Íàçâàíèå: Aircraft engineering (Ìîðîçîâà Ì. À.)
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Composite. airframe structures
PRACTICAL DESIGN INFORMATION AND DATA
In the past decades considerable progress in advanced composite technology has been made. However, the full potential in the design, manufacturing and especially the application of composites has not been realized. The use of com- posites in heavily loaded primary structures has been limited, mainly due to lack of hands-on experience and confidence. This book is intended to advance the technical and practical knowledge of advanced composites, emphasizing the design and manufacture of airframe structures. All aspects at composite design will be discussed in a thorough and rigorous fashion which includes guide-lines, observations, design factors, pros and cons of design cases, and troubleshooting techniques. However, neither the basic chemistry of materials nor laminate strength (or stress) analysis will be discussed in detail. Such information can be found in numerous composite books and published papers.
Composite structures are not just an extension of their metal counterparts and should not be considered only as piece-by-piece replacements (the BLACK ALUMINUM approach) that merely save structural weight by their lower materi- al density advantage. The designer's ingenuity and resourcefulness is needed to develop innovative concepts, which will reach the ultimate goal of composite structures that meet the requirements of durability, damage tolerance, maintai- nability, reparability, crashwothiness, low weight and cost effectiveness.
Early interface and support from producibility at the predesign level is critical in composite design to insure that cost-effective producibility features. Engineering design should also seek interface and criteria from tool design, production, manufacturing, industrial engineering, and quality assurance. It is recommended that during the composite design process on-board support guidelines are combined with previous counterpart metal ex-perience. It must be remembered that composite airframe structural design encompasses almost all the engineering disciplines, and engineers who go to the computer worksta- tion or drawing board today need hands-on information and data that are tech- nically sound and emphasize rational design and technical analysis
Combinations of different materials which result in superior products started in antiquity and have been in continuous use down to the present day. In early history mud bricks were reinforced with straw to build houses; more re- cently man-made stone was reinforced with steel bars (reinforced concrete) to build modern buildings, and bridges, etc., and now composites of matrix rein- forced with fibers are used to build airframe structures.
Modern composites owe much to glass fiber-polyester composites devel- oped since the 1940's, to wood working over the past centuries, and to nature over millions of years. Numerous examples of composites exist in nature, such as bamboo which is a filamentary composite. Through the years, wood has been a common used natural composite whose properties with and against the grain vary significantly. Such directional or anisotropic properties have been mastered by design approaches which take advantage of the superior properties while sup- pressing the undesirable ones through the use of lamination. Plywoods, for exam- ple, are made with an old number of laminae. Such a stacking arrangement is necessary in order to prevent warping. In the language of modern composites, this is referred to as the symmetric lay-up or zero extension-flexure coupling (ortho- tropic).
Progress in composites
The emergence of boron filaments gave birth to a new generation of com- posites in the early 1960s. The composites that employ high modulus conti- nuous filaments, like boron and carbon (or graphite), are referred to as ad- vanced composites. This remarkable class of materials is cited as a most prom- ising development that has profoundly impacted today's and future technologies of airframe design. The term composites or advanced composite material is de- fined as a material consisting of small-diameter (around 6 to 10 microns), high- strength, high-modulus (stiffness) fibers embedded in an essentially homogene- ous matrix as shown in Fig. 1.1.1. This results in a material that is anisotropic (it has mechanical and physical properties that vary with direction).
Serious development work with advanced composite materials started in the mid'60s with the boron fibers embedded in an epoxy resin matrix. Since that
time, a host of new materials has been added, including three types of carbon and graphite fibers, organic material fibres such as Kevlar, and new matrix ma- terials which include polyimides, thermoplastics, and even metals such as alumi- num, titanium, and nickel. Due to the remarkable specific properties of compo- site materials, component weight savings of up to 30\% have been achieved. However, the resulting structures are generally much more expensive than the metal counterpart. High raw material costs, extensive processing and quality as- surance procedures and the fact that the major emphasis is on maximum weight savings have led to these high costs. To accomplish the objective of cost and weight savings, design approaches should emphasize structural simplification, reduced part count, and elimination of costly design features.
Importance of composites in airframe design
Composite materials are ideal for structural applications where high strength-to-weight and stiffness-to-weight ratios are required. Aircraft and spacecraft are typically weight-sensitive structures (see Fig. 1.1.5), in which composite materials can be cost-effective. When the full advantages of compo- site materials are utilized, both aircraft and spacecraft will be designed in a manner much different from the present.
The study of composites actually involves many topics, such as manufac- turing processes, anisotropic elasticity, strength of anisotropic materials, and mi- cromechanics. Truly, no one individual can claim a complete understanding of all these areas. In this book, the emphasis is hence on practical design rather than on theroretical analysis. Adequate references to me latter found in either composite publications or references listed in this book.
