Composite Materials in Aerospace Applications - International

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1 International Journal of Scientific and Research Publications, Volume 4, Issue 9, September 2014 1 ISSN 2250-3153 Composite Materials in Aerospace Applications Nikhil V Nayak* * U.G. Student, Mechanical, B.V.Bhoomaraddi College of Engineering & Technology Abstract- Fiber-reinforced polymer composite materials are fast temperatures. Ceramics outstrip metals and polymers in their gaining ground as preferred materials for construction of aircrafts favorable melting points, ability to withstand high temperatures, and space crafts. In particular, their use as primary structural strength and thermal expansion properties, but due to their materials in recent years in several technology-demonstrator brittleness they are often unsatisfactory as structural materials. front-line aerospace projects world-wide has provided confidence This lead to the exploration of composites. One may define a leading to their acceptance as prime materials for aerospace composite as material as a materials system which consists of a vehicles. This paper gives a review of some of these mixture or combination of two or more micro constituents developments with a discussion of the problems with the present mutually insoluble and differing in form and/or material generation composites and prospects for further developments. composition. Examples of composites are steel reinforced Although several applications in the aerospace vector are concrete (metals + ceramics), vinyl-coated steel (metals + mentioned, the emphasis of the review is on applications of polymers), fiber reinforced plastics (ceramics + polymers). composites as structural materials where they have seen a Emergence of strong and stiff reinforcements like carbon significant growth in usage. A brief review of composites usage fibre along with advances in polymer research to produce high in aerospace sector is first given. The nature of composite performance resins as matrix materials have helped meet the materials behaviour and special problems in designing and challenges posed by the complex designs of modern aircraft. The working with them are then highlighted. The issues discussed large scale use of advanced composites in current programmes of relate to the impact damage and damage tolerance in general, development of military fighter aircraft, small and big civil environmental degradation and long-term durability. transport aircraft, helicopters, satellites, launch vehicles and missiles all around the world is perhaps the most glowing Index Terms- Composite materials; aerospace applications. example of the utilization of potential of such composite materials. I. INTRODUCTION II. THE AEROSPACE STRUCTURES AND FEATURES T he range of materials can be classified into the categories: Metals, Polymers, Ceramics and inorganic glasses and composites. Metals lose their strength at elevated temperatures. Important requirements of an aerospace structure and their effect on the design of the structure are presented in table 1. High-Polymeric materials in general can withstand still lower

2 International Journal of Scientific and Research Publications, Volume 4, Issue 9, September 2014 2 ISSN 2250-3153 Further, the structure has to meet the requirements of fuel The composites offer several of these features as given below: sealing and provide access for easy maintenance of equipments. Light-weight due to high specific strength and stiffness Passenger carriage requires safety standards to be followed and Fatigue-resistance and corrosion resistance these put special demands of fire-retardance and crash- Capability of high degree of optimization: tailoring the worthiness on the materials and design used. For spacecraft the directional strength and stiffness space environmentvacuum, radiation and thermal cycling-has to Capability to mould large complex shapes in small cycle time be considered and specially developed materials are required for reducing part count and assembly times: Good for thin-walled or durability. generously curved construction Two key developments in scientific-technological world Capability to maintain dimensional and alignment stability in have had a tremendous influence on the generation and space environment satisfaction of the demands raised by the aerospace community: Possibility of low dielectric loss in radar transparency one, the advances in the computational power and the other, Possibility of achieving low radar cross-section composites technology using fiber reinforced polymeric materials. These composites also have some inherent weaknesses: Laminated structure with weak interfaces: poor resistance to out-of-plane tensile loads III. USE OF COMPOSITES IN AEROSPACE STRUCTURE Susceptibility to impact-damage and strong possibility of It is to be realized that in order to meet the demands in table internal damage going unnoticed 1, it is necessary to have materials with a peculiar property-set. Moisture absorption and consequent degradation of high The use of composites has been motivated largely by such temperature performance considerations. Multiplicity of possible manufacturing defects and variability in material properties.

