Composite Materials for Future Aeroengines

Transcription

1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 41St., New York, N.Y The Society shall not be responsible for statements or opinions advanced in papers or in discussion at meetings of the Society or of its Divisions or Sections, or printed in its publications. ^y^^ Discussion is printed only if the paper is published in an ASME Journal Papers are available ],L from ASME for fifteen months after the meeting. Printed in USA Copyright 1989 by ASME 89-GT-313 Composite Materials for Future Aeroengines G. E. KIRK Chief Engineer Engineering Research Rolls-Royce plc Derby, England ABSTRACT Aeroengines will only satisfy the future market requirement if advances are made in material and manufacturing technology. Current aeroengine materials are reaching the limits of their development but composite materials have the potential to meet the increased requirements. The use of resin composites has increased and with further improvements could be used more extensively. Metal and ceramics composites are being considered where higher temperature capability is required. However there are a number of problems which are being addressed so that these newer composites can be so used to their full potential with confidence. INTRODUCTION The aero engine market requires that engines are constantly improved in order to remain competitive and to meet increased operational requirements. Advances are obtained by improved design methods and the application of new technology. For many years improvements in materials and aerothermal technology coupled with improved engine configurations have provided the powerplants required by the market. The potential for aerodynamic efficiency improvement is becoming less and improvements will have to obtained by operating at higher pressures and temperatures with further configuration developments. Significant improvement in material properties are required to meet these demands at a time when the conventional gas turbine materials are reaching the limits of development. Composite materials have the potential to meet the increased requirements. This paper identifies the future engine needs and reviews the potential material candidates. ENGINE REQUIREMENTS There are a number of factors that have to be considered in order to obtain the optimum design of engine which are common to civil and military engines. Their relative importance depends upon the engine duty. Safety Engines are required to meet the design standards dictated by the regulatory authorities. This factor is not negotiable and cannot be traded in order to improve the other factors. Thrust This is the prime requirement for an engine. The engine is required to achieve specified thrust requirements with adequate margin to cater for deterioration in service. Weight Engine weight contributes to the total aircraft weight and hence limits the amount of fuel or payload that can be carried and also the range. In civil applications this limits the revenue that can be obtained for the flight. In addition there is an increased fuel burn due to increased take off and cruise thrust. Fuel Burn The total mission fuel burn is a function of the fuel consumption in each of the flight regimes. Engines may spend a considerable proportion of their running hours at low power, during taxying and descent, especially on a short haul aircraft. It is therefore important to minimise fuel consumption at all power levels. Presented at the Gas Turbine and Aeroengine Congress and Exposition June 4-8, 1989 Toronto, Ontario, Canada

2 E Life Cycle Cost Engines contribute to the Life Cycle Cost, (LCC), of aircraft in three major respects; the level of the contribution depends on the type of operation of the aircraft. (a) Fuel Cost The fuel burn and price determine the fuel cost and fuel price is subject to many social and political pressures. On long range civil applications the fuel costs can represent fifty per cent of the engine related costs and just over a quarter of the total LCC for the aircraft. The fuel element of LCC rises dramatically with increased fuel price for long range civil aircraft. For the short and medium range aircraft the contribution to LCC of fuel cost is less significant. (b) Initial Price The initial price can be influenced by many marketing factors but has to be related to the development and factory cost of the engine. The price of aircraft and engines represents a large investment and the cost of servicing such undertakings represents a significant burden on the operating costs. (c) Maintenance Cost Maintenance costs are determined by the life of major components, the reliability of the engine, and the price of spare parts. Maintenance costs can be related to the number of components in an engine. Environmental In addition to meeting the minimum requirements of the civil regulatory authorities engines are having to meet more stringent noise and emission requirements imposed by Local Airports who are basing their landing fees on such factors in addition to landing weight. AEROENGINE FUTURE REQUIREMENTS by powerplants to the reduction of aircraft fuel for large transport aircraft. These improvements have been achieved by improvements in thermal and propulsive efficiencies. The thermal efficiency is a measure of the turbomachinery efficiency in converting fuel energy into the kinetic energy of the air at the jet and is a function of the component and cycle efficiencies. Propulsive efficiency is the efficiency with which the kinetic energy at the jet does useful work on the aircraft. The contribution of each improvement and the future prediction is shown in Figure Straight Low Medium HighUltra-high jet bypass bypass bypass -^ bypass -- % SFC 20. improvement Component efficiency (1958) (1960) (1963) (1973) (1981) (1983) (1988) Avon Conway Spey RB211 RB211 RB211 RB211-22B -524B4/D D Certification year Cycle efficiency Propulsive efficiency Figure 2 Civil engine cruise SFC trends Component efficiencies are currently very high, around 88-90% for all components and are reaching the theoretical limits. Further significant improvements will be difficult to obtain. Some benefits may be obtained by having materials which remain stiff at higher temperatures allowing rotors and vanes to remain concentric. Datum 't I 70 1 Mach 0.8 cruise: component polytropic efficiency=88% 10* Engine ^,* technology ** Improvement ^1 * Engine fuel * consumption Thermal efficiency v (Stotchlom etric) Cruise TET K 50 Airf rame technology, size, operating cc4g uration 60-\\\\\\\ \ \\ ^V ^^^ ^^^ A burn pft fuel 70^^ burn er seat Year M Figure 1 Fuel efficiency comparison - long range transport aircraft Future Civil Engines A major requirement for civil engines will be to continue reducing the Life Cycle Cost to which a prime contributor is fuel burn. Figure 1 shows the contribution made 30 Compressor outlet 700/ temperature K 20 y^ Overall pressure ratio Figure 3 Thermal efficiency va psi ooz Cycle efficiency may be improved by operating at higher Turbine Entry Temperatures, TET, higher overall pressure ratios and with reduced parasitic losses by minimising cooling air to. blades, vanes and 2

