Characterisation and testing of carbon fibre PEEK for linerless composite cryogenic storage tanks
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Cryogenic storage tanks, also known ’cryotanks’, are an important component of space launch vehicles. Cryotanks can form an integral part of the overall structure of the launch vehicle and also represent a significant proportion of the overall weight. Because of the high value of structural weight reduction in launch vehicles, many major space launcher manufacturers are researching moving from metal cryotanks to linerless composite cryotanks; that is, moving towards composite cryotanks without metallic liners. The work presented in this thesis investigates the suitability of Carbon Fibre (CF) PolyEtherEtherKeytone (PEEK) in the manufacture of linerless composite cryotanks due to its favourable specific strength, toughness, manufacturability and chemical resistance. Numerous challenges to the production of linerless composite cryotanks have been identified in the literature, including permeability, large scale manufacturing, joining, damage detection and damage tolerance. This thesis specifically focuses on the issues of permeability, joining by induction welding and thermal stress in CF-PEEK. The effects of cryogenic cycling, manufacturing method, PEEK matrix type, fibre type, cryogenic temperatures, pressure, and thickness on the permeability of CF-PEEK laminates are investigated. Mechanical testing is carried out on a hat stiffened, CF-PEEK, induction welded panel to evaluate the performance of the welded assembly and results are compared with similar testing of an adhesively bonded panel. An experimental and computational investigation into the heating of CF-PEEK is also carried out to identify the parameters that influence induction heating of CF-PEEK. Due to its importance in damage tolerance at cryogenic temperatures and permeability, an experimental investigation into thermal stress in CF-PEEK has also been carried out. The work investigates methods of reducing thermal stress given in the literature and whether a reduction in thermal stress results in reduced damage at cryogenic temperatures. The results of the permeability study provide a novel insight into the permeability and leak rate of CF-PEEK and show that the leak rate of CF-PEEK make it a suitable candidate material for use in linerless composite cryotanks. The work on induction welding addresses the lack of data relating to demonstrator scale welded assemblies in the literature, demonstrates that continuous induction welding is a suitable assembly technique for primary structures and identifies process-induced artefacts that may occur during induction welding. The work on induction heating provides a novel insight into the heating behaviour of CF-PEEK and demonstrates that finite element modelling is a valuable tool in process development of induction welding. The results of the thermal stress investigation verify that a reduction in thermal stress is possible by subjecting CF-PEEK to thermal post-processes and demonstrate, for the first time, that a reduction in thermal stress leads to a reduction in damage initiation at cryogenic temperatures. The work presented in this thesis provides valuable information relating to permeability, induction welding and thermal stress of CF-PEEK materials, which will inform the future development of CF-PEEK linerless composite cryotanks. The impact of the use of CF-PEEK in linerless composite cryotanks would be to reduce the weight and ultimately the environmental and economic cost of space travel, enabling future space exploitation and exploration.