Over the past decades, a variety of composite materials have been devel- oped which offer mechanical properties that are competitive with common alu- minum and steel but at fractions of their wieght. Fig. 1.1.7 gives a comparison of the properties of several different composites with conventional metallic ma- terials. It is possible for the designer to locate and orient the reinforcement in sufficient quantity and in the proper direction, even in very localized areas to withstand the anticipated loads. With composite materials it is possible can be arranged in such a way to create structures such as the forward swept wing of the
X-29A fighter, as shown in Fig. 1.1.8., which have aerodynamics characteristics that would not be possible if the structrures were composed metal. Historically, aluminum materials have been the primary materials for aircraft and spacecraft construction. Today, structural weight and stiffness requirements have exceeded the capability of conventional aluminum, and high-performance payloads have demanded extreme thermo-elastic stability in the aircraft design environment. To achieve the best composite structure design, composite designers should be trained to obtain basic knowledge as well as experience about metal structures. As matter of fact, composite designers should not consider composite materials to be a panacea, because in some areas of airframe structure the use of metal material is still the most cost effective choice. As mentioned previously, compo- site material costs are high compared with common airframe metals. Design costs are also higher with these new materials because of higher costs for analy- sis, components testing, certification and documentation testing. Furthermore, production and prototype tooling
Costs are high compared with conventional metals. Quality control, espe- cially non-destructive inspection (NDI), is another high-cost operation. Howev- er, it is possible to lower costs by:
• Innovative design concepts which consider of producibility
• Lowering part counts
• Elimination of costly fasteners or the use of fewer fasteners
• Use of automation methods to cutdown manufacturing costs
• The maturation of composite technology is still in progress, but the bass of understanding has broadened significantly. Fig. 1.1.9 illustrates the diversity of developmental experience obtained and use on advanced commercial transport structures. However, the application of advanced composite materials in civil aircraft has generally lagged behind military usage because:
• Cost is a more important consideration to commercial aircraft manufac-
• Safety is a more critical concern, both to the airframe manufacturer and
government certifying agencies
• A general conservatism due to financial penalties from equipment
The use of composite materials in military fighter aircraft construction has fluctuated in the past and is expected to change further in the future. The devel- opment of advanced composites in the 1960s resulted in a quantum jump in weight saving potential. This trend will continue with the introduction of new high-strain and high-toughness composite materials such as toughened thermo sets, or thermoplastics, which have been in development for many years and have produced results not possible with no toughened thermo sets [n summary, use of composites is based on demonstration that:
Significant weight savings can be achieved
• Use of composites can reduce cost, or can be cost effective
• Composite structures have been validated by tests as meeting all struc- tural requirements under aircraft environmental conditions.
• Cost-weight trade studies should conducted as part of design activity to determine appropriates use of composites versus metals
Structural weight reduction is the key advantage in using composite mate- rials. The relatively high raw material cost of composites can be offset by care- fully evaluating design and manufacturing processes to minimize the cost of fa- brication, inspection and repair. Obviously, the strongest of materials pound for pound, composites draw most of their strength from their hidden fibers, which come in many types and can be arranged in various patterns, some in three di- mensional shapes by braiding or weaving. These complex patterns can produce shaped with enormous strength in all directions. New contributions are being made by specialized industries, as shown in Fig. 1.1.11, that until now have not been involved in the airframe manufacturing business.
That the use of composites is now becoming almost commonplace illu- strated by the extensive use of the lighter fiber-reinforced materials in transport airframe construction and their progressively increased use. Transport airframe manufacturers are extending use from non-critical areas to the more critical areas of secondary structure, including flight control surfaces and primary em- pennage structures. It is likely that use will soon include wing and fuselage
structures where the greatest pay-off from weight and cost savings would be immediately appreciated.
Since the beginning of the 1980's, an all- or mostly-composite airframe has almost become a must in the developing and manufacturing of business air- craft, as shown in Fig. 1.1.13, as well as general aviation aircraft. Design ap- proaches which differ from those of most transport airframes and used to reduce cost and structural weight. These innovative designs and manufacturing tech- niques are pioneers in composite airframe structure development
Characteristics of composites
The most commonly used advanced composite fibers are carbon and gra- phite, Kevlar and boron, Carbon fibers are manufactured by pyrolysis of an or- ganic precursor such as rayon or PAN (Polyacrylonitrile), or petroleum pitch. Generally, as the fiber modulus increases, the tensile strength decreases. Among these fibers, carbon fiber is the most versatile of the advanced reinforcements and the most widely used by the aircraft and aerospace industries. Products are available as collimated, preimpregnated (prepreg) unindirectional tapes or wo- ven cloth. The wide range of products makes it possible to selectively tailor ma- terials and configurations to suit almost any application.
Matrix materials used in advanced composites to interconnect the fibrous reinforcements are as varied as the reinforcements. Resins or plastic materials, metals, and even ceramic materials are used as matrices. Today, epoxy resin is the primary thermo set composite matrix for airframe and aerospace applica- tions. In all thermo set materials, the matrix is cured by means of time, tempera- ture, and pressure into a dense, low-void-content structure in which the rein- forcement is aligned in the direction of anticipated loads.
