Composite Materials M. Knight D. Curliss Air Force Research Laboratory
I. Characteristics II. Constituent Materials III. Properties of Composites IV. Analysis of Composites V. Fabrication of Composites VI. Uses of Composites Composites
GLOSSARY Advanced composites Composite materials applicable to aerospace aerospace constructi construction on and consisting consisting of a highstrength, high-modulus fiber system embedded in an essentially homogeneous matrix. isotropic; c; having having mechanica mechanicall and/or and/or Anisotropic Not isotropi physical properties that vary with direction relative to a natural reference axis inherent in the materials. Compositee laminate laminate in which which all Balanced Balanced laminate laminate Composit ◦ ◦ laminae at angles other than 0 and 90 occur only in ±pairs. Constituent In general, an element of a larger grouping. In advanced composites, the principal constituents are the fibers and the matrix. Cure To change the properties of a thermosetting resin irreversibly irreversibly by chemical reaction. Single homogeneo homogeneous us strand strand of material material,, essentia essentially lly Fiber Single one-dimensional in the macrobehavior sense. Interface Boundary between the individual, physically distinguishable constituents of a composite.
Having uniform uniform propertie propertiess in alldirections. alldirections. The Isotropic Having measured measured properties properties are independe independent nt of the axis of testing. Lamina Sing Single le ply ply or laye layerr in a lami lamina nate te made made of a seri series es of layers. Laminate Unit made by bonding together two or more layers or laminae of materials. Matrix Essentially homogeneous material in which the reinforcement system of a composite is embedded. Having three three mutually mutually perpendi perpendicularplanes cularplanes Orthotropic Having of elastic symmetry. Material having having identical identical properproperTransversely isotropic Material ties along any direction in a transverse plane. Woven fabric composite Form of composite in which the reinforcement consists of woven fabric. 1, or x, axis Axis in the plane of the laminate that is used asthe0◦ refere referencefor ncefor design designati ating ng theangle theangle of a lamina lamina.. 2, or y, axis Axis in the plane of the laminate that is perpendicular to the x axis. 3, or z, axis Reference axis normal to the plane of the laminate x , y axes.
455
456
FIGURE 1 Cross section of a graphite fiber–reinforced epoxy polymer.
A COMPOSITE MATERIAL is described in this chapter as a material composed of two or more distinct phases and the interfaces between them. At a macroscopic scale, the phases are indistinguishable, indistinguishable, but at some microscopic scales, the phases are clearly separate, and each phase exhibits hibits the charac character terist istics ics of the pure pure materi material. al. In this this chapchapter, we are only describing the characteristics, analysis, and processing of high-performance high-performance structural composite material materials. s. This special special class class of composit composites es always always consists consists of a reinfo reinforci rcing ng phase phase and a matrix matrix phase. phase. The reinfo reinforci rcing ng phase is typically a graphite, glass, ceramic, or polymer fiber, and the matrix is typically a polymer, polymer, but may also be ceramic or metal. The fibers provide strength and stiffness to the composite component, while the matrix serves to bind the reinforcements together, distribute mechanical loads through the part, provide a means to process the material into a net shape part, and provide the primary environmental resistance of the composite component. In Fig. 1, 1, we can see the distinct cross section of graphite fibers in an epoxy matrix.
Composite Materials
Composites occur very commonly in nature. Some of the best examples are wood, bone, various minerals, mollusk lusk shells shells,, and insectexos insectexoskel keleto etons. ns. In wood, wood, thecellulos thecellulosee fibers of the cell wall are “glued” glued” together by the lignin matrix. matrix. Bone is composed composed of calcium calcium hydroxyap hydroxyapatit atitee crystals in a protein matrix. Mollusk shells are composites of calcium carbonate layers in various geometries bound together by a multilayer matrix. Insect exoskeletons exoskeletons bear a striking resemblance to man-made fiber-reinforced ber-reinforced composites. Some insects even exhibit apparent “layers” layers” of fibrous chitin embedded in a protein matrix, where the orientation of the fibers varies from layer to layer, much as we might design a man-made fiber-reinforced ber-reinforced composite. This example of a natural composite can be clearly seen in Fig. 2. 2. Modern materials engineers have used the composite composite concept concept— —reinfo reinforce rcemen mentt in a matrix matrix— —to create create a class of modern materials that offers opportunities signifi nificantly cantly greate greaterr than than those those of more more common common engine engineeri ering ng materials. Composites can be made of a such a wide variety of materials that it is impractical to discuss each one individually. One of the principal characteristics of all composites is that they have a reinforcement phase distinct from the matrix phase. The individual characteristics of the two phases combine to give the composite its unique properties. Classes Classes of material materialss commonly commonly used for reinforc reinforcemen ements ts are glasse glasses, s, metals metals,, polyme polymers, rs, cerami ceramics, cs, and graphi graphite. te. The reinforcement can be in many forms, such as continuous fibers or filaments, chopped fibers, woven fibers or yarns, partic particles les,, or ribbon ribbons. s. The criter criteria ia forselectin forselecting g the type type and form of reinforcement vary in accordance with the design requirement for the composite. However, However, certain general qualities are desirable, including high strength, high modulus, light light weight, weight, environ environment mental al resistan resistance, ce, good elongaelongation, low cost, good handleability, and ease of manufacture. ture. By far, far, the mostwidely usedreinforcemen usedreinforcementt is E-glass. E-glass.
I. CHARACTERISTICS Many materials can be classi fied as composites. They are composed of several distinctly different and microscopically identifi identifiable substances. Composites are widely used in many industries and applications today, driven by the need for strong, lightweight materials. The composites reduce weight and allow for designs that tailor the mechanical properties of the material to meet the loading requirements of the structure. In addition, composites are replacing replacing traditio traditional nal engineeri engineering ng material materialss in many indusindustrial, trial, recreatio recreational, nal, architec architectura tural, l, transpor transportati tation, on, and infrasinfrastructure applications.
Scanning electron microscope microscope (SEM) image of a FIGURE FIGURE 2 Scanning bessbeetle (Odontotaenius disjunctus ) elytra fracture surface.
