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Hierarchically structured fibre reinforced polymer (FRP) composites are a new generation of structural materials with high potential for tailored design. The increased degree of freedom in material selection and design is the main advantage of these materials over conventional composites. Ho wever, the available literature still lacks a comprehensive study on the structure-property relation ship and the interactions between the constituents in these composite materials.This work aims at exploring various aspects of new hierarchically structured carbon fibre polymer composites (CFRP) with the goal of understanding the interplay between the different components in relation to their mechanical properties and fracture. Multiwall carbon nanotubes (CNTs) and a phase separating thermoplastic modifier (polyoxymethylene (POM)) are the main structural elements ¨used to form the microstructure of the studied ma terials.The principal approach adopted in this work is to establish an initial understanding of the structure-property relationship in binary ( POM/epoxy or CNT/epoxy) and ternary (POM/CNT/epoxy ) bulk resin blends. This involves a study of the phase morphology, dynamic mechanical properties, and fracture toughness of various types of bulk resin blends with different compositions. At the next step, fiber reinforced composites based on the previously studied matrix blends are produced a nd characterized. Considering the challenges involved in the processing of the POM modified matrix blends into the corresponding composites at high temperature, a new manufacturing technique based on resin transfer molding is developed and further optimized. The knowledge acquired during the study of the matrix blends is employed to explain morphological observations as well as fracture properties and damage behaviour monitored during quasi-static tensile loading of the produced laminates. A correlation between the microstructure and phase morphology of the matrix and the properties of the laminates is established. It is shown that phase separated POM particles are able to enhance fracture toughness of bulk epoxy resins, as long as the particulate morph ology is dominant. Fracture toughness and ultimate mechanical properties of the bulk resin materials ¨are shown to be highly sensitive to the phase morphology of the phase-separated blend. Therefore, any external factors affecting phase separation of the thermoplastic modified blends can dr astically influence resulting properties. For ¨instance, it is illustrated that the presence of¨ fiber reinforcement changes the phase morphology of the matrix and, hence, affects the transfer of the toughness improvements of the bulk resin to the laminates.Inclusion of CNTs is als shown to influence the phase morphology of the bulk resin as well as the microstructure of the resulting hierarchical laminates. It is shown that CNTs limit mass diffusion during phase separation of the thermoplastic phase, causing redu ction in the particle size of the resulting thermo plastic particles. This in turn affects the fracture toughness of the bulk resin blends and damage development in the resulting CFRP laminate. In this part of the work, a new approach for incorporation of CNTs in the POM modified matrices is introduced, in which the phase separating thermoplastic particles are surrounded by clusters of CNT agglomerates.

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The overall objective of this project was to create novel carbon fiber composites with an excellent penetration impact resistance through hybridization with high-performance polymer fibers. The first step consisted of choosing the best matrix: polypropylene (PP) and epoxy were investigated. By changing the matrix from epoxy to PP, aramid and carbon fiber-reinforced composites absorbed 2.8 and 2.0 times more energy, respectively. The absorbed energy ratio aramid/carbon was 2.3 for PP, while 1.6 for epoxy. This study confirmed an important role of the matrix in design of impact resistance composites. PP was the logical matrix choice for the rest of the study, as the higher ratio has a higher potential for synergistic effect in fiber hybridization. The next step was to investigate the low-velocity impact behavior of a wider range of high-performance polymer fibers. Aramid, polybenzobisoxazole (PBO) and polyarylate (PAR) weaves were used with PP as matrix. PBO-PP absorbed the most energy, almost twice as much as aramid-PP and PAR-PP. Damage mechanisms were about the same: they all showed fiber fibrillation, fiber pullout and delaminations. The major difference lied in the shape of the failed region under the penetration impact. Aramid-PP failed in a “+” shape, while PBO-PP and PAR-PP failed in an “X”-shape. In the third and final step, carbon fiber composites are hybridized with each of the three high-performance polymer fibers. A positive hybrid effect was found when carbon is hybridized with PBO (up to 27%) and PAR (up to 10%), whereas aramid-carbon hybrids show no hybrid effect. The performance of the latter simply followed the linear rule of mixtures. All hybrids failed in a “+” shape. It is hypothesized that this shape is the reason for the observed hybrid effects: non-hybrid PBO and PAR fail in an “X”-shape, while carbon fails in a “+” shape. By combining these fabrics, competition arises between the two failure shapes, which results in an increased absorbed energy.

