Progress in the application of resin matrix composites for civil aviation engines

High thrust-weight ratio, low fuel consumption and low pollutant emission are important directions for the future development of aero-engines. Research and development of high-performance materials to replace traditional metal materials and achieve structural weight reduction is a key way to promote the progress of aero-engine. Resin matrix composites have the advantages of light weight, high strength, good structural designability, good fatigue resistance, excellent damping and vibration reduction performance, and easy to be integrated into the whole, which has become an ideal structural material for aircraft engine cold end components. The amount of resin matrix composites is also an important sign to evaluate the advancement of aircraft engines. With the improvement of composite design level, raw material performance, manufacturing process and reliability verification method, the application of resin matrix composite in civil turbofan engine has gone through a development road from less to more, from secondary to main, and from part to whole. At present, resin matrix composites have been widely used in many important parts of multi-type turbofan engine, such as fan blades and fan casing. By referring to the research and application experience of foreign aviation engine resin matrix composites, based on self-innovation, it brings both opportunities and challenges to accelerate the application of resin matrix composites in Chinese civil aviation engines.

I. Application of resin-based composite materials in civil aviation engines abroad

In recent years, General Electric, Pratt & Whitney and Rolls-Royce have made great progress in the application of resin matrix composite engine components. Pratt & Whitney, for example, was the first to prepare a fan rectification cone on a JT9D engine in 1970 using a fiberglass/epoxy composite. In order to further reduce the weight, JT9D-TR4 engine rectifier cone was prepared using arylon fiber/epoxy resin composite material in 1981. Later, resin matrix composites were widely used in Pratt & Whitney engines, such as the carbon fiber/epoxy resin fan blade pad prepared by resin transfer molding process of PW4084 engine, PW4168 double horse resin composite fairing and carbon fiber/epoxy resin composite thrust reversal device and other nacelle parts. FIG. 1 lists the application site, material system and preparation process of resin-based composites for domestic turbofan engines. FIG. 1 to 12 in turn are the ECU box of turbofan engine, inlet muffler liner, fan blade, intake rectifier cone, intake fairing, engine view door, thrust reverser, compressor fairing, outer culvert, outlet guide blade, fan casing, engine nacelle and other components. The following will be a detailed analysis of the application and development of the typical resin-based composite components of foreign civil aviation engines.

1 Application of resin-based composite in civil turbofan engine

1.1 Fan blades

In the 1970s, Luo Luo Company first tried to apply carbon fiber resin matrix composite material to RB211 engine fan blades. However, due to the low toughness of the composite matrix used, the engine failed to pass the bird strike test of the fan blade, resulting in the use of traditional titanium alloy fan blades.

With the development of aircraft engines with low quality, high intake efficiency and large bypass ratio and the improvement of the performance of resin-based composites, in the 1990s, General Electric Company selected HexPly 8551-7 ductile epoxy resin as matrix and IM7 carbon fiber as reinforcement fiber. GE90 engine intake fan blades were prepared by unidirectional prepreg molding process. The blade surface is coated with polyurethane anticorrosive coating to improve the blade corrosion resistance. The leading edge of the blade is bonded with 3M AF191 adhesive to enhance the impact resistance of the blade. The blade root has a self-lubricating Teflon wear layer (Figure 2(a)). Since then, GE's GEnx and GE9X engines have used resin-based composite fan blades.

2 Composite fan blades of GE90(a) and LEAP(b) series aeroengines

For the medium and small thrust engines suitable for single-aisle airliners, the traditional CFM56 series engines adopt titanium alloy fan blades and alloy steel metal casing. To further reduce engine mass and fuel consumption, CFM International, a joint venture between General Electric of the United States and Sinekma of France's Cifwind Group, has developed the LEAP family of engines. LEAP series engine fan blades adopt 3D integral braid technology to prepare fiber preforms with 3D interleaved structure and approximate zero dimensional error (Figure 2(b)). Fiber infiltration and resin curing were achieved by resin transfer molding process. (Figure 3) The three-dimensional fiber braided structure can effectively improve the impact resistance of the blade.

