Acknowledgment
5.1 Hybrid Composites S. Padma Priya
5.1.1 Introduction
Composites are materials made by the synergistic assembly of two or more constituting materials (matrix and reinforcement), engineered in such a way that they form a single component and yet can be distinguished on a macroscopic level. Matrix encases the reinforcements and acts as a binder for the fi bers.
Additionally, the matrix holds the reinforcements in a fi xed position and the reinforcement enhances the properties of the composite system by imparting its mechanical and physical properties. Composite materials can be engineered with desired properties by choosing and varying the concentration of diff er- ent types of matrices and reinforcements. Th e durability of composites is very high; they have very high strength to weight ratios, are resistant to environmental corrosion, and provide ease of use concept.
Composites are materials known from ancient days, when bricks were made (mud reinforced with straw). Until today, we have applications for components of aerospace materials. Mother Nature is the fi rst maker of composites and some examples of natural composites include wood, bone, etc.
Composites are classifi ed into several broad categories depend- ing upon the type of matrices and reinforcements used. Depending on the types of matrices used they can be classifi ed as
1. Metal matrix composites (MMC) 2. Ceramic matrix composites 3. Polymer matrix composites
Depending upon the reinforcement types they can be classi- fi ed as
1. Fiber reinforced composites 2. Fabric reinforced composites 3. Particulate composites
In order to further enhance the properties of the composite materials, hybrid composites were developed. Th e behavior of hybrid composites appears to be simply a weighted sum of the individual components in which there exists a more favorable bal- ance between the advantages and disadvantages. Th e hybrid com- posites off er advantages regarding structural integrity and sustained load under crash and impact conditions. Hybrid com- posites are infl uenced by a large number of microstructural parameters such as type of reinforcement, volume proportion of reinforcement, weave pattern of the fabric used as reinforcement, volume fraction of the reinforcement, etc. Th ere are several types of hybrid composites and can be characterized as: (1) interply or tow-by-tow, in which tows of two or more constituent types of fi ber are mixed in a regular or random manner; (2) sandwich hybrids, also known as core-shell, in which one material is sandwiched between two layers of another; (3) interply or lami- nated, where alternate layers of the two (or more) materials are stacked in a regular manner; (4) intimately mixed hybrids, where
5
Composite Systems Modeling—Adaptive Structures:
Modeling and Applications and Hybrid Composites
5.1 Hybrid Composites ...5-1 Introduction • Future Directions
Acknowledgment ...5-7 5.2 Design of an Active Composite Wing Spar with Bending–Torsion Coupling ...5-7
Introduction • Multicell Cross-Section Spar Design • Results • Concluding Remarks
References ...5-16
the constituent fi bers are made to mix as randomly as possible so that no overconcentration of any one type is present in the mate- rial; (5) other kinds, such as those reinforced with ribs, pultruded wires, thin veils of fi ber, or combinations of the above.
Hybridization can be done through many ways as follows:
1. Reinforcements 2. Matrices
3. Both reinforcements and matrices
5.1.1.1 Reinforcement Hybridized Composites
Th ese are the composites, which consist of two or more types of reinforcements embedded in a single matrix. Th e reinforcements can be of diff erent types as follows.
5.1.1.1.1 Synthetic (Man-Made) Fibers
Th is composite is composed of two types of reinforcements and both of them are synthetic. Composites of such types have been developed in order to achieve superior mechanical prop- erties and high heat resistance capabilities. Various types of synthetic reinforcements have been developed such as glass fi ber, carbon fi ber, Polyetheretherketone (PEEK), rayon, nylon, Kevlar, polyester, acrylic, olefi n, vinyl, aramid, etc. Th e fol- lowing fi bers can be hybridized (e.g., carbon fi ber/PEEK hybrid fabric, polyethylene [PE]/glass fabric, carbon fi ber/aramid fi bers) in diff erent (weight or volume) ratios according to the desired end property. For instance, if less weight however, a strong composite is required, a hybrid fabric can be consid- ered. Th is would have one fabric composed of diff erent types of fi bers in it and some examples for this type of reinforcement are carbon/aramid, aramid/glass, and carbon/glass.
Of the materials mentioned above the PEEK material is very much in demand. PEEK is a linear aromatic thermoplastic based on the following repeating unit in its molecules.
O
O O
n C
Ketone Ether
Polyether ether ketone
Ether
Continuous-carbon fi ber reinforced PEEK composites are known in the industry as aromatic polymer composite (APC).
