Comparison of Novel Resin Transfer Moulding, Pre-Preg Autoclave and Metallic CNC Manufacturing for UAV Spars ICMAC Belfast 18 th March 2009 Andrew Mills, James Hurley SAS Cranfield University a.r.mills@cranfield.ac.uk
UAV Airframe Challenges 50 years of manned fighter aircraft development has stalled Lockheed - BAE SYSTEMS F35 may be the last of the genre Need for many UAV variants with rapid design and development cycle Typically 50-100 production for MOD Targets 70% reduction in airframe manufacturing cost compared to Typhoon Near equivalent capability range and payload mass Minimal maintenance requirement damage resistant structure Embedded sensors and communication systems
Current UAV Composites Process Technology - Global Hawk Wings - Alliant Techsystems (ATK) Aluminium fuselage Low temperature cure pre-preg Four CFC spars I beam
Boeing X45 A Recent UAV demonstrators and production aircraft predominantly use aluminium fuselage frames with oven cure prepreg skins and all CFC wings X45 uses novel One shot wings with removed foam core inside carbon fabric preformed skins - vacuum infusion Expected to be replaced with lighter weight structures in series production
BAE Systems - Future UCAV - Taranis Jointly funded by the UK MoD and UK industry, and will bring together a number of technologies, capabilities and systems to produce a technology demonstrator based around a fully autonomous intelligent system. Ground testing of Taranis is expected to take place in early 2009 with the first flight trials taking place in 2010.
Cranfield SoE / BAe SYSTEMS EPSRC FLAVIIR DEMON 2.5m Wing Span Flight control through aerodynamics no control surfaces
Technology Challenge Manufacture of lightweight carbon fibre composite structure is highly effective and affordable for simple geometry components, such as wing skins, control surfaces and access panels BUT For components which have more shape complexity, the cost of moulding and assembly can be prohibitively costly: particularly Tooling cost for bonding or co-curing of small parts Thickness tolerance required to enable parts to be bolted together For current UAV aircraft internal fuselage structure high speed machining of aluminium is preferred The weight saving potential for highly loaded, complex shape internal structure Bulkheads, frames, spars - using carbon fibre composite encourages the investigation of new composites moulding technology The main requirements are: Maximize the components stiffness - to maintain the weight reduction provided by the prepreg autoclave process Provide accurate moulded thickness so as to allow bolted or bonded assembly without thickness shimming
F22 - Titanium Usually aluminium Heavy since no stiffness tailoring Requires fastener assembly Current Metallic Frame Technology
Pre-preg Hand Lay Up Autoclave Very high cost material Processing Labour intensive. High mechanical properties Thickness tolerance +/- 5% Complex assembly adhesive shims Damage resistance provided by fasteners Titanium 25 to fit each
Frame Study Objectives Produce sections of fuselage frame using: Best practice prepreg autoclave moulding Novel high mechanical performance, high quality resin transfer moulding technique (RTM) Comparison of Laminate mechanical performance Moulded component dimensional tolerance Fibre waviness - particularly in flange radii Full scale manufacturing cost for both techniques
RTM Version TEST ELEMENT MATERIALS AND MANUFACTURE Demonstrate novel RTM moulding for a high quality, high fibre volume fraction test component Novel fabric and moulding process were selected Impregnation technique used a matched tool, with high vacuum capability Peripheral flow to provide rapid wetting of the carbon fabric reinforcement, Allows high fibre volume fraction skins (54-55% FvF) to be produced using fully consolidated fabric performs Technique enables the moulding of unlimited length components, since resin is flowing a maximum distance of half the web width
Preforms - Novel Carbon Fibre Hexcel Composites Reinforcement developed novel low cost UD carbon fibre tapes No tow structure complications from textile process provides near straight fibres No crimp introduced by a fabric manufacturing process, such as weaving or warp knitting (NCF.) Thermoplastic veil provides flow path and ensures sufficient preform bulk avoids resin rich surfaces prevents preform displacement in RTM provides intra ply toughness Epoxy powder binder rigidises and allows preform compaction Vacuum bag compaction at 130C produces stiff preforms which can be trimmed with stable edges
High Speed Dry Tape Laying Wing Skin 6m radius
Preform Assembly Lay up - 16 layers, quasi isotropic 50% 45 25% 0 25% 90 Main challenge Avoidance of fibre wrinkling in the flange radii Simple vacuum forming table folding causes wrinkle since binder does not flow as per prepreg resin Lamination - four separate ply stacks, each folded over the male inner tooling block in four layer stages. Centre section of the mould was then eased in to position. Tool closure - by press to fully close the tool cavity to achieve the required fibre volume fraction, the mould assembly heated to 130 C for 30 minutes
RTM Frame Manufacture Mould Tool key features Consolidated, fitted skin De-moulding the infused pre-form composite tool vacuum seal to 20mBar protects from air entrapment Multiple part with removable centre insert Five part cavity machined to have a 4.70mm skin thickness at the resin curing temperature of 160C
Infusion Processing - RTM 1. Tool was vacuum tested in heated press to ensure vacuum level of 20mBar 2. 600grms of epoxy RTM-6 resin was degassed at 10mb for 60 minutes @80 C 3. Vacuum was applied to mould cavity for thirty minutes (@20 millibar) 4. Mould temperature raised to 120 C and resin pumped at 2Bar 5. Resin injection took 58 minutes 6. Outlet pipe was clamped and 1 bar back pressure was applied to the tool 7. Mould temperature was raised to 160 C for two hours for resin gelation and cure 8. Tool cooled and demoulded
Net Shape Moulded Frame No trimming or resin deflashing required - only lug insert hole machining Fibre Volume Fraction 54% Fibres follow tool corner without wrinkling
Dimensional tolerance - Thickness +/- 0.06mm Resin shrinkage is easy to adjust for Length and width cavity dimensions at 160C Easily calculated
Full Scale Moulding Process Recommendations Preform assembly cost minimisation essential Requires a flat full thickness lay up, fully consolidated by vacuum bag Simple heated double diaphragm process over the male tool block Using a binder which melts at <150C and allows intra ply shearing All other aspects are ideally suited to full scale production having a capability to produce a complete net shape part.