457
Composite Materials
E-glass offers excellent strength, compatibility with common matrix polymers, and is very low in cost. Various types of graphite fibers are commonly used in aerospace and the recreational products industry, where light weight and maxim maximum um materi material al perfor performan mance ce are very very import important ant to the designer. The matrix binds the reinforcement together and enhances the distribution of the applied load within the composite. Polymeric materials are widely used as matrix materials. Two Two general classes of polymers are used: thermosets and thermoplastics. Thermosets are initially low low molecu molecularweigh larweightt molec molecule uless that that areoften viscou viscouss liqliquids at room temperature— temperature —what we commonly think of as “resins.” resins.” Their low viscosity and fluid behavior make them very suitable to low-cost processing. The thermoset resins undergo chemical reactions when heated (or initiated by some other energy source such as UV light, electron beam, or microwave) and form a high molecular weight cross-linked polymer. In contrast, thermoplastics are high molecular weight linear polymers that are fully formed prior to processing as a composite matrix. When heated to temperatures well above their glass transition temperature, T g , they soften and exhibit a viscosity low enough to flow and consolidate the composite. In general, they must be heated to much higher temperatures than thermosets, exhibit much higher melt viscosity, and require higher pressures to form well-consolidated composite laminates. Thermoplastics offer some advantages such as reprocessability, recyclability, and, in general, eral, higher higher toughness toughness.. Howeve However, r, thermopl thermoplasti astics cs also have have several limitations that have restricted their wider acceptance as matrix materials for fiber-reinforced composites. posites. Thermoplas Thermoplastics tics have have lower lower solvent solvent resistanc resistancee than thermose thermosets ts and require require more expensive expensive processprocessing equipment, there are fewer commercially available thermoplastic matrix preforms available than for thermosets, and modern toughened thermosets offer similar performance to thermoplastic matrix composites. For such economic and performance reasons, thermoplastics are not widely used as thermosets for advanced composite matrix polymers. Other matrix materials are metals, ceramics, glasses, and carbon. They perform the same function in composites as the polymer matrix. These materials (with the exception of carbon) are still experimental, and their combined fraction of the composite matrix materials market is insignifi insigni ficant. Carbon has been used since the 1970s for exotic high-temperature ablative applications such as rocket motor nozzles. The Properties of Composites and Analysis of Composites sections of this article are general and apply to these developmental composite materials. The Processing and Applications sections, however, are concerned only with polymer matrix composites.
The matrix infl in fluences uences the service service temperat temperature, ure, serservice enviro environmen nment, t, and fabricat fabrication ion process process for composit composites. es. Compat Compatibi ibilit lity y with with the reinfo reinforce rceme ment nt is a consid considera eratio tion n in selecting the matrix. The matrix must adhere to the reinforcemen forcementt and be capable capable of distrib distributin uting g the loads loads applied applied to the composite. The properties of a composite can be tailored by the engineer to provide a wide range of responses, which increases their usefulness. Composites can be made to exhibit some interesting responses when loaded: They can be design designed ed to twist twist and bend when loaded loaded in plane and to extend or contract when loaded in bending. Analysis Analysis approache approachess are availa available ble for predictin predicting g these these responses. There are many processes for the fabrication of composite posites. s. These These often often result result in reduct reduction ion in number number of parts, parts, reduct reduction ion in produc productio tion n time, time, andsavings andsavings in overa overall ll manumanufactu facturin ring g cost. cost. The number number of indust industrie riess using using compos composit ites es and the various uses of composites continues to grow. It is dif ficult cult to forese foreseee what what thefuture thefuture of this this class class of mater material ialss will be.
II. CONSTITUENT MATERIALS A composite can contain several chemical substances. There There are additiv additives, es, for example,to example,to improveprocess improveprocessabil ability ity and serviceab serviceabilit ility y. However However,, the two principa principall constitue constituents nts that that arealways arealways presen presentt in advanc advanced ed compos composite itess arethe matrix and the reinforcement. reinforcement. Generally, they are combined without chemical reaction and form separate and distinct phases. phases. Ideally Ideally,, the reinforc reinforcemen ementt is uniforml uniformly y distrib distributed uted throug throughou houtt thematrix thematrix phase.The phase.The combin combinati ation on of theproperties of the reinforcement, the form of the reinforcement, the amount of reinforcement, and matrix properties gives the composite its characteristic characteristic properties. The matrix phase contributes to several several characteristics of the composite. The matrix provides some protection for the reinforc reinforcemen ementt fromdeleterious fromdeleterious environm environmenta entall conditions such as harmful chemicals. The matrix plays an important role in determining the physical and thermophysical properties of the composite. In continuous filament, unidirectionally reinforced composites, the properties transverse to the filaments are strongly infl in fluenced by the properties of the matrix. The distribution of the applied load throughout the composite is in fluenced by the properties of the matrix. Table I shows typical values of selected properties of common matrix materials. The properties are tensile strength, F tu , Young’ oung’s modulus, modulus, E t , total total strain strain (or strain strain-t to-failure), ε , coef ficient of thermal expansion, α , and specifi specific gravity. It can be seen that there is a wide variation in these values between types of matrix materials.