PEEK is a semicrystalline polymer, and amorphous PEEK is pro- duced when the melt is quenched.
Th e presence of fi bers in PEEK composites tends to increase the crystallinity to a higher level because the fi bers act as nucle- ation sites for crystal formation. Increasing crystallinity increases both the modulus and the yield strength of PEEK but reduces its strain to failure.
PEEK’s maximum continuous-use temperature is 250°C.
PEEK is the foremost thermoplastic matrix that may replace epoxies in many aerospace structures. Its outstanding property
is its high fracture toughness, which is 50–100 times higher than that of epoxies. Another important advantage of PEEK is its low water absorption, which is less than 0.5% at 23°C compared to 4%–5% for conventional aerospace epoxies. Being semicrystal- line, it does not dissolve in common solvents.
5.1.1.1.2 Natural Fibers
A composite in which its reinforcements come from natural resources is considered natural. Natural fi bers, because they are naturally renewable, can be imbedded into biodegradable poly- meric materials, which can produce a new class of biocomposites.
Recently, society has realized that unless the environment is pro- tected, it will be threatened due to the loss of natural resources.
Conservation of forests and optimal utilization of agricultural and other renewable resources like solar energy and seawater has become almost mandatory. In this view, many attempts have been made to replace high-strength synthetic fi bers with natural fi bers.
A single natural fi ber may not impart all the properties for the fi nal composite, thus it is hybridized with other types of natural fi bers. Some of the known natural fi bers are wool, cotton, silk, linen, hemp, ramie, jute, coconut, pinewood, pineapple, angora, mohair, alpaca, kapok, fl ax, sisal, bast fi ber, kenaf, wood fi ber, bamboo, banana, etc. To tailor the properties of a natural fi ber reinforced composites, it is hybridized with the second type of natural fi ber. Researchers have conducted studies on thermal con- ductivity and specifi c heat of jute/cotton, sisal/cotton, and ramie/
cotton hybrid fabric-reinforced composites.
Small inorganic particles have also been used as fi llers to pre- pare particulate composites. Th e most relevant reason to use fi ller in the composite is to reduce the overall cost of the mate- rial. Fillers are also used to increase fl ame retardancy, surface hardness, esthetic appeal, and the thermal properties of the composites. Some of the types of fi llers that have been used are fl y ash, silica, mica, granite powder, wood fl our, aluminum oxide, carbon black, etc. Using such fi llers and natural fi bers or fabric, a new class of hybrid composites has been produced. One of the reinforcements is fi ller while the other is the fi ber or the fabric, and here the fi ller particles are embedded between the fi brous reinforcement and the matrix. Other research develop- ments underway have used sisal/saw dust in conventional com- posites and they possess an ease of processing, are environment friendly, and are economically aff ordable.
5.1.1.1.3 Natural and Synthetic Fibers
Th is type of composite consists both of natural and synthetic reinforcements in the composite system. Natural fi ber rein- forced composites do not always fulfi ll all the properties required for some technical applications. In such cases, hybrid- ization with small amounts of synthetic fi bers allows these natural fi ber composites to be more suitable for technical appli- cations. Th e most common synthetic fi ber used to hybridize the natural fi ber as a reinforcement is glass fi ber. Some exam- ples of these types of fi bers are sisal/glass, silk/glass, pineapple fi ber/glass, jute/glass, etc. With the addition of a small amount of glass fabric to the natural fi ber reinforced composite, the
mechanical strength and the chemical resistance of the compos- ite has been shown to increase considerably (see Table 5.1 and Figure 5.1).
5.1.1.1.4 Miscellaneous Reinforcements
Miscellaneous reinforcement is also done to achieve required properties in the composites and some of them are synthetic fi ber–particulate (glass along with fl y ash) composites used to increase the abrasion resistance and compression strength of
the composites, blending various levels of glass to wollastonite allows for tailored composites with high-strength properties or good dimensional stability. Fiber metal laminates (FML) off er signifi cant improvements over currently available materi- als for aircraft structures due to their excellent mechanical characteristics and relatively low density and some examples for these types of composites are aluminum 2024 alloy;
carbon fi ber/epoxy (Ep), and aluminum 2024 alloy/glass fiber/
Ep composites.
