1、Best Practices Entry: Best Practice Info:a71 Committee Approval Date: 2000-03-09a71 Center Point of Contact: GSFCa71 Submitted by: Wil HarkinsSubject: Fiber-Reinforced Polymer Composite Material Selection Practice: Material selection is an important aspect of design. Often the success of the design
2、is critically dependent on a material or materials performing as desired. This is especially true in the case of advanced composite materials with polymer matrices reinforced with carbon or aramid (Kevlar) fibers. The important considerations necessary for a proper selection of a fiber-reinforced po
3、lymer composite material in NASA spacecraft and satellite structures include fiber material, fiber reinforcement form, fiber volume, matrix material, ply lamination, processing, cost, database, health and safety factors and end-item properties.Programs that Certify Usage: N/ACenter to Contact for In
4、formation: GSFCImplementation Method: This Lesson Learned is based on Reliability Guideline Number GD-ED-2210 from NASA Technical Memorandum 4322A, NASA Reliability Preferred Practices for Design and Test.Benefit:Proper selection of the fiber, fiber-reinforcement form, and polymer matrix will produc
5、e a material system that 1) satisfies design property requirements thermal/physical/mechanical), 2) facilitates Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-fabrication processes (lay-up and cure) and 3) minimizes program risks (cost, schedule, an
6、d technical).Implementation Method:OverviewUnlike a monolithic, homogenous material or an alloy, a composite is composed of two or more materials that retain their identity on the macroscopic level. Materials composing a composite can be classified as a reinforcement or strengthening phase and a mat
7、rix or binder phase. Reinforcement materials can be ceramics, polymers, or wires. Reinforcement forms can be continuous fibers, discontinuous or chopped fibers, whiskers, particles, platelets, etc. Matrix materials can be polymers, metals, or ceramics.The primary consideration of this guideline is f
8、iber reinforcements and thermoset polymer matrices in the most common product form, a prepreg (pre-impregnated and partially cured) sheet or ply. This is done out of practical considerations, since these composites possess the highest structural efficiency (specific properties) and are the most high
9、ly developed in terms of processing methods and material characterization (data base). Much of the information in the guideline is, however, relevant to other manufacturing forms and methods, such as Resin Transfer Molding, Filament Winding, Fiber Placement, Pultrusions, and Injection Molding.The co
10、ntent by volume of fibers in the composite is a critical parameter from which the composite derives thermo/physical/mechanical properties. A body of science called micromechanics references 1-4 exists to predict the properties of composites as a function of fiber volume given the material properties
11、 of the reinforcement and matrix. Micromechanics will not be discussed in this article.Fiber reinforcement in a ply can be unidirectional or multidirectional. The latter applies to woven and non-woven fabrics. Choice of reinforcement form is a degree of freedom that can result in better processing a
12、nd labor savings in part fabrication.Plies with either or both reinforcement scheme are stacked and cured to make a laminate. The ply fiber angles in the laminate are oriented to satisfy application design requirements (stiffness, strength, thermal expansion, etc.). Laminate design/analysis methods
13、will not be covered in this article. Information on this subject can be found in many publications references 5-8.The matrix phase is typically the material that most affects the processing, and most directly the curing of the composite. Choices for polymer matrices include a variety of epoxies, pol
14、yimides, and others. The choice of polymer matrix determines to a great degree the operational temperature limits for the composite. The matrix phase also affects other physical properties, such as outgassing and moisture diffusion.Provided by IHSNot for ResaleNo reproduction or networking permitted
15、 without license from IHS-,-,-Lastly, there are additional considerations that include economic, experience (flight history), and safety considerations that factor into the selection of a composite material. Table 1 summarizes relevant selection considerations for a fiber-reinforced composite with a
16、 polymer matrix. A more detailed discussion of the selection considerations follows.Table 1. Composite Selection Considerations 1. Fiber Considerations a. Thermo/physical/mechanical properties and relevance to end applicationb. Ply thickness and tow size availabilityc. Ply flexibility and part curva