458
Composite Materials TABLE I Matrix Materials Materials Epo Epoxy
Polyim lyimid idee
Poly olyest ester
Polysu lysulf lfo one
Polyether ethe etherr keton etonee
Al 202 2024
Ti 6-4 6-4
6.2–103
90
21–69
69
69
414
924
2.8–3.4
2.8
3.4–5.6
2.8
3.6
72
110
Pro Propert perty y E tu (MPa) t
E (GPa) t
ε (%)
4.5
7–9
0.5–5.0
50–100
2.0
10
8
α (10−6 m m−1 K−1 )
0.56
0.51
0.4–0.7
0.56
0.5
24
9.6
Specific gravity
1.20
1.43
1.1 –1.4
1.24
1.2
2.77
4. 4 .43
There is great variety in polymers typically used for composite matrix materials. As discussed earlier, thermosets and thermoplastics make up the two general families of engineering polymers; but there are many different polymers within each family that exhibit very diverse propertie properties, s, depending depending on their their chemical chemical composit composition. ion. Thermosets are generally named for the characteristic reactiv activee group group of the resin resin (e.g., (e.g., epoxy epoxy,, maleim maleimide ide), ), wherea whereass thermoplastics thermoplastics are generally named for either their building block (“ (“mer” mer” unit; e.g., polystyrene, polyethylene, polypropylene, polyvinyl chloride) or for a characteristic repeating chemical group within the thermoplastic polymer (e.g., polysulfone, polyimide). It is more appropriate to refe referr to the the matr matrix ix poly polyme merr as a resi resin n syst system em,, the the syst system em being a mixture of the base polymer (or thermoset resin and curing curing agents). agents). Diluents, Diluents, fillers, llers, tougheners tougheners,, and other modifi modifiers are sometimes added to the resin system to alter viscosity, increase toughness, modify reactivity of the thermosets, or change other properties of the base polymer system. Because there are so many starting combinations, it is easy to see how there can be a wide variation in the properties of materials in the same general class (e.g., based on the same basic polymer, but with different additives). additives). The other principal constituent of a composite is the reinforcement. There are several types of materials, and their various forms are used as reinforcements. The continuous fiber has been used most extensively for the development of advanced composites. This form of reinforcement provides the highest strength and modulus. It can be used to make other forms such as woven
fabric, chopped fibers, and random fiber mats. These reinforcement forms typically reduce the mechanical performance compared to unidirectional fibers, but offer benefits in fabrication. Glass, graphite, and polymeric fibers are generally produced as bundles of many filaments of very small diameter. Metal, boron, and ceramic reinforcements are usually single fibers. After fabrication, fibers are processedwith processedwith surface surface treatmen treatments ts for protectio protection n during during handli handling ng and weavin weaving g and also also for chemi chemical cal compat compatibi ibilit lity y withthe matrix matrix systems. systems. After After forming forming and treating treating,, the filaments aments are typica typically lly wound wound on spools spools for use by manuf manufacacturers in fabricating fabricating composites, producing unidirectional preforms, or weaving into various geometries of textile preforms. Table II lists the properties of some of the fibers, measured in the longitudinal direction (along the axis of the fiber), used in composite materials: tensile strength F tu , Young’ Young’s modulus modulus E t , coef coef ficient cient of expansio expansion n α , strain-t strain-toot failure ε , diameter, and density ρ . Mechanical properties transverse to the longitudinal axis are not shown. Because of the small diameter of the fibers, transverse properties are not measured directly. directly. Variations in the fiber properties can be caused by several factors. There can be variations in the composition of the starting material such as in E-, S-, and C-glass fibers. There can be variations in processing such as in the way the processing temperature is changed to vary the strength and modulus of graphite fibers. Also, the dif ficulty of performing mechanical testing on fibers contributes to uncertainty and scatter in the measured properties of fi of fibers.
TABLE II Fiber Materials Materials Property E tu (MPa) t
E (GPa) α (10−6 m m−1 K−1 ) ρ (g cm−3 ) −3
Diameter (10 ε t (%)
m)
Boron
Carbon
Graphite
Aramid
Alumina
Silicon carbide
E-glass
S-glass
2.8–3.4
0.4–2.1
0.81–3.6
2. 2 .8
1.4
3.3
3.4
4.6
379–414
241–517
34–552
124
345–379
427
69
83
4.9
−0.09
−0.09
−4.0
3.4
.40
5.1
3.4
2.5–3.3
1.55
1.55
1.60
3.90
3.07
2.55
2.5
0.05–0.2
0.008
0.008
0.013
0.38 –0.64
0.14
0.005 –0.01 0.013 3
0.00 0.009 9–0.010
0.67
1.0–2.0
0.4–2.0
2.5
0.4
0.6
4.8
5.4
Composite Materials
The reinfo reinforce rceme ment nt is themain load-b load-bear earing ing phase phase of the composite. It provides strength and stiffness. There is a direct direct relati relations onship hip betwee between n an increa increase se in volum volumee fracti fraction on of reinforcement and an increase in strength and stiffness stiffness of the composite material. This relationship depends on the assumption of compatibility with the matrix and on the existence of good bonding to the fibers. The reinforcement and matrix are combined either before or at the time of fabrication of the composite. This depend dependss on the fabri fabricat cation ion proces process. s. A common common practi practice ce in making continuous-fi continuous- fiber-reinforced laminates is to combine the constituents before fabrication into a continuous tapelike ” preform that is used much like broadgoods in “tapelike” that shapes are cut out of the preform and fabricated into parts. To produce this preform product, fibers are combined with resin, typically by drawing the fiber bundle through a resin or resin solution bath. Several bundles of resin-impregnated fibers are then aligned and spread into very thin layers (0.127 mm thick) on a release ply backing. The resin is usually partially cured during production of the preform to reduce its “tackiness” tackiness” and improve the handleability of the preform. This tapelike preform is known as prepreg, or unidirectional tape. It is an expensive method for producing a preform, but the preform is a continuous, well-characterized, well-characterized, well-controlled method to combine the matrix resin and the reinforcing fiber. After prepregging, the material is usually stored in a freezer to retard the chemical reaction until the material is used. If the matrix system is a thermoplastic polymer, then no reaction can occur, and the material may be stored inde finitely at room temperature. These layers of unidirectional fibers and resin are used to make laminates by stacking many layers in directions speci fied by the engineer. The number of “ of “plies” plies” in a laminate and the direction of fi of fibers in each layer is dependent on the mechanical properties required for the part. The next two sections, Properties of Composites and Analysis of Composites, describe how an engineer would design design a compos composite ite lamina laminate te to have have the proper propertie tiess needed needed for an applic applicati ation. on. It is exact exactly ly this this tailor tailorabi abilit lity y that that makes makes composites attractive attractive for engineering applications.
