In the advanced polymer composites reinforced by 3D woven fibre architectures, tows areinterlaced into through-thickness direction to overcome the problems encountered in theapplications of traditional 2D laminates, such as poor interlaminar toughness anddelamination resistance. The understanding of the influence of fibre architectures on thefatigue performance of 3D woven composites is essential in providing guide for the designof fibre architecture. This PhD project is an in-depth study into the fatigue damagemechanisms of 3D woven composites reinforced by two kinds of fibre architectures,namely, 3D modified layer-to-layer (MLL) and 3D angle-interlocked (AI). 3D X-raycomputed tomography (CT) has been used as the main tool to non-destructively evaluateand quantify the evolution of fatigue damage, with an attempt to link macro behaviour withlocal micro (damage) microstructure. Part I is focused on a post-failure study on both typesof materials to identify their respective failure mechanism, using the combination of 2D(optical surface and SEM cross-sectional) imaging and 3D (X-ray CT) imaging. Somecharacteristic features are found in both materials: firstly, fatigue damage progresses by theinitiation of transverse cracks within weft yarns and subsequent propagation as interfacialdebonding crack until the catastrophic failure occurs in a localised area; secondly, bothmaterials display a high resistance to ultimate failure. However, a distinctive damage modeobserved in MLL composites is the extensive development of debonding cracks, whichresult in larger scale of damage (~10μm) than those in AI composites (1-2 μm). Part IIpresents an investigation of evolution of fatigue damage in 3D woven MLL compositesfollowed by an X-ray time-lapse experiment. An innovative algorithm was developed toenable automatic classification of damage, providing insight into the competition andinteraction of different damage modes. Fatigue damage is regularly distributed throughoutfatigue life, with a geometrical dependency on the repeating unit cells. Damageinteractions have been identified, indicating a high level of damage tolerance. Aquantitative analysis has been carried out to examine and compare the growth of differenttypes of damage as a function of fatigue cycles. Transverse cracks initiate at almost thebeginning the fatigue life (0.1%) and govern the growth of weft/binder debonds, but don’tcompromise fatigue life, whereas interply debonds have a large growth towards the end offatigue life and facilitate the ultimate failure. Other types of damage occurring in the resinhave a trivial effect on the fatigue life. Part III carries out a systematic study to find out thebest approach to detect the fatigue damage in the 3D AI composites. Different strategieshave been employed in each scan, including imaging the cracks with the load applied, withcontrast enhanced by phases contrast and staining. The image contrast was not effectivelyenhanced by applying phase contrast imaging, but significantly improved by staining. Withthe application of in-situ loading, the visibility of transverse cracks is highly improved,while longitudinal debonding cracks still cannot be resolved. Overall, the best approachwas found to be high resolution ROI (region of interest) scanning in combination withstaining, in terms of practical feasibility, scan time and image quality.

Three dimensional (3D) woven composites have attracted the interest of academia and industry thanks to their damage tolerance characteristics and automated fabric manufacturing. Although much research has been conducted to investigate their out-of-plane "through thickness" properties, still their in-plane properties are not fully understood and rely on extensive experimentation. The aim of this work is to study the electromechanical behaviour of three different fibre architectures of 3D woven composites "orthogonal (ORT), layer-to-layer (LTL) and angle interlock (AI)" loaded, in three different orientations "warp (0º), weft (90º) and off-axis (45º)", in quasi-static tension. Stress/strain response is captured as well as damage initiation and evolution up to final failure. The ORT architecture demonstrated a superior behaviour, in the off-axis direction, demonstrated by high strain to failure (~23%) and high translaminar energy absorption (~40 MJ/m3). The z-binder yarns in ORT suppress delamination and allow larger fibre rotation during the fibre "scissoring motion" that enables further strain to be sustained. In-situ electrical resistance variation is monitored using a four-probe technique to correlate the resistance variation with the level of damage induced while loading. Monotonic and cyclic "load/unload" tests are performed to investigate the effect of piezo-resistivity and residual plasticity on resistance variation while damage is captured by X-ray scanning during interrupted tests at predefined load levels. In addition, this study investigates the potential of using 3D woven composites in joint assemblies through open-hole tension and "single fastener double-lap joint" bearing strength tests. 3D woven composites in the off-axis orientation, especially ORT, demonstrate a potential for overcoming some of the major challenges for composite joints' applications which are the pseudo-ductility, stress redistribution away from the notch and notch insensitivity. Finally, the study proposes a micro-mechanics based damage model to simulate the response of 3D orthogonal woven composites loaded in tension. The proposed model differs from classical damage mechanics approaches in which the evolution law is obtained by retrofitting global experimental observations.