An important element in determining the material behavior is the composi- tion of the matrix that binds the fibers together. The selected matrix formulation determines the cure cycle and affects such properties as creep, compressive and shear strengths, thermal resistance, moisture sensitivity, and ultraviolet sensitiv- ity, all of which affect the composite's long-term stability. Characteristics of a selection of composite matrices include
• most widely used
• best structural characteristics
• maximum use temperature of 200° F (93 °C)
• easy to process
• toughened versions now available
• maximum use temperature of 350°F (180°C)
• easy to process
• toughened versions becoming available
• variety of matrix types
• can be used up to 500 – 600°F (320°C)
• difficult to process
• relatively poor structural characteristics limit usage to non-structural
• easy to process
• same limitations as polyesters
• more difficult to process
• provide higher use temperature than polyesters and epoxies
• low smoke generation
• greater improved toughness
• unique processing capabilities
• have processing difficulties
The major advantages of the thermoplastic matrix over thermoset are
• high service temperature
• shorter fabrication cycle
• no refrigeration required for storage
• increased toughness
• low moisture sensitivity
• no need for a chemical cure
A detailed discussion which compares thermoplastic and thermoset compo- sites is contained in Chapter 2.2.
The matrix can also be affected by exposure to a water. Since it is the ma- trix, and not the fiber (except for Kelvar), that exhibits these hydroscopic charac- teristics, the matrix-sensitive properties are seriously reduced, especially at high temperatures by exposure to moisture. For airframe structures, which experience rapid changes of environment, this loss of mechanical performance due to mois- ture absorption must be accounted for in design.
Kevlar (Aramid) is the trade name for a synthetic organic fiber. A density of 0.052 lb/in3 gives Kevlar a specific tensile strength higher than either boron or most carbon fibers. When compared to other composite materials such as carbon and boron, Kevlar has poor compressive strength. This inherent charac- teristic of Kevlar results from internal buckling of the filaments. However, Kev- lar demonstrates a significant increase in resistance to damage compared to oth- er composite materials. Kevlar fibers are hygroscopic and this fact must be con- sidered in designing with Kevlar.
High performances advanced composites are often used in stiffness- critical applications. Thus, when developing new materials, the tendency is to maximize longitudinal module while maintaining acceptable levels of strength, impact resistance, strain-to-failure, and fracture toughness. Tensile properties are fiber-dominated; therefore, the choice of fiber is dictated by the application.
Compressive properties in unidirectional laminates are both fiber- and matrix-dependent. While compressive module is related to the fiber, compres- sive strength is dictated by the neat matrix shear modulus. But for homogene- ous, isotropic materials, the neat matrix shear modulus is related to matrix tensile modulus. Therefore, having relatively high matrix strength will prevent or mi- nimize interplay cracking in the composite under impact conditions, and will al- so insure acceptable transverse properties. Fracture toughness is important in matrices to minimize the propagation of cracks and defects, especially at cross- ly interfaces.
Retention of compressive strength and strain after impact is important property in high performance composites. It should be emphasized that, al- though damage prevention is important, damage containment is even more cru- cial. Therefore, to prevent impact-generated cracks from propagating and caus- ing excessive delaminating, adequate in-terlaminar fracture toughness is re- quired in composites used for the airframe structures.
Guidelines for the synthesis of improved matrices have evolved primarily from experiential data which highlights weaknesses;
• design criteria which considers the most dangerous threat to perfor- mance degradation
• limitations in process technology
• evaluations of the relationships between neat matrix properties and composite properties
Metal/matrix composite (MMC) materials have very high tensile and compressive strength and stiffness compared to most carbon/epoxy materials as shown below:
• Boron/Aluminum – Simple members for high tension or compression load. Beef-
up aluminum member for additional strength
• Boron/Titanium — Higher strength structures
• Borsic/Aluminum — Higher strength structures
• Borsic/Titanium — Higher strength and high temperature applications
• Carbon/Aluminum — Aerospace
• Carbon/Magnesium — Aerospace
Much of the MMC development work has been government funded; the major characteristics of MMC's are
• good strength at high temperature
• good structural rigidity
• dimensional stability
• light weight
• and processing flexibility
Composites vs. metals (Aluminum alloys)
For some time it seemed as if composite materials would replace aluminum as the material of choice in new aircraft designs. This put pressure on aluminum developers to improve their products. One result was aluminum-lithium (The first aluminum-lithium alloy, called 2020, was actually developed in the 1950s for the U.S. Navy RA-5C Vigilante). One of the main efforts of the developers is to save weight and cost compared to composites because the conventional aluminum manufacturing facilities can be used on aluminum-lithium.