TABLE 5.1 Properties of Selected Commercial Reinforcing Fibers
Fiber
Typical Diameter (mm)a
Specifi c Gravity
Tensile Modulus (GPa)
Tensile Strength (GPa)
Strain to Failure (%)
Coeffi cient of Th ermal Expansion
(10−6/°C)b
Poisson’s Ratio Glass
E-glass 10 2.54 72.4 3.45 4.8 5 0.2
S-glass 10 2.49 86.9 4.30 5.0 2.9 0.22
PAN carbon
T-300c 7 1.76 231 3.65 1.4 −0.6, 7–12 0.2
AS-1d 8 1.80 228 3.10 1.32
AS-4d 7 1.80 248 4.07 1.65
T-40c 5.1 1.81 290 5.65 1.8 −0.75
IM-7d 5 1.78 301 5.31 1.81
HMS-4d 8 1.80 345 2.48 0.7
GY-70e 8.4 1.96 483 1.52 0.38
Pitch carbon
P-55c 10 2.0 380 1.90 0.5 −1.3
P-100c 10 2.15 758 2.41 0.32 −1.45
Aramid
Kevlar 49f 11.9 1.45 131 3.62 2.8 −2, 59 0.35
Kevlar 149f 1.47 1.79 3.45 1.9
Technorag 1.39 70 3.0 4.4 −6
Extended-chain PE
Spectra 900 38 0.97 117 2.59 3.5
Spectra 1000 27 0.97 172 3.0 2.7
Boron 140 2.7 393 3.1 0.79 5 0.2
SiC
Monofi lament 140 3.08 400 3.44 0.86 1.5
Nicalon (multifi lament)h
14.5 2.55 196 2.75 1.4
Al2O3
FiberFPf 20 3.95 379 1.90 0.4 8.3
Al2O3–SiO2i Fiberfrax
(discontinuous)
2–12 2.73 103 1.03–1.72 −1.72
a 1 mm = 0.0000393 in.
b 1 m/m per °C = 0.556 in./in. per °F.
c Amoco.
d Hercules.
e BASF.
f DuPont.
g Teijin.
h Nippon carbon.
i Carborundum.
5.1.1.2 Matrix Hybridized Composites
Th e matrix is also one of the important constituents of the composites . Hybridizing a matrix with another matrix also means toughening of the matrix. Such matrices when used to prepare the composites also can lead to a hybrid composite.
Toughening of the matrix is carried out in order to reduce the brittleness of the binder matrix and as a consequence to improve the (mechanical and chemical resistance) properties of the com- posites. An unmodifi ed matrix usually consists of single-phase materials, while the addition of modifi ers turns the toughened matrix into a multiphase system. When modifi er domains are correctly dispersed in discrete forms throughout the matrix, the fracture energy or toughness can be greatly improved. In the case of metals, diff erent types of alloys have been used to produce the composites. Polymer blends, by defi nition, are physical mixtures of structurally diff erent homopolymers or copolymers. In polymer blends or polymer alloys, the mixing of two or more polymers provides a new material with a modifi ed array of properties. For polymers, there are as known there are two types of matrices: thermoplastic (polymethyl methacrylate, polycarbonate [PC], polybutylene terephthalate [PBT], polysty- rene [PS], etc.) and thermosets (Ep, unsaturated polyester, etc).
Toughening of thermoplastics is usually carried out by blending them with diff erent types of thermoplastic materials and elasto- mers in order to improve their mechanical properties and the fl exibility of the matrix. Examples include blends of PC and a
thermoplastic copolyether/ester/elastomers, which is shown to have a signifi cant eff ect on the properties. Polyoxymethylene/
elastomer/fi ller ternary composites have been prepared, in which a thermoplastic polyurethane and inorganic fi ller, CaCO3, were used to achieve balanced mechanical properties for poly- oxymethylene. Two other thermoplastics have also been blended together to achieve composites with high-performance proper- ties as well as improve the mechanical properties of the com- posite: polyamide (PA) dispersed in a PE thermoplastic as a matrix for glass fi bers. Researchers have also found that there is a signifi cant increase in the degradation properties of thermo- plastic blends with the addition of ceramic materials (PE-co- ethyl acrylate with a polyisobutyl methacrylate polymer reinforced with ceramic oxide powders). Some other examples of hybrid thermoplastic matrices are PC/acrylonitrile-butadi- ene-styrene ABS blend, PS/PC, PVA/polymethyl methacrylate, etc. Th ough a variety of thermoset materials have been used to prepare matrix hybrid composites, toughening of these com- posites by using Ep and unsaturated polyester are probably the most widely used combination.