Rowe Rowe started work on a prototype engine mechanism called the SuperFan in Bristol, England, in early January 2020. The engine uses all resin matrix composite fan blades and casing. Fan blades are cured by laying carbon fiber/ductile resin prepreg. The leading edge of the blade is coated with titanium alloy similar to that of the GE90 fan blade to resist corrosion and foreign body impact. The company expects the engine to reduce the overall weight of the aircraft by 700kg when commissioned, making it more fuel efficient than the first Trent and reducing carbon dioxide emissions by at least 25 percent.

AP series engine resin matrix composite fan blade manufacturing process

1.2 Fan casing

When the blades are fractured due to impact or fatigue during engine operation, the fan casing can contain the lost blades to avoid damage to other parts of the aircraft. Therefore, fan casing is an important part to maintain the safety and reliability of aircraft service.

The fan blades of early turbofan engines were mostly made of titanium alloy, which had a large impact energy on the fan casing once they fell off. Fan casing is mostly made of aluminum alloy, titanium alloy or high-strength alloy steel to increase structural thickness and improve containment effect, which is called hard containment. Later, the composite structural casing was developed with the annular metal casing as the lining and several circles of aramid fiber braided strips as the protective layer. Fragments were captured by the aramid fiber layer, which was easy to undergo large deformation and absorb energy, so it was called soft inclusion.

As the bypass ratio of engine fan increases, the proportion of fan part in the total weight of engine increases, and the requirement of high performance and lightweight becomes more and more urgent. With the mature use of composite fan blades for GE90 series engines, GE developed an all-composite fan casing for subsequent GEnx engines. Toray's TORAYC T700 carbon fiber is braided into a 7.62mm thick preform in three directions of 0° and ±60° using an automated two-dimensional triaxial braid technology. CYCOM PR520 epoxy resin was cured by resin transfer molding process. Snekma, a subsidiary of France's Cefe Group, has also used reinforced fiber 3D braided technology and resin transfer molding process to produce LEAP series engine composite fan cases.

1.3 Sound Lining

Intake fan noise has become the main source of modern high bypass ratio aeroengine noise. Laying sound lining in the inlet is one of the important ways to reduce the noise of aeroengine. The perforated plate honeycomb structure of the sound liner can be seen as a number of parallel Helmholtz resonance structures. When the resonance frequency matches the noise frequency, the noise reduction effect is achieved. The traditional single-degree-of-freedom sound lining has narrow noise absorption band, while the multi-degree-of-freedom sound lining can broaden the sound absorption band, but it also has the problems of complex processing technology, large size and heavy structure.

Based on the above problems, Hearst developed Acousti-Cap embedded honeycomb, as shown in Figure 4. The surface perforated flexible material (such as polyether ether ketone, PEEK) is folded into the shape of baffle cap and embedded into the honeycomb cavity for adhesive positioning, so as to play the role of acoustic diaphragm in the two-degree-of-freedom sound lining. The cellular acoustic impedance characteristics can be adjusted by the following factors: (a) the number of diaphragms in the cellular cavity; (b) Location of the septum cap in the honeycomb cavity; (c) Barriers with different types of acoustic impedance characteristics. Compared with the traditional multi-degree-of-freedom interlining, the honeycomb interlining has thinner thickness, smaller installation space and higher overall structural strength. Currently, the honeycomb has been used in multiple aero-engines produced by GE, Lo Lo, CFM International, etc., which not only reduces mass but also achieves up to 30% noise attenuation.

4 Schematic diagram of embedded honeycomb manufacturing

1.4 Bushing

The traditional resin matrix composite matrix has low temperature resistance and is usually used in the cold end structure and external cladding parts of aeroengine. The development of high temperature resistant resin matrix represented by polyimide resin makes it possible to apply resin matrix composite to the hot end parts of aeroengine. Polyimide bushing is one of the typical applications of resin-based composites in compressor and other parts requiring high temperature resistance. Polyimide composites for lining not only meet the requirements of long-term working temperature of about 280℃ and short-term temperature resistance of more than 400℃, but also have good thermal dimensional stability, self-lubrication, low friction coefficient, excellent wear resistance and mechanical properties. DuPont (USA, DuPont Company) has developed the Vespel family of polyimide composites, which include graphite filled polyimide composites (e.g. Vespel SP-21, SP-22, etc.) and carbon fiber fabric reinforced polyimide composites (e.g. Vespel CP-8000, etc.). CP-0664, etc.). This series of polyimide composite materials has been applied to the adjustable static cotyledon bushing of multi-type aero-engine compressor such as Luo Luo BR710 and Pu & Whitney PW6000 series.