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While composite materials are undergoing rapid improvements by industry and academia year on year, the accurate characterization of their properties remains a challenge even today. The biggest challenge for unidirectional (UD) composites is posed by the longitudinal stress concentrations observed under the grips in tensile testing, which can lead to premature failure and inaccurate determination of tensile strength. The standard practices for composite tensile testing, especially for UD composites, remain open-ended in terms of specimen geometry. This research aims at developing a thorough understanding of the stress states involved in tensile testing. These stress states can lead to premature failure in conventional designs and based on their understanding, the improvement obtained by using novel continuous tab geometry is explored. A continuous tab is designed as a sandwich type hybrid of material being tested (test layer) with a continuous tab layer on each side. The materials considered for this research are UD carbon fiber reinforced polymer (CFRP), glass fiber reinforced polymer (GFRP) and a hybrid of these to materials. UD S-glass/epoxy and E-glass/epoxy composites are considered as continuous tab materials. To meet the objectives of this research work, finite element analysis (FEA) was conducted on specimen with varying geometrical and material parameters. Firstly, the influence of conventional and continuous tab geometries on the stress states were analyzed and compared. The next step was to obtain an ideal geometry for the continuous tab specimen such that it does not experience longitudinal stress concentration in test layer. To enable this, FEA simulations were conducted for different material combinations and thicknesses for both the test layer and continuous tab layer. Comparative analysis performed on the tensile test specimen model with conventional end-tabs and continuous tabs showed that continuous tabs were effective in reducing longitudinal stress concentrations under the grips. The continuous tab also showed improvement in terms of transverse and shear stress states present in specimen due to gripping force. The results from the analysis using different thickness of specimen and tab, with carbon fiber/epoxy as test layer and S-glass/epoxy as continuous tab layer, showed that longitudinal stress concentrations under grips reduced as the ratio of thickness of test layer vs. continuous tab layer (defined as thickness ration) decreased. Also, for thickness ratios lower than 0.5 for the above material, no longitudinal stress peaks were observed under the grips. Furthermore an analytical equation was proposed to determine longitudinal position of the stress concentrations’ maxima in relation to the thickness ratio. This equation, while limited by the current study dataset, should help in predicting the position of the maximum longitudinal stress for a given thickness ratio and save valuable time and effort for engineers in choosing the appropriate thickness of the test and continuous tab layers. Longitudinal stress concentration variations were also studied for different stiffness ratios of test layer with respect to continuous tab layer, but were observed to not follow any trend. Further analysis would be required for understanding the effect of stiffness ratio on the longitudinal stress concentration.