17、tured. Sizing and surface treatments for matrix bonding and wettinge. Cost, availability, lead time, and stable supply source2. 3. Reinforcement Considerations a. Part curvatureb. Ply thicknessc. Laminate ply orientationsd. Machininge. Weaving styles (drape) and weaving vendorsf. Cost, availability,
18、 lead time, and stable supply source4. 5. Resin Considerations a. Fiber sizing compatibility and wettingb. Cure temperature and related items: laminate residual stresses, tooling expansion, upper use temperature, composite glass transition temperature (Tg), and microcrackingc. Prepeg handling charac
19、teristics: tack, drape, outlifed. Flow characteristics and processing methode. Mechanical properties: shear and tensile strength, modulus and strain compatibility with the reinforcing phasef. Physical properties: outgassing, moisture absorption/diffusivity/swelling, othersg. Toxicity and health conc
20、ernsh. Cost, availability, lead time, and stable supply source6. 7. Other Considerations (Composite Level) a. Material characterization data baseb. Flight historyc. Cost, availability, lead time, and stable sourceProvided by IHSNot for ResaleNo reproduction or networking permitted without license fr
21、om IHS-,-,-Fiber SelectionThe designer or material specialist has a wide range of fibers from which to make a selection. Often a fiber is selected because of physical properties. For example, graphite or carbon fibers are electrically and thermally conductive, while aramid (Kevlar) and glass fibers
22、are non-conductive. In certain applications, such as an antenna reflector, electrical conduction is required. Hence, graphite (carbon) fibers are generally chosen for reflector-type applications. In other applications, for example a radome, radar transmissibility is desired. Here, Kevlarand glass fi
23、bers are the materials of choice.Fiber selection should also consider mechanical and thermal properties. The salient mechanical properties are modulus and strength. Those for thermal properties include coefficient of thermal expansion (CTE) and thermal conductivity. Table 2 presents typical properti
24、es of some commercially available fibers presently utilized for space and spacecraft structures.Table 2. Typical Fiber Properties (Axial Direction) Trade Name/ TypeYoungs Modulus (Msi)Tensile Strength (Ksi)CTE (PPM/F)Thermal Conduct. (BTU/hr- ft-F)Density (Lb/in3)T300 33.5 530 -0.3 5 0.064 AS4 33.5
25、530 0.065 IM7 41.1 710 -0.5 9 0.065 T50 56.4 350 -0.55 40 0.0654 UHMS 64 550 0.067 P75S 75 300 -0.72 107 0.072 P100S 105 325 -0.8 300 0.078 Kevlar49 18 525 -2.2 5.3 0.052 E-glass 10.5 500 2.8 0.56 0.094 S2-glass 12.6 665 3.1 0.090 Quartz 10 500 0.3 0.0795 K1100 130-145 350-550 -0.9 550-676 .0777-.08
26、13 M46J 63.3 611 -0.5 0.0665 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-M50J 69 569 -0.55 57 0.0672 M55J 78.2 583 -0.61 90 0.690 M60J 85.3 569 -0.61 88 0.0694 XN-50 75 530 -0.8 100 0.0773 XN-70 105 530 -0.9 180 0.0780 XN-80 114 530 -0.9 235 0.07
27、80 Often figures of merit (FOMs) are used in fiber selection. FOMs are ratios of composite material properties references 5,6, for which the fibers may be unidirectional or cross plied depending on the application. Some typical FOMs are E/r, F/r, E/r/a, and Ek/r, where E, F, r, a, and k denote Young
28、s modulus, strength, density, coefficient of thermal expansion, and thermal conductivity, respectively.Broadly speaking, most structural applications fall into two categories: strength critical and stiffness critical. The choice of fiber must be attuned to the driving design requirement of the appli
29、cation. For example, in primary structure, strength is usually the dominant factor influencing fiber selection. Therefore, F/r is the appropriate FOM for fiber selection. Whereas in secondary structure having vibration frequency and/or deflection requirements, stiffness may be the dominant selection
30、 factor. In this case E/r is the pertinent FOM. For dimensionally stable applications, E/r/a is the meaningful FOM. For thermal applications, Ek/r is a relevant FOM.Tensile strength and modulus are controlled principally by the fibers in the composite. Compressive and shear properties, however are d
31、erived from both the fiber and matrix and the interface (bond) between them.The thickness of a ply or layer is also determined by the fibers, more specifically the fiber diameter and the number of fibers in bundle or tow. Thin plies (2.5 mils or less) require low tow counts (500 to 1000 filaments),
32、which are not available for some fibers.Ply flexibility, which is a function of fiber modulus and ply thickness, should be considered with respect to part curvature. For example, the brittleness and thickness of a ply with ultra-high modulus carbon fiber may preclude its use in fabricating a deeply
33、curved part.The choice of fiber also can impact the type of reinforcement form possible. For example, ultra-high modulus graphite fibers may present weaving difficulties, which can preclude the availability of certain fabric styles with tight weaves. The availability of fabric woven with high-modulu
