With the progressively more widespread exploitation of post-buckled skin-stiffened structures in the aerospace industry, aimed at making them more weight efficient, assessing the failure of such structures is of utmost importance. Additionally with the trend of moving towards fastener free connections, one of the potential failure modes of skin-stiffened structures is the skin-stiffener separation that leads to a sudden and drastic reduction in the load carrying capacity of the structure. Hence, it is imperative that structures are designed against such failure modes, being crucial to have predictive models for the initiation and propagation of delaminations at the connection interface.

Given this motivation, the present thesis investigates the development of a novel multi-domain semi-analytical cohesive zone approach to predict the initiation and propagation of damage along a two-dimensional interface. In the multi-domain method the main structure being analyzed is decomposed into multiple smaller domains for computational advantage, with each domain having its own set of continuous approximation functions based on the Ritz method. A penalty formulation governs the inter-connection of domains. To extend the multi-domain framework to model damage, the interface connection is modelled as a cohesive zone with the traction-separation law governing its response. The area connection from the multi-domain method is modified to include a damage-dependent penalty stiffness term, whose degradation is controlled by the traction-separation law of the cohesive zone model. Lastly, the energy dissipated to create a crack is included in the formulation. Since the framework is based on the Ritz approach, the appropriate choice of continuous approximations to model damage is investigated, presenting their limitations and the proposed methods to overcome them.

Finally, the results obtained from the multi-domain framework were verified against finite elements for a multitude of cases, which in turn had been validated by experimental data. The results were subsequently followed by discussions and the caveats of the developed framework. Results indicate that the developed multi-domain semi-analytical cohesive zone method predicts the results within 4.3% of finite element results, while taking 75% less time on average.

Finally, having somewhat successfully modelled the initiation and propagation of damage along a two-dimensional interface, this thesis establishes a robust basis for future research involving the use mixed-mode behaviours with multi-linear cohesive laws, whose formulations can just be slotted into the current framework. All of this forms a solid foundation to efficiently model damage initiation in larger structures and, more concretely, skin-stiffener separation.

Fibre-reinforced composites are increasingly used in aircraft structures as an alternative to traditional metallic components due to their lighter weight and tailorable mechanical properties. However, certifying these materials for critical aircraft components still relies heavily on extensive mechanical testing, which is both resource-intensive and expensive. This reliance highlights the need for virtual testing frameworks that can accurately predict trans-laminar failure behaviour in composite laminates and assess large damage capabilities to create damage-tolerant designs critical for the certification process. Addressing this need, this thesis introduces a novel global-local modelling approach for analyzing trans-laminar damage growth in notched composite laminates. This approach aims to facilitate large-scale structural analysis with reduced computational demands, making it suitable for comprehensive damage assessment in composite aircraft structures.

This research investigates progressive failure analysis in notched composite laminates, with a particular focus on simulating stable damage growth and arrest within the transition region of tapered laminates. A multi-fidelity modelling approach was adopted using flat laminates as a baseline for evaluating damage growth behaviour in the transition regions. Consistent ply stack orientations in both tapered and flat laminates allow for direct comparisons of their distinct failure behaviours. The methodology includes a low-fidelity model that utilizes cohesive elements to simulate self-similar crack growth initiating from notch tips. For more detailed predictions, a high-fidelity model incorporating a discrete ply-by-ply approach with cohesive interactions was used to capture both intra-laminar and inter-laminar damage mechanisms. In this high-fidelity approach, two intralaminar damage models are compared: a user-defined continuum damage model (VUMAT) and a built-in Hashin damage model available in ABAQUS. The VUMAT model applies specific softening laws and element sizes for each failure mode to ensure accurate energy dissipation, while the Hashin model serves as a simpler built-in alternative.

The results show that, despite simplifications, low-fidelity models provide reasonable estimates of failure loads and stiffness in the elastic region, aligning closely with high-fidelity models and existing experimental data. The high-fidelity model effectively captures complex damage modes, with the VUMAT model outperforming the Hashin model in accuracy. The global-local approach proves reliable, offering results comparable to a globally refined meshing approach. It shows potential for use in analyzing larger, computationally demanding structures. Overall, the findings also highlight the importance of fibre-aligned meshing and precise cohesive zone modelling in predicting damage initiation and progression in notched composite laminates, providing insights for designing damage-tolerant composite structures.

Experimental and numerical characterisation of a novel filler material was carried out, followed by an investigation of the elastoplastic behaviour in a butt joint. The study aims at understanding the behaviour of the novel material when subjected to loads as expected in the butt joint.

Skin-stiffened structures are commonly used in aerospace applications, because of the excellent strength-to-weight ratios. For this, it is important to understand the behaviour of all the materials which go into the structure. The filler material under consideration is a novel material developed for the application of a butt-joint section.

The current study is made in two phases. First, the filler material in characterised for plasticity experimentally, followed by a numerical characterisation stage, wherein the tensile tests are replicated using Abaqus. After this, short butt-joint sections were taken and subject to 3-point bending tests. FEM simulations were carried out for the 3-point bending test in order to study the state of stresses and strains occurring on the filler section.

In a nutshell, a novel filler material was first characterised experimentally as well as numerically, with special emphasis on a real-world application.