III. PROPERTIES OF COMPOSITES In many of the applications in which composite materials are used, they can be considered to be constructed of several layers stacked on top of one another. These layers, or laminae, typically exhibit properties similar to those of orthotropic materials. Orthotropic materials have three mutually perpendicular planes of material property symmetry. Figure 3 shows a lamina with its coordinate system andtwo of theplanes theplanes of symmet symmetry ry.. We will will first discuss discuss the
459 properties of the lamina and some factors that in fluence them. Next, the properties of laminates will be discussed. The lamina is made of one thickness of reinforcement embedded in the matrix. The elastic and strength properties ties of the reinfo reinforce rcemen mentt and the elasti elasticc and streng strength th propproperties of the matrix combine to give the lamina its properties. In addition to the properties of the constituents, the amount of reinforcement, the form of the reinforcement, and the orientation and distribution of the reinforcement all infl influence the properties of the lamina. The reinforcement provides the strength and stiffness of the composite. Increasing the amount of reinforcement reinforcement increa increases ses the streng strength th and stiff stiffnes nesss of the compos composit itee in the direction parallel to the reinforcement. The effect of the form form of the reinfo reinforce rcemen mentt is not as simple simple.. Howe Howeve ver, r, some some general observations can be made. Laminae reinforced by long, continuous, parallel fibers have greater strength and stiffness than laminae reinforced by short, randomly oriented fibers. Woven fiber– ber–reinforced laminae usually have greater strength perpendicular to the principal fiber direction than do unwoven fiber– ber–reinforced laminae. The strength and stiffness of laminae reinforced by unwoven continuous fibers bers decrea decrease se as the angle angle of loadin loading g change changess from parallel to the fibers to perpendicular to the fibers. Table III shows typical values for some properties of composite materials made of unwoven continuous fiber reinforcements. The table shows the strength and elastic properties of a laminate made of several laminae stacked on top top ofone anot anothe herr withall withall the the fibers bers aligne aligned d in thesame direction. The properties in the direction parallel to the fibers are much greater than the properties in the direction perpendicular to the fibers. This variation of properties with with the orient orientati ation on of the lamina lamina axis axis is called called anisot anisotrop ropy y. The single lamina serves as a building block. The engineer can select the orientation and number of each of the laminae in a laminate and design the laminate such that it has the required response. This designing of a laminate has some interesting implications that the engineer should understand. Two important factors are balance and symmetry. Balanc Balancee and symmet symmetry ry simpli simplify fy the analys analysis is of the lamlaminate and give it more conventional conventional response characteristics. tics. Balanc Balancee in a lamina laminate te means means that that for each each laminawith laminawith a positive angle of orientation there must be a lamina with an equal negative angle of orientation. Both laminae must have the same mechanical and physical characteristics. This is important in controlling the laminate’ laminate ’s overall response to loading both in service and in fabrication. Symmetry means that for every lamina above the midplane of the laminate there is a lamina an equal distance below the midplane that is of the same type with the same orientation. Symmetry also infl in fluences the laminate response to loads.
460
Composite Materials TABLE III Typical Typical Properties Properties of Composite Composite Materials: Materials: Laminates Laminates Reinforce Reinforced d With Unidirectio Unidirectional nal Continuous Fibers Property
Unit
E-glass epoxy
Aramid epoxy
Graphite epoxy
Boron epoxy
MPa
1100
1380
1240
1296
Parallel to the fibers Tensile strength σ xT Tensile modulus
E xT
Poisson’s ratio νxy Total strain ε T Compressive Compressive strength
σ xc
GPa
39.3
75.8
131
207
—
0.25
0.34
0.25
0.21
%
2.2
1.8
1.21
0.66
MPa
586
276
1100
2426
Compressive modulus E xc
GPa
39.3
75.8
131
221
Shear strength τ xy
MPa
62.0
44.1
62.0
132
Shear modulus G xy
GPa
3.45
2.07
4.83
6.2
MPa
34.5
27.6
41.4
62.7
Transverse to the fibers Tensile strength σ yT Tensile modulus
E yT
Compressive Compressive strength σ yc Compressive modulus
E yc
GPa
8.96
5.5
6.2
18.6
MPa
138
138
138
310
GPa
8.96
5.5
6.2
24.1
Specific gravity
—
2.08
1.38
1.52
2.01
Fiber volume V f f
%
∼50
∼60
∼62
∼50
FIGURE 3 Lamina coordinate axis and planes of symmetry.
461
Composite Materials
FIGURE 4 Orientation and location of laminae in a laminate.
If a laminate is not balanced and symmetrical, it will twist or bend when in-plane loads are applied. Laminates may also extend or contract when bending loads are applied. Whether the results are good or bad depends on whether they were planned or unplanned during the design of the laminate. Figure 4 shows how the laminae are oriented and stacked in a laminate.
IV. ANALYSIS OF COMPOSITES Composite materials are complex. The properties of the constitu constituents ents are differe different, nt, and the fiber properti properties es are anisotropic. The composite may also be constructed by layers, with the fiber directions varying layer to layer. Analysis of the mechanical properties of such laminates is a sophisticated process; research into better methods to predict composite performance is being pursued. However, ever, acceptable engineering analysis methods have been developed developed that allow structural parts to be designed with composite materials. Further research is required to develop sound engineering methods to predict failure in composite materials, especially when subjected to severe environments that may degrade the matrix, the reinforcement, or the interfaces of the composite material. In this section, a brief summary of the currently accepted
