## 1 Introduction  
Adhesive joints with mechanical fasteners are often used in aircraft and space vehicle structures for their high stiffness and reliability. Mechanical fasteners are used on adhesive joints to prevent bending failures due to offset loads and peel failures on the adhesives. Figure 1 shows the peel failure mode.  
[PERSON] [1] analyzed the shear stress on the adhesive of a lap joint. [PERSON] [2] improved the analysis by using a nonlinear stress strain curve including plastic strain. [PERSON] and [PERSON] [3] analyzed the linear stress strain relation, in which they included the geometric nonlinear effects of an offset load. The peel stress, however, was similar to that of [PERSON]'s analysis [1]. [PERSON] [4] and [PERSON] [5] improved [PERSON] and [PERSON]'s analysis but again found no significant difference in peel stress from [PERSON]. [PERSON] et al. [6] reviewed the peel resistance of adhesives materials, and [PERSON] [7]  
Figure 1: Peel failure mode.  
evaluated the fracture toughness in relation to bond-line thickness and peel-arm thickness. [PERSON] et al. [8] experimented and investigated numerically into the shear strength behavior of aluminum alloy adhesive lap joints. [PERSON] et al. experimented and investigated between plastic behavior of adhesives, bondline thickness and lap shear strength [9], [PERSON] et al. [10] studied the effect of surface treatment on the shear strength of aluminum adhesive single-lap joints using lap shear tests and surface inspection. [PERSON] et al. [11] tested and inspected the lap shear strength and fatigue behavior of a friction stir spot welded to dissimilar magnesium-to-aluminum joints with adhesive.  
On the other hand, mechanical fasteners without adhesive have been used for mechanical joints. Mechanical fasteners using bolt-nut connections, however, face problems of loosening clamping force and low fatigue strength due to the spiral shape of the bolt-nut thread, wherein a high stress concentration factor occurs in the first bolt thread [12], and the load distribution in the threads is uneven. Slight pitch differences have to be introduced to increase fatigue strength and prevent loosening [13]. Also, for conventional mechanical joints using pure metal fasteners, there could be a problem of heat shorts on the fuselage structure [14]. Adhesive bonding without a peel-stop fastener has the advantage of not weakening the components to be joined; however, the reliability is generally not as high as mechanical fasteners. Therefore, adhesive bonding with peel-stop fasteners is often used when very reliable joints are required.  
Despite the research mentioned above, no reliable or quantitative design method has yet been devised for single lap joints without peel stop fasteners. Of course, there is no mathematical model for a single lap joint with peel stop fasteners for the size, strength, and stiffness of the peel-stop fasteners. Thus, the authors decided to investigate the effect of the stiffness of peel-stop fasteners by testing fasteners of various shapes and materials. An understanding of the mechanism of strengthen using peel stoppers has worth and can lead to future mathematical models.