Prepreg Version Material - Standard high strength fibre Unidirectional prepreg from Hexcel Composites - AS4 fibre 8552 resin 268 g.s.m fibre areal weight Lay Up 16 layers, 4 layers 0, 4 layers 90, 8 layers +/-45. Eight two ply layer stacks, trimmed using a template and fitted into the female tool cavity De-bulked every second layer to ensure tight conformance to the flange radius PTFE coated glass sheet was fitted onto the final ply and silicone rubber block intensifiers were fitted into the corners to eliminate ply wrinkling at the flange radius.
Autoclave Cure Whole assembly envelope bagged and cured at a temperature of 180 C for 120min using a ramp up rate of 2 C minute and a 4 bar pressure 4.2mm thickness +-0.3mm 58% FvF
Flange Radii Component Quality Intensifier blocks ensure excellent conformance of the fibre paths to the flange radius However - blocks cause a local thickness reduction of around 0.6mm under their surface area because of a local increase in bag consolidation pressure Inner (bag surface) General surface undulation tow visibility but good thickness consistency +- 0.3mm Process Improvement Recommendations for Production Double diaphragm forming Shape lay-up in one shot after hot vacuum compaction Full surface area intensifiers (moulded inner surface) Improve the evenness of the component s bag surface
Laminate Mechanical Performance Tensile and Compression Strength and Modulus Moduli Results Results normalised for FvF Novel tape RTM and pre-preg have identical tensile moduli Tape RTM laminate shows a 4% lower compression modulus Strength Results Tensile - Tape RTM shows 7% reduction Compression - Tape RTM shows 6% lower reduction compared to prepreg Translation of fibre stiffness through tape manufacturing, laminating and infusion processing is extremely good
Manufacturing Cost Modelling - Pre-preg Autoclave c.f. Fabric RTM c.f. Aluminium CNC Detailed spreadsheet based process cost models set up Total production volume of 100 aircraft assuming five similar parts 500 parts Full scale fuselage frames - size 1.25m length, 0.25m width and a mean thickness of 5mm, with 50mm width edge flanges and three full length stiffeners of height 50mm and thickness 5mm Prepreg process assumed a production approach of hand lay up full ply stack flat lay up and double diaphragm flange forming. RTM process assumed high speed tape lay up and vacuum forming of the flanges Aluminium process assumes 2024 grade aluminium CNC machining of integral component
Cost Model Result for RTM Spar 1.25m Length
Cost Modelling RESULTS FLAVIIR developed novel tape materials and RTM processes showed a realistic cost saving of 29% compared to current (low cost) prepreg autoclave technology. However Pre-preg frames additionally incur an extremely high assembly cost since they require adhesive shimming for skin and rib/longeron attachment RTM frames are net shape, which eliminates the highly labour intensive and fixture cost shimming process. Assembly cost using the RTM frames is expected to be much lower and similar to the cost of metallic frame assembly
Cost Model Observations Pre-Preg Materials Cost Lay Up Mould Tool 25 20 4 RTM (%) 16 26 10 At 500 parts similar size and shape with interchangeable insert blocks For a lower production volume, tooling cost becomes more dominant @100 parts 19% 49%
Novel RTM Frame Conclusions New low cost reinforcement 70% less than standard low performance fighter aircraft prepreg Manufacturing cost around 30% lower than prepreg / autoclave manufacture and 47% higher than Aluminium CNC machining at 500 total production Assembly cost for RTM would be greatly reduced compared to prepreg Automated lay up with closed mould processing provides near equivalent stiffness and strength to prepreg materials and manufacturing Mechanical performance reduction and lower fibre volume fraction 54% compared to prepreg at 58% Lead to 2% weight increase Net shape moulding to +/-0.1mm is feasible Materials and moulding technique are expected to be affordably applicable to un-kinked frame type, high performance carbon fibre composite component, with good production volume
Thanks to: EPSRC BAE Systems Hexcel Composites Bombardier Shorts