3 International Journal of Scientific and Research Publications, Volume 4, Issue 9, September 2014 3 ISSN 2250-3153 Even after accepting these weaknesses, the projected significant amount of composites as highlighted in the figure benefits are significant and almost all aerospace programmes use below. All this is, of course, not without its share of hassles. namely, design, analysis and manufacturing. These should Challenges of using composites on such a large scale are many. provide fast transfer of information including graphics and The composites are not only new but also non-conventional: they accurate analysis methods for a reasonable prediction of complex are anisotropic, inhomogeneous, have different fabrication and behavioural patterns of composites. It is only by harnessing the working methods and also different controls for quality vast computational power for various purposes that the aircraft assurance. They have a complex material behavior under load structural design of today can meet the challenges posed by the requiring new and complicated analysis tools. Moreover, the required performance. behaviour is not always predictable by analysis and this makes reliance on several expensive and time consuming tests unavoidable. IV. MATERIALS FOR AEROSPACE COMPOSITES The routes to meet these challenges have evolved around use The materials systems which have been considered useful in of the advances in computer technology and analysis methods to aerospace sector are based on reinforcing fibers and matrix resins implement schemes based on computer aided design, computer given in table 2 and 3, respectively. Most aerospace composites aided engineering, finite element methods of analysis and use prepregs as raw materials with autoclave moulding as a building computer interfaces amongst all aspects of development, popular fabrication process. Filament winding is popular with

4 International Journal of Scientific and Research Publications, Volume 4, Issue 9, September 2014 4 ISSN 2250-3153 shell like components such as rocket motor casings for launch preferred. Resin injection moulding also finds use in special vehicles and missiles. Oven curing or room temperature curing is components such as radomes. Some of the popular systems are used mostly with glass fibre composites used in low speed small given in table 4 along with the types of components where they aircraft. It is common to use composite tooling where production are used in a typical high-performance aircraft. rates are small or moderate; however, where large number of components are required, metallic conventional tooling is Table 2. Reinforcing fibers commonly use in aerospace applications.

5 International Journal of Scientific and Research Publications, Volume 4, Issue 9, September 2014 5 ISSN 2250-3153 Table 3. Polymeric matrices commonly used in aerospace sector. composite modeling such as microscopic modeling, macroscopic V. CONCERNS WITH COMPOSITE USAGE modeling, mixed modeling, discrete reinforcement modeling and The concern in use of composites arises mainly due to sub modeling. However, the most common use in finite element demands of high degree of reliability and safety of aerospace simulations of composite material are layered shells,layered- structures as against the complexity of composite behaviour and solids, stacked solid elements and stacked or layered continuum consequent difficulties in building prediction models. This shells. creates an excessive reliance on testing at all stages; design and The objective of ABAQUS analysis and simulation of development, proving and certification, and in-service inspection unidirectional E-glass is to predict the mechanical properties and and repairs. The costs of such testing are sometimes enormous mechanical response of unidirectional E-glass such as tensile, and this had led to some skepticism in use of composites. Two compression and thermal response and then will be compared major issues in this regard are briefly discussed below. and verified with experimental results. This option is for orthotropic materials and used specifically 5.1(a) Simulation for plane stress, such as in laminated shell. In this study, the simulation was undertaken in framework of It requires specification of E1, E2, 12, G12, G13 and G23 ABAQUS commercial finite element package. Finite element where E1 represent the Longitudinal Modulus, E2 is Transverse modeling of composites is depending on the purpose of the Modulus, 12 is major Poissons Ratio and G12, G13 and G23 analysis. In ABAQUS, there are several techniques for are in-plane Shear Modulus.

