## 1 Introduction
Additionally, a numerical and experimental investigation on the mechanical behavior of CFRP/aluminum bolted joints was introduced in [38], focusing on the effect of the environmental conditions (temperature and moisture) on the mechanical performances of the bolted joint. Hence, different tests were presented by considering extremely hot, hot and humid, and extremely cold environmental conditions. In [39], the thermal effects on a single lap hybrid metal/composite bolted joint were investigated. Indeed, metal and CFRP are characterized by different thermal expansion coefficients, resulting in uneven load-deformation characteristics and three-dimensional stress field around the bolt hole. The self-piercing rivet technique is used to join sheet materials [40; 41]. Unlike bolted joints, this method does not require a pre-drilled hole, allowing faster joining performed in just one operation. However, this operation has a critical impact on the fatigue life and on the static strength of hybrid CFRP/aluminum components [41; 42; 43]. In particular, in [44] the possible damage induced by self-piercing riveting of CFRP with aluminum, which include delamination and fiber/matrix failures in the CFRP region, were investigated, and the influence of the damage on the joint strength was assessed. In [45], the tensile and fatigue properties of CFRP/aluminum self-pierce riveting joints were evaluated. Different joint configurations were investigated, and the influence of the rivets' geometrical parameters on the joint failure was studied. In general, the choice of the particular joining technique strongly depends on the specific application. In order to take advantage of the benefits of the different joining techniques, hybrid solutions can be found whose performance is enhanced by combining adhesives with rivets or bolts [46; 47; 48; 49]. In [50], an experimental investigation on single-lap joined CFRP laminate and AA2024-T6 aluminum sheet was introduced. Bonded, self-pierce riveting, and hybrid SPR/bonded joints were considered. In particular, an appreciable synergism of the bonded and self-pierce riveting joints could be observed in the hybrid solution, which exploited high resistance levels due to the bonding and the high failure energy due to the self-pierce riveting. In [51], a semi-analytical method was developedfor the analysis of composite bolted/bonded single lap joints, under coupled in-plane and bending loads. The method was based on [PERSON] and [PERSON] beam theories to evaluate the laminate and the bolt displacements, respectively. Several test cases were introduced, including analyses of bolted joints, bonded joints, and hybrid joints. Additional analyses were carried out for the hybrid configuration, considering a debonding in the bonded interface between the laminates. A few works can be found in the literature on the mechanical behavior, including damage, of hybrid composite-metal structures, composed of differently joined subcomponents. An investigation on a complex bolted structure can be found in [52], where the mechanical behavior of a hybrid composite/aluminum wing-box was assessed. The structure was composed of a number of single-lap bolted joints with a titanium fastener, which were numerically studied by introducing a three-dimensional finite element model (FEM), able to account for material non-linearity such as progressive damage, and plasticity models, respectively, in the composite and metal regions. Two-node connector elements were used to simulate the fasteners behavior. Both twisting and bending loading conditions were considered, together with an applied increased temperature, in order to locally investigate the behavior of the bolted joints. Experimental results were used to validate the numerical model. However, more investigations are mandatory to fully understand the effect of the joints on the mechanical behavior of a complex structure composed of different subcomponents, as well as the effects of key structural components on the damage behavior of hybrid complex structures.