Numerical Assessment of Defects Effect on Boltless Composites Longitudinal Joints

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. Nevertheless, difficulties in assessing the quality of joining line after manufacturing restrict the applicability of boltless joint to non load-critical structural components. This thesis aims to numerically evaluate the effect of manufacturing-induced defects, such as weak bond or disbond, on the overall performances of the structural configuration. Longitudinal joints of the fuselage of Airbus A350-XWB aircraft family are used as reference design, since they are currently carried out through single-lap bolted technique. To simulate an HLT joint, a Finite Element (FE) model of a Wide Single Lap Shear (WSLS) specimen, previously adopted as test setup during BOPACS (Boltless assembling Of Primary Aerospace Composite Structures) project, is implemented in the commercial FE software Abaqus. Damage modeling exploits the Cohesive Zone Model (CZM) approach, in which fracture energies drive damage initiation and evolution behavior. The computational loading scenario focuses on quasi-static non-linear analysis. To assess the influence of defects on joint load-carrying capability, three different classes of joining line are investigated, namely a brittle and a ductile adhesive for adhesive bonding application, and a thermoplastic polymer matrix for welding technology. Globally, all types of boltless joints experience ultimate strength reduction whenever damage occurs in the joining line, but each material exhibits a different decreasing trend depending on its inherent mechanical properties. In addition, two methodological approaches, which exploit only the numerical outcomes of FE quasi-static analyses, are proposed as predictive methods for fatigue response. Fatigue limits for a constant fatigue life are predicted by exploiting the Similarity Principle of stress peaks distribution. Constant Life Diagrams (CLDs) are numerically calculated by following a reverse algorithm based on experimental fatigue data extrapolated from BOPACS test campaign. On the other hand, fatigue initiation loads and fatigue lifetime are predicted by exploiting the Fatigue Crack Growth (FCG) approach and numerical integration of Paris's law. Based on these investigations, it is finally concluded that predictions of fatigue response can be preliminarily assessed by exploiting numerical outcomes of quasi-static simulations, in conjunction with limited experimental data used as starting points.