6 International Journal of Scientific and Research Publications, Volume 4, Issue 9, September 2014 6 ISSN 2250-3153 It is typically unspecified that a unidirectional E-glass fiber good stead, however, and no further significant reduction in or lamina can be treated as transversely isotropic. For strength or growth of damaged is observed under in-plane loads. transversely isotropic lamina, the independent elastic constant The current philosophy to handle impact damage problem is as becomes five because E2= E3, G12 = G13 and 12 = 13. follows: (i) design the structure to have alternate load paths to have damage tolerance against impact of moderate severity. This 5.1(b) Experimental is generally taken care by designing the structure as a framework The experimental work is successfully done which consists of stiffening members or as boxes; (ii) lower the design of tensile test and thermal-stress test. This experiment required allowable strength values to an extent where the barely visible only simple rectangular-shape test specimen where it is prepared impact damage (BVID)can be sustained even at the highest load using hand lay up process. During the experiment, the surface of and for all the time with no degradation in performance; (iii) any clean plate flat surface was waxed to facilitate easy removal of damage that exceeds the BVID level (i.e. visible damage) may the laminate before apply mix of resin on the waxed surface. lower the intermediate performance and should be repaired Then, cut the first fiber layer into required dimension and placed immediately. The basic safety of aircraft with damage is ensured on the top of that and apply the resin again. Make even the resin due to (i) and (iv) the structure may not cater to very severe using serrated roller and brush and removed all trapped air in impact. resin and fiber. Repeat this step for the next layer until 6 layers. There is, of course, a penalty in lowering the allowables but Finally, cover the layers with waxed flat surface and put load on for the present systems, this is considered to be not too excessive the top of it to produce a better surface. Specimen was cured at in view of the similar reduction of allowables required for room temperature for 24h in ambient condition. fastener holes. With improved processing to get large parts Then, it was cut into the specimen dimension which is 25 integral with stiffeners and other complex shapes and with mm x 250 mm. availability of high strength fibres the limitations due to impact The tensile test was undertaken using Material Test System damage would be more perceptible and prohibitive. (MTS) machine. Another consequence of the impact damage issue which the aeronautical community is, perhaps, not yet fully exposed to is in 5.1(c) Impact damage and damage tolerance terms of the inspection intervals and defining levels of repairs The laminated structure of the composites and the fiber- etc. when the presently developed aircraft go in full service. matrix interfaces provide weak interfaces for delamination and Extensive studies and gathering of experience through testing is debondingto take place. This is further aggravated by practical presently underway to tackle this problem. structural features such as discontinuous plies to create thickness changes and sharp bends required in stiffening members. of 5.2 Environmental degradation particular concern is the proneness exhibited for damage due to The presently used epoxy resins absorb about 5-6% moisture impact. The issue is not merely the reduction in strength by weight when fully saturated. This leads to about 1.5-1.8% (particularly in compression) but also that the damage is inside moisture weight gain in carbon-epoxy composites with the usual the material and not visible at the structure. This is particularly so 60% fiber volume fraction. In practice, under the normal where the impact is due to blunt objects at low to medium operating conditions, the maximum equilibrium moisture gain in velocities. Common instances are dropping of tools, hail-stones, an aircraft component can be about 1.0-1.4%. This moisture gain runway debris and impacts and jolts while handling (even before can cause (a) swelling and dimensional changes, (b) lowering of the assembly of the air craft). Such hidden damage can be the gas transition temperature (Tg) of the resin matrix, and (c) extensive- both in terms of planar dimensions and through the degradation of matrix dominated properties of composites such thickness. The damage mostly occurs as delamination, but may as shear and compression strengths. sometimes be accompanied by fiber-breaks in back plies which are not visible from outside. In the shop, such damages can be found by ultra-sonic C-scan method and a barely visible impact damage can cause a reduction in compressive strength by almost 60%. The fatigue resistance of carbon composites stands it in

7 International Journal of Scientific and Research Publications, Volume 4, Issue 9, September 2014 7 ISSN 2250-3153 The dimensional changes and weight gain by itself are components for aerospace quite expensive and tedious. generally not significant in many aircraft structures but may be of Moreover, associated with the already complex behaviour of considerable significance where extreme precision is required composites particularly in the long run. such as in antennae panels and in aircraft structures is the Apart from the moisture absorption, the other significant degradation of the shear and compressive strength properties- aspects relate to the UV degradation and radiation effects in the particularly at high temperatures close to Tg which in itself is long term. These are particularly important in space structures. now reduced due to moisture absorption. The design of a The current studies on the subject have provided some solutions structural component, therefore, generally proceeds by reducing to these problems even though the concern about long term allowables for moisture degradation. behaviour exists. This single issue of environmental degradation due to moisture absorption has made development of composite Table 4. Typical composite material systems in aerospace.

