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This thesis presents a new modeling framework and application methodology for the study of aircraft structures. The framework provides a ‘cradle-to-grave’ approach to structural analysis of a component, where structural integrity encompasses all phases of its lifespan. The methodology examines the holistic structural design of aircraft components by integrating fatigue and damage tolerance methodologies. It accomplishes this by marrying the load inputs from a fatigue analysis for new design, into a risk analysis for an existing design. The risk analysis incorporates the variability found from literature, including recorded defects, loadings, and material strength properties. The methodology is verified via formal conceptualization of the structures, which are demonstrated on an actual hydraulic accumulator and an engine nacelle inlet. The hydraulic accumulator is examined for structural integrity utilizing different base materials undergoing variable amplitude loading. Integrity is accomplished through a risk analysis by means of fault tree analysis. The engine nacelle inlet uses the damage tolerance philosophy for a sonic fatigue condition undergoing both constant amplitude loading and a theoretical flight design case. Residual strength changes are examined throughout crack growth, where structural integrity is accomplished through a risk analysis of component strength versus probability of failure. Both methodologies can be applied to nearly any structural application, not necessarily limited to aerospace.

Les impacts sur structures composites peuvent fortement diminuer leur résistance résiduelle sans laisser de marque visible sur la surface extérieure. Dans le domaine aéronautique, un seuil minimum de détection de l’endommagement d’impact est défini, basé sur l’indentation permanente laissée par l’impact. En deçà de ce seuil, la structure doit résister à un chargement défini : c’est la notion de tolérance aux dommages d’impact. Dimensionner numériquement une structure composite en tenant compte des aspects détectabilité et tolérance aux dommages nécessite donc de savoir modéliser à la fois l’impact, l’indentation permanente et la résistance résiduelle sous compression.Ces travaux se focalisent sur la modélisation numérique des composites stratifiés formés de plis unidirectionnels. L’objectif est d’établir un modèle prédictif de la tenue résiduelle après impact. Une étude expérimentale a été réalisée afin d’étudier le scénario d’endommagement à l’impact et sous compression après impact (CAI), et de fournir des résultats expérimentaux pour valider les modèles numériques.Une modélisation par éléments finis avec une approche de type Discrete Ply Model (DPM) est effectuée, basée sur des travaux précédents. Le modèle d'impact est amélioré et validé sur différentes séquences d'empilement pour assurer la robustesse du modèle. Des essais de flexion trois points spécifiques sont réalisés pour apporter une meilleure compréhension de la formation de l'indentation permanente. Un nouveau modèle d'indentation permanente est alors proposé et appliqué dans le modèle d'impact. Enfin, un modèle de CAI est construit pour prédire la résistance résiduelle. Les trois étapes : impact, indentation et CAI sont combinées au sein d’un unique modèle. / Impacts on composite structures can greatly reduce their residual strength without leaving a visible mark on the outer surface. In aeronautics, a minimum detection threshold of the impact damage is defined, based on the permanent indentation left after impact. Below this threshold, the structure must withstand a defined load: it is the notion of impact damage tolerance. The numerical design of a composite structure taking into account aspects such as detectability and damage tolerance thus requires to know how to represent impact, permanent indentation and residual strength under compression.This work focuses on the numerical modeling of composite laminates made of unidirectional plies. The objective is to develop a predictive model of post-impact residual strength. An experimental study was conducted to investigate the damage scenario during impact and compression after impact (CAI), and provide experimental data to validate the simulations.A finite element modeling with a Discrete Ply Model (DPM) approach is performed based on previous work done at the laboratory. The impact model is improved and validated on different stacking sequences to ensure the robustness of the model. Specific three-point bending tests are performed to have a better understanding of the formation of permanent indentation. A new model of permanent indentation is then proposed and applied in the impact model. Finally, a model is built to predict CAI residual strength. The three steps: impact, indentation and CAI are combined into a single model.

