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Knowledge of material microstructures and processing characteristics enables enhanced and novel approaches to be developed for manufacturing composite components. Select from the options above for an overview of work undertaken in ACCIS under this theme.
The way in which reinforcements conform to a mould is critically important with regard to the costs of manufacture, the fibre orientations and ultimate performance of the composite structures and the development of a range of defect types. We are studying the detailed deformation characteristics of the various reinforcement materials we use with regard to both in-plane and out of plane deformations, and developing robust tools that can be utilised in design. The understanding of reinforcement deformations is also vitally important in the development of novel preforming processes such as dry tow placement and in studies of defects and how residual stresses develop during the manufacturing processes.
Virtual Fabric Placement (VFP) is a computational tool being developed to give designers of composite components the ability to lay-up a virtual ply or stack of plies onto a virtual tool before a tool is fabricated. By providing this ability, the designer will be able to design not only the component itself, but also the lay-up sequence required to produce that component. Manufacturing instructions take the form of an animation, illustrating how the ply is fitted to the tool to achieve the required lay-up. Also, input for laser guidance equipment can be generated automatically from the lay-up design. The virtual fabric placement tool can also be used to support the development of robotic lay-up of woven fabrics as it can define the routes that a robot actuator must take.
Research is investigating the distortions that arise during high temperature cure of composite structures due to different mechanisms such as material anisotropy, cure shrinkage, tool-part interaction and material variability. Our industrial partners are interested in distortions in components such as rotorblades, wing spars and engine nacelle structures. Work is also being undertaken to extend the deep hole drilling technique to composites. This has been successfully developed to allow the residual stresses in laminated plates up to 22mm thick to be evaluated.
The defects that may be found in RTM laminates can have a significant effect on the strength of mouldings. What may appear to be minor defects (such as a surface resin rich zone) can lead to significant reductions in strength. Techniques have had to be developed to allow the manufacture of laminates with controlled and reproducible defects to allow an in-depth study of these phenomena.
In order to understand the Resin Transfer Molding (RTM) process as applied to real components we are studying the internal fibre architecture, the resin and binder distribution, the type and incidence of defects within the parts, the sources of defects in the manufacturing process, the likely effects on performance of those defects and approaches to improved processing and performance. Techniques such as CAT scanning can show the internal structure and the use of fluorescent resins (right hand image) can help identify near-surface effects.
The squeeze casting technique is an impregnation process where the liquid matrix phase is forced into a reinforcing preform to achieve the finished composite. This offers the opportunity to tailor the composite microstructures by managing the preform architectures. A two-level pressurizing process is being optimised to minimise the deformation of the preform, namely, infiltration at lower pressure and consolidation at higher pressure. This is particularly important for those preforms with hierarchical architectures. In order to achieve complete impregnation, a vacuum assisted infiltration process may be adopted. (An Al MMC with controlled short fibre distribution is shown right).
Current composites manufacturing techniques lead to the production of significant amounts of waste prepreg material. This adds to the cost of production, and must be disposed of as active waste to special landfill sites. Processes are being identified by which the maximum value can be extracted from the waste prepreg, ideally by direct reprocessing into added value components. At the same time the overall economics and marketing issues are also under consideration.
A wide range of interfacial interactions between reinforcing materials and their matrices have been studied using transmission electron microscopy and analysis. In particular diffraction contrast imaging, electron diffraction and chemical profiling by high spatial resolution electron energy loss spectroscopy have been exploited. The composites studied include mechanically alloyed particulate reinforced Ti/Mg matrices, diamond fibre- reinforced Ti alloys, boron particle-reinforced Ti6Al4V, oxide dispersion-strengthened alloys, SiC reinforced Al alloys and SiC/SiC basket weave composites.
Ultrasonic array transducers are increasingly widely used in Non-Destructive Testing (NDT). They have obvious advantages when used simply to emulate inspections performed with conventional transducers such as increased inspection speed and flexibility. More importantly, an array enables the ultrasonic wavefield to be controlled in ways that could not be physically realised with a single-element transducer. The inherent anisotropy and multi-scale heterogeneity of composite materials presents acute challenges for conventional ultrasonic NDT. For this reason, current inspections are generally based on unfocused normal incidence ultrasound and are limited to detecting in-plane delaminations. However, an array enables the angle-dependent ultrasonic velocity in composite material to be accommodated hence opening up the possibility of higher-resolution focused images to detect more subtle defects, such as resin-rich regions, voids, ply-drops and fibre waviness.
Further information: Ultrasonics and Non-destructive Testing (NDT) Group
High performance composites normally fail suddenly and catastrophically, which is an undesirable characteristic for many structural applications. In order to overcome this issue and obtain ductility or pseudo-ductility of composites, as one of approaches, a lab-scale aligned discontinuous fibre prepregging rig with the HiPerDiF (High Performance Discontinuous Fibre) method has been developed. Since aligned discontinuous fibre composites with shorter length than the critical fibre length exhibit ductile properties as predicted in modelling work, the new rig allows a range of further interesting studies as a key technology to investigate new ductile composite materials.
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