Diff erent kinds of modifi ers have been studied to improve the toughness or ductility of cured thermoset resins. Th ey can be classifi ed as liquid rubbers, engineered thermoplastics, reactive diluents, and inorganic particles. Some of the following exam- ples illustrate this type of toughening: Ep toughened with polymethylmethacrylate (PMMA), Ep toughened with PC, Ep toughened with PBT, Ep with reactive liquid rubber such as Ep
200 400
Resin impregnated strands (Kevlar 29)
aramid
(Kevlar 49) aramid
Carbon (T-300)
(Kevlar 49 (dry)) S glass
Polyethelene Boron
Carbon (cellon GY-70)
Carbon (P-100) Ordered
polymer fiber PBT
Goal
PBO
E glass
Titanium Aluminum
HSLASteel
SiC (ceramic fiber)
Alumina (fiber FP)
600
Specific stiffness (tensile modulus/density, in ⫻ 106) Specific stiffness (tensile strength/density, in ⫻ 106)
800 1000 1200 1400
00 2 4 6 8 10 12
FIGURE 5.1 Specifi c properties of advanced fi bers.
phenol liquid with cashew nut shells, unsaturated PE with Ep, unsaturated PE toughened with PMMA, unsaturated PE with PBT, etc. Signifi cant increases in some mechanical- and chemi- cal-resistant properties were found for all of the above combina- tions of cured thermoset resins.
5.1.1.3 Reinforcement and Matrix Hybridized Composites
Composites of this type consist of multiphase materials (four phases) in which both the matrix and the reinforcements have been hybridized. In one way, it can be said that it is the combina- tion of the above two types. Th is area of research is a new and challenging concept because as the incorporation of materials increases, there are more interphases in the composite system and if there are any diff erences in the interfaces of the materials, it could lead to the failure of the complete system. Care should be always taken to meet these future challenges in order to achieve a material with superior mechanical properties. It should also be noted that the viscosity of the toughened matrix resin be suffi - cient enough to wet the hybrid reinforcements. One of the exam- ples that has been developed is silk/glass fabric and used as a reinforcement in thermoplastic-toughened Ep resin to achieve high mechanical properties.
Th e potential for the usage of hybrid composites far outweighs any negative aspects since there are a vast number of plastic/
plastic, plastic/metal aramid-reinforced aluminum laminate (ARALL), metal/metal (MMC), metal/ceramic, ceramic/ceramic (space shuttle outer tile), and ceramic/plastic composite combina- tion systems yet to be explored, investigated, developed, and in use.
5.1.1.4 Hybrid Composite Applications
Hybrid composite materials (HCM) represent the newest of the various composite materials currently under development.
Th e hybrid composite category covers both the hybridizing of a composite material with other materials (either other composites or base unreinforced materials) and composites using multi- ple reinforcements. Further, this category covers the use of multiple materials (at least one of which is a composite) in structural applications and highlights the multiple uses and advantages of composite materials.
Hybrid composites can be divided into fi ve major subcatego- ries: (1) HCM, (2) selective reinforcements, (3) thermal manage- ment, (4) smart skins and structures, and (5) ultralightweight materials.
5.1.1.4.1 HCM
HCM are defi ned as a composite material system derived from the integrating of dissimilar materials, at least one of which is a basic composite material. A typical example of a HCM is a rein- forced polymer composite combined with a conventional unre- inforced homogenous metal. Th e HCM blends the desirable properties of two or more types of materials into a single material system, which displays the benefi cial characteristics of the sepa- rate constituents. An existing example of a hybrid com posite is
ARALL, which consists of high-strength aluminum alloy sheets interleafed with layers of aramid fi ber-reinforced adhesive as illustrated in Figure 5.1. Th e ARALL hybrid composite (Figure 5.2) is baselined on several secondary structural composites of fi xed-wing subsonic aircraft . A second example of a hybrid com- posite is a carbon–carbon composite (CCC) with a single side application of the refractory metal rhenium. Th is carbon–car- bon–rhenium material is being developed for thermal manage- ment heat pipes on space-based radiator systems. Other examples include interpenetrating polymer networks (IPNs), which are hybrid resin matrices consisting of thermoset and thermoplastic resin combinations. Still another HCM concept involves multi- ple reinforcement types within a common matrix such as chopped fi bers and continuous fi bers within a polymer matrix.