2. The application advantage of resin matrix composite material in civil aviation engine

2.1 conducive to the optimization of aero-engine structure

The fan blades of modern turbofan engines may be subjected to a centrifugal load of about 100t during operation. The centrifugal load increases with the increase of fan diameter, rotation speed and blade mass. Using resin-based composites to reduce the mass of fan blades can increase the design margin of fan blade size and speed. Due to the excellent designability of composite materials, composite fan blades have an efficient aerodynamic profile with S-shaped swept-back wide chord, so the use of fewer blades still has a higher intake efficiency. At the same time, compared with the hollow structure of titanium alloy fan blade, it can achieve 10%~15% weight reduction.

With the improvement of composite fan blade design level and material performance, after several generations of development of GE90, GEnx and GE9X, GE engine composite fan blades show a trend of decreasing number, thinning thickness and stronger performance. The GE90, GEnx and GE9X engines respectively have 22,18,16 composite fan blades. Among them, the latest GE9X engine fan blade adopts carbon fiber with higher stiffness as the reinforcement body, which can make the fan blade longer, thinner and more efficient. In addition, alloy steel with better impact resistance is used to replace the titanium alloy leading edge edge of the fan blades of GE90 and GEnx engines. The trailing edge is glass fiber composite material with special structure. The impact resistance of the fan blades is further improved by local strengthening measures at the front and rear edges of the blades, which can make the overall thickness of the blades thinner. Therefore, although the diameter of the GE9X fan is 3.4m, the engine fan is lighter, faster, more aerodynamic efficient and has better overall performance. The application of composite fan blades led to the production of carbon fiber fabric/epoxy resin composite containment casing. The all-composite containment casing not only has light weight, but also possesses high structural stiffness and good elastic deformation, which can realize good containment of composite blades.

Simultaneous molding of material and structure is one of the characteristics of resin matrix composites which is different from metal materials. It provides the possibility for integrated design and manufacture of large complex components of aeroengine. Nessa, an American company, has ditched the traditional concept of separate subsystem design and developed an integrated propulsion system on the LEAP-1C engine on Comac's C919 jumbo jet. These include an all-in-one composite intake fairing and an integral composite "O-shaped" sliding thrust back device (Figure 5). In addition, it is also a typical case of the application of composite integral forming technology to change the traditional engine inlet splicing type sound liner into annular non-splicing type sound liner.

5. LEAP 1C engine all-in-one fairing and "O-type" sliding thruster

2.2 conducive to improving the economy of aero-engine

Reducing engine mass is an important way to improve fuel efficiency and thrust-weight ratio of aero-engines. Resin matrix composite material is light and strong, which can effectively reduce the quality of engine structure when applied to fan blades, containment casing and other components. For example, the CFM56-7B engine has 24 titanium alloy fan blades with a total mass of 118kg, while the LEAP series engine has 18 composite fan blades with a total mass of only 76kg. Compared with the metal blades and casing of the same size, the overall weight can be reduced by 455kg using resin-based composite materials.

Using composite material co-curing, co-adhesive and other integral forming technologies to prepare large and complex structural parts, on the one hand, the engine performance can be improved through structural optimization, on the other hand, the number of sub-parts can be reduced, and the structural weight gain and performance loss caused by component assembly and connection can be reduced. Compared with the multi-block spliced aluminum alloy lip and inlet of traditional engine, LEAP-1C engine integrated seamless composite intake fairing can avoid the decrease of intake efficiency caused by discontinuity of flow field. The integrated composite "O-type" sliding push-back device replaces the typical two-piece "D-type" door design, which not only improves the push-back efficiency, but also realizes the transformation from the original heavy hydraulic drive system to the advanced electronic drive system, and solves the problem that the "D-type" door interlock mechanism needs continuous maintenance. Overall, integrated propulsion systems enhance engine aerodynamic performance, reduce engine mass through structural integration, reduce fuel consumption, improve reliability, and facilitate maintenance, all of which will effectively improve engine economy and reduce aircraft operating costs.