approach to performing stress analysis of composites is presented. The emphasis has been focused on unidirectional unidirectional fiberreinforced composites. The lamina or ply form of advanced composites has been developed into the basic unit for analysis. Most of the structural applications of advanced composites involve involve material in a laminated form. The laminates are constructed of plies or laminae laid up to a designed confi con figuration (see Fig. 4). 4). The approach to the analysis of composites starts with the lamina lamina and itselastic itselastic proper propertie ties. s. Then Then these these are relate related d to the geometry of the lay-up for the laminate. The elastic properties and orientation of the laminae are used to calculate the modulus and stiffness of the laminate. The constitu constitutiv tivee relations relationship hip and a selected selected failure failure criterionare criterionare used to estimate failure. In developing the analysis of the lamina, several assumptions were made. It was assumed that (1) the fibers and matrix were bonded together, (2) the lamina was void free, (3) the lamina’ lamina ’s thickness was small in comparison with its width and length, (4) the lamina was a homogeneous orthotropic material, and (5) the fibers were uniformly distributed within the matrix. The lamina is analyzed as a macroscopic, homogeneous, orthotropic material in a plane stress condition. If the coordinate axes for the laminate are oriented parallel
462
Composite Materials
and transverse to the fiber axis (see Fig. 3), 3), the constitutive equation relating stress α and strain ε is
σ 1 σ 2
=
Q 11
Q 12
Q 12
Q 22
0
0
τ 12 12
0 0
ε1 ε2
Q 66
(1)
γ 12 12
where Q is called the reduced stiffness and is de fined as E 1
Q 11 =
1 − ν12 ν21 ν12 E 2
Q 12 =
1 − ν12 ν21
;
Q 22 =
E 2
1 − ν12 ν21
(2)
Q 66 = G 12
;
where E 1 is Young’ Young’s modulus in the direction parallel to the fibers; E 2 is Young’ Young’s modulus in the direction perpendicular to the fibers; ν12 and ν21 are the major Poisson’ Poisson ’s ratio and minor Poisson’ Poisson ’s ratio, respectively; and G 12 is the in-plane shear modulus. Equation (1) can be inverted to give the form
ε1 ε2
=
γ 12 12
S11
S11
S12
S22
0
0
0 0 σ 2
(3)
S22 = 1/ E 2 S66 = 1/ G 12
(4)
E 1 = E f f V f f + E m V m ν12 = ν f V f f + νm V m
η=
σ y τ x y
¯ ¯ ¯
= Q 12
Q¯ 12 Q¯ 22
Q¯ 16 Q¯ 26
Q 16
Q¯ 26
Q¯ 66
Q 11
k
k
ε x ε y γ x y
(6) k
The Q¯ terms terms arethe compon component entss of thestiffne thestiffness ss matrix matrix for the lamina referred to an arbitrary axis. They are de fined as Q¯ 11 = Q 11 cos4 θ + 2( Q 12 + 2 Q 66 ) sin sin2 θ cos2 θ 4
+ Q 22 sin θ
sin2 θ cos2 θ Q¯ 22 = Q 11 sin4 θ + 2( Q 12 + 2 Q 66 ) sin 4
Q¯ 12 = ( Q 11 + Q 22 − 4 Q 66 ) sin sin2 θ cos2 θ 4
Equation (4) relates the compliance coef ficients to the engineer gineering ing consta constants nts.. These These can be determ determine ined d by mechan mechan-ical testing. Also, estimates of the engineering constants can be made by using equations developed by micromechanics. chanics. In this approach, approach, the propertie propertiess of the constitue constituents nts are used in equations for the engineering constants. These are
P / Pm = (1 + ξ η V f f )/(1 − η V f f )
σ x
4
+ Q 22 (sin θ + cos θ )
τ 12 12
where the S terms are the compliance coef ficients and are defi defined as S11 = 1/ E t t ; S12 = −ν12 / E 1 ;
+ Q 22 cos θ
σ 1
S66
lamina in the laminate coordinate systems. This is done throug through h a transf transform ormati ation. on. By a combin combinati ation on of mathem mathematatical transformation and substitution, the following relationship between stress and strain for an arbitrary lamina k is developed:
(5)
( P f / Pm ) − 1 ( P f / Pm ) + ξ
where V f f , V m are the volume fraction of the fiber and matrix, respectively; respectively; ν f , νm are Poisson Poisson’’s rati ratio o ofthe fiberand matrix, respectively; respectively; P is the compos composite ite modulu moduluss E 2 , G 12 , or G 23 ; P f is the corresponding fiber modulus E f f , G f , or ν f , respectively; Pm is the corresponding matrix modulus E m , G m , or νm , respectively; and ξ is a factor related to the arrangement and geometry of the reinforcement; for square packing ξ = 2, and for hexagonal packing ξ = 1. Because not all laminae in a laminate are oriented with the fibers parallel or transverse to the laminate coordinate axis x – y , there must be a way to find the properties of the
(7)
Q¯ 66 = ( Q 11 + Q 22 − 2 Q 12 − 2 Q 66 ) sin sin2 θ cos2 θ 4
4
+ Q 66 (sin θ + cos θ )
sin2 θ cos3 θ Q¯ 16 = ( Q 11 − Q 12 − 2 Q 66 ) sin 3
sin θ cos θ + ( Q 12 − Q 22 + 2 Q 66 ) sin Q¯ 26 = ( Q 11 − Q 12 − 2 Q 66 ) sin sin2 θ cos θ 3
sin θ cos θ + ( Q 12 − Q 22 + 2 Q 66 ) sin where θ is the ply angle according to the Tsai convention (see Fig. 4) 4). Counterclockwise rotations are positive positive and clockwise rotations are negative. The constitutive relationships relationships for the lamina and linear small deformation theory were used to develop the analysis for composite structures. Some assumptions that were made made are as follo follows: ws: (1) The lamina laminaee are bonded bonded togeth together er,, and they do not slip relative to one another when load is applied; (2) the normals to the undeformed midplane of the laminate are straight, and they remain so with no change in length after deformation; (3) the thickness of the the plat platee is smal smalll comp compar ared ed with with the the leng length th and and widt width; h; and and (4) the strain in the thickness direction is negligible. The in-pla in-plane ne strainis strainis assume assumed d consta constant nt forall thelaminae thelaminae.. The stress varies from lamina to lamina. As a simpli fication, the force and moment resultants were defi de fined. The force resultants N x , N y , and N xy were defi defined as the sum of the laminae stresses per unit width. The moment resultants M x , M y , and M xy were defi defined as the sum of the respective tive stress stresses, es, timesthe timesthe area area over over which which they they act, act, multip multiplie lied d by the approp appropria riate te momentarm. momentarm. The in-pla in-plane ne strain strainss at the
463
Composite Materials 0 midplane, ε x0 , ε y0 , and γ xy , and the curvatures, κ x , κ y , and κ x y , are related to the resultants as shown in Eq. (8).