The early demonstrations of the 25-35\% weight savings composites offer over aluminum constructions plus a substantial reduction in the number of parts required for each application represents a major attraction of these composites. The obstacles to a wider use today of composite materials are their high acquisi- tion cost compared with aluminum, the labor-intensive construction techniques required and substantial capital costs involved in buying a new generation of production equipment. However, the labor-intensive construction can be solved by automation (see later discussion in this Chapter) of the manufacturing process which is the key technology in developing composites. The use of tape-laying machines, for example, can cut the time and cost of constructing composite components by a factor of ten or more.
The use of composites in the U.S. began in the early 1970s under USAF funding and in the late 1970s NASA instituted a series of programs aimed at de- veloping composite technology and succeeded in placing primary and secondary structural designs in commercial services. As a result, aircraft manufacturers became more comfortable with the materials and more efficient construction techniques were developed; the increased demand led to lower costs of compo- site materials.
Metals are isotropic, having structural properties which arc the same in all directions. Composites arc anisotropic (see in Fig. 1.3.1), a single ply having very high strength and stiffness in the axial direction but only marginal proper- ties in the crosswise direction. Cross-plying based on load and function enables composites to meet and surpass the properties of metals. However, composites can be laid up to be quasi-isotropic (having nearly isotropic properties).
Composites versus metals
Composites differ from metals as their
• Properties are not uniform in all directions
• Strength and stiffness can be tailored to meet loads
Possess a greater variety of mechanical properties
• Poor through the thickness (i.e., short transverse) strength
• Composites are usually laid up in essentially two-dimensional form, while metal may be used in billets, bars, forgings, castings, etc.
• Greater sensitivity to environmental heat and moisture
• Greater resistance to fatigue damage
• Propagation of damage through delaminating rather than through- thickness cracks
Advantages of composites over metals
• Light weight
• Resistance to corrosion
• High resistance to fatigue damage
• Reduced machining
• Tapered sections and compound contours easily accomplished
• Can orientate fibers in direction of strength/stiffness needed
• Reduced number of assemblies and reduced fastener count when co cure or co-consolidation is used
• Absorb radar microwaves (stealth capability)
• Thermal expansion close to zero reduces thermal problems in outer space applications
Disadvantages of composites over metals
• Material is expensive
• Lack of established design allowables
• Corrosion problems can result from improper coupling with metals, es- pecially when carbon or graphite is used (sealing is essential)
• Degradation of structural properties under temperature extremes and wet conditions
• Poor energy absorption and impact damage
• May require lightning strike protection
• Expensive and complicated inspection methods
• Reliable detection of substandard bonds is difficult
Defects can be known to exist but precise location cannot be determined
Structural efficiency of composites vs. metals
In general, composite materials are most effectively utilized when they are preferentially oriented. That is, the laminas are oriented so that the majority of the fibers arc placed in the principal load direction and the proportion of the transverse or angle plies is determined by the relative values of the biaxial load components or torsional stiffness requirements. The range of weight reduction potentials of five classes of composite materials, compared to 7075-T6 aluminum sheet (B-values). To quantify the extremes of these directional characteristics, both unidirectional and quasi-isotropic (0/±45/90) properties are shown in the figures. For shear strength or stiffness 100\% of +45 laminate properties are shown, as they represent the upper limit for shear properties. The ratio of the specific strength or stiffness of aluminum to that for the extremes of composite properties is also shown. The average of these is identified as the mean ratio, and the range of ratio is also illustrated. It is postulated that the mean ratio represents the most likely measure of the weight reduction to be expected from the use of composites. It is evident from the values shown that substantial weight savings are possible through the judicious use of composites. It is also evident that the selection of the optimum composite material depends on the ap- plication requirements in terms of stiffness and loading type. In general, the high strength Carbon/Epoxy (C/EP) has the best balance of properties. Howev- er, the intermediate modulus C/EP is less costly and is being used extensively where modulus is not the determining factor.
Design for low cost production
Low cost production starts with designing components so that they can be built using techniques which are feasible and available. Cost has to be controlled as a design parameter when building composite airframe structures in the same way as it was controlled in the design of their metal counterparts. The design
definition and drawing stages are vital, but the initial production phases are no less important to cost control.
Novel materials and processes can involve cost pitfalls. Unforeseen prob- lems with production and quality control may be added to the extra outlay for pioneering techniques, so that apparently cost-effective solutions are not neces- sarily so. Conventional technology, e.g., metal superplastic forming techniques which can be used to form thermoplastic composite laminates, should always be re-examined closely before going on to try some new technology. High- investment, high-waste fabrication methods, such as integral machining, must be evaluated carefully and weighted against the alternatives. The repeatability of quality must be realizable, not just promised, and the intended economies in labor must actually occur.
The benefits of reduction of the number of parts is shown below:
(1) Fewer parts mean less of everything required to manufacture a prod-
• Number of tools
• Production planning
• Production floor space
• Engineering time, drawings changes
• Production control records and inventory
• Number of purchase orders, suppliers, etc.
• Number of bins, containers, stock locations, etc.