From the point of view of mechanization and automation of manufacturing and testing, the technology of automatic prepreg cutting, laser positioning overlay and three-dimensional fiber preform weaving has been more mature at present. Recently, in the "Super Fan" engine prototype mechanism building project of Lo Lo Co., the composite fan blades and casing were manufactured by automatic wire laying (FIG. 6(a)) and automatic tape laying (FIG. 6(b)) respectively. Three-dimensional laser measurement technology (FIG. 6(c)) and underwater ultrasonic flaw detection technology (FIG. 6(d)) are also applied in the inspection process of fan blade shape and size measurement and internal defect detection. The improvement of the mechanical automation level not only improves the work efficiency, ensures the standardization and accuracy of the composite component manufacturing and testing process, but also reduces the reject rate and labor costs, which is conducive to reducing the manufacturing cost of aero-engine.

6 Automated manufacturing and testing of Rolls-Royce's "Super Fan" engine prototype

The excellent fatigue resistance and durability of composites can significantly reduce the repair and maintenance cost of components in service. Data show that the installed GE90 composite fan blades have been used for a total of 7.5 million flight hours without special inspection and on-site maintenance. Despite having experienced up to 100 bird strikes during its service, only three composite blades had to be completely replaced, demonstrating good reliability and economy. In addition, the root of the composite fan blade has a self-lubricating Teflon wear-resistant layer, which eliminates the need to add lubricants when the blade is loaded into the dovetail groove, eliminating the cost of regular lubrication and maintenance.

2.3 Help to improve the environmental protection of aviation engines

With the increasing requirement of environmental protection in today's world, aero-engine exhaust emission and noise level have become the focus of attention of aero-engine manufacturers. Resin matrix composites can effectively reduce the structural quality of the engine, reduce fuel consumption, reduce the exhaust emissions of the aircraft engine, and improve its environmental protection. Compared with the CFM56 series, the LEAP series uses a large number of composite components to reduce fuel consumption and carbon dioxide emissions by 15 percent and nitrogen oxide emissions by 60 percent.

Engine noise is the main source of aircraft noise. With the increase of the bypass ratio of turbofan engine, the intake fan noise in the engine noise proportion increases gradually. The traditional inlet silencing board is spliced and pieced, which causes the inlet wall acoustic impedance discontinuity and weakens the silencing effect. As shown in Figure 7, the early A320 aircraft engine inlet sound liner of Airbus in Europe was stitched with 3 pieces of 15cm, and the later A340 600 aircraft engine was stitched with 2 pieces of 7.5cm wide. After adopting resin matrix composite integral forming process, A380 aircraft engine is ring without splicing sound lining.

7 Civil aviation engine inlet splicing and non-splicing silencer panels

The International Civil Aviation Organization first put forward the requirements for aircraft noise control in Chapter 2 of Annex 16 of the Convention on International Civil Aviation in 1972, which is called Chapter 2 noise control standard. In 1977, Chapter III noise control standard was implemented, in which requirements were put forward for the cumulative noise of aircraft with different takeoff masses, such as over flight, sideways, approach and the above three. Later, ICAO proposed the more stringent Chapter IV and Chapter 14 noise control standards, with cumulative noise 10dB and 17dB lower than Chapter III noise, respectively. The United States Federal Aviation Administration defines the second, third, fourth and fifth stage noise control standards (Stage2/3/4/5) respectively according to the noise standards in each chapter of ICAO (as shown in Figure 8).

