No description available.

The use of fibre-reinforced polymer matrix composite materials is growing at a faster rate than GDP in many countries. An improved understanding of their processing and mechanical behaviour would extend the potential applications of these materials. For unidirectional composites, it is predicted that localised absence of fibres is related to longitudinal compression failure. The use of woven reinforcements permits more effective manufacture than for unidirectional fibres. It has been demonstrated experimentally that compression strengths of woven composites are reduced when fibres are clustered. Summerscales predicted that clustering of fibres would increase the permeability of the reinforcement and hence expedite the processing of these materials. Commercial fabrics are available which employ this concept using flow-enhancing bound tows. The net effect of clustering fibres is to enhance processability whilst reducing the mechanical properties. The effects reported above were qualitative correlations. Gross differences in the appearance of laminate sections are apparent for different weave styles. For the quantification of subtle changes in fabric architecture, the use of automated image analysis is essential. Griffm used Voronoi tessellation to measure the microstructures of composites made using flow-enhancing tows. The data was presented as histograms with no single parameter to quantify microstructure. This thesis describes the use of automated image analysis for the measurement of the microstructures of woven fibre-reinforced composites, and pioneers the use of fractal dimensions as a single parameter for their quantification. It further considers the process-property- structure relationships for commercial and experimental fabric reinforcements in an attempt to resolve the processing versus properties dilemma. A new flow-enhancement concept has been developed which has a reduced impact on laminate mechanical properties.

The aerospace industry is steadily increasing its use of polymer-matrix composites (PMCs) in airframe structures as it seeks to benefit from the high specific in-plane strength of laminated structural PMCs. However, PMC laminates suffer from low interlaminar shear strength due to their weaker polymer-matrix. Minimising risks of delamination is of paramount importance towards improving the safety of PMC structures. Multi-scale composites that are reinforced by both continuous fibres and nano-particles were identified as a potential solution for improving toughness and reducing risks of delamination in PMCs. An important challenge in the fabrication of multi-scale PMCs is to ensure that nano-particles are dispersed uniformly within the matrix. This is only achieved through minimal filtration of nano-particles during processing. The short resin flow lengths enabled by the resin film infusion (RFI) process make this process a prime candidate for the fabrication of multi-scale PMCs. The main objective of this thesis is to validate the possibility of using out-of-autoclave RFI for fabricating multi-scale carbon fibre composites featuring epoxy resins modified with carbon nanotubes (CNTs). The work is accomplished in 5 phases. In phase 1, preliminary work investigates the fabrication of PMCs with and without CNTs, using out-of-autoclave RFI. Results show that the types of reinforcement and matrix have strong effects on the porosity and interlaminar strength of PMCs. These results ushered the need for more thorough investigation and understanding of the RFI process, beyond what is available in the literature. Phases 2 to 4 focus on understanding how the choices of materials and types of stacking configuration can affect parts made using RFI. Phase 2, the in-situ characterisation of resin saturation during RFI is performed. Results enable a detailed analysis of the way in which resin flows around and inside yarns. Phase 3 consists in the characterisation of void formation during RFI. Two types of voids are observed: flow-induced voids resulting from either the merging of resin flow fronts or the drainage from capillary action; and gas-induced voids resulting from resin volatiles going out of solution and remaining in the resin matrix. In this work, the greatest source of porosity was caused by volatiles. In phase 4, the distribution and filtration of CNTs during RFI processing is characterised. Results show that processing choices can limit filtration and that clustering of CNTs prevents a uniform dispersion of CNTs in PMCs. Finally, the possibility of using RFI for making a multi-scale PMC demonstrator part is investigated. The work culminated with the successful fabrication of a delta-stringer panel. This thesis makes several important contributions to the knowledge pertaining to multi-scale PMC processing and performance, and to RFI. Firstly, it provides a robust description of RFI processing beyond was it available in literature, through in-situ observations of resin flow and void formation. Secondly, it assesses the viability of RFI for producing multi-scale PMCs featuring CNTs. In-situ observations of RFI processing enabled the identification of mechanisms leading to a loss of CNT dispersion during processing, partly explaining the minimal improvements in the interlaminar properties of composites observed when adding CNTs to the matrix. Thirdly, the fabrication of a delta-stringer panel made of a multi-scale PMC was successful, making it the first validation of the scalability of out-of-autoclave RFI processing for manufacturing multi-scale PMCs. The work presented herein contributed to the dissemination of knowledge; one conference paper was presented at ICCM20 (20th International Conference on Composite Materials), and another was presented at CANCOM2017 (10th Canadian-International Conference on Composites), and one journal article written in collaboration with project partners was submitted to Composites Science and Technology.

