Volume 21 - Composites Part 2 pot

Fig. 13 Tensile modulus comparison—fiber-epoxy tape versus fabric Properties such as tack, flow, gel time, and drape are critical to proper selection of material form. Tack should be adequate to allow the prepreg to adhere to prepared molding surfaces or preceding plies for a lay-up, but light enough to part from the backing film without loss of resin. Tack qualities can be specified to require the prepreg to remain adhered to the backing until a predetermined force is applied to peel it off. Prepregs with excessive tack generally are difficult to handle without disrupting resin distribution and fiber orientation or causing a roping (fiber bundling) of the reinforcements. Constituents are not reproducible because undetermined amounts of resin are removed when the release film or backing is separated from prepreg. In general, all the disadvantages of wet lay-up systems are inherent to overly tacky prepregs. Prepregs with no tack are either excessively advanced, have exceeded their normal storage life, or are inherently low in tack. Such materials cannot attain adequate cured properties and should be discarded. Exceptions are silicones and some polyimides, which can only be prepared with no tack. Lay-ups with these materials are limited to those situations where lower mechanical properties can be tolerated in exchange for improved heat resistance or electrical properties. A lack of tack in thermoplastic prepregs does not interfere with their consolidation, provided that they can be heated to the melting point of the polymer during processing. Flow is the measure of the amount of resin squeezed from specimen as it cures (under heat and pressure) between press platens. Flow measurement indicates the capability of the resin to fuse successive plies in a laminate and to bleed out volatiles and reaction gases. Flow can be an indicator of prepreg age or advancement. It is often desirable to optimize resin content and viscosity to attain adequate flows. In some cases, prepreg flow can be controlled by adding thickening or thixotropic additives to the resin. Gel time, the measure of the time a specimen remains between heated platens until the resin gels or reaches a very high viscosity stage (Ref 11), can be an indicator of the degree of prepreg advancement. The useful life of prepregs is limited by the amount of staging or advancement. Most prepregs are formulated to attain a useful life of ten days or more at standard conditions. Life can be prolonged by cold storage, but each time the prepreg is brought to thermal equilibrium at lay-up room temperatures, useful life is shortened. Gel time measurements are used as quality control verifications (Ref 11). Drape is the measure of the formability of a material around contours, which is critical to fabrication costs. Tape drapability is typically measured by the ability of a prepreg to be formed around a small-radius rod. The pass/fail criterion for drape is the ability to undergo this forming without incurring fiber damage. This measurement translates to the ability of fabrication personnel to form the prepreg to complex tools. Of the physical properties mentioned, drape is one property where tapes differ from other prepreg forms. Tapes are typically less drapable than fabric forms of prepreg, and this difference must be considered when specifying a prepreg form for manufacture. It is essential that prepregs for structural applications be staged to desirable tack and drape qualities. The combination of manageable tack and drape is sometimes best attained from woven satin fabric-reinforced prepregs. Cross-plied or multiplied prepregs are sometimes used to provide transverse strengths for lay-ups of broad goods. The term “broad goods” refers to wide prepreg tape (>305 mm, or 12 in.) that consists of one or more plies of tape oriented at 0° or off- axis to each other. Reference cited in this section 11. B.D. Agarwol and L.J. Broutman, Analysis and Performance of Fiber Composites, John Wiley & Sons, 1980 Fabrics and Preforms Multidirectional Tape Prepregs When a number of tape plies are laminated at several orientations, the strength of the composite increases in the transverse direction. As the number of oriented plies is increased, the isotropic strength is approached asymptotically. Multidirectional tapes can be manufactured with multiple plies of unidirectional tape oriented to the designer's choice. These tapes are available in the same widths and package sizes as unidirectional tape, with varying thickness. Up to four or