8 International Civil Aviation Organization and United States Federal Aviation Administration noise control standards at each stage

Benefiting from the non-splicing sound lining of the composite inlet and the high efficiency and low noise design of the intake fan blade, the operation noise of the GE9X engine is lower than the noise requirement of the fifth stage and has an 8dB margin. In addition, the European Aviation Safety Agency carried out flight noise tests on Airbus A321neo aircraft fitted with LEAP-1A engines. Data showed that the overflight, lateral and approach noises of A321neo aircraft were 83.3, 88.3dB and 94.7dB respectively, all lower than those of Airbus A321 aircraft equipped with CFM56 engines, meeting the noise control requirements of phase 4.

3. The new trend of the application of resin-based composite materials in civil aero engines

3.1 Hybrid technology of micro and nano materials

Aero-engine nacelles and fan blades are vulnerable to foreign bodies at different speeds during maintenance and service, resulting in internal layered failure and even penetrating failure. At present, the 3D braiding or stitching technology of fiber reinforcement is usually used to introduce braiding or stitching fibers in the thickness direction of composite materials to improve the interlaminar fracture toughness and impact resistance of composite materials. However, the high manufacturing cost of 3D fiber reinforced structures and the degradation of in-plane properties caused by fiber damage during processing are also important problems restricting their wide application. Compared with traditional metal materials, the electrical conductivity of composite materials is weak, and the structural damage of engine components is easy to occur when subjected to lightning impact, which endangers flight safety. Based on the above two points, multi-scale micro-nano particle hybrid technology is used to improve the interlaminar fracture toughness and electrical conductivity of resin matrix composites, and the development and application of structural and functional integrated components has attracted the attention of many scholars.

At present, the most used nano fillers are carbon nanotubes (CNT), graphene, graphite oxide, carbon black, nanofibers and so on. After Bhanuprakash et al. added modified graphite oxide nano filler into epoxy resin matrix, the interlaminar fracture toughness of glass fiber/epoxy composite Ⅰ, Ⅱ was greatly improved. This is consistent with the conclusion obtained when the filler is added into the epoxy composite material of carbon fiber fabric. Srivastava et al. added 3%(mass fraction, same below)CNT, graphene and carbon black particles into the carbon fiber epoxy resin composites, and the results showed that the samples with graphene and CNT had the maximum fracture toughness in type Ⅰ and type Ⅱ interlayer, respectively. Although the method of directly mixing nano-filler into the resin matrix is simple and convenient, the resin viscosity increases when the amount of nano-filler is high, resulting in difficulties in process implementation, and the agglomeration of nano-filler leads to poor impregnation of resin and easy to produce defects in the parts. Therefore, the researchers improved the interlaminar fracture toughness of the composites by introducing nanofiber cloth or resin film or powder with high nano-filler content. Shin et al. mixed up to 9%CNT into resin and introduced it into unidirectional and fabric carbon fiber laminate in the form of resin film containing nanotubes. The results show that the type II interlaminar fracture toughness of the two laminates increases first and then decreases, and the maximum value is found when the CNT mass fraction is 3%. The authors suggest that the bridging effect of CNT retards crack propagation and improves the fracture toughness of type Ⅱ interlayer. Abidin et al. significantly improved the interlayer fracture toughness of type Ⅰ carbon fiber composite by adding resin powder containing carbon nanotubes between the layers. Ou et al. inserted low-density carbon nanotube yarn between the interface of carbon fiber/epoxy resin composite material, and its type I interlayer fracture toughness increased by 60%.

In addition, polysulfone, polystyrene, nylon 66, polyacrylonitrile, polyvinyl butyrate, polyurethane and other polymer materials can be prepared by electrostatic spinning nanofiber cloth inserted into the composite layer to improve the fiber reinforced resin matrix composite Ⅰ, Ⅱ interlayer fracture toughness. It is found that the polystyrene nanofiber cloth can not only improve the interlaminar fracture toughness but also enhance the in-plane properties of the composite laminates. The introduction of nylon 66 between layers resulted in a slight increase in laminate thickness and a slight decrease in tensile strength. The polysulfone nanofiber cloth was miscible with epoxy resin after curing and heating up, and separated in the form of microspheres after cooling. The weak bonding between the polysulfone nanofiber cloth and epoxy resin matrix resulted in the decrease of bending strength and modulus of composite laminates. In order to solve the above problems, the mixed use of different nano fillers such as polysulfone/carbon nanotubes, polyacrylonitrile/alumina, polyacrylonitrile/carbon nanotubes can avoid the negative effect of single nano fiber cloth on the inner performance. Handschuh et al. inserted thermoplastic polyurethane Namib into the prepreg layering of the leading edge of composite blades to improve the toughness of composite materials and the impact resistance of the blades. The results show that after toughening treatment, no apparent delamination damage occurs in composite blades, but without toughening treatment, the delamination and fracture damage are serious.

