During the initial period of this project few international laboratories had experience of processing and fabrication of long-fibre CMCs. Glass and glass-ceramic matrix composites were initially developed at AEA Harwell (later AEA Technology) and the process later transferred to Pilkington Research. Warwick University had initiated a parallel project, with Rolls-Royce collaboration, in developing novel borosilicate glass and MAS glass-ceramic matrix constitutions based on their expertise in the glass-ceramic field. Based on this combined experience, we now have a precise formulation for modified pyrex glass and non-stoichiometric MAS compositions together with optimised CMC processing schedule.
The major part of this project was subsequently conducted on commercially-available BMAS glass-ceramic matrix tiles hot-pressed at AE Technology, Harwell. In the assessment of basic properties it became clear that the Harwell Process had not been optimised and the data generated in the project enabled a marked improvement in process route.
The combined knowledge of constitution, processing and CMC fabrication, for glass and glass-ceramic systems, developed as part of this programme represents a European 'state of the art' which is now internationally competitive.
Within the difficult area of CMC tensile testing involving high stresses, high anisotropy and high temperatures, with a need for sensitive strain monitoring, an additional feature has been the development of a high-speed CNC diamond machining facility to enable rapid, high-precision, prototyping of test specimen geometries.
The testing programme has also generated useful feedback to test-machine and component manufacturers, with instrumentation operating at the limits of mechanical and thermal stability, for example fluctuations in Instron load cell output due to thermal drift which is critical under 'constant load' (creep) control.
Simple models have been developed for monotonic and cyclic stress/strain response and for creep. The main purpose in developing analytical models for the stress/strain response of engineering solids is to predict this response in relation to the properties of microstructural components (e.g. moduli or creep rates of fibres and matrices together with interfacial shear stresses and debond energies). Alternatively these individual phase or interface properties may be analysed from the observed composite deformation in combination with the models. In this project the simple modelling approach has been substantiated within a wide range of experimental test data.
At the initiation phase of the project glass and glass-ceramic matrix composites were considered to be ideal models for generic CMC behaviour but also to have potential as real high temperature materials. Few of these applications are ow relevant; in the lower temperature regime there is competition from Ti-based alloys and intermetallics and at higher temperatures the major problem is interface oxidation and, ultimately, fibre stability. Within this project a major contribution has been made to understanding these degradation mechanisms and hence redefining design limits within differing stress states, time and temperature dependencies. Hence a major motivation is subsequent project planning on CMC materials development has been that of oxidation-stable interfaces which retain the necessary debond/shear property.
The application of GMCs and GCMs in their current microstructural states remains a possibility for low density rigid structures which demand corrosion resistance to moderate temperatures not accessible to PMCs. Economic issues relating to fibre and processing costs are likely to be critical.
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