Composite materials are widely used in demanding applications in aerospace and other industries. In order to understand the complex behaviour of the composite materials and their components, standardised test methods are used. One example is the double cantilever beam (DCB) test in which the test specimen is loaded in an opening, i.e., tensile mode. Failures in composite materials loaded compression are different from those in tension, for example, kink band or buckling-like failures can occur. In this project, several DCBs are designed and a new fixture which allows for compression testing of a DCB is developed for an existing Instron testing machine. The fixture overcomes a known problem of tensile peak causing the failure of the adhesive at the inner surfaces of the DBC by applying additional compressive loads along the outer surfaces of the DBC. The compressive forces can induce the desired kink band formation so that researchers can better study the failure mode. The conceptual development of the new DCBs and the new fixture are presented. Several prototypes of the specimens and the fixture are modelled using the three-dimensional (3D) computer-aided design software Creo Parametric 2.0.  One of the fixtures is selected to further study. The different DCB specimens are studied in order to obtain information about the kink band using 3D finite element analysis with the software programme Abaqus CAE. The selected fixture is analysed to determine if there are any areas of concern. Finally, the behaviour of the compression stress along the DCB using two pairs of forces is studied. Unfortunately, it is determined that the tensile peak experienced by the adhesive cannot be eliminated by the application of two pairs of compressive loads, one at the free end and the other in the vicinity of the tensile peak. Several suggestions are made for future work which might serve to reduce the tensile peak; e.g., the movable force couple is applied as a surface load instead of a point load. For this, the fixture will have to be modified with a new geometry, although the DCB could be the same. This will allow further work to focus on the combined behaviour of the tensile peak and the fixture.

The compressive strength of unidirectional (UD) carbon fibre reinforced plastics (CFRPs) is often only 60-70% of their tensile strength owing to premature failure associated with kink-band formation. The sudden and complex nature of kink-band formation has been hindering the progress in experimental studies on the evolution of damage in compressive failure. A better understanding of the damage mechanisms associated with kink-band formation can help to design more reliable composite structures. Therefore, the principal aim of this project is to identify, in three dimensions (3D), the key damage mechanisms underlying the initiation and propagation of kink bands in UD carbon fibre/epoxy composite. A new manufacturing method is developed to fabricate high-quality UD T700/epoxy cylindrical rods for axial compression tests and high-resolution imaging of kink bands by post mortem and in situ X-ray computed tomography (CT). The morphology of kink bands is visualised in 3D by segmenting fibre breaks at kink-band boundaries and representative longitudinal splits. The geometrical parameters of each fully developed kink band are consistent through the specimen. Radiographs obtained from ultra-fast synchrotron imaging show that a kink band initiates and propagates across the specimen in less than 1.2 ms. A scenario of kink-band failure is proposed: fibre buckling and longitudinal splitting occur prior to fibre breakage, which forms kink-band boundaries and eventually the morphology of multiple kink bands develops suddenly. 3D tomographs of the fast and unstable kink-band formation could not be captured in the axial compression experiments. Therefore, a testing method of loading notched UD carbon fibre (T800, T700 and T300)/epoxy beams using a four-point bending (FPB) fixture is developed to enable monitoring of more stable initiation and propagation of kink bands by in situ X-ray CT. Kink-band formation is significantly slowed in the FPB tests. Fibre micro-buckling accompanied by splitting, could initiate the formation of kink bands. In the T700/epoxy system, the early initiation stage of fibre micro-buckling without fracture is captured, and the critical radius of curvature of unbroken fibres prior to fracture is ～130micro metre. Unloading causes significant recovery of fibre curvature (radius of curvature ～280 micro metre) and a reduction of 10-20º in fibre rotation angle within the kink band. The results show that in situ 3D characterisation of kink bands is essential as fibre buckling is a 3D phenomenon, resulting in development of both in-plane and out-of-plane kink bands. Understanding of kink-band formation in 3D will help to establish strategies to improve the compressive strength of CFRP composites by depressing kink-band formation; in this respect lateral constraint conferred by strong interfaces is a key aspect.

Il est couramment admis que la déformation plastique des phases MAX est dueau glissement de dislocations dans les plans de base s'organisant en empilements et murs. Cesderniers peuvent former des zones de désorientation locale appelées kink bands. Cependant, lesmécanismes élémentaires et le rôle exact des défauts microstructuraux sont encore mal connus. Cemanuscrit présente une étude expérimentale multi-échelle des mécanismes de déformation de laphase MAX Ti2AlN. A l'échelle macroscopique, deux types d'expériences ont été menés. Des essaisde compression in-situ à température et pression ambiantes couplés à la diffraction neutroniqueont permis de mieux comprendre le comportement des différentes familles de grains dans le Ti2AlNpolycristallin. Des essais de compression sous pression de confinement ont également été réalisés dela température ambiante jusqu'à 900 °C. À l'échelle mésoscopique, les microstructures des surfacesdéformées ont été observées par MEB et AFM. Ces observations complétées par des essais denanoindentation ont montré que la forme des grains et leur orientation par rapport à la directionde sollicitation gouvernent l'apparition de déformations intra- et inter-granulaires ainsi que lalocalisation de la plasticité. Finalement à l'échelle microscopique, une étude détaillée par METdes échantillons déformés sous pression de confinement a révélé la présence de configurations dedislocations inédites dans les phases MAX, telles que des réactions entre dislocations, des dipôleset des dislocations hors plan de base. À la vue de ces résultats nouveaux, les propriétés mécaniquesdes phases MAX sont rediscutées. / It is commonly believed that plastic deformation mechanisms of MAX phases consistin basal dislocation glide, thus forming pile-ups and walls. The latter can form local disorientationareas, known as kink bands. Nevertheless, the elementary mechanisms and the exact role ofmicrostructural defects are not fully understood yet. This thesis report presents a multi-scale experimentalstudy of deformation mechanisms of the Ti2AlN MAX phase. At the macroscopic scale,two kinds of experiments were performed. In-situ compression tests at room temperature coupledwith neutron diffraction brought new insight into the deformation behavior of the different grainfamilies in the polycrystalline Ti2AlN. Compression tests from the room temperature to 900 °Cunder confining pressure were also performed. At the mesoscopic scale, deformed surface microstructureswere observed by SEM and AFM. These observations associated with nanoindentationtests showed that grain shape and orientation relative to the stress direction control formationof intra- and inter- granular strains and plasticity localization. Finally, at the microscopic scale,a detailed dislocation study of samples deformed under confining pressure revealed the presenceof dislocation configurations never observed before in MAX phases, such as dislocation reactions,dislocation dipoles and out-of-basal plane dislocations. In the light of these new results, mechanicalproperties of MAX phases are discussed.