Faithful replication of genomic DNA is crucial for the survival of a cell. In order to achieve high-level accuracy in copying its genome, all cells employ replicative DNA polymerases that have intrinsic high fidelity. When an error occurs on the template DNA strand, in the form of lesions caused by diverse chemicals, reactive oxygen species, or UV light, the high-fidelity replicative DNA polymerases are stalled. To bypass these replication blocks, cells harbor multiple specialized translesion DNA polymerases that are error-prone and therefore able to accommodate the lesions and continue DNA synthesis. As a result of their low fidelity, the translesion polymerases are associated with increased mutagenesis, drug resistance, and cancer. Therefore, the access of the translesion polymerases to DNA needs to be tightly controlled, but how this is achieved has been the subject of debate. This Thesis presents the development of a co-localization single-molecule spectroscopy (CoSMoS) method to directly visualize the loading of the Escherichia coli replicative polymerase on DNA, as well as the exchange between the replicative polymerase and the translesion polymerases Pol II and Pol IV. In contrast to the toolbelt model for the exchange between the polymerases, this work shows that the translesion polymerases Pol II and Pol IV do not form a stable complex with the replicative polymerase Pol IIIα on the β-clamp. Furthermore, we find that the sequential activities of the replication proteins: clamp loader, clamp, and Pol IIIα, are highly organized while the exchange with the translesion polymerases is disordered. This exchange is not determined by lesion-recognition but instead a concentration-dependent competition between the replicative and translesion polymerases for the hydrophobic groove on the surface of the β-clamp. Hence, our results provide a unique insight into the temporal organization of events in DNA replication and translesion synthesis.

This thesis presents the work of an experimental investigation into the impact damage resistance and damage tolerance for symmetrical and unsymmetrical composite honeycomb sandwich panels through in-plane compression. The primary aim of this research is to examine the impact damage resistance of various types of primarily carbon/epoxy skinned sandwich panels with varying skin thickness, skin lay-up, skin material, sandwich asymmetry and core density and investigate the residual in-plane compressive strengths of these panels with a specific focus on how the core of the sandwich contributes to the in-plane compressive behaviour. This aim is supported by four specifically constructed preconditions introduced into panels to provide an additional physical insight into the loading-bearing compression mechanisms. Impact damage was introduced into the panels over a range of IKEs via an instrumented drop-weight impact test rig with a hemi-spherical nosed impactor. The damage resistance in terms of the onset and propagation of various dominant damage mechanisms was characterised using damage extent in both impacted skin and core, absorbed energy and dent depth. Primary damage mechanisms were found to be impacted skin delamination and core crushing, regardless of skin and core combinations and at high energies, the impacted skin was fractured. In rare cases, interfacial skin/core debonding was found to occur. Significant increases in damage resistance were observed when skin thickness and core density were increased. The reduction trends of the residual in-plane compressive strengths of all the panels were evaluated using IKE, delamination and crushed core extents and dent depth. The majority of impact damaged panels were found to fail in the mid-section and suffered an initial decline in their residual compressive strengths. Thicker skinned and higher density core panels maintained their residual strength over a larger impact energy range. Final CAI strength reductions were observed in all panels when fibre fracture in the impacted skin was present after impact. Thinner skinned panels had a greater compressive strength over the thicker skinned panels, and panel asymmetry in thin symmetrical panels appeared to result in an improving damage tolerance trend as IKE was increased due to that the impact damage balanced the in-plane compressive resistance in the skins with respect to the pre-existing neutral plane shift due to the uneven skin thickness.

Fibre reinforced composites have higher specific strength and stiffness in comparison to metals. However, composites are susceptible to impact damage resulting in degradation of mechanical properties especially compression strength. Numerous studies have been conducted to improve the impact damage tolerance of composite laminates using modified resin systems, thermoplastic matrices, 3-D fibre architectures and through thickness reinforcement. This work is primarily focussed on incorporating non dissolvable polypropylene fibres (PP) in a thermoset matrix for improving the damage tolerance. Commingling and wrapping techniques have been investigated. PP fibres have been incorporated at the preform stage and hence do not adversely affect the viscosity of the resin during infusion. The healing effect of PP fibres on impact damaged composite laminates when heating is introduced has also been studied. High velocity impact test results showed that using commingled glass/PP fibres increased the total energy absorption of composite laminates by 20% due to the extensive plastic deformation of the PP fibres and through the use of toughening mechanisms in the form of resin cracking and delamination. It has been found that PP fibres provide protection to the glass fibres during low velocity impact loading, so fewer fibre breakages occur which lead to improved residual properties compared with pristine glass laminates. Compression after impact (CAI) tests showed that the residual strength as a percentage of non-impacted strength increased with percentage of PP fibres used. For impact of 20-50J, glass/epoxy laminates retained 32 45% of their compressive strength while laminates with 7%, 13% and 18% PP fibres retained 37 50%, 42-52% and 43-60% of their compressive strength, respectively. It was also observed that glass/PP woven laminates had better compressive strength retention (62 83%) than the glass/PP non-crimp laminates (37-50%). Composite laminates with high-modulus PP fibres (Innegra) exhibited higher residual compression strengths in comparison to laminates with lower modulus PP fibres. For 15-50J impact, glass/Innegra laminates showed residual compression strength of 50 63% in comparison to 39-60%; laminates without thermoplastic fibres exhibited 33 43% residual compression strength. Modulus of thermoplastic fibres appears to be important at higher energy levels. Healing of damaged commingled laminates produced a significant reduction in the damage area and a corresponding increase in CAI strength after heating at 200ºC; CAI strength of healed laminates is about 85% of undamaged samples in comparison to 60% for non-healed samples. A novel micro-wrapping technique, developed in this work, demonstrated significant reduction in damage area (46%) in comparison to the commingling method. Core wrapped laminates had higher residual strength (43-60%) than glass laminates (33-43%). Better PP distribution in core-wrapped composites helped to decrease the PP rich areas and the impact damage did not propagate easily in comparison to commingled composites. However due to the reduction in damage area, impact energy absorption in core wrapped laminates was lower than for commingled.