Alumina particles filled Ultra-high Molecular Weight Polyethylene (UHMWPE), with Al2O3 contents 0, 1, and 2.5 wt% were milled for up to 10 hours by the mechanical alloying (MA) process performed at room temperature to produce composite powders. Compression molding was utilized to produce sheets out of the milled powders. A partial phase transformation from orthorhombic and amorphous phases to monoclinic phase was observed to occur for both the un-reinforced and reinforced UHMWPE in the solid state, which disappeared after using compression molding to produce composite sheets. The volume fraction of the monoclinic phase increased with milling time, mostly at the expense of the amorphous phase. The melting temperature decreased as a function of milling time as a result of modifications in the UHMWPE molecular structure caused by the milling. At the same time, for a given alumina composition the activation energy of melting increased with milling time. Generally, the crystallinity of the molded sheets increased with milling time, and this caused the yield strength and elastic modulus to increase with milling time for a given alumina composition. However, the tensile strength and ductility remained about the same. / Master of Science

No description available.

Matrix assisted laser desorption/ionization mass spectrometry (MALDI MS) and the corresponding visualization technique MALDI MS Imaging (MSI) have emerged as important analytical tools in biochemical sciences, e.g., for drug development or to trace the metabolomic changes in cancerous tissues. Initially developed for the detection of high molecular weight compounds (HMWC; M > 1000 Da), in recent years the reliable and reproducible detection of low molecular weight compounds (LMWC; M < 1000 Da) has gained high attention, e.g., in the research fields of metabolomics and lipidomics. By using a protective matrix, the MALDI technique is capable of soft ionization of analytes to prevent their fragmentation or degradation. This matrix is responsible for the spatial separation of the analyte molecules, their protection from the strong laser shots, and their ionization. Commonly used matrices are small organic matrices (SOMs; M < 500 Da), which are utilized in HMWC analytics and recently also in LMWC analytics since they show sufficient absorption of the laser radiation, high crystallinity, and good ionization efficiency. However, their utilization can cause several drawbacks: (i) High background interferences below m/z = 1000 (not MALDI silent), which is disadvantageous specifically for LMWC analytics; (ii) low vacuum stability, which is especially problematic for standard instruments operated under high vacuum (HV); (iii) challenging homogeneous thin-layer coating, potentially causing inconsistent measurement conditions; and (iv) usually no suitability for dual polarity mode experiments, i.e., carrying out positive and negative mode measurements with the same matrix. Polymeric materials are promising candidates for MALDI silent matrices, as the large variety of possible molecular layouts potentially allows to meet all prerequisites of a MALDI matrix: (a) Sufficient ultra-violet (UV) laser radiation absorption, implemented by introducing conjugated π-electron systems in the polymer backbone or side chains; (b) high ionization efficiency, enhanced by adding acidic and/or basic functional groups to the polymer’s molecular structure, potentially also allowing dual polarity mode measurements; (c) MALDI silence, enabled by the high molar mass of the polymer chains; (d) high vacuum stability, also granted by the polymer’s molar mass; and (e) homogeneous thin-films, achieved by multiple available coating methods. Yet, despite their high potential only a handful of polymeric matrices were reported in literature and so far, investigations to develop conscious design strategies are missing. The target of this thesis is to contribute to the field of MALDI silent matrices by developing and investigating different polymers as macromolecular MALDI MS and MSI matrices for LMWC analytics. Therefore, two different strategies were explored: (i) Investigating conjugated polymers, and (ii) polymerizing SOMs. For the first strategy, five conjugated polymers were tested as MALDI matrices for the detection of various LMWCs. Among these, four were found to be excellent matrices, with sufficient ionization efficiencies and rare dual polarity mode suitability and allowed LMWC detection with low background interferences (MALDI silent). A high crystallinity of the matrix (SOM) is reported to be crucial to ensure successful measurements, yet conjugated polymer matrices (CPMs) are semi-crystalline, i.e., they contain crystalline and amorphous domains. Hence, the analytes are expected to be incorporated in the crystalline domains of the CPMs, depending on their degree of crystallization. Therefore, two amorphous CPMs were synthesized and tested, showing similar matrix performances (e.g., ionization efficiencies, dual polarity mode, MALDI silence) as a structurally related semi-crystalline CPM. This indicates that the analytes are incorporated in the amorphous parts of the CPM. The second strategy towards polymeric matrices (PMs) is the polymerization of standard SOMs. As the matrix performance of the corresponding SOMs is known, the performance of the respective polymerized SOMs (P(SOMs)) can be validated against this benchmark. At the same time, polymerization can induce the properties needed to enable efficient LMWC analytics. Therefore, two standard SOMs were modified and polymerized, resulting in P(SOMs), which were vacuum stable and MALDI silent, and showed similar optical properties, analyte scopes and ionization efficiencies in benchmark tests with their respective SOMs. For the fast and facile comparison of the matrix performances of PMs and standard matrices, the graphing software OriginPro was used to visualize, process, and evaluate the acquired mass spectra. To automatize these tasks, a script was programmed using the OriginPro-native programming languages LabTalk and OriginC: X Functions.