• Material handling equipment, receiving docks, inspection stations, etc.
• Accounting details and calculations
• Service parts, catalogs, and training
• Production equipment, facilities, training
(2) A part that is eliminated:
• Costs nothing to make, assemble, move, handle, orient, store, purchase,
clean, inspect, rework, service
• Fewer jams or automation breakdowns
Fewer failures, malfunctions, or needed adjustments
Composites, with many of their qualities determined during manufacture, encourage designers to take more account of production engineering; however, cost assessment should be given the same emphasis. Early composite products, such as fiberglass, can still be considered as alternatives to graphite/carbon for minimum-cost structures where there is no critical stress requirement.
Composite construction enables the designer to make extensive use of con- figurations, such as the sine wave beam, which have always proved expensive in metal. In composite wing skins, tighter manufacturing tolerances can be ob- tained to meet the requirement of manufacturing assembly, and then checked by robotic scanning – all at great cost savings. The number of fasteners has been reduced which eliminates many problems:
• Fasteners are difficult to feed
• Fasteners tend to jam
• Fasteners require monitoring for presence and torque
• Fasteners are expense (especially installation costs)
• Fasteners allow possible galvanic corrosion when composites are attached to metals. It is no secret that designers need to design composite structures with manufacturing cost effectiveness in mind. Studies in Concurrent Engineering indi- cate that a large percentage of the final cost of a product is determined in the ear- ly phases of the product life cycle. However, advanced composites are expen- sive to manufacture, to the point that fabrication cost is currently a major issue affecting the ultimate widespread use of these materials. There is a general con- sensus that several areas can play a key role in the reduction of the high cost of manufacturing composite structures; these include:
• part design
• materials selection
• cost-effective manufacturing processes
• automated systems
In the near term, engineers must show that in many applications, the high performance capabilities of composites justify the high costs. In long term, to make real progress in driving prices lower and also in meeting the ultra-light
structural weight expectations, revolutionary manufacturing technology must be developed in parallel with innovative design concepts.
Cost factors which are often given insufficient attention include plant modifications such as;
• air-conditioning ventilation
• safe health-conscious handling of composites
• new tooling
• cleaning of equipment
• protective clothing or operatives
• special training
Production methods vary as to the cost involved due to processing time, consumables and tooling. The types, shapes, and sizes of components for which each method is most suitable must be addressed. Cocuring and reusable self- sealing rubber bags optimize the use of autoclaves, for instance.
All of the factors discussed above obviously impact costs. The designer needs to know which aspects of a design are the cost drivers as the design is handled in the manufacturing, inspection or maintainability areas. At the early stage of a design, decisions are made which will influence 90-95\% of the total cost, including operations and maintenance costs. The preliminary design phase gives the designer the maximum opportunity to influence the direction and cost of the entire project. The designer must develop an open approach to other dis- ciplines and be assertive in demanding their involvement and informed and considerate response.
Concurrent Engineering is the buzz word being used today which de- scribes a new communication method that involves all of the disciples shown below:
• Design (focal pointy
• Materials and Processes
• Stress Engineering
The mechanism which is described by concurrent engineering is not new and it is rather something that we lost it along the way. Prior to the 1960's, con- current engineering was the accepted practice and the disciplines of design and manufacturing were co-equals in the definition of a product. In the mid-60's, however, manufacturing was de-emphasized and product innovation hig- hlighted. Simultaneously worldwide competitors went in the opposite direction and the results are very visible.
Concurrent engineering is the process where all participants in a project are communicating from the start. Project development is performed in sequen- tial fashion, with the designer starting at the beginning with the loads group. The loads group passes the baton to the structural analyst who passes their re- quirements to the designer. The designer continues to pull the requirements into a cogent package and finally tooling, manufacturing, and other disciplines are given the go-ahead. It is not, however, until the design is sufficiently developed that the designer feels comfortable about discussing it with most other discip-
Considering that 70\% of the product cost is determined in the first 5\% of the design process, concurrent engineering changes the project development from a sequential to a simultaneous involvement of the necessary disciplines. It means that the designer takes materials, structures, tooling, manufacturing, in- spection, maintainability, and cost problem into consideration to a greater de- gree than takes place in the sequential system. This method makes the designer more aware of manufacturing and quality assurance needs during the design phase. No one expects the designer to become thoroughly familiar with all of the other disciplines; this would be an impossible task. What is intended, how- ever, is to bring the other disciplines into the design process (see Fig. 1.4.1) at the beginning so they can evaluate the concepts as they develop.
The important aspect of this system is that data is reviewed as it is needed and required, not when the generator of the data or design is ready to release it. It is presumed that if the various disciplines have access to data and design con-
cepts at earlier stages, even as it is developing, it will open interdisciplinary di- alogue and result in products of higher quality and lower cost. Each discipline is able to measure and evaluate the impact of a change on their area of expertise.
