148 C.Soutis Progress in Aerospace Sciences 41 (2005)143-151 variations [21].In traditional prepreg technology,the resin has already infiltrated the fibres and processing mainly removes air and volatiles.consolidates and cures. RTM in its simplest form involves a fabric preform being placed in an enclosed cavity and resin forced into the mould to fill the gaps under pressure and cure.The RFI method utilises precast resin tiles with thickness ranging from 0.125 to 0.25 in.This approach reduces the number of consumables used,but is very process- sensitive relying on the resin being of sufficiently low permeability to fully impregnate the fabric before cure advances too far.The use of an autoclave or press to apply pressure varies.The RFI process is being applied within the Advanced Composites Technology (ACT) Programme in conjunction with traditional autoclave Fig.1.V22-Osprey tilt-rotor plane (courtesy of Bell Textron, processing.Heat is the energy source to activate the USA). resin cure,but some resin systems can be activated by radiation.Wright Paterson claim that thermal oven processing could save 90%of autoclave processing time and energy and hence 50%cost.There is also a radiation curing process developed jointly by NASA and Advanced Composites Group (ACG)and of innovative electron beam cured structures being devel- oped by Foster Miller,Lockheed Martin and Oakridge National Laboratories in the USA [7]. The vacuum-assisted RTM is a liquid resin infusion process and is currently considered by the aircraft industry to be the favoured low-cost manufacturing process for the future.It is an autoclave-free process that Fig.2.Aircraft wing rib element produced by RTM. has been identified as reducing the cost of component processing.It is reported that dimensional tolerance and mass measurements are comparable with stitched RFI and membrane is then evacuated and the vacuum is autoclave panels.A conventional blade stiffened test maintained until the resin has cured.Quite large,thin panel (3ft x 2ft with 4-in high blades 0.5in thick)has shell mouldings can be made in this way at low cost.The been manufactured recently at NASA by using the majority of systems suitable for vacuum-only processing VARTM method,achieving a reasonable quality. are cured at 60-120C and then postcured typically at Further cost reduction when manufacturing with 180C to fully developed properties.In 1991,the composites will be achieved by reducing the assembly evaluation of this method started at the Phantom Works cost,by moving away from fastening (drilling of using the resin system LTM10 (low-temperature mould- thousands of holes followed by fastener insertion and ing)and they created a small allowables database for sealing)towards bonding and to assembly with less or their X36 fighter research aircraft study.In 1996. no expensive jigging.Bell Textron among others are McDonnel Douglas characterised LTM45 EL for the building and developing a number of structures(for the Joint Strike Force(JSF)prototype and generated design V22 and B609)where they are applying state-of-the-art allowable data.In 1998,Boeing also produced LTM45 composites technology/processes to achieve a unitised EL data.LTM10 applications demonstrated for com- approach to manufacturing and assembly.There are of plex parts with a 140F cure under vacuum include a course significant certification challenges with an adhe- serpent inlet duct.A box using LTM10 was shown at the sively bonded joint for a primary aircraft structure 1998 Farnborough Airshow.A research programme at application that need to be addressed. NASA Langley is looking at the development of 180C material properties using low-temperature curing resins. The main advantages of LTM systems are the potential 4.Applications to use autoclave free cures,the use of cheaper tooling and reduced springback of parts. In the pioneering days of flight,aircraft structures RTM and RFI are the predominant curing processes were composite being fabricated largely of wood being developed today of which there are several (natural composite),wire and fabric.Aluminium alloysand membrane is then evacuated and the vacuum is maintained until the resin has cured. Quite large, thin shell mouldings can be made in this way at low cost. The majority of systems suitable for vacuum-only processing are cured at 60–120 1C and then postcured typically at 180 1C to fully developed properties. In 1991, the evaluation of this method started at the Phantom Works using the resin system LTM10 (low-temperature mould￾ing) and they created a small allowables database for their X36 fighter research aircraft study. In 1996, McDonnel Douglas characterised LTM45 EL for the Joint Strike Force (JSF) prototype and generated design allowable data. In 1998, Boeing also produced LTM45 EL data. LTM10 applications demonstrated for com￾plex parts witha 140 F cure under vacuum include a serpent inlet duct. A box using LTM10 was shown at the 1998 FarnboroughAirshow. A researchprogramme at NASA Langley is looking at the development of 180 1C material properties using low-temperature curing resins. The main advantages of LTM systems are the potential to use autoclave free cures, the use of cheaper tooling and reduced springback of parts. RTM and RFI are the predominant curing processes being developed today of which there are several variations [21]. In traditional prepreg technology, the resin has already infiltrated the fibres and processing mainly removes air and volatiles, consolidates and cures. RTM in its simplest form involves a fabric preform being placed in an enclosed cavity and resin forced into the mould to fill the gaps under pressure and cure. The RFI method utilises precast resin tiles with thickness ranging from 0.125 to 0.25 in. This approach reduces the number of consumables used, but is very process￾sensitive relying on the resin being of sufficiently low permeability to fully impregnate the fabric before cure advances too far. The use of an autoclave or press to apply pressure varies. The RFI process is being applied within the Advanced Composites Technology (ACT) Programme in conjunction withtraditional autoclave processing. Heat is the energy source to activate the resin cure, but some resin systems can be activated by radiation. Wright Paterson claim that thermal oven processing could save 90% of autoclave processing time and energy and hence 50% cost. There is also a radiation curing process developed jointly by NASA and Advanced Composites Group (ACG) and of innovative electron beam cured structures being devel￾oped by Foster Miller, Lockheed Martin and Oakridge National Laboratories in the USA [7]. The vacuum-assisted RTM is a liquid resin infusion process and is currently considered by the aircraft industry to be the favoured low-cost manufacturing process for the future. It is an autoclave-free process that has been identified as reducing the cost of component processing. It is reported that dimensional tolerance and mass measurements are comparable withstitched RFI autoclave panels. A conventional blade stiffened test panel (3 ft  2 ft with 4-in high blades 0.5 in thick) has been manufactured recently at NASA by using the VARTM method, achieving a reasonable quality. Further cost reduction when manufacturing with composites will be achieved by reducing the assembly cost, by moving away from fastening (drilling of thousands of holes followed by fastener insertion and sealing) towards bonding and to assembly withless or no expensive jigging. Bell Textron among others are building and developing a number of structures (for the V22 and B609) where they are applying state-of-the-art composites technology/processes to achieve a unitised approachto manufacturing and assembly. There are of course significant certification challenges with an adhe￾sively bonded joint for a primary aircraft structure application that need to be addressed. 4. Applications In the pioneering days of flight, aircraft structures were composite being fabricated largely of wood (natural composite), wire and fabric. Aluminium alloys ARTICLE IN PRESS Fig. 2. Aircraft wing rib element produced by RTM. Fig. 1. V22-Osprey tilt-rotor plane (courtesy of Bell Textron, USA). 148 C. Soutis / Progress in Aerospace Sciences 41 (2005) 143–151