Using nano fillers with good electrical conductivity to "grow" or cover the fiber surface can not only improve the interlaminar fracture toughness of the composite, but also effectively improve the electrical conductivity of the composite structure. Bhanuprakash et al. covered the surface of carbon fiber with GO and modified GO respectively, and the epoxy resin composite type I interlayer toughness and electrical conductivity were improved to varying degrees. Pozegic et al. showed that by growing CNT on carbon fiber surface, the in-plane conductivity of carbon fiber epoxy resin composites increased by 330%. Due to the formation of conductive seepage channel in the direction of thickness between layers, the increase of conductivity in the direction of thickness is more obvious (up to 550%), which is consistent with the results of significant improvement of conductivity in the direction of thickness of composite materials in the literature. Duongthipthewa et al. applied graphene to the surface of carbon fiber with carbon nanotubes, the in-plane and thick-direction conductivity of carbon fiber/epoxy resin composites increased by 300% and 190%, respectively, and the impact performance increased by 71%. For different resin substrates, Russello et al. prepared thermosetting epoxy resin and thermoplastic polypropylene resin thin layer composites by using carbon fiber fabrics grown on the surface of carbon nanotubes, and the results showed that the conductivity of the latter improved more.

3.2 3D printing technology

3D printing, also known as additive manufacturing, is one of the hot trends in the development of material manufacturing technology. The traditional manufacturing method realizes the preparation from raw material billet to parts through the "subtraction" process such as turning milling, planing and grinding. 3D printing creates parts from the bottom up using an "additive" process that builds up layer by layer. In accordance with the increase of material system can be divided into the material status and forming method curing light stereo shape constituency, fused deposition molding, laser sintering, layered entity manufacturing, etc. Compared with the traditional manufacturing process, additive manufacturing can realize the rapid and accurate forming of small batch customized complex parts, reduce the waste of raw materials, save the cost of mold and labor, simplify the manufacturing process and shorten the manufacturing cycle.

At present, the application of 3D printing technology in the aerospace field is increasing year by year. The aeroengine components prepared by 3D printing technology mainly include fuel nozzles, turbine blades and other metal components. In recent years, preliminary research has also been carried out in the direction of 3D printing resin and its composite materials. NASA Langley Research Center of the United States used photocuring stereoforming technology to manufacture a model of a pure resin-made variable thickness acoustic liner, and the swept impedance tube was used to verify the acoustic attenuation characteristics of the variable thickness acoustic liner (Figure 9(a)). The Glenn Research Center used Stratasys Ultem9085 resin system to prepare the sound liner and perforated engine view door by melting deposition (FIG. 9(b), (c)). The wind tunnel experiment shows that the sound absorption performance of the interlining manufactured by this technology is similar to that of the traditional honeycomb core/perforated panel interlining. In order to improve the mechanical properties of the parts, 10% cut AS4 carbon fiber was added into Ultem1000 resin system, and the compressor inlet guide blade was prepared by melt deposition method (FIG. 9(d)). It was found that the tensile strength and modulus were increased by 23% and 38%, respectively, compared with the system without the addition of short-cut carbon fiber resin, but the porosity reached 25% and the brittleness was greater. This is because after carbon fiber filling Ultem1000 resin system, the melt viscosity can only meet the process requirements at 420℃. High temperature will make the volatile components such as low molecular weight water vapor in the resin system and the gas wrapped in the extrusion process expand, resulting in higher porosity of the exit guide blade. The author believes that by controlling the molecular weight distribution of Ultem1000 resin system, the viscosity of Ultem1000 resin system can reduce the porosity of the parts to a certain extent.