3 structures. In addition higher TET results in a lighter core engine but there may be a noise penalty due to the higher jet velocity and higher cost due to more sophisticated materials being required. The benefit of improved cycle efficiency is shown in Figure 3. Reducing the specific thrust of the engine by increasing the airflow to generate a given thrust increases the propulsive efficiency. requirements must be achieved at the same or lower initial cost of current engines. The third element in civil engine life cycle cost is the maintenance and reliability costs. These have reduced over the years due to improved component life, reduced numbers of parts, better repair capabilities and engine condition monitoring. This trend needs to be maintained and improved Thermal y o Mach 0.8, , ISA efficient 0 Turbojets Low bypass 0.8 \ turbofans SFC- LB/HR/LB key ^h 0.4 S^ne n9 Iher Current turbofans Infln' Figure 4 Propulsive efficiency This is the most effective way of reducing fuel consumption, Figure 4, and also gives the potential for lower noise due to the lower jet velocity. This has resulted in configurations being considered as shown in Figure 5. The trend of moving from low bypass to high bypass ratio engines will continue. These ultra high bypass ratio engines may be the ducted fan engine or the propfan. It is possible that each configuration will be needed but within different market sectors, Ref 1. UHe Future Military Engines Future military engine requirements will continue to be for high thrust to weight ratio, twice that the current engines, and small frontal area. The choice of specific thrust and hence bypass ratio will be a compromise depending upon the mission required but will be in the range 0.3-1:1. The technology requirements will be to provide TET over 2000 C and the highest pressure ratio consistent with forward speed to give a small light engine with good cycle efficiency, Ref 1. The engine will need to operate at this higher TET with less cooling air to reduce parasitic loss. The military engine has a particular requirement to provide supersonic capability. It will continue to require a reheat system which needs to be light and have the capability of operating with minimum cooling air, but at higher temperatures. The progression of military engine development is shown in Figure 6. ^i^? k 1r 1^ y ix Spey 202 RB199 RB E4 I - "' RB541 RB509 8 JAM1 19A8 Figure 6 Military engine evolution Mid 1990 Future ETG 3311]V I»UE 1 71 RB529 Figure 5 Civil engine evolution The technology requirements for future engines are summarised in Figure 7. It can be seen that many requirements are common and that technology only becomes specific where configuration differs. Another major element in the Life Cycle Costs equation is the initial price. The improvements to date in thrust weight ratio and fuel consumption have all been attained but at increased price which reduces their benefit on LCC. The future technical