five plies of tape, with each ply typically being 0.125 mm (0.005 in.), can be plied together in various orientations to yield a multidirectionally reinforcing tape. Figure 14 depicts the difference between unidirectional and multidirectional tapes. Fig. 14 Unidirectional versus quasi-isotropic lay-ups By using a preplied quasi-isotropic prepreg, the fabricator can avoid a substantial lay-up cost. However, preplied prepregs are typically more costly than unidirectional prepregs because of the additional work necessary to ply the tape. Multioriented prepreg performance can be accurately predicted from test data that have been generated on these configurations. Tables 5 and 6 show typical mechanical property data for these lay-ups compared with other structural materials. Table 5 Comparative strength/weight versus material form Strength, 0° Strength, 0°/±45°/90° Strength/density, 0° Strength/density, 0°/±45°/90° Material (a) MPa ksi MPa ksi Density, g/cm 3 10 6 cm 10 6 in. 10 6 cm 10 6 in. Graphite High-strength, low modulus 2.2 0.32 0.73 0.11 1.55 14.3 5.63 4.8 1.9 High-strength, intermediate modulus 2.4 0.35 0.80 0.12 1.52 … … … … Low-strength, high modulus 1.2 0.17 0.43 0.06 1.63 15.1 5.94 2.7 1.1 S-glass 1.8 0.26 0.76 0.11 1.99 9.2 3.6 3.9 1.5 E-glass 0.82 0.12 0.52 0.075 1.99 4.2 1.7 2.7 1.1 Aramid 1.5 0.22 0.39 0.057 1.36 10.9 4.29 2.9 1.1 Aluminum … 0.41 0.059 … 2.77 … 1.5 0.59 … Steel … 2.1 0.30 … 8.00 … 2.6 1.0 … (a) In epoxy-resin matrix Table 6 Comparative stiffness/weight versus material form Stiffness, 0° Stiffness, 0°/±45°/90° Stiffness/density, 0° Stiffness/density, 0°/±45°/90° Material (a) MPa ksi MPa ksi Density, g/cm 3 10 6 cm 10 6 in. 10 6 cm 10 6 in. Graphite High-strength, low modulus 0.15 0.022 0.046 0.0067 1.55 0.98 0.39 0.30 0.12 High-strength, intermediate modulus 0.17 0.025 0.065 0.0094 1.52 1.14 0.45 0.43 0.17 Low-strength, high modulus 0.20 0.029 0.052 0.0075 1.63 1.25 0.49 0.33 0.13 S-glass 0.055 0.0080 0.0025 0.0036 1.99 0.28 0.11 0.13 0.051 E-glass 0.041 0.0059 0.018 0.0026 1.99 0.21 0.083 0.09 0.035 Aramid 0.073 0.011 0.025 0.0026 1.36 0.59 0.23 0.19 0.075 Aluminum … 0.069 0.010 … 2.77 … 0.25 0.098 … Steel … 0.19 0.028 … 8.00 … 0.24 0.094 … (a) In epoxy-resin matrix Cross-plied tapes offer controlled anisotropy, that is, properties can be varied and modified in selected directions, but these tapes are generally more expensive than unidirectional tapes because of the additional manufacturing steps. This disadvantage is often overcome, however, by the cost savings from using a preplied tape in part lay-up. Properties are controlled by the number of plies of tape oriented in critical directions. Figures 15 and 16 show typical changes in tensile properties and when ply orientation is changed. Fig. 15 Tensile modulus of elasticity of carbon-epoxy laminates at room temperature Fig. 16 Ultimate tensile strength of carbon-epoxy laminates at room temperature Fabrics and Preforms Tape Manufacturing Processes Tape manufacturing processes fall into three major categories: hand lay-up, machine-cut patterns that are laid up by hand, and automatic machine lay-up. Hand Lay-Up. Historically, tapes have primarily been used in hand lay-up applications in which the operator cuts lengths of tape (usually 305 mm, or 12 in.) and places them on the tool surface in the desired ply orientation. Although this method uses one of the lower-cost forms of reinforcement and has a low facility investment, it results in a high material scrap rate, fabrication time/cost, and operator-to-operator part variability. The scrap factor on this type of operation can exceed 50%, depending on part complexity and size. Auxiliary processing aids should be used extensively to expedite the lay-up operation and to use molds and tools more efficiently. It is customary to presize the laid-up ply before it is applied to the mold. Usually, an auxiliary backing is fixed in position on the lay-up tool, which is sometimes equipped with vacuum ports to anchor the backings. Plies are oriented to within ±1° using tape-laying heads, or manually, using straight edges, drafting machine dividing heads (Ref 4) or ruled lines on the table (Ref 4). Indexes or polyester film templates also can be used to reduce the lay-up times on molds. The presized plies are first laid up and oriented on the templates. When the mold is available for the lay-up, the plies are positioned on them and transferred. Positioning is achieved by using the references used for indexing. Reference posts for the templates are sometimes located on the