34、s fibers has improved greatly in the recent past.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Lastly, fiber selection must consider wetting, bonding, and material compatibility of the matrix resin. A coating or sizing on the fibers is applied for
35、these purposes. In material selection, one should be aware of the importance of a proper coupling agent between the fibers and resin. This is particularly important for carbon fibers. Coupling agents for carbon fibers are usually proprietary formulations of the fiber producer or the prepreg vendor.R
36、einforcement FormChoices of fiber reinforcement forms include unidirectional and multidirectional. Selection aspects of each form are discussed below.The most commonly used product containing unidirectional fiber reinforcement is prepreg tape. Unidirectional tape has collimated bundles of fibers cal
37、led tows, which run in the length or long direction of the tape. Unidirectional tape gives the ability to tailor the fiber orientations from layer to layer in a laminate. This results in design flexibility. Also, the widest choice of fibers are available in unidirectional tape.Unidirectional tape is
38、 available in a range of widths. The choice of tape width can facilitate lay-up and can promote efficient material usage. An example of the latter is less wasted material from cutting ply pattern details. When laying up unidirectional tape, the continuity of the fibers should be maintained. End-to-e
39、nd butt splices that result in fiber discontinuity should be avoided. On the other hand, butting the sides of adjacent layers with parallel fibers is permissible.Use of unidirectional tape to produce parts with deep, double curvature can prove difficult. Lay up may be facilitated by cutting the tape
40、 into narrow strips. However, labor increases in doing so.The most common multidirectional reinforcement form is woven fabric. Fabric weave styles can have drastically different draping characteristics, which is an important characteristic in making doubly-curved parts and those with integral flange
41、s and bends. Harness-satin weaves are more drapable than plain weaves. Other desirable characteristics of fabrics include bidirectional properties at a minimum gage, resistance to microcracking (matrix splitting between fibers), and good machining characteristics.In a woven fabric, the fibers will b
42、e curved to some degree or another depending on the weave style. Fiber curvature results in decreased composite moduli and strengths, especially in compression. Weaving also reduces the volume available for fibers in the composite. As a result, laminates made from woven fabric composites are less st
43、ructurally efficient than those made from unidirectional tapes.Recent manufacturing advancements have made possible ultra-thin unidirectional and fabric reinforced composites. These materials have been manufactured in a thickness of one mil or less for unidirectional prepreg and about two mils for f
44、abric prepreg. Such ultra-thin composites are Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-especially attractive for lightly-loaded, minimum gage structures. Here, the weight of the structure and, importantly, the weight savings potential of the c
45、omposite are directly proportional to layer thinness. Other benefits of ultra-thin composites include less micro cracking under thermal cycling and the potential for more homogeneous laminate stacking sequences for a given laminate thickness. The latter usually results in improved strength due to a
46、more thorough interspection of ply-angle orientations in the laminate stacking. The disadvantages of ultra-thin composites are increased handling difficulty, cost, and lead time.Fabric prepregs may offer labor efficiency in lay ups. For example, a 0/90 (Fiber angles are 0 degrees and 90 degrees) lam
47、inate needs 50% fewer plies to be laid up using an orthogonally woven fabric, since fibers in two directions are obtained with every ply applied. Some fabric prepregs are available with hybrid fibers, such as carbon warp-yarns and Kevlarfill-yarns, which may provide more potential design solutions.T
48、heoretically, the possibilities of fiber type and weave style are unlimited. Practically, off-the-shelf choices are limited. Certain non-standard fabrics made from high- and ultra high-modulus fibers can be obtained from specialty weavers. The design use of such fabrics should be tempered by cost an
49、d lead-time considerations.Matrix SelectionThe choice of prepreg matrix resin is of critical importance in fabrication. The composite part quality is extremely dependent on the resin matrix and its handling and cure processing characteristics. The matrix vendor is usually the vendor that makes the prepreg, i.e., incorporates the fibers in the matrix.Handling characteristics are most importa