Other HCMs include new composite materials such as nano- composites, functionally gradient materials (FGMs), hybrid mate- rials (Hymats), IPNs, microinfi ltrated macrolaminated composites (MIMLCs), and liquid crystal polymers (LCPs), which may force development of previously uneconomical process routes if they off er the path to a technical solution for advanced system capabil- ity. Th ese materials present opportunities for reducing the number of stages in turbine engines and in so doing may be economically benefi cial even at a higher material cost because the smaller number of stages leads to greater economy of use.
HCM technology is in its infancy in comparison to that of the other types of composite materials. Whereas ARALL and IPNs have been used for the past decade, the other types of HCM are truly embryonic. From nearly all aspects of research, these HCM off er great potential for structural applications; however, their widespread use remains a decade or more in the future.
5.1.1.4.2 Selective Reinforcement
Selective reinforcement is the category of hybrid composites that provide reinforcement to a structural component in a local area or areas by means of adding a composite material. An example of this is the use of superplastic forming-diff usion bonding (SPF/
DB) as a means of integrating a titanium-reinforced MMC into a base titanium structure. As part of the initial design approach, consideration must be given to the tooling required for place- ment of the reinforcing material within the structure. Th is is accomplished by building into the form tooling areas in which the MMC material is placed that permit SPF expansion of the
Aramid/epoxy
Aramid/epoxy
7075-T5
7075-T5 7075-T6 sheet
0.03 mm
NOM.
1.3 mm
FIGURE 5.2 ARALL is an example of an HCM in production.
base titanium to the MMC during processing and allowing diff usion bonding to occur, thus accomplishing integral rein- forcing of the fi nal structure.
Th e selective reinforcement design approach allows the aero- space designer to utilize the more costly, higher performance materials only where they are required and the less expensive materials in areas where they can perform the job. Th is approach leads to an optimization of both cost and performance in the most ideal case. Similarly, problems associated with use of the reinforcing composite, such as low mechanical joint strength, can be eliminated by not reinforcing the area where the mecha- nical joining occurs. In reality, while this approach can be very effi cient, it requires the introduction of multiple materials to solve the design problem and may in turn result in increased fabrication costs and risks.
5.1.1.4.3 Th ermal Management
In the fi eld of thermal management, composite materials and HCM can be innovatively constructed to eff ectively limit the maximum temperature of structural hardware and rapidly transfer heat from hot areas to cool areas. Th is capability arises from the unmatched thermal conductivity of graphite fi ber, which is higher (in the fi ber direction) than that of oxygen-free high-conductivity copper (OFHC). Th e graphite fi bers act as heat paths and, by suitable arrangement in the structure of interest, can remove heat by transmitting it along its length. In contrast, the matrix materials can act as thermal insulators so that thermal conduction through the thickness is lower by orders of magni- tude than in-plane. Th is allows designers to develop a structure that is a thermal insulator in some directions but a thermal con- ductor in others.
5.1.1.4.4 Smart Skins and Smart Structures
Smart skins and smart structures are related in that each contains embedded, nonstructural elements. A smart structure contains sensors that monitor the health of the structure itself, such as fi ber optics to determine temperature and structural deformations or cracks. A smart skin contains circuitry and electronic compo- nents that enable the skin to double as part of the electronic system of the parent vehicle, be it an aircraft or a missile.
Smart structure technology, like that of smart skins, is still evolving. Th e present technology consists of the incorporation of sensors into structural elements in the material processing stage to better control cure (or consolidation), and so on.
5.1.1.4.5 Ultralightweight Materials
Th e category of ultralightweight materials includes the emerg- ing family of liquid crystal ordered polymers, which by virtue of their molecular structure, exhibit extremely high specifi c strength and specifi c stiff ness. Th ese highly directional materi- als are similar to composite materials in that the long, ordered molecular chains within the polymer act very much like rein- forcing fibers in a composite material. Examples of these materials are poly-p-phenylene benzobisthiazole (PBZT) and
poly-p-phenylene benzobisoxazole (PBO). Another example is gel-spun PE. Th ese polymers exhibit extremely high specifi c strength and specifi c stiff ness and can potentially be used as reinforcing fi bers in composite laminates, as rope or cable, or as a self-reinforced thin fi lm structure. Th ese ordered polymers, in the form of thin fi lms, fi nd use in shear webs and skin applica- tions for aircraft . Th ese thin fi lms can also be processed into honeycomb for lightweight structural applications. It has been estimated that a shear web made of PBZT would be one-eighth the weight of an aluminum web and one-sixth the weight of a graphite Ep web.