## 3 Results and Discussion  
### The Effects of Cover Plates  
The slip factors and slip loads are listed in Table 7. Two sets of tests were performed, as shown in the table. Each kind of test was repeated two times using two identical specimens to grant the reliability of the tests. The load-displacement curves of the two tests series are shown in Figure 5. The load-displacement curve of the G-S connection can be described by the (c) type curve from Figure 3. The load-displacement curve of the G-G connection can be described by the (b) type curve from Figure 3. The initial stiffness of the G-S curve was larger than that of the G-G curve. These characteristics can be explained by the adhesion theory of friction [26]. The friction force consists of two components, namely adhesion and deformation [26]. The adhesion force is the rupture resistance of the junctions, which is induced between the two contact surfaces. The deformation force is the resistance that is caused by the plastic deformation of interlocking asperities when the contact surfaces slide with each other. In the G-G connection test, the connecting plates and cover plates are all GFRP, the hardness is the same, and one side cannot be embedded into the other. The asperities on the surface of the GFRP plates were in an elastic contact state. The deformation of asperities in polymers is primarily elastic. The amount of the junction induced by the plastic deformation of the asperities was small. Thus, barely no scratches were formed after shear tests and only minor plastic deformation points could be seen, as shown in Figure 6. The significant proportion of the friction force with polymers came from the deformation force [26]. The surface of the stainless-steel plate was harder than that of the GFRP plate. The asperities on the surface of the stainless-steel could penetrate and plough into the GFRP surface, which produced grooves if the shear strength of the GFRP was exceeded. In Figure 5, the load-displacement curve of the G-S connection was above the curve of the G-G connection. This is due to the larger deformation force of the G-S connection. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Connection** & **Connecting Plate Materials** & **Cover Plate Materials** & **Relaxation Time** & **Specimen** \\
**Methods** & **(Surface Treatment)** & **(Surface Treatment)** & **(Days)** & **Specimen** \\ \hline \hline \multirow{3}{*}{G-G} & \multirow{3}{*}{GFRP (untreated)} & 0 & A-0-1/2 \\  & & & 1 & A-1-1/2 \\ \cline{1-1}  & & & 7 & A-7-1/2 \\ \cline{1-1}  & & & 30 & A-30-1/2 \\ \hline \multirow{3}{*}{G-S24\#} & \multirow{3}{*}{GFRP (untreated)} & 0 & B-0-1/2 \\ \cline{1-1}  & & Stainless-steel (\#24 grit & 1 & B-1-1/2 \\ \cline{1-1}  & & blasting) & 7 & B-7-1/2 \\ \cline{1-1}  & & & 30 & B-30-1/2 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Specimens in the pre-tension force relaxation tests. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Specimen** & **Slip Load \(F\) (kN)** & **Slip Factor \(\mu\)** \\ \hline G-G-1 & 2.64 & 0.17 \\ G-G-2 & 2.78 & 0.18 \\ Average value & 2.71 & 0.18 \\ \hline G-S-1 & 3.16 & 0.20 \\ G-S-2 & 3.17 & 0.20 \\ Average value & 3.17 & 0.20 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: The slip load and slip factor values of tested connections with different cover plates. According to [PERSON] [26], the friction factor, \(\mu_{a}\), of the adhesion force with polymers can be represented by the following equation:  
\[\mu_{a}=\frac{\tau_{0}}{p}+\alpha \tag{3}\]  
where, \(\tau_{0}\) is the shear strength of the polymers, \(p\) is the contact pressure on the actual contact area, \(\alpha\) is a constant, and the value of \(\alpha\) for polymers is 0.2. In the polymers, the \(\tau_{0}\)/\(p\) is much smaller than \(\alpha\)[26], the friction factor can be approximately 0.2 when the deformation force is ignored. The experimental average value of the slip factor with a G-G connection was 0.18, which is close to 0.2. This demonstrates that the deformation component can be ignored in G-G connections.