8 International Journal of Scientific and Research Publications, Volume 4, Issue 9, September 2014 8 ISSN 2250-3153 Some other aerospace applications are illustrated above: Fig. A : Two Seater transport aircraft Fig. B : Space launch vehicles (Space Shuttles) Fig. C : Satellites Fig. D : Advanced helicopters (Military & Civilian) becoming available. The developments seem to be in two VI. ADVANCES IN MATERIALS FOR COMPOSITES directions: one, for aircraft applications, is aimed basically at 6.1 Reinforcements higher strength (>5 GPa) with concurrent improvements in The carbon fiber technology continues to improve modulus to a moderate level (>300 GPa) and the other, for space harnessing the versatility of carbon fibre and new varieties in applications, is aimed at high modulus (>500 GPa) with terms of better combinations of modulus and strength are moderate strength (3.5 GPa). The higher failure strain for the

9 International Journal of Scientific and Research Publications, Volume 4, Issue 9, September 2014 9 ISSN 2250-3153 fiber is expected to result in composites with better damage The higher properties of basic fibers (such as carbon) cannot, tolerance. The developments in aramid fibers also aim at higher however, be fully exploited in the composite without concurrent modulus with concurrent increase in strength. However, the developments in the matrix materials and the intermediate major thrust in improving reinforcements for composites comes products such as prepregs or performs. It is to be noted here that from the requirements of multidirectional weaving. Several the carbon fiber composites which use a carbon fiber with a processes (weaving, knitting, braiding) have been developed for strength of 3 GPa as reinforcement result in an allowable stress this purpose and performs with multidirectional woven fibers of only 0.3 GPa in a composite. Significant scope thus exists for have now been made. Simplification and cost reductions appear translating high fiber properties into high performance of to be the major motives for further developments. composites. 6.2 Matrix Resins A significant effort in improving composites is focused on improving matrix materials. The two major concerns mentioned VII. CONCLUSIONS earlier viz. impact damage tolerance and hygro thermal Hence we can finally conclude that: degradation, provide the main motivation for improvement. A Composite materials offer high fatigue and corrosion major direction of improvement appears to be an improvement in resistance. the toughness, which should result in higher resistance in to Composite materials have high strength to weight ratio. delamination and against impact. High failure strain of matrix So they are best suited for various aerospace applications. resin would help in translating the higher performance of the improved fiber to the composite. Higher resin shear modulus would help in achieving better transfer of load from fiber to resin REFERENCES and again to fiber and should therefore improve compression [1] Composite airframe structures by Michael C. Y. Niu. strength. For polymeric materials a possible figure of 5 [2] Designing with advanced fibrous composites by L. J. Hart Smith, Douglas GPashould be achievable as against the current resins with shear A./C. company workshop on new materials and process for mechanical modulus of about 2 GPa. As far as hygro thermal degradation is design 1988 Brisbane 11-13 Aug (1877). considered, newer systems based on cynate ester look very [3] L.J. Hart Smith designing to minimize peel stresses in adhesive bonded promising and some of these have already found some joints in delamination and debonding of materials ASTM STP 876 (eds). W. application. Another route being investigated is the use of S. Johnson ASTM (1985) 238-266. thermoplastic resins and their blends. Poly-ether-ether-ketone [4] L. J. Hart Smith The design of repairable advanced composite structures soc. Automotive engineers trans., 851830 (1985). (PEEK) has been considered very promising, but the industry [5] M. F. Earo& J. H. Stannes Current research in composite Structures at needs to resolve the problems associated with high temperature NASAS Lagleyresearch center intern. Conf, composite materials and (> 350 0C) processing of a material. Current approaches to new structures India Jan 6-8 (1988). resins appear to be directed towards producing polymeric [6] J. E. Mecarty, R. E. Harton, Damage tolerance of composites intern. Conf. systems which can be processed in the way composites industry aeronautical sciences 15th congress England (1986). is used to (such as autoclave curing up to 180 0C).

10 International Journal of Scientific and Research Publications, Volume 4, Issue 9, September 2014 10 ISSN 2250-3153 AUTHORS First Author Nikhil V Nayak, U.G. Student, Mechanical B.V.Bhoomaraddi College of Engineering & Technology

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