Design for production
The maturation of composites technology has increased at an accelerating pace over the last decades. The development has taken the technology from that of a laboratory curiosity into real hardware application in the aerospace, automo- tive, marine, sports, and medical fields. The structural configurations have ranged from simple flat laminates and tubes to complex stiffened skins and sandwich structures with compound curvatures.
Designing for production is designing for manufacturability. The result is the generation of a design which exploits a manufacturing method and yields a product of high quality and low cost. This occurs because of the interdiscipli- nary involvement of manufacturing and their various representatives with the designers. The following is describes the goals of the product development cycle:
(a) Design for customer (Quality)
• Design a functionally and visually appealing product
• Provide mechanical reliability for the long haul
(b) Design for manufacturability (Cost)
• Product can be made cost-effectively
(c) Reduce design cycle (Time)
• Do a better job in a shorter length of time
• Minimize liabilities
Manufacturing and producibility engineers must be involved at the start to not only advise the designer of manufacturing approaches which may stream- line the design, but also to initiate early investigations to seek out better me- thods for producing the component. If manufacturing and producibility engi- neers are not involved until later in the design process, the schedule will not per- mit changes based on their input and the product is stuck with whatever was de- cided on at the start.
The assembly process also benefits from the early involvement of the manufacturing and producibility engineers. The size of details to be assembled and the location of manufacturing break points will directly influence the weight and cost to the product.
The use of composites has generally been justified on the basis of over- riding requirements such as weight reduction, stiffness, fatigue damage tolerance, or zero co-efficient of thermal expansion. These requirements have generally been sufficiently important to justify the high cost of the labor-intensive processes which are characterized by large amounts of hand labor. There, of course, have been exceptions, such as filament wound railroad box cars and rotor blades, but generally these applications are unique.
It is time now to step off the threshold and move the technology of ad- vanced composites manufacturing into higher volume production and into the factory environment. To do this it is necessary to reduce highly-labor-intensive procedures, increase the production rate, and decrease the amount of scrap created.
In metal design, tooling cost was all-important and so was analysis of the tooling. The case with composites manufacturing is somewhat different. The actual labor and material cost (see Fig. 1.4.2) of the part itself is usually the greatest single item in its cost, while the amortization of the tooling, engineering, and other considerations is much less important. However, tool durability and the high cost of some composite tooling concepts must be considered in the design
Some concerns have suffered tremendously because of that ingenious in- stinct of the engineer to create and design something new instead of using an ex- isting part or assembly, which, with reasonable ingenuity, may be made to serve. The natural barrier against using something created by someone else must be broken down, a true mark of a good designer is the ability to see reasons why an- ything formerly used can be employed again, instead of the tendency to think of reasons why it is not once again functionally satisfactory. Standard types of equipment and standard structural parts must be used. The use of standard parts involves:
• Developing a modular designs
• Designing parts for repeat use
• Using off-the shelf components
• Standardizing and rationalizing Simplicity in design consists of the fol- lowing:
• Designing for ease of component fabrication
• Designing for ease of assembly
• Designing for commonality
interfacing with other disciplines
including sufficient details in the preliminary concept to enable other dis- ciplines to evaluate the design
taking tooling into consideration
using simple processes and operations
Simplifying work instructions
Simplifying processes, operations, material handling
Simplifying tooling and fixtures
Automation is the key to the practical application of composites technology and it has already been extensively applied to simple structures such as tubes and straight constant-cross-section configurations. Fabricators of these struc- tures have been able to exploit the filament or tape winding, and pultrusion processes. Certainly any design which exploits the braided or woven structures processes can also be said to be automated. This is also true of simple press and injection molding methods.
However, laminate and sandwich structures of the size and complexity normally en-countered in the airframe industry have proven difficult to auto- mate. This results in excessively labor-intensive processes with resultant high part acquisition costs. Several companies are currently developing automated tape laying systems.
However, all of the systems merely automate the lay up of the laminate. A completely integrated system is needed which will permit a
hands-off fabrication of wing covers and fuselage. The cure cycle control is also being automated and will permit the automatic monitoring and control of cure cycles.
The degree of automation which can be contemplated will be determined by the early participation of the manufacturing and producibility engineers. The designer is not expected to be up-to-date on the most current manufacturing sys- tems available. The designer, however, is expected to involve those aware of the new systems in the development of the design.
The type of basic automation that immediately offers high cost savings in composite construction is the use of high-speed tape laying machines. More and more composite materials are being prepared under automatic processing, and filament-wound structures are amenable to it, but accurate timing is crucial if curing requirements are to be met. A relevant axiom is «never underestimate the difficulties of automating the role of a skilled operator, and do not try to imitate his skill directly». After all, the diminishing labor pool and highly intensive processes dictate that for advanced composites technology to grow it must au- tomate.
