The work presented in this thesis focuses on the numerical assessment of defects effect on potential boltless design for fuselage panels longitudinal joints, typically identified as High Load Transfer (HLT) configuration. Nowadays, the state-of-art joining technique for primary aircraft composite structures, such as fuselage barrels, is still mechanical fastening. This conventional approach limits the full exploitation of potential benefits, achievable by composite structures, in terms of weight and cost reduction. Therefore, alternative boltless joining technologies open new possibilities to further improve the airframe design of future aircraft generations. Two major techniques are currently under investigation, namely adhesive bonding and welding, which are employable to join thermosetting- and thermoplastic-based laminated composites, respectively.