The aim of this study was to develop a new single-part healing system for self-healing composites. The self-healing approach to composite repair has been developed in the last two decades and means that a damaged area can be repaired by material already housed within the structure. The background and development of self-healing has been reviewed. The two main self-healing mechanisms are discussed. To date only two part self healing systems have been examined. These require diffusion of the separate constituents to a single location in order to effect cure and restore strength. Single part adhesives do not have this disadvantage and are therefore very attractive. Several candidate single-part adhesive or resin systems were considered and discussed according to the critical requirements of a self-healing system. A series of experiments was undertaken to evaluate the possibility of candidate adhesive systems being effective for self-healing by focusing on the determination of storage stability and bonding efficiency. The results of storage stability testing showed that the stability of cyanoacrylate and polyurethane adhesives was poor. However silane and polystyrene cements showed good storage stability. Very low bonding efficiency was achieved with polystyrene cement but a 22% strength recovery was obtained with the silane 3-[tris(trimethylsiloxy)silyl]-propylamine. Suggestions for further research into single-part healing systems are also given.

Polymer matrix composites reinforced by metal fibers are observed to present an onset of the insulator-to-conductor transition through previous experimental studies. Analytical studies revealed that the percolation threshold occurs when fiber volume fraction reaches the critical value. The numerical study based on Monte Carlo simulations are performed to investigate such a relation. In this work, the conductive fillers are modeled as a three dimensional (3D) network of identical units randomly distributed in the polymer matrix. For the simplest case, straight fibers are used in the simulation. The effects of the aspect ratio and fiber length on the critical volume fraction are also studied. Linearization is made to the logarithm of simulation results. Next, in order to study the effects of emulsion particles and the emulsion particle sizes on the percolation behavior, cubic particles are aligned in the sample model. The gap width to particle size ratio is fixed at 1/10. The calculated critical volume fraction is used in the power-law function to predict the electrical conductivity of the polymer composites. Due to the insensitivity of the thermal conductivity to the percolation threshold, a combination of two empirical equations is used to predict the range of overall thermal conductivity.

The aim of this study was to develop a new single-part healing system for self-healing composites. The self-healing approach to composite repair has been developed in the last two decades and means that a damaged area can be repaired by material already housed within the structure. The background and development of self-healing has been reviewed. The two main self-healing mechanisms are discussed. To date only two part self healing systems have been examined. These require diffusion of the separate constituents to a single location in order to effect cure and restore strength. Single part adhesives do not have this disadvantage and are therefore very attractive. Several candidate single-part adhesive or resin systems were considered and discussed according to the critical requirements of a self-healing system. A series of experiments was undertaken to evaluate the possibility of candidate adhesive systems being effective for self-healing by focusing on the determination of storage stability and bonding efficiency. The results of storage stability testing showed that the stability of cyanoacrylate and polyurethane adhesives was poor. However silane and polystyrene cements showed good storage stability. Very low bonding efficiency was achieved with polystyrene cement but a 22% strength recovery was obtained with the silane 3-[tris(trimethylsiloxy)silyl]-propylamine. Suggestions for further research into single-part healing systems are also given.

Polymer matrix composites (PMCs) offer several advantages compared to traditional metallic counterparts when employed in high-performance products that need to be lightweight, yet strong enough to sustain harsh loading conditions - such as aerospace components and protective structures in military applications- armours, helmets, and fabrications retrofitted to transport vehicles and bunkers. These are often subjected to highly dynamic loading conditions under blast and ballistic impacts. Severe impact energy involved in these dynamic loading events can initiate discrete damage modes in PMCs such as matrix cracking, matrix splitting, delamination, fibre-matrix debonding, fibre micro-buckling and fibre pull-out. Interaction of these damage modes can severely reduce the load carrying capacity of such structures. This needs to be understood to design structures with improved resistance to such loading.