mold; corresponding holes in the templates fit exactly over the posts. In some cases, the templates are shaped so that they fit only one way in the mold. The plies are rubbed out from the templates onto the mold, the mold is removed, the bleeder systems are laid up, and the assemblies are bagged and cured. Machine-Cut Patterns. More advanced technology uses machine-cut patterns that are then laid up by hand. This method of manufacture involves a higher facility cost but increases part fabrication output and reduces operator error in lay-up. The right-sized pattern can be automatically cut in one or more ply thicknesses using wider tapes of up to 1500 mm (60 in.), which are potentially more economical to fabricate. The cut is normally done on a pattern-cutting table, where up to eight plies of material are laid up. Various templates are located on top of the lay-up, and the most economical arrangement is determined by matching templates. The patterns are then cut and stored until required. Cutting of plies can be done by laser, water jet, or high- speed blades. The machine-cut method is often used in modern composites shops and is best suited for broad goods and wide tapes. A typical cutting machine is shown in Fig. 17. Fig. 17 Gerber cutting machine Automatic Machine Lay-Up. Numerically controlled automatic tape-laying machines, especially in the aerospace industry, are now programmed to lay down plies of tape in the quasi- isotropic patterns required by most design applications. In addition to being able to lay down a part in a short time and with reduced scrappage, robotics also lend consistency to lay- down pressures and ply-to-ply separations. These advantages are rapidly causing the aerospace industry to switch from hand lay-up operations. Automatic tape layers are evolving from being able to handle only limited tape widths and simple tool contours to being able to fabricate large, heavily contoured parts. Additional information is provided in the article “Automated Tape Laying” in this Volume. Reference cited in this section 4. G. Lubin, Handbook of Composites, Van Nostrand Reinhold, 1982 Fabrics and Preforms Prepreg Tow Another form of prepreg is a towpreg, which is either a single tow or a strand of fiber that has been impregnated with matrix resin. The impregnated fiber is typically wound on a cardboard core before being packaged for shipment. Because a towpreg is potentially the lowest-cost form or prepreg, it is of significant interest to designers. It also lends itself to potentially low- cost manufacturing schemes, such as filament winding. Towpreg is being considered by filament winders as a way to combine the advantages of low-cost part manufacture and high-performance matrix resins. The fibers that are typically used are shown in Table 7. Table 7 Fiber tow characteristics Before impregnation Yield/tow Filament size Material m/kg yd/lb μm μin. Graphite (1000–12,000 filaments/tow) 300–1200 150–600 5–10 200–390 Fiberglass (2450–12,240 filaments/tow) 490–2400 245–1200 4–13 160–510 Aramid (800–3200 filaments/tow) 2000–7850 980–3900 12 470 Manufacture. Most towpregs are converted in a solvent-coating process (Fig. 18) in which base resin is first dissolved in a mix containing 20 to 50% solvent and resin. The dry fiber is then routed through the solvent-resin mix and dried in a tower consisting of one or more heated zones. Resin content is controlled either by using metering rolls after impregnation or by adjusting the solvent-resin ratio. This drying step reduces volatiles and advances the resin so that the towpreg will not adhere to itself during unspooling in part manufacture. Towpregs can also be manufactured in a hot-melt operation by filming resin on substrate paper, impregnating strands between two layers of filmed paper, and then advancing the resin to an intermediate point between freshly mixed and cured (B-staging) on a prepreg line. However, this tends to result in a higher-cost towpreg. Fig. 18 Typical towpreg manufacturing process Forms. Table 8 shows typical form parameters that a manufacturing shop might specify. A designer must evaluate the size and complexity of the part being designed before selecting material parameters. Resin content will determine part mechanical performance and thickness by determining fiber volume, assuming that little or no resin is lost in the curing process. Tow width, which is important in establishing ply thickness and gap coverage, can be modified during lay- down. Package size can be important to manufacturing personnel, especially when more than