01 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Summary of test deflection results.  
Figure 5: Test loading site diagram.  
In the initial stage of the curve, the linear state is approximately straight, the effect of the ECC thickness variable on the stiffness of the member is significantly greater than the effect of the bolt spacing variable on the stiffness of the member, and the stiffness of the member increases with the increase in the ECC thickness while it decreases with the increase in the bolt spacing. After the first turn of the curve, when the member enters the working stage with cracks, the curve only changes in slope, the influence of each parameter on the stiffness of the structure is still maintained, the influence of the two variables on the stiffness of the member is almost increased (due to the large shear stress of the bolts with larger spacing at this stage and the early deformation, which leads to the relative slip of the ECC and the steel plate and interlaminar damage), and the stiffness decreases.