## 1 Introduction  
Adhesive joints are increasingly used for joining composite structures or dissimilar materials in aerospace and automotive industries. Adhesive joints have unique advantages compared with traditional mechanical fasteners, which have to deal with the problems of loosening clamping force and fatigue strength, due to the spiral shape of the bolt-nut thread, wherein high-stress concentration factor occurs at the first bolt thread [1] and the load distribution in threads are uneven [2]. Composite materials have been increasingly used in aerospace, military, automobile, and other industries. Special characteristics of these materials, such as the significant strength-weight ratio, wear resistance, and thermal resistance, have further promoted their application scope, as they are substituting conventional materials. Furthermore, the manufacturing process of composite materials is changing. For example, the 3D printing of curvilinear fiber-reinforced composite structures (CFRCS) is a novel approach in this area with fast development [3]. Various methods have been utilized to connect composite parts, among which the single lap joint, double lap joint, and steep single lap joint are the most well-known methods. Therefore, defining its connections is one of the most important aspects of designing a composite structure [4]. According to the statistics, almost 70% of the failures in structures occur in their joints [5]. These challenges make this area more interesting for researchers from different points of view. [PERSON] and [PERSON] presented an analytical solution for a composite single-lap joint based on the mechanics of composites and anisotropic laminated plate theory [6]. The method was developed to define the location and magnitude of the peel and shear stress distributions in the adherents and adhesive. Additionally, an FEA code was used to validate the proposed methodology results. [PERSON] and [PERSON] [7] compared the analytical and numerical results of single lap joints under tensile force. At first, the classical Goland and Reissner method was reversed and modified in an analytical solution. Then the results were compared with a 2D finite element model. Stress analysis for a composite adhesively bonded lap joint was proposed by [PERSON] [5]. Then the analytical results of a single lap joint and a double lap joint were compared with their corresponding FEM results. [PERSON] et al. [8] considered 3D stress analysis in composite joints. They developed and combined equations presented by [PERSON] [9] via Hypersizer to derive a new way to calculate inter-laminar stress. The resulting equations were numerically solved and compared with the FE model. This method is applicable to a wide range of adhesive joints. Classic composite joints are made of two or three adhesive adherents bonded together. The adherents in these joints can be the same (fiber-reinforced polymer (FRP) to FRP) or dissimilar (FRP to steel). A classic double lap joint consists of three adherends bonded two by two with adhesive layers, shown in Figure 1. The thickness of the middle adherend can be the same as the outer ones. Joint modeling was carried out assuming geometrical symmetry based on the Reissner-Mindlin stacking plates theory. Finally, the results were validated relative to a finite element solution. [PERSON] and [PERSON] [10] experimentally, numerically, and analytically investigated and compared the stress of the composite double lap joints. The stress of a bonded-bolted Al-GFRP double lap joint was experimentally and numerically explored by [PERSON] and [PERSON] [11]. Their results were compared with an adhesively bonded joint and a bolted joint. The strength of the hybrid joint was higher than both simple joints against fatigue, energy absorption, and static tensile loads. An analytical solution was proposed by [PERSON] et al. to calculate peel stress in a double-lap joint under tensile force using classic laminate theory [12]. A finite element model in ANSYS validated the accuracy of the equation. [PERSON] et al.