9. Aeroengine 3D printed resin and its composite components

Impossible Objects' carbon fiber/polyether ether ketone system can withstand 250℃. The performance of the parts is two-thirds of the traditional aluminum alloy, but the quality is only half of the aluminum alloy, which can be used in the 3D printing process of aircraft parts.

3.3 Metamaterial technology

The concept of metamaterials was first proposed by scholars in the field of electromagnetics. Metamaterials are defined as composites composed of periodic or aperiodic artificial microstructural units. Such materials can exhibit extraordinary physical properties that natural materials do not possess. In recent years, more and more attention has been paid to the research of metamaterials in the field of acoustics. Noise capture and attenuation can be realized through the design of special meso-structure, which provides a new idea for the future design of aeroengine sound lining.

When the noise frequency is low, the perforated plate honeycomb sound liner needs to increase the structural thickness to meet the demand of noise reduction. However, the limited installation space of the liner inside the aeroengine restricts the performance of the traditional sound liner in reducing the low frequency noise of the aeroengine. The unique structural designability of metamaterials helps to resolve this contradiction. Li et al and Chen et al respectively designed two spatial spiral structures, which, combined with micro-perforated panels, can effectively reduce the thickness of the structure and expand the absorption range of low-frequency noise (FIG. 10(a), (b)). Yang et al. achieved the balance among structural thickness, absorption frequency domain and absorption capacity of low frequency by means of bending and folding cavity (Figure 10(c)). Tang et al. introduced the metamaterial form of oblique microporous partition into the traditional honeycomb sound lining structure to meet the requirement of wide-frequency and efficient absorption of random incident sound waves (Figure 10(d)). Jiang et al. proposed the concept of unidirectional metasurface composed of Helmholtz resonators of different depths and acoustical metamaterials of approximately zero refractive index. When sound waves enter the structure, they are "captured" and achieve energy dispersion in continuous reflection and absorption (Figure 10(e)). By placing the unidirectional metasurface between the aero-engine sound liner and the interior surface of the short cabin, the noise can be spatially limited to the area where the sound liner acts, thus achieving a synergistic effect and promoting noise absorption. The complex and fine structure is the key factor for the performance of metamaterials, but the traditional material processing technology is difficult to manufacture. At present, researchers have used 3D printed Kevlar fiber/thermoplastic resin to prepare corrugated core sandwich structures similar to those described in the literature. Therefore, combined with new technology such as 3D printing and the characteristics of light weight and high strength of resin-based composites, it brings the possibility for the development and application of new metamaterial sound lining.

Figure 10. Design of metamaterial structures for noise absorption and capture of different types

4. Enlightenment of resin-based composite material application in civil turbofan engine

4.1 Advanced structural design is the pilot of the application of resin matrix composites for aeroengines

The anisotropy of composite materials brings not only rich choices but also great challenges to the design of aero-engine composite components. Taking composite fan blades as an example, problems such as aerodynamic performance, foreign body impact and structural fatigue at high rotational speed in service should be considered in the design process. Therefore, based on the interdisciplinary integration of composite material structural mechanics, aerodynamics, failure analysis and other disciplines, it is particularly important to form the control of the internal mesostructure of the parts, grasp the overall dimensional accuracy, and coordinate the multi-angle comprehensive design concept of structure and aerodynamic performance. The emergence of new 3D printing technology and metamaterial structure provides a new direction for the design of composite components of aeroengine. We should give full play to the characteristics of new technology and new structure, expand the design idea and optimize the design idea of complex components. In addition, attention should be paid to the technical maturity of the material system and the accessibility and efficiency of the molding process during the structural design of the parts. Realize "design - material - manufacturing" integrated planning, play the leading role of structural design.