3D woven composites, due to the presence of through-thickness fibre bridging, have the potential to improve damage tolerance and at the same time to reduce the manufacturing costs. However, the ability to withstand damage depends on weave architecture as well as the geometry of individual tows. A substantial amount of research has been performed to understand in-plane properties as well as the performance of 3D woven composites exposed to impact loads, but there is limited research on the damage tolerance and notch sensitivity of 3D weaves and no work is reported on the damage tolerance of 3D weaves with a weft binding pattern. In view of the recent interest in 3D woven composites, the influence of weft binder on the tensile, open hole tensile, impact resistance and subsequent residual compressive strength properties and failure mechanisms of 3D woven composites was investigated against equivalent UD cross-ply laminate. Four different 3D woven architectures; layer-to-layer, angle interlocked, twill angle interlock and modified angle interlock structures were produced under identical weaving conditions. All the above mentioned tests were performed in both the warp and weft directions on 3D woven and UD cross-ply laminates. Stress concentration and yarn waviness due to through-thickness reinforcement led to lower mechanical properties compared with the UD cross-ply laminate. However, improved in-plane and damage tolerance properties of 3D woven composites under tensile loads were achieved by modifying the weave architecture. The influence of the weave architecture and binder yarn orientation on the notch insensitivity and damage tolerance of 3D woven composites was less significant for compressive loads. Despite the lower undamaged compression strength of 3D woven structures, their residual compressive strength was found to be superior to their equivalent UD cross-ply laminates. The lower rate of strength reduction in the 3D woven fabrics laminates was attributed to a crack bridging mechanism, effectively inhibiting delamination propagation.

Laser cladding is a more recent approach to repair of aviation components within a damage tolerant framework, with its ability to restore not simply the geometric shape but the static and fatigue strength as well. This research analysed the fatigue performance of Ti-6Al-4V that has undergone a laser clad repair, comparing baseline specimens with laser clad repaired, and repaired and heat treated specimens. First an understanding of the microstructure was achieved by use of BSE imagery of the substrate, clad repaired region and post heat treated regions. The substrate of the material was identified with large grains which compared to a repaired clad region with a much finer grain structure that did not change with heat treatment. Next, performance of the specimens under tensile fatigue loading was conducted, with the clad specimens experiencing unexpectedly high fatigue performance when compared to baseline samples; the post heat treated specimen lasting significantly longer than all other specimens. It is theorised that the clad may have contributed to an increase in fatigue resilience due to its fine microstructure, when compared to the softer, more coarse substrate. The heat treatment is likely to have relaxed any residual stresses in the specimens leading to a reduction in any potential undesirable stresses, without impacting the microstructure. Residual stress analysis using EDD was unproductive due to the unexpected coarse microstructure and did not provide meaningful results. Fractography using the marker-band technique was explored with some success, proving a feesable method for measuring fatigue crack growth through a specimen post failure. Unfortunately fatigue crack growth throughout the entire fatigue life was not possible due to the tortuous fracture surface and potentially due to the fine micro-structure of the clad, resulting in interrupted marker-band formation. Future research shall expand on this work with a greater focus on residual stress analysis and its impact on fatigue.

Master's thesis deals with the damage tolerance analysis of wing-to-fuselage joint of L 410 NG airplane. Thesis includes determination of the load distribution to the individual attachments of wing-to-fuselage joint, residual strength analysis and residual fatigue life analysis of the most loaded attachment lugs, calculation of fatigue crack growth curves in the attachment solids and inspection program proposal. This analysis was performed using FE model of the wing and central part of the fuselage and AFGROW software.