4 Civil engines Military combat engines Low LCC - High thrust to Higher thrust weight ratio requirements Demands - Small frotal area Higher component efficiency - Lower LCC Higher pressure ratios Higher turbine entry temperature High Low Fewer compressor/turbine stages Specific Lower parasitic airflow ecifi c t tgus thrust Reheat systems Results Large sysi emshrust Higher component loading diameter fans Smaller number parts y Higher temp gas path components Stiff lightweight casings Fewer cooled components Figure 7 Engine Technology requirements MATERIAL REQUIREMENTS The initial gas turbines utilised the materials available at the time, steel and aluminium. This gave way to the use of titanium and nickel alloys as the temperature requirements increased. Improved process development has enabled titanium and nickel based alloys to be used at even higher temperatures, and has led to the development of powder technology and single crystal castings especially in turbine applications. Potential still exists for further development in metallic materials but they will still not be able to meet all the future requirements of increased strength, stiffness, temperature capability and reduced density. All materials need to have predictable behaviour to ensure a safe product and guaranteed component life. Materials and manufacturing costs need to be reduced. The anticipated higher aerodynamic loading will require material with higher specific strength, stiffness. Increased engine temperatures will require material properties to be maintained and to have the capability of operating with reduced cooling air. The large structures required in civil engines will need to be light and low cost if these configurations are to be viable which again will require high stiffness and low density. Figure 8 summarises the major requirements for future materials. The next major advances will have to come from the use of composite structures. Compared with metallics they exhibit reduced density, increased specific strength and stiffness, improved temperature capability and oxidation resistance. There should also be the opportunity to reduce cost, by net shape forming. A comparison of material specific strength is shown in Figure 9. KKg Kg aoo- \ 700,C/Epoxy 600 -CIPMRts Existing material Future resin composite Future metal composite xglass matrix composites Future ceramic composite 0 ^ uglass ceramic matrix composites Ti MMC s Carbon/carbon 200 ^! } \ \ Silicon carbide/ 100 \ \, Silicon carbide NiNiM MC lsoo Undirectional properties C Figure 9 Material specific strength RESIN COMPOSITES v^,... z Polymers reinforced with glass, carbon, ceramic or organic fibres exhibit the highest specific strength and stiffness of available materials. Consequently, resin composites have replaced conventional metallic fabrications in aero engine applications. The replacement of aluminium by resin composites has typically yielded a 20-30% weight saving and 25% reduction in cost. In the early 1960s Rolls-Royce produced the RB162, a small lift engine shown in Figure 10. The compressor incorporated the glass fibre reinforced, epoxy novalac and bismaleimide polymer composites in the compressor casing, stator vanes and rotor blades. Approximately forty engines were in service in Trident 3B aeroplanes and completed a total of over cycles in operation. Predictable behaviour at high stress levels Lower manufacturing costs Increased strength Increased stiffness Increased temperature capability Reduced density Oxidation resistance CONIPOS^rts Figure 8 Material requirements 2 De 1987 ETG 33048V issue Figure 10 RB162-86/05 Composite experience

5 Li The use of resin composites has since increased, mainly in non load bearing applications such, as cowlings, compressor fairings and more extensively in the nacelle, for civil engines, Figure 11. Figure 11 Use of composite components Current applications have used epoxy resin which has a maximum continuous operating temperature of around 150 C. An increase in temperature is possible by using higher oxidative stability organic polymers, for example polymide PMR 15 which is likely to give operating temperatures of 280 C. This higher temperature capability will allow resin composites to be used within the core of the engine, turbine fairing, air off-take ducts, and the higher temperature nacelle parts. There are still a number of issues which are being addressed before all these applications can be realised. The organic polymers require higher processing temperatures which can lead to increased residual stress in the composite. The brittle nature of the cross linked thermo setting systems currently in use has resulted in matrix micro cracking and a probable deterioration in properties. The lack of erosion resistance will be a problem but suitable high temperature coatings are being developed. Components in the main gas path are subject to impacts due to foreign objects being ingested and improvements in impact resistance and impact modelling will therefore be required. For resin composites to be used in more arduous roles it is necessary for new design criteria and analysis techniques to be developed, based on an understanding of the physical phenomena involved. This requires more sophisticated modelling techniques, an understanding of fatigue behaviour, innovative component design, and improved process control and inspection techniques. METAL COMPOSITES The specific properties of metal matrix composites are not as good as those of resin c3mposites, though they do offer significant advantages in terms of temperature capability and can provide a significant improvement over monolithic metals. The use of continuous fibre reinforcement offers increased strength and stiffness and improvement in temperature capability. These benefits apply to aluminium and magnesium matrix composites, but these materials may have limited engine application due to their temperature capability It is considered that titanium metal matrix has considerably more potential in aero engines, offering weight savings of 10-20% in the compressor by direct replacement of monolithic titanium. There are a number of critical factors which are being addressed before metal matrix composites can be used with confidence. The primary requirement is the availability of a suitable fibre which must be thermally stable, offer high strength, stiffness and be compatible with the matrix. The ones currently available are the large diameter silicon carbide and boron monofilaments and this size of fibre limits the range of component shapes that can be produced. The new fibre may still require a coating to prevent interaction between the fibre and matrix. The cost of components in these materials is likely to be significantly higher than the equivalent monolithic parts but the savings in weight and improved capability may offset this disadvantage. For major rotating parts lifing rules will need to be established which address fatigue and creep behaviour. CERAMIC COMPOSITES Reinforced Ceramics Engineering ceramics have the potential of good high temperature mechanical properties, low density, and high temperature oxidation and corrosion resistance. These properties make engineering ceramics extremely attractive in turbines where increased TET and the reduction of cooling air are primary objectives. The monolithic ceramics silicon nitride and silicon carbide offer high strength and stiffness up to about 10 C and 1600 C respectively and engine components have been tested in these materials. However, two basic problems prevent the wide spread incorporation of monolithic ceramics into engines. Inherent brittleness and lack of toughness - which may be alleviated by the incorporation of whisker or particulate reinforcement and some success has been achieved with these systems. Poor defect tolerance - which leads to poor reliability. This problem may be overcome or limited by the improvement in material process control and manufacture or by the introduction of second phases, whiskers or particulates. The full potential of ceramics may only be realised when continuous reinforcement is incorporated. Silicon carbide/silicon

