## 4 Test Results and Analysis of Time-History Curve
### Vertical Displacement Time-History Curve  
According to the analysis of the test results, the vertical displacement time-history curves of the W2, W3 and W4 specimens had a certain similarity, so the W4 specimen was taken as a typical result for analysis. The displacement time-history curves of W4 at the impact height of 1 m and the impact height of 1.5 m are shown in Figure 14.  
It can be seen from Figure 14 that the vertical displacement of the measuring point of W4 under the two different impact heights changed within a short recording time from 0.1 s to 0.2 s. Since measuring point 1 was located at the most unfavorable force position in the center, and measuring points 2, 3, 4 and 5 were arranged on the circumference of a circular contact area with a diameter of 250 mm, the vertical displacement of measuring point 1 was greater than that of other measuring points.  
Figure 13: Strain time-history curve of W4 specimen; (**a**) Impact at a height of 1 m; (**b**) Impact at a height of 1.5 m.  
Furthermore, the vertical displacement of measuring points 1-5 was measured. It seems that the maximum displacement values were measured at point 1 in theory when the center of gravity of the sandbag coincided with the center of the specimen. This is because measuring point 1 was under the most unfavorable position, and the data of point 1 in Figure 14 confirm this result. In addition, the difference in the displacement value between point 1 and the other points was not large because the power of the low-speed impact was small. Moreover, the displacement values at points 2-5 should be the same in theory. However, there was a deviation in the results due to the effect of a test error and the effect of the dimensions of the specimen. This situation was normal and reasonable.  
Moreover, the vertical displacement value recorded after 0.2 s was slightly higher than the vertical displacement value recorded at the initial moment, which was due to the elastic deformation performance of the wall. The recorded results show that there was a dynamic process of slow recovery of deformation after the composite wall was subjected to a low-speed collision. In addition, the vertical displacement change of each measuring point under the impact height of 1.5 m was greater than that under the impact height of 1 m.  
By comparing the test results of W2, W3 and W4, it can be seen that the measured maximum vertical displacement of the W3 specimen was smaller than that of W2. The analysis results indicate that the concrete strength of the W3 specimen was higher than that of W2, which led to an increase in the overall strength of the composite wall and enhanced the impact resistance of the W3 specimen to a certain extent.  
It can be seen from the vertical displacement curve that the vertical displacement change of the W4 specimen was higher than that of W2. The analysis result shows that W4 was not equipped with light steel mesh, which resulted in the impact resistance of W4 being weaker than that of W2. In addition, the vertical displacement trend of the W2 and W3 specimens under different impact heights was similar to that of W4.