Aerospace manufacturers increasingly rely on composite materials for the most advanced aircraft, due to their superior performance and tailorability. As the adoption of such structures grows, so does the occurrence of various kind of damages throughout their service life. Therefore, there is the necessity to develop robust, reliable and repeatable procedures to fully restore the structural integrity of composite components. An additional challenge is posed by composite structures that include functional inserts: the functionality of such inserts needs to be re-instated on top of the integrity of the overall structure, while ensuring a seamless repair finish. This thesis considered two damage scenarios in a CFRP fuselage access panel whose edges are wrapped with a functional material, and defined the most suited repair procedure to tackle them.
The first scenario is represented by damages located in the CFRP structural part of the fuselage access panel. The scarf repair method was identified as the optimal one for this instance, research effort was therefore directed towards its improvement. Indeed, such technique does not currently allow to achieve a fully flush surface, as a mismatch between the repaired area and the undamaged one remains noticeable. Eliminating such unevenness is crucial for stealth and eventually aerodynamic reasons, and therefore needs to be investigated. Two repair configurations were implemented: the first one consists of re-milling the surface once the repair is completed, while the second one relies on a thinner repair patch that — once properly aligned — allows for a flush surface. The first option allowed for an improvement of over 80\%, reducing the surface unevenness from more than 1/2 of a millimeter to less than a 1/10, while ensuring a smooth, continuous surface finish. At the same time, it proved capable of meeting all the mechanical requirements, performing closely to a reference repair configuration in several tests. On the other hand, the second option only partially improved the surface finish, but fell short of the fatigue life requirement by a large margin and also showed a significantly poorer mechanical performance compared to the reference repair and the other configuration.
The second scenario is represented by small, cosmetic damages located in the functional edge of the fuselage access panel. It was identified that such damages are best addressed with repair procedures based on a filler compound. Guidelines to define such repair compounds were defined. Then, two repair procedures based on the use of these compounds were thought out: the first one is similar to conventional filling repair processes, while the second one relies on a bespoke tool to inject the repair compound. The former was implemented and produced promising results: it allowed to precisely restore the original profile and achieve a seamless surface finish. Damages affecting both the structural and functional parts were also briefly addressed, paving the way for future developments.

The current generation of commercial aircrafts extensively use composite materials such as Carbon Fibre Reinforced Polymers (CFRP) in both exposed and primary structures. These materials lack through-thickness reinforcement and are hence susceptible to out-of-plane impact damages. Barely visible impact damage caused by low energy impacts poses a unique problem, since delaminations, de-bonding and cracking may be present below the surface layers without any indication of damage on the surface. The focus of this research is on one such scenario, multiple site low energy hail impacts, while the aircraft is on the ground. Taking into account the relevant parameters, a hail impact envelope was established both in terms of the initial kinetic energy and peak impact force. Further, contradictions found in literature over the influence of the compressive strength of hailstones were resolved. These were accomplished with the aid of a state-of-the-art finite element model and experiments in the laboratory. With help from a custom designed and assembled impact force measurement experimental setup a relation was established between impacts carried out with steel impactors and those with simulated hail ice impactors. Based on this relation, predictions are made on which hailstones have the potential to cause damages to CFRP structures.

Transparency and translucency are essential features of modern architecture. Glass products have been widely used in facade applications and recently their use has expanded to the load-bearing structure due their large in-plane compressive strength. However, the fragile nature of glass requires special treatment and attention to detail to avoid fracture caused by tensile stresses. Connections between glass components are very critical to the structural integrity of the system. Bolted connections, which are often used to transfer forces between glass elements, require drilling the glass. This process may cause additional flaws and stress intensifications around the holes and thus reduces the bearing capacity of glass elements. Adhesive connections offer an alternative approach for the connecting joints and enable a more uniform distribution of stresses. Laminated connections have recently been developed that combine high strength and transparency. This work focuses on the Transparent Structural Silicone Adhesive (TSSA), produced by Dow Corning, that is used for the realization of laminated connections. TSSA connections have been used in several projects worldwide; however, the hyperelastic and viscoelastic nature of the material has not been fully investigated. In this work, the mechanical response of TSSA laminated connections under static and cyclic loading is investigated by means of experimental, analytical and numerical studies.

Firstly, the shear behaviour of TSSA laminated circular connections is characterized by means of monotonic and cyclic loading tests at different frequencies. The hysteretic behaviour of the material under loading cycles, which is caused due to its viscoelastic nature, is analyzed. The adhesive exhibits significant stress-softening under repeated cycles that becomes more severe as the maximum load increases. Energy dissipation analysis is conducted to understand the nature of this phenomenon with the aim to simplify the cyclic behaviour of the adhesive for modeling purposes.

Secondly, TSSA laminated circular connections are subjected to monotonic and cyclic tensile loading of increasing maximum load. The development of the whitening phenomenon is studied for both cases. The stress level when whitening appears for the first time and the way it propagates to the adhesive surface show some consistency both for the cases of static and cyclic loading. The occurrence of the stress-softening phenomenon is also recorded, in order to observe whether it appears within the working limit of the connection.

Thirdly, the deformation behaviour of the adhesive is described analytically based on hyperelastic prediction models. Conventional test set-ups, such as uniaxial and biaxial tension tests, are combined with the simple shear test results obtained within the framework of this thesis, for the material characterization of TSSA. The hyperelastic material parameters are calibrated by a simultaneous multi-experiment-data-fit based on the nonlinear least squares optimization method. The softening behaviour observed in shear tests is modeled based on a simplified pseudo-elastic damage model, which is supported by most finite element software. A first attempt is also made to model the actual softening response of the adhesive. A less conservative approach proposed by Guo, also based on the theory of pseudo-elasticity, proved to give a good approximation of the actual cyclic response of the adhesive.

Finally, TSSA laminated connections on the edge of the glass are experimentally and numerically investigated. The edge bonded specimens are tested in shear and the stress distribution of the adhesive is analyzed by means of a three-dimensional finite element model. The distribution of stresses in the adhesive is non-linear showing significant stress peaks towards the free edges of the adhesive. A parametric study is conducted to relate the magnitude of the stress peaks with the eccentricity of the applied load.