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - N x
N y
N x y M x
M y
M xy
A11
A12
A16
A12
A22
A26
A16
A26
A66
B11
B12
B16
B12
B22
B26
B16
B26
B66
B11
B12
B16
B12
B22
B26
B16
B26
B66
D11
D12
D16
D12
D22
D26
D16
D26
D66
ε x0
strains, curvatures, forces, or moments are known in a given situation. The The defi definiti nition onss for for the the elem elemen entsof tsof the the [ A], [ B ],and[ D ] matrices are
ε y0
n
Ai j =
γ x0y
( ¯
Q i j )k (h k − h k −1 )
κ x
Bi j =
κ y
1
n
( ¯ ) 2 1 ( ¯ ) Qi j
k
h 2k − h 2k −1
(10)
Qi j
k
h 3k − h 3k −1
(11)
k =1
κ x y
n
Di j =
(8) where N x , N y , and N x y are force resultants; M x , M y , and M xy are moment resultants; [ A] is the in-plane stiffness matrix for a laminate; [ B ] is the coupling stiffness matrix for a laminate; [ D ] is the bending stiffness matrix for a laminate; ε x0 , ε y0 , and γ x0y are the strains at the laminate geometric mid-plane; and κ x , κ y , and κ xy are the curvatures of the laminate. Examination of Eq. (8) shows that the [ A] matrix is the coef ficients for the in-plane strains. The [ B ] matrix relates lates the curva curvatur tures es to the force force result resultant antss and the in-pla in-plane ne strains to the moment resultants. The [ D] matrix relates the curvatures to the moment resultants. Equation (8) can be partially or fully inverted, depending on whether the
(9)
k =1
3
k =1
Figure 5 shows how k and h are defi defined for the laminae. The force resultants and moment resultants are de fined as
N x
N y
h /2
σ y
=
−h /2
N xy
σ x
dz
(12)
τ x y
and
M x
M y
h /2
σ y z dz
=
M x y
FIGURE 5 Relationship of laminae to the laminate coordinates.
σ x
−h /2
τ xy
(13)
464
Composite Materials
FIGURE 6 Force resultants on an element.
where σ x , σ y , and τ xy are the stresses in the laminate coordinate system and z is the distance from the midplane in the direction normal to the midplane. Figures 6 and 7 show how the force and monment resultants act on an element in the laminate. Equation (8) is the constitutive equation for a general laminated plate. Signifi Signi ficant simplifi simplifications of Eq. (8) are possible. If the [ B ] is made zero, the set of equations for the stress and moment resultants is uncoupled. “Uncou-
pled” pled” means that in-plane loads generate only in-plane responses, and bending loads generate only bending responses. The [ B ] can be made zero if for each lamina above above themidplane themidplane there there is a laminawith laminawith thesame proper proper-ties, ties, orientat orientation, ion, and thickness thickness located located at the same distance distance below the midplane. This is signi ficant not only in simplifying the calculations but also in the physical response to load and in fabrication. If the [ B ] is zero, the laminate will not warp when cured, and no bending will be induced
FIGURE 7 Moment resultants on an element (following the right-hand rule).
465
Composite Materials
when the laminate is under in-plane loads. Equation (8) becomes
A11
A12
A16
A12
A22
A26
A26
A66
D11
D12
D16
D12
D22
D26
N x
N y
=
N x y
A16
(14)
(15)
ε x0 ε y0
0 γ xy
and M x
M y
M xy
=
D16
D26
D66
k x0 k y0
0 k xy
In the preceding discussion, only the elastic properties of the laminate were considered. The elastic behavior of a laminate can be used to analyze the strength behavior of a laminate. To determine the strength of a laminate, we need a failure criterion for the lamina. It is assumed that the the resp respon onseof seof the the lami laminawillbe nawillbe the the samewhenit samewhenit isin the the laminate under the same stresses or strains. The strength of the laminate will be related to the strength of the individual lamina. The general approach is to determine the force and moment resultants or the mid-plane strains and curvatures for the laminate by using the laminate plate equation or an inverted form. The stress or strain is calculated for each lamina in the laminate axis system, and then it is transformed into the lamina axis system for each lamina and the failure criteria applied to determine if failure occurred in the lamina. If the first-ply failure concept for the laminates is applied, the laminate is considered to have have failed failed when when the first lamina lamina fails fails.. No single single approa approach ch hasbeen unive universa rsallyaccep llyaccepted ted forstrength forstrength analys analysis is of lamlaminates after first-ply failure.
V. FABRICATION OF COMPOSITES Fabrication of components from composite materials is somewhat different from that using traditional engineering materials in that the properties of a composite are highly dependent on the geometry of the reinforcement. The structural designer must consider the issues associated with processing the composite part to ensure that reinforcement reinforcement volume fraction, reinforcement geometry, geometry, and other material properties can be produced economically. cally. The diversity diversity of composite applications has stimulated the development of a wide range of techniques for fabricating fabricating structural composites. In fact, one of the principal reasons for the success of composites is the ease of fabrication fabrication and the many different processes with widely varying levels of sophistication and cost that are available for their production. Structural and decorative composites can be fabricated with techniques ranging from very crude hand lay-up processes without molds to very
sophisticated techniques with complex molds, woven 3D reinforc reinforcemen ementt preforms preforms,, and artifi artificial intelligence– intelligence–guided computer-controlled resin infusion and curing. The configuration of the part, along with the basic manufacturing considerations such as volume, production speed, and market conditions, determine whether a part will be built in open or closed molds, by compression molding, or by an automated automated system. system. Composite Composite fabricat fabrication ion technolo technologies gies can be classifi classified as either open or closed molding, the choice of appropriate technique being governed by factors mentioned earlier. We can group most of the processes into two classes: open molding and closed molding. The main distinction is that open molds are one piece and use low pressure or no pressure, and closed molds are two pieces and can be used with higher pressure.
