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Traditional routes to the manufacture of timber based composite materials such as particleboard rely on platen pressing a premix of carefully chosen and prepared timber particles and an adhesive to produce flat sheets. Historically such routes have made very inefficient use of forest resources, and because of the planar nature of the composite produced, the finished articles have enjoyed a reputation of being of poor quality and "cheap". The work documented in this thesis sought to examine alternatives to the traditional manufacturing methods in terms of both raw materials and processing route, and to establish technical, economic, and environmental boundaries for the use of the alternatives. Having settled upon extrusion as the most likely technique with which to proceed, the investigation was carried forward by the construction of a series of experimental instruments with the joint aims of refining the technique to a realistic working level and investigating the underlying mechanical principles behind the process. Recourse to the literature of polymer rheology, fluid mechanics, and soil mechanics gave the basis for a crude mechanism hypothesis, and iterative improvements in the instrumental techniques yielded data with which this could be refined. Although some correlation between the extrusion behaviour of this system with other more easily quantifiable systems was found, there is still considerable work required in defining the dynamic changes in the material properties as the raw material is extruded. Small scale production experiments proved successful, however, and an elementary financial model of a suitable production facility indicated that the techniques developed could be used as the basis of an environmentally acceptable, economically viable, low technology industry.

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A composite skid landing gear design investigation has been conducted. Limit Drop Test as per Federal Aviation Regulations (FAR) Part 27.725 and Crash test as per MIL STD 1290A (AV) were simulated using ABAQUS to evaluate performance of multiple composite fiber-matrix systems. Load factor developed during multiple landing scenarios and energy dissipated during crash were computed. Strength and stiffness based constraints were imposed. Tsai-Wu and LaRC04 physics based failure criteria were used for limit loads. Hashin s damage initiation criteria with Davila-Camanho s energy based damage evolution law were used for crash. Initial results indicate that an all single-composite skid landing gear may not be feasible due to strength concerns in the cross member bends. Hybridization of multiple composites with elasto-plastic aluminum 7075 showed proof of strength under limit loads. Laminate tailoring for load factor optimization under limit loads was done by parameterization of a single variable fiber orientation angle for multiple laminate families. Tsai-Wu failure criterion was used to impose strength constraints. A quasi-isotropic N = 4 (pi/4) 48 ply IM7/8552 laminate was shown to be the optimal solution with a load factor under level landing condition equaling 4.17g s. LaRC04 predicts that failures will be initiated as matrix cracking under compression and fiber kinking under in-plane shear and longitudinal compression. All failures under limit loads being reported in the metal-composite hybrid joint, the joint was simulated by adhesive bonding and filament winding, separately. Simply adhesive bonding the metal and composite regions does not meet strength requirements. Filament wound bolted metal-composite joint shows proof of strength. Filament wound composite bolted to metal cross member radii is the final joining methodology. Finally, crash analysis was conducted as per requirements from MIL STD 1290A (AV). Crash at 42 ft/sec with 1 design gross weight (DGW) lift was simulated using ABAQUS. Plastic and friction energy dissipation in the reference aluminum skid landing gear were compared with plastic, friction and damage energy dissipation in the hybrid metal-composite design. Damage in composites was modeled as progressive damage with Hashin s damage initiation criteria and Davila-Camanho s energy based damage evolution law. The latter meets requirements of aircraft kinetic energy dissipation up to 20 ft/sec (67.6 kJ) as per MIL STD 1290A (AV). Weight saving possibility of up to 49% over conventional metal skid landing gear is reported. The final design recommended includes Ke49/PEEK skids, 48 ply IM7/8552 cross member tapered beams and, Al 7075 cross member bend radii bolted to the filament wound composite tapered beam. Concerns in composite skid landing gear designs, testing requirements and future opportunities are addressed.

The application of laser generated ultrasonics was first demonstrated in the mid-seventies and has shown good potential when applied to isotropic materials. However, its use with composite materials is still in the early stages of development. This study explores the potential for application of laser generated Rayleigh and Lamb waves in graphite/epoxy composites. Numerical results are obtained by the solution of the wave equations using assumed solutions, and enforcing the boundary conditions. Experimentally, Rayleigh and Lamb waves were generated by a Q-switched ruby laser in the ablation regime and detected by piezoelectric pinducers which permitted accurate phase velocity measurements. The Rayleigh wave velocity was measured at various directions relative to the fiber direction and results were found to agree closely with numerical predictions. The increase of surface wave velocity using thin plates could be useful for the application of delamination detection in thick composites and an increase of Rayleigh wave attenuation could indicate damages caused by impact. Also, surface waves can reflect from small surface cracks. Therefore, laser generated surface waves, particularly along the fiber direction, have high potential for application in non-destructive testing. Lamb wave experiments were conducted in aluminum plates and gave distinctive signals, but there were some difficulties in detecting the precise arrival of each Lamb wave mode for the graphite/epoxy composite plates. / Graduation date: 1992

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.

An investigation is presented into the behaviour of carbon fibre composite joints subjected to dynamic loading rates in the range of 0.1 m/s to 10 m/s. The research is focused on the response of single fastener joints and more complex structural arrangements involving multiple fasteners and complex loads. Fasteners play a crucial role in the joining of aerospace components due to their ease of installation and inspection and their resistance to creep and environmental degradation. A consequence of the operating environment of aircraft is that many critical load cases involve impact and crash. These loading events are characterised by high loading rates, high kinetic energy and possibly loads well above the static design case. The properties of composite materials change with loading rate, so it is likely that the behaviour of bolted composite joints may also vary significantly. Dynamic behaviour of bolted joints is an area of research that has been given little attention to date. The few available papers on the topic are limited to the investigation of ideal bearing loads and include some contradictory results. The research developed a detailed understanding of the behaviour of bolted joints in composite structures through a combined numerical and experimental investigation. A set of quasi-static and dynamic single fastener joint tests was conducted to develop an understanding of the complex failure mechanisms present in bolted composite joints. Simple structural tests were developed to investigate the interaction of multiple bolts in a joint. High speed camera footage, full-field strain measurement and CT scanning techniques were all used to develop an understanding of the changes in the failure process with increased loading rate. Finite element analyses used implicit and explicit dynamic algorithms to model the tests. The finite element analysis contributed to the understanding of the experimental results as well as providing a predictive tool to minimise the need for further testing. A method of incorporating detailed information about bolt failure into large scale structural models was investigated and developed. The original contributions of this thesis involve novel dynamic joint testing including dynamic pull-through and structural tests. CT Scanning was utilised in a novel way to investigate the complex failure modes within a bolted joint. Novel finite element techniques were developed for modelling bolted joints at both a detailed level and a simplified level for structural analyses. These contributions significantly improve the current understanding of bolted joint failure, both quasi-statically and dynamically, and will allow for more efficient design of bolted composite structures for crash and impact loads.