4.2 High-performance raw materials are the basis for the application of resin-based composites for aeroengines

The continuous progress of advanced reinforcement materials and resin systems is an important basis to promote the application of resin matrix composites in aeroengine. From the point of view of reinforced fiber, the reinforced fiber used in aero-engine components has undergone a transformation from traditional glass fiber to high performance carbon fiber and aramid fiber. From the perspective of resin system, the demand for high performance of epoxy resin, double horse resin and polyimide resin is increasing day by day. At present, the application trend of resin matrix composite materials in high speed rotating parts such as fan blades and high temperature parts in aero-engine promotes the development of resin system for engine to high toughness and high temperature resistance. While exploiting the performance potential of fiber and resin, the third phase functional components represented by micro and nano materials are developed to achieve a balanced unity of strengthening mechanical properties and expanding functional properties of composite materials. From the perspective of 3D printing technology, there are few mature material systems suitable for different 3D printing processes. Therefore, it is of great significance to realize the standardization, standardization development and verification of 3D printing material system and ensure the advanced and mature performance of raw materials for the development and application of 3D printing aeroengine resin matrix composite components.

4.3 Efficient and low-cost manufacturing is the key to the application of resin-based composites for aeroengines. 4.3 Efficient and low-cost manufacturing is the key to the application of resin-based composites for aeroengines

High efficiency and low cost manufacturing technology is the key to ensure the stable mass application of composite components for aero-engines. For example, the accuracy of composite fan blade size and shape has a crucial impact on engine intake efficiency and aerodynamic stability of blade operation. At present, automatic prepreg cutting, laser positioning overlay technology and 3D weaving technology of fiber preformed body used in composite material blade production not only ensure the accuracy of blade size, but also greatly reduce the subsequent trimming and other processes, saving the expenditure of manpower and material resources. Mechanical automation has become an important symbol of the development level of resin matrix composite molding technology. Therefore, according to the requirements of precise and efficient production of resin matrix composite parts for aero-engine, different disciplines such as materials, machinery and control should be integrated to actively carry out the development and application of automation equipment, so as to achieve multi-direction crossover, promote win-win results among different specialties and promote automatic manufacturing of resin matrix composite materials. In addition, for small-batch complex structural parts, 3D printing technology can reduce the waste of raw materials without tooling manufacturing, and avoid large investment in tooling manufacturing, raw material reserve and other links. Thus, it is helpful to reduce the high manufacturing cost of aero-engine components.

4.4 High reliability verification and evaluation is the guarantee for the application of aeroengine resin matrix compositesIn the process of service, the resin-base composite parts for aero-engine will be subjected to the harsh working conditions such as damp-heat aging, fatigue chatter and foreign body impact. Foreign aeroengine composite fan blades have experienced fatigue test, deicing system test, surge and rotation stall margin test, swallow bird test and other experimental certification. Composite materials for the tolerance effects evaluation should also through planar target, full size tolerance experiment and experiment examination. It should be pointed out that the composite materials are different from the traditional metal materials in the aspects of forming quality evaluation and failure mode analysis, so the evaluation standard of the original metal materials can not be completely copied. By materials, components, parts as the accumulation of the experimental data and the actual service condition state collection, establish and improve the resin used for aviation engine base material validation database and the corresponding evaluation mechanism, promote the application of resin matrix composites in aircraft engine.

After ten years of development, resin-based composites have been widely used in civil turbofan engines. Resin matrix composites not only reduce the structural quality of aeroengine, but also play a positive role in engine reliability, economy and environmental protection. Its dosage has become an important symbol to measure the advancement of aero engine. Foreign countries have accumulated a lot of experimental data and service experience in the application of aero-engine composite materials, and have obtained more mature applications in multi-type aero-engine. In comparison, there is some gap in research and application of resin matrix composite components for civil aviation engine in our country. We need to catch up in many aspects such as structural design, raw materials, manufacturing process, verification and evaluation. It is believed that with the progress of resin matrix composite technology in our country, the application of resin matrix composite in civil turbofan engine will usher in a breakthrough development.

Disclaimer: The article is reprinted from Materials Engineering by Ma Xuqiang, Su Zhengtao (Beijing Aeronautical Materials Research Institute of Air China). This article is only for sharing, does not represent the position of the company, in case of infringement, please contact to delete, thank you!