THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47th St., New York, N.Y. 10017 The Society shall not be responsible for statements or opinions advanced In papers or discussion at meetings of the Society or of its Divisions or Sections, 94•GT-475 or printed in its publications. Discussion is printed only if the paper is pub- lished in an ASME Journal, Papers are available from ASME for 15 months after the meeting. Printed in U.S.A. Copyright © 1994 by ASME Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1994/78873/V005T12A012/2405346/v005t12a012-94-gt-475.pdf by guest on 26 September 2021 CURRENT AND FUTURE MATERIALS IN ADVANCED GAS TURBINE ENGINES G. A. Kool Materials Department National Aerospace Laboratory NLR Amsterdam, The Netherlands 11111111111111111111111 ) ABSTRACT as during development of new, untried aero engine materials. Gas turbine engines are constructed of components with From the operator's point of view, safety and cost are two major excellent strength and stiffness, a minimum density, a high concerns in engine operation. These two points will be discussed temperature capability for long times, and at affordable cost. in more detail. Material behaviour under service conditions and Metallic materials are the centrepiece in fulfilling these material costs must be considered extensively where conventional requirements. Future gas turbine engines will have to have higher materials are replaced and new materials are implemented. thrust-to-weight ratios, better fuel efficiencies and still lower costs. This will require new and advanced lightweight materials Safety with higher temperature capabilities. Engine problems account for only a small share of air transport This paper discusses some of the presently applied materials in accidents [I], namely 12 % between 1959 and 1989 (Fig. 1). the fan, compressor and turbine sections of gas turbines, and Considering the period 1976-1983 for transport aircraft, only a reviews the material developments that are occurring and will be little more than 25 percent of the failures involved discs (Fig. 2). necessary for the near and long term futures. Because of the storage of kinetic energy in disc fragments, disc failures produce the most serious consequences. Of the 52 cases recorded, 12 were classified category three: significant damage to INTRODUCTION . In civil and military aircraft the propulsion is based on the gas turbine engine. In principle the gas turbine ingests air from the atmosphere and compresses it several times in the compressor. engine-related aircraft accidents Fuel is added and the mixture is burned giving a high pressure, 1959-1989 high velocity gas stream. Part of the energy in the gas stream is used to rotate a turbine section which in turn drives the engine lam and inappopriate compressor. However, the largest part or the energy can be used crew response to drive a propeller, a fan, or give thrust by itself. 25% The goals for the gas turbine engines of the 21st century are significantly higher thrust to weight ratios, better fuel efficiencies uncontained and lower life cycle cost. These general requirements translate engine failures into the need for materials with increased strength and stiffness, 48% reduced density, and higher temperature capability for longer maintenance-related times. Major concerns once these materials are available will be human talon 8% the design, development, manufacture, testing and inspection, and the repair of required components at affordable cost. cone= failure modes This paper identifies some presently applied engine materials in N. (inducing simulaneous the fan, compressor and turbine sections, and reviews material losses at thrust on more than developments for the near and far term. one engine) thrust reverser 8% 4% ENGINE MATERIAL REQUIREMENTS Safety and costs have to be considered continuously during the Fig I Engine problems account for 12 percent of air transport selection and application of current, well-known materials as well accidents during 1959- 1989 Met 13 Presented at the International Gas Turbine and Aeroengine Congress and Exposition The Hague, Netherlands — June 13-16, 1994 uncontained failures commercial transports 1976-1983 122 blades 52 discs 28 spacers cat. 3 cat. 4 7 3 Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1994/78873/V005T12A012/2405346/v005t12a012-94-gt-475.pdf by guest on 26 September 2021 cal. 2 46 cat. 1 69 20 15 category 1: nacelle damage category 2: minor aircraft damage category 3: significant aircraft damage category 4: severe aircraft damage Fig. 2 Uncontained engine failures [Ref 1] Engine components Turbine lost in flight section — Combustion Fan section section ( Compressor section forward Accessory drive section Fig. 3 CF6-6 engine cutaway [Ref. 2] 2 the aircraft resulting in damage to the primary structure, rapid POLYMER-MATRIX COMPOSITES (PMCs) depressurization, fire, slight