6 L carbide (SiC/SiC) is a leading candidate material. Small diameter fibres with high strength and stiffness, low density and thermal stability during component fabrication and under operating engine conditions are required. It is unlikely that current ceramic fibres will be able to provide adequate long term reinforcement at temperatures greater than 1200 C and most are expected to be limited to 1100 C. New high temperature thermally stable fibres are therefore required. Fibre coatings are required to limit the fibre matrix chemical interaction and to provide mechanically weak boundaries for toughening. The long term stability of these coatings with the fibre and matrices is also important in maintaining the material properties. SiC/SiC is currently produced by Chemical Vapour Infiltration, CVI, methods. Other fabrication techniques such as Sol Gel, directed oxidation methods are being developed. Considerable work is underway to ensure that adequate reliability, producability and surface finish can be achieved in components with complex geometry. Inspection and possible repair techniques must be developed before components can be offered for service. Reheat and exhaust unit components have been successfully tested under engine conditions. It is anticipated that SiC/SiC components in these applications will be incorporated into production engines in the near future. Glass and Glass Ceramic Matrix Composites Glass matrix and glass ceramic matrix composites offer a number of advantages for aeroengines:- 1 Low density, one third to half that of metallic alloys of equivalent temperature capability. 2 High specific properties. 3 Low expansion, which may be be controlled from zero to values close to metal expansion coefficients. 4 High temperature capability. A 10 C capability may be available for glass ceramic matrix composites. Silicon carbide reinforced pyrex is being considered for use in compressor rotor blades, stator vanes, casings and fairings. Reinforced glass ceramic systems are being considered for turbine aerofoils and casings and may find applications in reheat systems and exhaust ducts. Glass viscosity limits the temperature capability of silicon carbide reinforced pyrex and the availability of suitable thermally stable fibres currently limits the temperature capability of reinforced glass ceramics. Other areas which must be addressed are material and component behaviour and development of cost effective component manufacturing routes. An understanding of the fibre matrix interaction and associated failure modes is required. Improved design techniques, predictive modelling, joining and test technology are also required. Carbon-Carbon Carbon-carbon is a class of composites having high specific strength and stiffness, low density and, in an inert atmosphere, the ability to operate at temperatures of 3000 C. The limitation with this material, however, is its oxidation in air at temperatures above 0 C. The potential applications are reheat exhaust systems and turbine components. Studies have shown a potential 50% weight saving compared with existing nickel based alloy components and significant improvements in engine cycle by the elimination of cooling air. The provision of a coating to prevent oxygen ingress which is durable and erosion resistant is required. The current approach is to apply multi layer silicon carbide. Oxidation inhibitors are also required within the carbon matrices and fibres, such that fracture of the overlay coating does not result in castrotophic and immediate failure of the composite. Current material exhibits low interlaminar shear strength, high processing costs, poor erosion capability, moisture susceptability and contact load damage. Despite these problems successful development rig and engine testing has been completed on prototype carbon-carbon reheat and turbine components. This testing yielded valuable design information and confirmed the short term viability of the material in these applications. As a result further reheat and turbine applications have been designed and will be tested in the near future. SUMMARY AND CONCLUSION The future demands of the market for aero engines requires significant improvement in material properties. These potential improvements can be realised by extending the use of the currently widely used resin based composites and introducing the advanced glass, metal and ceramic composites. If the full potential of composites is realised then the percentage usage in engines could be as shown in Figure 12. There are a number of technical problems which are being addressed in order to allow these new materials to be incorporated in a satisfactory manner. All composite materials need to have established scientifically based material behavioural understanding, improved component analysis methods and design criteria to give optimum components and predictable characteristics. Manufacturing cost must be reduced by lower raw material cost and by improved processes, improved process control minimising component inspection and testing.

7 I Metal matrix composites Weight /.. Titanium Ceramic matrix composites - x/ 10 Resin based. _,/ composites C Year Tyne Spey RB211 RB E4 Figure 12 Predicted trends in jet engine material usage Resin composites need increased temperature capability, improved impact resistance and modelling, coatings for erosion resistance and also better repair procedures. The advanced glass, metal and ceramic composites need improvements in fibre technology, coatings, the control of fibre matrix interface, and advanced fabrication techniques. REFERENCES 1 Sadler J H R - Trends in Civil Aircraft Propulsion I Mech E., Aerotech Denning and Mitchell - Trends in Military Aircraft Propulsion I Mech E., Aerotech 1987 ee vsoon 7