Composites go from strength to strength

Tom Shelley reports on some of the latest enhancements in fibre reinforced composites for use in aerospace, defence and automotive applications Breakthroughs continue to be made in polymer/polymer, conventional and metal matrix composites. Strengths, energy absorption to failure and pullout forces continue to be enhanced while manufacturing costs look likely to fall. While many of the latest developments started out in aerospace, more than a few are proving of interest to automotive engineers, and one of the most striking, could soon be found in extreme cold weather gear for both military and civilian personnel. A whole series of breakthrough developments were revealed at a symposium on, "Composites - the Cutting Edge" part of the Material Congress 2004 just hosted by the Institute of Materials, Minerals and Mining. Leaving aside the high strength and energy absorbing composites for a moment, one of the most commercially significant developments was revealed by Sabrine Höffele, who delivered a paper about a material that incorporates beads of aerogel in a fabric matrix. This at the same time, protects the beads from being crushed, while retaining flexibility. Aerogels are silica gels, which are mostly air, and are extremely good thermal insulators, but easily crushed. Although invented by Dr Samuel Kistler in 1931, their usefulness has hitherto mainly been limited to space applications. Ms Höffele's research group is headed by Dr Stephen Russell (s.j.russell@leeds.ac.uk) and is in the Centre for Technical Textiles, part of the University of Leeds. The material weighs 140g/sq m and has a thermal resistance of 3.717 Tog/cm as compared to its nearest rival, an ?Italian product which is rigid, also based on aerogels, weighs 550 g/sq m and has a thermal resistance of 7 Tog/cm. The material is patent pending. Ms Höffele told Eureka that the material can be washed and dries very quickly. A mountaineering friend who tested it in his boots is said to have reported that when using it, it was the first time he had had dry feet. The silica gel beads are held in pre-formed channels in a non woven viscose fabric. Thermoplastic polymer fibre reinforced thermoplastics started out, we understand, as a military research project in the USA, but Professor Ton Peijs, of Queen Mary University, London, wants to see these used in cars. He has been investigating polypropylene fibre reinforced polypropylene, with a view to making structural parts that are 100% recyclable, in order to help car makers reach the 2015 85% recyclability goal set by the European Union end of life directive. Polypropylene as moulded has a Young's Modulus of approximately 1.5GPa but drawing increases this to 17GPa. This is not as much as the 70GPa of glass but it does weigh only just over one third as much. Previous attempts to make such materials have involved melt infiltration of ultra high molecular weight PP fibres with low molecular weight PP but PP in the liquid state is very viscous. Researchers at the University of Leeds have made PP polymer-polymer composites by very carefully heating the fibres so that only the surfaces melt, and then compacting together. This works quite well but suffers from only having a 2 deg C processing window. An alternative approach is to co-extrude PP fibres with lower melting point 'skins', but Professor Peijs has made a breakthrough in finding a way to co-extrude skinned tapes. This allows the production of a composite which is 80% to 90% volume fraction fibre, with a compaction window of 140 deg C to 170 deg C. The tapes are 60 microns thick and 4 to 5mm wide. A prototype car undertray has been made to demonstrate the efficacy of the technology and Professor Peis considers the technique could also be used to make light weight boat hulls. Tapes are not of infinite length but compacted together in a brick wall type structure so that butt joints occur in different places. Another breakthrough with PP fibres, of potential application to all composites is the development of fibres that fatten when in tension. The idea is to increase resistance to fibre pull out, increasing strength, inhibiting crack propagation and enhancing energy absorption during failure. Virginia Simpkins presented a paper about a study on these materials being undertaken at the Bolton Institute. The technical term for such materials is "Auxetic". Polypropylene can be made auxetic by compacting ultra high molecular weight PP powder, with a particle size of 30 to 120 microns. It is then sintered at 160 deg C for 20 minutes and extruded at 160 deg C. Experiments with resins modified to reduce natural adhesion between fibres and matrix showed pullout forces that were nearly doubled, and three and a half times higher energy absorptions. Mrs Simpkins told her audience that work was already under way to "Develop auxetic materials at the molecular level" and extend the idea to carbon fibres. Potential applications of the materials range from energy absorbing materials in cars to light weight body armour. Further information about the idea can be found on an Auxetic Materials Network at www.auxetnet.org. To round off the day, Dr Hua-Xin Peng a lecturer in the University of Bristol Department of Aerospace Engineering described how the properties of metal matrix composites could be enhanced by using reinforcements other than particles and pieces or mats of fibre. He showed micrographs of reinforcement structures made up of balls of fibres, three dimensional networks with connecting pores and branched whiskers. The intention, he told his audience, is to steer crack propagation. He said that balls of fibres could in some cases increase energy absorption during failure by 40 to 50% and increase UTS. The 3D reticulated foam reinforcement structure, he said, improved stiffness and greatly reduced wear rates. He suggested that it might be a good material with which to make brakes. From these presentations, it may be deduced that fibre reinforced composites have to the potential to achieve even higher performances that they already do, and while aerospace and defence may well be the initial users of the most advanced products, automotive engineers and boat builders will be following very close on their heels. Institute of Materials, Minerals and Mining Auxetic Materials Network Sabrine Höffele email Professor Ton Peijs email Virginia Simpkins email Eureka says: Fibre reinforced composites have to the potential to achieve even higher performances that they already do, and while aerospace and defence may well be the initial users of the most advanced products, automotive engineers and boat builders will be following very close on their heels. Pointers * A new textile product achieves unprecedented thermal insulation by incorporating aerogels in pockets to prevent their being crushed * Co-extruded tapes allow the manufacture of thermoplastic polymer fibre reinforced thermoplastic composites with 80% to 90% fibre * Auxetic fibres, which fatten when they are pulled, increased pullout forces and energy absorption during failure and inhibit crack propagation * The properties of metal matrix composites can be enhanced by infiltrating aluminium into novel reinforcement structures