injuries to the passengers. Three Extensive experience with composites has been gained over the were classified category four: severe damage to the aircraft past decade. Kevlar aramid fibre containment casings have been ending with crash, loss of the aircraft, serious or fatal injury to standard on CF6-engines since 1980. The outlet guide vanes for the passengers. the CF6-88C2, 88 in number, have been made with graphite Disc separations are usually fatigue-related, whether induced by epoxy since 1985. At present GE is researching a composite engine operating cycles (e.g. start/stops) or high-cycle dynamic forward fan for the 6E90, the GE engine for the next generation modes of the rotor or rotor/stator interactions, sometimes of of jetliners. At Pratt and Whitney, composites are standard on aerodynamic origin. The origin of fatigue cracking and fracture commercial engines for guide vanes and for the ducting where often lies in the design of the part or in material deficiencies. One acoustic liners are required, as in the area of the fan tips. Fibrous Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1994/78873/V005T12A012/2405346/v005t12a012-94-gt-475.pdf by guest on 26 September 2021 of the rare major accidents which was caused by a material defect materials have good damping characteristics for noise. Rolls- occurred in the summer of 1989 in Sioux City, Iowa, USA, [2]. Royce are looking at PMR-15 for core fairings now made with After burst of a CF6-6 fan disc (Fig. 3) from the tail engine of a titanium on R821I-engines, as well as graphite-epoxy bypass DC-I0 the engine component fragments hit the aircraft's hydraulic ducts on Rolls' Tay-engine, used in the Fokker 100. control systems. The disc failure from the tail section engine was Like the GE90 fan, most of today's production composites are followed by an emergency landing of a virtually uncontrollable made with epoxy resins. The epoxy resin serves as the binder, or aircraft, which crashed at the airport 45 minutes after the disc matrix material that holds high-strength graphite (or other) fibres failure. A semi-circular fatigue crack with a radius of 14.5 mm in place. Epoxy-matrix/carbon fibre composites can be used at resulted in the disc burst. Fatigue cracking originated from a relatively low service temperatures to a maximum of 120-150 °C. cavity (1.4 mm long and 0.4 mm deep) in the Ti-6A1-4V disc This is far to low for the more critical, hot segments of a jet bore. This cavity was located in an area with a hard phase engine. The NASA-developed polyimide PMR-15 is capable of inclusion containing microcracks and microporosity. withstanding thousands of hours of use at temperatures between The above illustrates that high standards or even higher ones 290 and 345 °C. Almost all high-temperature parts now in are required for new materials in design, development, production are found on military engines and made of graphite- manufacturing processes and inspections. Safety is not negotiable polyimide composites [4]. GE has used PMR-15 resin for the and cannot be traded in order to improve other factors. main duct of the F404 engine, which powers the F-18 fighter, since 1988. More than 250 engines with PMR-15 ducts have been Costs built. Also in production at GE are PMR-15 air splitters and inner The primary goal of an engine manufacturer and operator is. ducts for the F110 for the F-I6 fighter. Pratt and Whitney is in safe and reliable operation. Ultimately this can be translated into production with a graphite-polyimide aerodynamic exhaust feather satisfied customers and financial benefits. The second objective is on its F100-PW-229 for the F-15 and F-16 fighters. To realize to minimize the operational cost. The main cost factors for long the full advantage of PMCs in aircraft engines, however, new range civil engine operation are [3]: composite materials must be developed with: • faio[fl depreciation, interest and insurance. The - improvements in the stability of polymer matrices coupled capital cost is significantly influenced by the number of spare with improvements in polymer/fibre interfaces engines and modules required to support operation. Factors to _ reduced proressing costs decrease cost are: - oxidation resistant coatings that will enable PMC use at - low removal rates temperatures up to 425 °C - short shop turnaround times Polymer research at NASA Lewis [5] has produced major - pooling of spare engines advances in high-temperature polymer-matrix composites. The new polymer V-CAP, which is given a recently developed • Maintenance costs (II %'r material and labour; both on-wing nitrogen-postcure treatment, has a useful lifetime double that of and in the shop. In general, maintenance costs can be related PMR-I1-50 composite and five times that of PMR-15 (Fig. 4). to the number of components in an engine. In case of KSSU This lifetime is based on 10 % weight loss in air at 370 °C.