## 3 Results and Discussion
### The Effects of the Grit-Blasting Surface Treatment  
Three sets of nine specimens were applied with shear tests. The slip load and slip factor values acquired are shown in Table 8. The load-displacement curves obtained from the shear tests of each connection method are shown in Figure 7. The load-displacement curve of the G60#-S24# connection in Figure 7 can be described by the typical (b) type curve from Figure 3. The specimen showed a gradual change in the load as the displacement was increased. The load-displacement curves of the G-S24# and G-S60# connections in Figure 7 can be described by the (a) type curve in Figure 3. These two types of specimen showed a major slip and reduction in the load. The reduction rate of the load with the G-S24# connection specimens was much larger than that of the G-S60# connection specimens. The slip load increased with the increase in the grit size (from #60 to #24). As previously explained, the main component of the friction force with the GFRP connections is likely the deformation force. The grit-blasting surface treatment increases the number of asperities and roughness on the faying surface of stainless-steel cover plate. The deformation force increased with the increase in the number of asperities and the roughness of the harder stainless-steel cover 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. As is shown in Figure 8, the grooves were formed  
Figure 5: The load–displacement curves of specimens with different cover plates. Figure 6: Plate surfaces of G-G connection after shear test. (a) GFRP connecting plate surface after shear test; (b) GFRP cover plate surface after shear test. on the GFRP connecting plate and the GFRP powder ploughed by embedded asperities were left on the stainless-steel cover plate surface. A load drop did not occur in the load-displacement curve of the G60#-S24# connection compared to the load-displacement curves of the G-S24# and G-S60# connections, which may be related to the ratchet mechanism [26]. The ratchet mechanism describes a situation where two rough surfaces contact each other and plastic deformations do not happen. The asperities of one mating surface climb up and down on the other mating surface. The motion mode of the ratchet mechanism resembles a Coulomb friction model. The roughness of the surface of the GFRP plate increased after being treated by grit-blasting. The rough surface of the GFRP plate contacted the rough surface of the stainless-steel plate and the asperities being produced under contact stress elastically deformed. The ratchet mechanism appeared in the G60#-S24# connection, which accounts for the maintenance of the connection bearing capacity. As shown in Figure 9, barely any trace could be found after the shear test due to the ratchet mechanism that is elastic deformation of the asperities recovered after unloading. The slip factor of the G60#-S24# connection was the highest and the slip factor of the G-S60# connection was the lowest among the three connection types. The difference between the slip factors of three connections was minimal. [PERSON] et al. [5] tested the GFRP slip-critical connection with stainless-steel cover plates. In their experiments, the surfaces of stainless-steel cover plates were treated by grit-blasting. They found that the slip factor was 0.449. This value is close to the results from our tests, as listed in Table 8. [PERSON] et al. [3] found that the slip factor value of the GFRP slip-critical connection with structural-steel cover plates was around 0.37, which is again similar to the results of our tests. It is clear that increasing the roughness of the faying surface using a grit-blasting treatment is a valid way to increase the slip load of GFRP slip-critical connections.