The future of the aerospace industry in large part relies on two factors: (i) development of advanced damage tolerant materials and (ii) development of advanced smart sensors with the ability to detect and evaluate defects at very early stages of component service life. Laminated composite materials, such as carbon fibre reinforced plastics (CFRP), have emerged as the materials of choice for increasing the performance and reducing the cost and weight of aircrafts, which leads to less fuel consumption and therefore lower CO2 emissions. However, it is well known that these materials exhibit fragile behaviour, poor resistance to impact damage caused by foreign objects and require a relatively slow and labour intensive manufacturing process. These factors prevent the rapid expansion of composite materials in several industrial sectors at the current time. Inspired by the use of rope throughout history and driven by the necessity of creating a lean manufacturing process for composites and enhancing their impact properties, the first part of this work has shown that enhanced damage tolerance and pseudo-ductile behaviour can be achieved with standard CFRP by creatively arranging the fibres into a 3D twisted helical configuration. Through an extensive experimental campaign a new method to arrange fibre reinforcement was presented and its effect investigated. The second part of this PhD work focused on developing a new smart sensor. A spiral passive electromagnetic sensor (SPES) for damage detection on CFRP and glass fibre reinforced plastics (GFRP) is presented in this work. A range of defect types in glass and carbon composite has been considered, such as delamination, perforated holes and cracks. Furthermore, throughout this work, the SPES has been exploited as a multi-sensing device allowing the ability to detect temperature and humidity variation, presence of ice and act as an anti/de-icing device.

Ces travaux s’attachent à décrire et quantifier les mécanismes de ruptures des compositestissés 3D sous chargement de traction quasi-statique et à mettre en place une méthode de simulationnumérique adaptée et robuste, pouvant à terme être appliquée en bureau d’études.Dans cette optique, une étude expérimentale a été menée afin de quantifier la propagation defissures dans ces matériaux. Celle-ci a permis de mettre en place un scenario de rupture, entirant parti de la multi-instrumentation des essais. L’étude a également été effectuée sur deséprouvettes de géométries et de tailles variées et a mis en évidence d’importantes variations dutaux de restitution d’énergie avec les conditions d’essai. Un formalisme d’analyse et de modélisationintroduisant des longueurs internes a ensuite été présenté et adapté aux mécanismes derupture des composites tissés 3D. Ce formalisme est étayé par la recherche des mécanismes àl’aide de l’analyse des faciès de rupture. Les longueurs introduites ont ainsi été mises en relationavec les paramètres du tissage. Une méthode d’identification des paramètres a été proposée etles conséquences de ce comportement sur le dimensionnement de pièces composites discutées.Enfin, le transfert de ces résultats a été effectué vers des simulations numériques robustes. Desméthodes de régularisation des modèles d’endommagement continu ont été présentées et évaluéesà l’aune de leur capacité à assurer, d’une part, la robustesse des résultats et, d’autre part,la bonne retranscription des effets d’échelle expérimentaux. La prise en compte de ces considérationsnumériques et physiques nous a amené à proposé un modèle d’endommagement Non-Local.Une méthode d’identification des paramètres et de la longueur interne à partir des données expérimentalesa été proposée. / This work aims to describe and quantify the failure mechanisms of 3D woven composites underquasi-static tensile loading and to implement an adapted and robust numerical simulationmethod, that can be applied in industry. To this end, an experimental study was carried out toquantify the propagation of cracks in these materials. Thus, a crack propagation scenario wasestablished, thanks to the multi-instrumentation used during the tests. The experimental campaignwas carried out on specimens of various geometries and sizes and highlighted significantvariations in the fracture toughness with the test conditions. A modelisation framework introducinginternal lengths was then presented and adapted to 3D woven composites. This frameworkis supported by the identification of the failure mechanisms subsequent to the analysis of thecrack profile. The introduced lengths were thus related to the weaving parameters. A method foridentifying the parameters was proposed and the consequences of this behaviour on the designof the composite parts discussed. Finally, these results were transferred to robust numerical simulations.Regularisation methods of continuous damage models were presented and evaluatedin terms of their ability to ensure, on the one hand, the robustness of the results and, on theother hand, the correct transcription of experimental size effects. Taking into account these numericaland physical considerations led us to propose a Non-Local damage model. A method foridentifying the parameters and the internal length on experimental data was proposed.