A. Open-Mold Open-Mold Processes Processes Open-mold processes such as spray-up, wet hand lay-up, autoclave, filament lament winding, winding, vacuum vacuum infusion, infusion, pultrusi pultrusion, on, or combinations of these techniques are the most common open-mold methods to produce composite products. Many Many products products are suited suited to these these manufact manufacturin uring g methods, methods, including aerospace structures, tanks, piping, boat hulls and structur structures, es, recreati recreational onal vehicle vehicle component components, s, commercommercial truck cabs and components, structural members, and plumbing applications (e.g., tubs, showers, pools, and spas). In spray-up and wet hand lay-up open molding, the mold surface typically has a high-quality finish and is the visual surface of the finished part. Common to all open molding techniques is mold preparation. To prepare the mold surface prior to spray-up, hand lay-up, or vacuum infusion, the mold is treated with a release agent to aid in composite part removal and then may be coated with a “gel coat” coat” (a color-tinted layer of resin that becomes the visual surface of the finished part). In spray-up fabrication, the thermoset resin is sprayed into the prepared mold simultaneously with chopped reinforcing fiber. The random sprayed-up mat of fiber and resin may then be compacted with hand rollers prior to cure to produce a more dense part. A hand lay-up component, the resin, and reinforcement (usually a fabric or random fiber mat) are laid into the mold, compacted with rollers,and rollers,and allowed allowed to cure. cure. Oftenhand lay-up lay-up is combined combined with spray-up techniques depending on the structural requirements of the part. Fiber volumes of 15 to 25% are typically achieved with these techniques. There are several variations of the basic process. A vacuum bag made of a nonporous, nonadhering material can be placed over the lay-up. Then a vacuum is drawn inside the bag. The atmospheric pressure outside the bag eliminates the voids
466
Composite Materials
and forces out entrapped air and excess resin. Another approach is to use a pressure bag. The bag is placed against the lay-up and the mold covered with a pressure plate. Air or steam steam pressu pressure re is applie applied d betwee between n the bag and the plate. plate. Vacuum acuum infusi infusion on is an open open moldin molding g proces processs that that is very very suitable suitable for large large component componentss for many many importantreasons. importantreasons. Vacuum infusion uses an airtight membrane over the entire part to provide vacuum pressure on the reinforcement and to prevent any volatile resin products from escaping into the atmosphere. The resin is introduced after the entire reinforcement is laid into the mold and the vacuum membrane is in place; this reduces some issues associated with the working time of the resin prior to cure. Finally, higher volume fractions of reinforcement are achievable since the reinforcement is compacted by vacuum pressure and only the minimum amount of resin necessary is added. Reinforcement volume fractions up to 70% have been reported. An open-mold technique that is widely used in the aerospace industry and is slightly different different from the preceding processes is autoclaving. One difference in this process is that the entire assembly (the lay-up and supporting unit) is placed inside an autoclave. An autoclave is a large pressure vessel that is used to provide heat and pressure to the lay-up during cure. Autoclaves are usually cylindrical, with an end that opens for full access to the interior. They have provision to pull vacuum on the layup assembly, and they often have multiple temperature sensors that are used to monitor the temperature of the part during cure. The curing takes place under pressure, 1–10 bar (15– (15–150 psi), and at elevated temperature. The lay-up assembly is slightly different ( Fig. 8). 8). The top surface of the lay-up is covered with a perforated or porous release film, and if necessary bleeder plies of dry cloth are added to absorb excess resin. Then the assembly is sealed within a nonporous sheet material and placed into the autoclave. The application of pressure and control of temperature is critical. This process offers better quality control than other low- or no-pressure molding processes.
Another process that is used extensively is filament winding. The concept of wrapping filaments around articles cles to impro improve ve their their perfor performan mance ce is very very old. old. The modern modern practice of filament winding was developed in response to the requirements for lightweight pressure vessels. Filament winding winding uses continuou continuouss reinforce reinforcement ment to maximize maximize the use of fi of fiber strength. Preimpregnated tape, or a single strandthat strandthat haspassed haspassed throug through h a resin resin bath, bath, is wound wound onto onto a mandre mandrell in a prescr prescribe ibed d patter pattern. n. Design Design and windin winding g techtechnique allow the maximum fiber strength to be developed in the direction desired. When the winding is completed, the assembly is cured either at room temperature or in an oven. After cure, the mandrel is removed. This process provides for a high level of fi of fiber content. The process of pultrusion is the opposite of extrusion. The reinforcement is passed through a resin bath and then pulled through a die that controls the resin content and final shape. The die can be heated to cure the resin, or the material can be passed through an oven for curing.
B. Closed-Mold Processes The closed-mold processes use a two-part mold or die. When the two parts are put together, they form a cavity in the shape of the article to be molded. The molds are usually usually madeof metal metal with smooth smooth cavity cavity surfaces surfaces.. Higher Higher pressures and temperatures than those in open molding are usually used. The processes produce very accurate moldings. Most of the processes are attractive for mass production. Matched die molding is a closed-mold process. There are variations to this process. The main variations concern the form of the starting material and the manner in which it is introduced into the mold. In some cases, the reinforcement is first made into a preform and placed into the mold and then a metered amount of resin is added — this this is know known n as resi resin n tran transf sfer er mold moldin ing, g, or RTM. TM. RTM is a widely used technique for production of components that require accurate dimensional tolerances, since the outer
FIGURE 8 Cross section of the composite laminate lay-up and vacuum bagging processing method.
467
Composite Materials
surface of the part is determined by the tool surface. In other cases, a resin– resin –reinforcement reinforcement mixture is made and a premeasured amount placed into the mold. The molding compound can be introduced automatically or manually. The molding temperatures range from 100 ◦ C (212◦ F) to 140◦ C (284◦ F). Pressures range from 7 to 20 bar. Cure cycles can be as short as minutes. The selection of a fabrication fabrication process depends on several factors, including the materials to be processed, the size and design of the article, the number of articles, and the rate of production. Processes differ in their capacity to use different forms of reinforcement and to achieve the proper distribution and amount of reinforcement. reinforcement. The chemistry and rheology of the resin are important factors in process selection. Closed molds require higher temperatures and pressures. The size and shape of the article to be produced affect the selection. Very large articles such as boat hulls and vehicle bodies and components are more easily and economically produced in open-mold processes. Small gears and precision electrical parts are more suitably produced in closed molds. Shapes that are surfaces of revolution are ideal for filament winding. Very large cylindrical containers have been fabricated by this process. In most openmold processes, the molds are made of low-cost materials and are easily fabricated but have shorter lives. Autoclave processi processing ng of composit composites, es, while considere considered d an open-mol open-mold d technique, requires accurate, robust tools because of the relatively high temperatures and pressures used in the autoclave. Autoclave techniques are well suited to large structural components for aerospace applications; hence, dimensional accuracy accuracy of the tools is critical. Open-mold, hand lay-up processes have higher labor cost. If one is making a large number of parts and requires high production rates, mold life and labor cost are important factors. tors. Open-mold Open-mold processes processes are usually usually more costly costly in these these two two areas areas than than closed closed-mo -mold ld proces processes ses.. Also, Also, some some closed closed-mold processes can be automated. Automatin Automating g the fabricat fabrication ion of advance advanced d composit composites es and improving processing science for composites are two current goals. The advantages of advanced composites are lighter weight, higher strength- and modulus-to-weight ratios, flexibility in design and fabrication, and usually fewer parts per component. Automating the fabrication process could result in a reduction in labor cost and an improvement in quality. The computer-aided manufacturing technology could be utilized to reduce the total labor hours. The application of higher precision control technology could improve quality and lower rejection rates. Work in processing science should result in increased understanding of the cure process, which will aid the development of resin systems and automating production cycles.