The selection of composite materials for specific applications is generally determined by the physical and mechanical properties of the materials, eva- luated for both function and fabrication. The functional considerations include items such as the strength, weight, hardness, and abrasion resistance of the fi- nished part. Fabrication considerations include cure cycle (time, temperature, pressure), quantity of parts, tooling costs, equipment, and availability of facili-
There are a number of standards and specifications which are intended to ensure repeatable results by carefully defining either the technical requirements of a material or the specific steps used in the manufacturing process.
(a) Military specifications: These are issued by the Department of De- fense (DoD) to define materials, products, or services used only or predominantly by military entities.
(b) Military standards: These provide procedures for design, manufactur- ing, and testing, rather than giving a particular material description.
(c) Federal specifications and standards: These are similar, except that they have come out of the General Services Administration, and are primarily for federal agencies. However, in the absence of military specifications and standards for a given product, federal specifications and standards are accepta- ble for use.
(d) Federal Aviation Regulations (FAR): In addition to military specifica- tions, the Federal Aviation Administration (FAA) has advisory circulars for composite materials and part fabrication methods which as acceptable for air- craft. The FAA advisory circulars discuss several areas:
• Test Plan — A unified program and schedule for tests that verify design allowables. These tests for composites might include a coupon test, static full- scale test for durability, environmental tests, stress analysis, and tests for sub- components of a major structure.
• Process Specifications — this includes both a material specification to be used to help select a commercial product and a process specification.
• Quality Assurance Plan — this details acceptance and repeatability tests for material and process inspection of fabricated parts.
• Report Submission — Includes final report submission, audit tests, how tests are accomplished, and who witnesses them.
(e) Company specifications — In many cases, companies feel that military standards and specifications do not reflect the most up-to-date materials and processing techniques. So companies develop specifications that will ensure all the requirements for fulfilling the military contracts.
(f) International standards — The International Standards Organization
Technical Committee 61 and its subcommittee 13 covers reinforced composites.
Among the latest efforts from the DoD itself are two materials specifica-
• MIL-P-46179A for thermoplastic composite, which covers polyamide-
imide, PES, PEEK, Polysulfone, PEI, PPS, etc.
• MIL-P-46187 for high-temperature thermoset composites
To date, the greatest problem is how to get users to agree on test measures and reduce the number of tests, especially since every company has its own pro- prietary products and has established its own specifications and handbooks. In the composites industries, agreed-upon composites standards are not only a must, but essential for cutting costs.
Requirements and specifications
When the type and use of the aircraft is defined, reference is made to the requirements of the customer involved, and in the case of the commercial air- craft, the requirements of the licensing agency. The minimum structural re- quirements for aircraft for the various agencies are presented in the following documents:
a) Civil aircraft
• Federal Aviation Regulations (FAR), Volume III, Part 23 — Airworthi- ness Standards: Normal, Utility, and Aerobatic Category Airplanes
• Federal Aviation Regulations (FAR), Volume HI, Part 25 — Airworthi- ness Standards: Transport Category
• British Civil Airworthiness Requirements, Section D — Aeroplanes
• Joint Airworthiness Requirements (JAR), JAR — 25 Large Aeroplanes
• FAA Advisory Circular 20-107A
• JAA Advisory Circular
(b) Military aircraft (U.S.A.) Air Force:
• MIL-A-008860A(USAF), Airplane Strength and Rigidity General Spe- cification For, 1971
• AFGS-87221 A, General Specification for Aircraft Structures, Air Force
• MIL-STD-1530A (11), Aircraft Structural Integrity Program, Airplane
Requirements, Military Standard, 1975
• MIL-A-8860B (AS) Airplane Strength and Rigidity General Specifica- tion Fo1987
General requirements do not always apply to new types of aircraft. Con- sequently, interpretations and deviations from the requirements are often neces- sary. These deviations and interpretations are then negotiated with the licensing or procuring agency. In some cases, special requirements may be necessary to cover unusual aircraft configurations. The manufacturer's requirements are usually the result of experience or advancement in the «state-of-the-art» by that manufacturer. The trend on commercial aircraft in recent years has been toward the establishment of specially designated «Special Conditions» for each indi- vidual aircraft design. The FAA specifies these conditions by negotiation with the airframe manufacturer.
Certification of composite airframe structures
Certification requirements of airframe structures are ultimately identical whatever materials are used. Certification of composites has become more com- plex than certification of conventional materials (aluminum alloys, etc.) because of special design considerations and increased material variability. Because the use of composites usually requires fabrication from perishable raw materials, more controls are required over them. The attitude of the certificating authori- ties is that use of new materials should not subject aircraft operators to higher levels of risk than they accept with existing materials. It is the composite design- er's responsibility to determine how this assurance is to be provided.