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The time-dependent properties of a unidirectional (UD) composite loaded in tension were investigated to improve lifetime and reliability assessment. It is current belief that the matrix material plays an important role in this strength degradation over time. The goal of this dissertation is to reveal the dominant mechanisms and to investigate a modeling as well an experimental approach towards this problem. As a result the visco-plastic properties of the NTPT 736LT epoxy resin were investigated by performing compression tests at different strain rates and temperatures. An algorithm in MATLAB was developed in order to obtain the required modelling parameters for an existing resin model (originally developed for RTM-6 epoxy resin by Morelle (2015)). Compressive creep tests were conducted to assess the matrix creep behavior at different stress levels. It was observed that the creep behavior of the matrix is strongly stress dependent and different in the pre- and post-yield region. There are indications that imply the onset of secondary and tertiary creep stages in the long term behavior. A creep subroutine in Abaqus was developed to model the matrix behavior over time. A three-dimensional finite element analysis was performed to assess the time-dependent evolution of the stress state surrounding a fiber break in a unidirectional carbon fiber-reinforced composite with hexagonal packing. The simulation was conducted for different macroscopic strains at a timescale of 20000s or 5.5h. The results indicate that the stress concentration factors decrease upon increasing macroscopic strain and increasing time, this was assessed to be due to yielding and relaxation of the matrix respectively. The redistribution of initial stress concentrations near the fiber break happens in a matter of hours due to the strong stress dependence of the creep strain. The 736LT resin was compared with an RTM-6 resin as a reference case, the latter exhibits a slightly higher yield strength and is less prone to creep. First results indicate that the effect of different yield strength on the stress concentrations remains limited when considering matrix relaxation. The same applies to the evolution of the ineffective length over time. The time and stress dependent parameters however, play a major role. At low timescales the stress-dependent parameter is dominant, at higher timescales it is the other way around. More research is required to validate these statements. The obtained results are useful input parameters for the improvement of state-of-the-art composite strength models, which will enable the prediction of time-dependent composite failure in tension. Simulated results of time-dependent fiber breaks can be compared to in situ synchrotron CT scans, of composite specimens under load, acting as a validation case.

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Despite long-lasting efforts by both industry and academia, accurate characterization, understanding, and finally mitigation of delamination in carbon fibre reinforced polymers remains a ‘stinging’ challenge. The complex nature of delamination is interleaved with its multivariable character. Concretely, the orientation of the plies adjacent to the delaminating interface, the stacking sequence, the specimen geometry, and the interaction with other damage and toughening mechanisms are just some of the influential factors in fracture toughness characterization. In parallel, assessment of these factors is extremely difficult owing to their interdependence and potential coupling. Due to these challenges, the standards ruling fracture toughness characterization remain bounded, while scientific views on both the qualitative and quantitative assessment of fracture toughness are far from reaching a consensus. Bearing in mind the significance of delamination resistance characterization, we aimed to contribute to the research and investigation of phenomena controlling the process of delamination initiation and growth in carbon fibre reinforced polymer composites (FRPC). In particular, the first objective of the research was to establish relationships between the fracture toughness values in mode-I loading and the fibre orientation at the delamination interface. In order to do so, Double Cantilever Beam (DCB) tests were performed for two different layups, namely [0°]22 and [(+45°/-45°)5/45°//-45°/(-45°/+45°)5], with the interface of interest present at the midplane. The tests were coupled with macroscale digital image correlation and in-situ edge microscopy, for crack tip monitoring and visualisation of the occurring phenomena. For the study to be executed in a systematic way, simultaneous focus was incorporated to unravel dependencies on the global stiffness matrix, something which brings us to our second objective. Mechanisms such as antielastic curvature and bending-twisting coupling, influencing the load distribution at the crack tip and consequently the shape of the crack front, have been repeatedly reported in the literature. However, since carbon composites are opaque, in-situ surface imaging techniques are incapable of tracking the shape of the crack front and hence characterization is often based on post-mortem analysis. To counteract this inability, a novel technique, micro-computed tomography, was implemented, allowing in-situ assessment of crack front shape during DCB testing. The results show significant differences between the two interface types. From the edge microscopy results, the crack path for the unidirectional [0°]22 specimens appeared straight with no crack migration or severe bridging phenomena. For the [±45°]-interface, however, phenomena such as crack migration to adjacent plies and secondary crack formation were evident. Furthermore, based on the in-situ DCB with micro-computed tomography results, 3D characterization of the crack front shape in various loading steps took place, revealing the crack front propagating faster in the middle of the specimens. A possible expansion of this study could be towards assessing different delamination interfaces, nature of plies and loading modes. These expansions are bound to provide a more comprehensive understanding of fracture toughness.