## 2 Experimental Details
### Specimen Configuration
#### 2.2.2 Surface Preparation  
A grit-blasting treatment was introduced in our tests to increase the surface hardness and roughness. Using compressed air, a high-speed jet of air was formed, which sprayed abrasives at high speed onto the surface of the specimen to be treated. The impact and cutting action of the abrasives helped to clean the surface to a certain degree, change the roughness, and enhance the mechanical properties. It was determined that the grit-blasting caused an increase in the Vickers micro-hardness values of all of the materials, which led to an increase in the ultimate tensile strength [13]. Based on the tribological theory, the friction factor can increase with the increment of the surface roughness and strength [14]. The surfaces of the test specimens were treated by grit-blasting using a HZ-1616r-1 sand blaster. The key parameters of the grit-blasting treatment are listed in Table 2.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Materials** & **Tensile Strength (MPa)** & **Elastic Modulus (GPa)** & **Vickers Hardness (kgf/mm\({}^{2}\))** \\ \hline Glass fiber reinforced plastic (GFRP) & 300 & 15 & / \\ Austenitic stainless-steel 30408 & 515 & 193 & 140 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Mechanical properties of plates.  
Figure 1: Specim size (unitmm).  
The grit-blasting treatment was carried out on the stainless-steel plate using #24 grit and #60 grit abrasive according to [PERSON] et al. (2015). The numbers #24 and #60 represent the size of the abrasive, the higher the number, the smaller the grit size and the smoother the surface will be after blasting ([PERSON] et al., 2016). Therefore, the surface roughness of the stainless-steel plates treated with the #24 grit abrasive will be greater than that of the stainless-steel plates treated with #60 grit abrasive. The different surface conditions captured by commercial camera under daylight are presented in Figure 2. When the surfaces of the plates become rougher, the diffuse reflection ratio on the surface will increase, and the mirror reflection ratio will decrease. Therefore, under the same light source, the visual perception of brightness is different, and the rougher the surface, the dimmer the appearance ([PERSON] et al., 2016). The blasting time for the GFRP plate should be reasonably controlled to prevent initial damage. The surface of the untreated GFRP plate was left in its original state, while the surface of the GFRP plate treated with the grit abrasive was glossy and somewhat rough.  
When implementing the grit-blasting treatment, only the faying planes were treated, and the non-working planes contacting bolt washers remained untreated, as shown in Figure 1. Several combinations of connecting methods were developed in this paper and their abbreviations are listed in Table 3. The optimal matching method of various connections were predicted.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Blasting Treatment** & **Abrasives Type** & **Nozzle Pressure (bar)** & **Blasting Angle (\({}^{\circ}\))** & **Distance of Spray Gun (mm)** & **Blasting Time (s)** \\ \hline Manual & Brown corundum & 8 & 90 & 100 & 30 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Main parameters of the grit-blasting treatment.  
Figure 2: Plate surfaces with and without grit-blasting treatment. (**a**) Untreated stainless-steel surface; (**b**) stainless-steel surface treated with 60# grit-blasting; (**c**) stainless-steel surface treated with 24# grit-blasting; (**d**) untreated GFRP surface; (**e**) GFRP surface treated with 60# grit-blasting; (**f**) GFRP surface treated with 24# grit-blasting.