## 3 Results and Discussion
### Characteristics of LSS in Hybrid Bonding between CFRP/Steel with Clearance-Filling Effect
#### 3.1.2 Bolting  
Figure 5 shows the results of evaluating the adhesive load according to the use of washer and structural adhesive when CFRP and steel were bonded by bolting. Type 1 denotes the case in which only bolts and nuts were used. The maximum lap shear load was approximately 2400 N, which was relatively low compared with other cases. However, the maximum lap shear load significantly increased to approximately 4200 N when the washer was used for both the bolt and nut (Type 4). Since the inclination of the bolt was constant when no adhesive was used, the strength improved because the deflection of the bolt decreased under the influence of the washer. Unlike riveting, in all cases of bolting, the adhesive strength was determined by the fracture of the steel. Therefore, the reduction in the deflection of the bolt prevented bending of the steel and minimized the pre-tension on the outer surface of the steel that resulted from bending; consequently, the bonding strength increased. When the washer was used on only one side as in Types 2 and 3, unlike the case in which the bolt and nut were used on both sides, the effect of preventing the bolt from bending reduced; consequently, the maximum lap shear load was lower than that when the washer was used on both sides. In particular, when a washer was used only on the bolt side, it had a significantly lower strength than when a washer was used on the nut side because the steel broke at the nut side; therefore, bending of the steel could be minimized by using a washer on the nut side. Consequently, the washer effectively prevented the bending of the bolt even if it was already tilted by the clearance. In addition, the structural adhesive was used to investigate the change in adhesive strength according to the initial inclination of the bolt when all washers were applied. The red bar in Figure 5 shows its location. When only washers were fixed using the structural adhesive (Type 5), they showcased a similar level of maximum lap shear load with a slight increase of 100 N compared to Type 4 (without the structural adhesive). Therefore, no special effect was observed upon attaching the washer to the adhesive surface. However, the highest maximum lap shear load (4800 N) was obtained when the structural adhesive was used on the washer bonding and entire-bolt contact surfaces (Type 6). In Type 6, the clearance between the bolt and hole was filled with adhesive. Compared to the case in which only the bolts and nuts were used, the maximum shear load was approximately doubled. In other words, in mechanical fastening by bolting the CFRP and steel, the washer prevents bending of the steel, and the adhesive prevents the initial inclination of the bolt. This improves the mechanical fastening strength between the CFRP and steel. Figure 6 shows a photograph captured after an LSS test of different specimens according to the use of the washer and adhesive during bolting. It shows the angle change during deformation of the bolt thread based on the vertical and horizontal lines. When only the nuts and bolts were used as in Type 1, the largest angle change was observed compared to other cases. In  
Figure 4: Fractured surface of LSS specimens depending on whether or not adhesive was used: (**a**) river only and (**b**) both river and adhesive. Type 2, where a washer was used only on the bolt side, deformation of the bolt thread was slightly alleviated due to the effect of the bolt-side washer. However, the steel showcased a deformation similar to that observed in Type 1. This reconfirms that if there is no washer on the nut side, the metal can be significantly deformed. However, the thread deformation angles of Types 4 and 6 were \(2.5^{\circ}\) and \(1^{\circ}\), respectively, which are the minimum levels in the LSS tests. In Type 4, bending of the steel was minimized by preventing the deformation of the bolt under the influence of the washer.