In July 1978, the FAA put out Advisory Circular AC20-107 (the document for complying with specific foreign countries certification requirements is AC21-2) on the certification of composite airframe structures. It is a brief doc- ument stating that the evaluation of a composite should be based on achieving a level of safety at least as high as that currently required for metal structures. It also emphasizes the need of testing for the effect of moisture absorption on static strength, fatigue and stiffness properties for the possible material property degra- dation of static strength after application of repeated loads. Typical test require- ments for composite structural certification are shown below:
• 150\% design limit load (DLL) test requirement of critical design condi- tion(s)-check civil or military certification requirements
• Fatigue testing — Damage tolerance on primary structures
• Design (Environmental effects dominate)
– Notched effect
In addition, damage tolerance (with particular reference to the effects of moisture and temperature) and crashworthiness must be addressed. Other data include flammability, lightning protection, weathering, ultraviolet radiation and possible degradation by chemicals and fuel; and also specifications covering quality control, fabrication technique, continuing surveillance and repair.
In 1982, the FAA published special "rules" which were applicable solely to the Lear Fan 2100 aircraft (American first all-composite airframe design), shown in Fig. 1.1.13(a). This airframe is made of advanced composite material and extensive use was made of bonding during assembly. The material and as- sembly technique is completely different from typical aluminum structures.
The following contains information the U.S. certification of the composite airframe structures (in case of conflict between the material herein and the detail specification and requirements, the certification specification and requirements shall prevail.):
Commercial airframe — Federal Aviation Administration (FAA)
Federal regulations require that all civil aircraft operated in the United States should receive an airworthiness certificate. The certification process is administered by the FAA Part 23. 25. 27 and 29 Type Certificate, if the aircraft complies with design and safety regulations and standards. An Airworthiness Certificate is issued when the FAA determines that the particular aircraft was built in accordance with the specifications approved under the Type Certificate. The Airworthiness Certificate remains effective so long as required mainten- ance and repairs are performed on the aircraft and its equipment in accordance with the FAA regulations.
The certification process, shown in Fig. 1.6.1 and Fig. 1.6.2, is rigorous and generally takes several years to complete. Several of the phases required for cer-
tification are briefly described below (for the actual number of phases and re- quirements consult directly with local FAA representatives):
1).Engineering Data Package – Preparation of this package includes drawings and specifications; load and structure analyses; structural, ground and flight test proposals and reports; flight and maintenance manuals; and an equipment list and parts catalogue. These reports essentially explain and record the aircraft's performance and constituent parts.
2).Quality Control Manual and Quality Control System – The FAA requires that the airframe Certification Program implement a quality control manual and sys- tem. This requirement is intended to ensure conformity with what is specified in the Engineering Package.
3).Tool Fabrication – The tooling required to manufacture and assemble the air- frame must be fabricated in a manner consistent with the structural drawings and specifications.
4).Flight Test Prototype No. 1 – A prototype, conforming to the Engineering Data Package, will be built and will be used as a flight test model to measure aerodynamic factors. Changes may be made to this prototype if the changes do not negatively affect safety, performance or flight characteristics and the changes can carried through to actual production.
5).Static Test Prototype No. 2 – An airframe consisting of a fuselage, main wing and tail (or canard or both) will be built and will be used for testing under static load conditions.
6).Fatigue Test Prototype No. 3 – An airframe will be built to test cyclic loads. The tests will determine the number of safe structure flight hours. The airframe will be intentionally damaged to determine the extent to which it can be damaged and still carry the critical load as defined by the FAA.
7).Environmental Test Assemblies – Several assemblies will be load tested before, during and after exposure to extreme temperature and humidity far beyond ex- pected in-service conditions.
8).TIA (Type Inspection Authorization) Inspection and Ground Test – The FAA will issue a TIA inspection after a review of aii technical data and upon a find- ing that the airframe is safe for FAA flight testing.
9).TIA Flight Test – Upon successful completion of the above phases, the FAA will conduct the flight test to determine that the airframe prototype meets regu- latory requirements.
10).Type Certificate Issuance – After completing the above phases, a final Engi- neering Data Package containing all engineering data is submitted to the FAA, and a final Type board meeting will be held to ensure that all agenda certifica- tion items have been completed and properly demonstrated for compliance. When all items are cleared, the FAA will issue a Type Certificate.
Certification of composites for aircraft requires meeting the specifications of the following documents;
a) Military specification MIL-A-8860A (USAF) through MIL-A-8870A (USAF)
b) Military specification MIL-A-8860B (AS) through MIL-A-8870B (AS) se- ries
c) Military specification MIL-A-87221 (USAF) – Aircraft Structures, General
(d) Military standard MIL-STD-1530A (USAF) – Aircraft Structures Integrity Program (ASIP), Airplane Requirements Certification of a military airframe is generally a process negotiated the user and the airframe manufacturer based on the applicable specifications:
• U.S. Air Force — use the above mentioned item (a), (c) and (d)
• U.S. Navy Air Force — use the above mentioned item (b) plus a stan- dard document similar to (d) which is now under study
e) For example, when certifying Air Force airframes, the above mentioned USAF Aircraft Structural Integrity Program (ASIP) is primarily used for full scale development of metal and composite structures. Fig. 1.6.3