Fabric Fabricati ation on proces processes ses for other other matri matrix x materi materials als are imimportant for the use and continued development of these composites. However, not as much work has been done in these areas. The use of these materials represents a small part of the overall uses of composite materials.
VI. USES OF COMPOSITES Composit Compositee material materialss have have been introduc introduced ed into almost almost everyindustr eryindustry y in some some form form or fashi fashion.Weshal on.Weshalll look look at some some of the advantages of using composites and discuss some of the industries that have made used of these materials. The wide range of property values attained with composites and the ability to tailor the properties is an advantage. Composite materials also generally have higher strength strength-- and modulusmodulus-to-we to-weightratios ightratios thantraditionalenthantraditionalengineering materials. These features can reduce the weight of a system by as much as 20 to 30%. The weight savings translates into energy savings or increased performance. Advanced composites exhibit desirable dynamic properties and have high creep resistance and good dampening characteristics. In fact, the superior fatigue performance of composite materials enables them to be used to repair metallic airframes with fatigue damage. Since composite materials can be manufactured into almost any shape, they allow great design flexibility and offer reduced parts count for articles. The opportunity to select the constituents, tailor them to obtain the required proper propertie ties, s, andthen throug through h designmake designmake theoptimum theoptimum use of the properties is a situation that makes composites very attractive to many industries. The matrix polymer can impart great chemical and corrosion resistance to composites. The transportation industry has made extensive use of composite materials. The light weight and high strength and the ability to easily manufacture aerodynamic shapes have resulted in lower fuel costs. The lack of corrosion of the materials and the low maintenance cost have reduced the cost of ownership and extended the service life of many parts and products. Examples of products in this industry include auto and truck bodies and parts, trailers, tanks, special-purpose vehicles, and manufacturing manufacturing equipment. Composites have added new dimensions to the design and construction of buildings. Their ease of manufacture, light weight, high strength, low maintenance, decorativeness, and functionality have had a signi ficant impact on the indust industry ry.. New-c New-cons onstru tructi ction on time time has been been reduce reduced d and more flexibi exibilit lity y has been been added added to the design design of struct structure ures. s. Composite materials affected the marine industry very early in their development, and their in fluence continues to grow. Lack of corrosion, low maintenance, and design flexibility have contributed to the acceptance of
468 composites. The ease of fabricating very large and strong articles in one piece has been another. another. In addition to pleasure boats, large military and commercial boats and ship hulls have been fabricated. Large tanks for fuel, water, and cargo have been used aboard ships. Composites have made the greatest impact in the sporting goods industry, replacingtraditio replacingtraditional nal material materialss at a revolut revolutionar ionary y pace. pace. Applications such as golf club shafts, fishing poles, tennis rackets, rackets, skiing skiing equipmen equipment, t, boating boating applicat applications ions,, and many many other other sport sportss equipm equipment ent produc products ts are now now produc produced ed almost almost exclusi exclusively vely using using advanced advanced composit composites. es. In most cases, cases, the change in material has translated into an improvement in performance or safety for participants. participants. The aerospace and military markets are the two areas that have accounted for the largest effort in the development and advancem advancement ent in composit compositee technology technology.. The need for stronger, stiffer, stiffer, and lighter structures was an opportunity for composite materials to demonstrate their superiority over more commonly used materials. Durability and low maintenance are additional assets. These increase the service life and reduce the cost of maintaining systems. The development of new and the improvement of existing fabrication processes have brought about a reduction in manufacturing cost. There have been reductions in the number of parts required to construct some components by using molding and composite materials. The unique features of composites have enabled designers to formulate advanced systems that could be made only of composite materials. New military aircraft almost exclusively utilize advanced composites for structure. Rocket motor cases, nozzles, and nose cones are missile applications. Radar domes, rotor blades, propellers, and many secondary structure components such as fairings, doors, and access panels are also fabricated from advanced composites. Numerous pressure vessels, armaments, and items of space hardware hardware are made of select selected ed compos composite ite material materials. s.
Composite Materials
The use of composite materials will continue to grow. As more engineers come to understand composites, more opport opportuni unitie tiess will will be recogn recognize ized d fortheir fortheir use. use. As theuse of composites increases, more developments will take place in the areas of constituent materials, materials, analysis, design, and fabricat fabrication. ion. Composite Composite material materialss offer offer tremendo tremendous us for tailorability, design flexibility, and low-cost processing with low environment impact. These attributes create a very bright future composite materials.
SEE ALSO THE FOLLOWING ARTICLES ADHESION DHESION AND ADHESIVES • BIOPOLYMERS • CARFIBERS • FRACTURE AND FATIGUE • METAL MATRIX COMPOSITES • POLYMERS, MECHANICAL BEHAVIOR • POLYMERS, THERMALLY STABLE • SANDWICH COMPOSITES
BON
BIBLIOGRAPHY Ashton, J. E., Halpin, J. C., and Petit, P. H. (1969). “Primer on Composite Materials: Analysis,” Technomic Publishing Company, Stamford, CT. Hull, Hull, D. (1981). (1981). “An Introduction Introduction to Compositive Compositive Materials,” Cambridge University Press, London. Jones,R. Jones,R. M. (1975) (1975).. “Mechanicsof Mechanicsof Composit Compositee Materials, Materials,” Scripta Scripta Book Company, Washington, D.C. Sih, G. C., and Hsu, S. E. (1987). “Advanced Composite Materials and Structures,” VNU Science Press, Utrecht, The Netherlands. Tsai, S. W. (1985). “Composites Design—1985,” Think Composites, Dayton, OH. Tsai, S. W., and Hahn, H. T. (1980). “Introduction Introduction to Composite Materials,” Technomic Publishing Company, Westport, CT. Whitney, J. M., Daniel, I. M., and Pipes, R. B. (1982). “Experimental Mechanics of Fiber Reinforced Composite Materials,” Society for Experimental Stress Analysis, Brook field Center, CT. Industry Overview: FRP Materials, Manufacturing Methods and Markets, (1999). Composites Technol. 5, 6–20.