one spool is used in the manufacturing process. In such cases, manufacturing personnel often try to match the sizes of spools that are used in order to minimize spool doffs (changes) and splices in the manufactured part. Table 8 Towpreg form parameters Parameter Typical range Strand weight per length, g/m (lb/yd) 0.74–1.48 (0.00150–0.0030) Resin content, % 28–45 Tow width, cm (in.) 0.16–0.64 (0.06–0.25) Package size, kg (lb) 0.25–4.5 (0.5–10) To determine the mechanical properties of a towpreg, it can be tested by a single-strand type of test or by winding tows on a drum to specified thicknesses and then laying up laminates from this wind. Mechanical properties of towpregs are comparable to those of tapes, if they are cured under autoclave conditions. Filament- sound structures that are not autoclave cured will typically have higher void contents than autoclave- cured parts. Applications. The two basic uses for towpregs are as a filler in hard-to-form areas and in joints of structural components such as I-beams (Fig. 19) and as a replacement for low-performance filament-winding resins in filament-winding operations. Using a towpreg as a filler material in areas where tape or fabric prepregs will not lay down involves hand lay-up. Fig. 19 Towpreg used a filler in an I-beam Most of the development in towpreg technology has been in the area of winding, particularly using a graphite- epoxy towpreg. The six-axis winding machine (Fig. 20) unspools the towpreg bundles and collimates them into a band of prepregs before laying down a unified band. The band of prepreg can be laid into complex cylindrical or nongeodesic forms, as shown in Fig. 21. This technology has the potential of making significant inroads into complex low- cost aerospace-grade part manufacture and may revolutionize the amount of composites and types of techniques used in aircraft fuselage manufacture. Additional information on towpreg is provided in the article “Filament Winding” in this Volume. Fig. 20 Six-axis winding machine Fig. 21 Complex structure wound with towpreg on six-axis winding machine Fabrics and Preforms Acknowledgments The information in this article is largely taken from the following articles in Composites, Volume 1, Engineered Materials Handbook, ASM International, 1987: • W.D. Cumming, Unidirectional and Two-Directional Fabrics, p 125–128 • F.S. Dominguez, Unidirectional Tape Prepregs, p 143–145 • F.S. Dominguez, Multidirectional Tape Prepregs, p 146–147 • F.S. Dominguez, Prepreg Tow, p 151–152 • F.S. Dominguez, Woven Fabric Prepregs, p 148–150 • F.P. Magin III, Multidirectionally Reinforced Fabrics and Preforms, p 129–131 • W.T. McCarvill, Prepreg Resins, p 139–142 Fabrics and Preforms References 1. Textiles, Vol 7.01 and 7.02, Annual Book of ASTM Standards 2. “Textile Test Methods,” Federal Specification 191a, 1978 3. C. Zweben and J.C. Norman, “Kevlar” 49/ “Thornel” 300 Hybrid Fabric Composites for Aerospace Applications, SAMPE Q., July 1976 4. G. Lubin, Handbook of Composites, Van Nostrand Reinhold, 1982 5. H. Lee and K. Neville, Handbook of Epoxy Resins, McGraw-Hill, 1967 6. L.S. Penn and T.T. Chiao, Epoxy Resins, Handbook of Composites, G. Lubin, Ed., Van Nostrand Reinhold, 1982 p 57–88 7. P.F. Bruins, Epoxy Resin Technology, Wiley- Interscience, 1968 8. K.L. Mittal, Ed., Polyimides, Vol 1, Plenum, 1984 9. A. Knop and L.A. Pilato, Phenolic Resins, Springer-Verlag, 1985 10. K.L. Forsdyke, G. Lawrence, R.M. Mayer, and I. Patter, The Use of Phenolic Resins for Load Bearing Structures, Engineering with Composites, Society for the Advancement of Material and Process Engineering, 1983 11. B.D. Agarwol and L.J. Broutman, Analysis and Performance of Fiber Composites, John Wiley & Sons, 1980 Fabrics and Preforms Selected References • F.K. Ko and G W. Du, Processing of Textile Preforms, Advanced Composites Manufacturing, T.G. Gutowski, Ed., John Wiley & Sons, 1997, p 157–205 • M.M. Schwartz, Composite Materials, Vol 2, Processing, Fabrication, and Applications, Prentice Hall, 1997, p 114–125 [...]... (1 .2) 32 (1 .25 ) 18.34 (2. 66) 21 . 10 (3.1) 150.5 (21 . 8) 20 . 62 (3.0) 145.1 (21 . 0) I-beam 2 Braid/lay-in Glass/glass 60 460 (18.1) 305 ( 12. 0) 31 (1 .2) 33 (1.3) 30.54 (4.43) 30.54 (4.43) 23 7.9 (34.50) 29 .44 (4 .27 ) 176.4 (25 .58) I-beam 3 Braid/lay-in Glass/carbon 65 447 (17.6) 305 ( 12. 0) 32 (1 .25 ) 33 (1.3) 44. 82 (6.5) 68 .26 (9.9) 29 2.0 ( 42. 3) 68.67 (9.96) 175.9 (25 .51) References cited in this section 26 D.E... 1.66 g/ cm3 and a fiber volume fraction of 75% Table 7 Properties of two-dimensional braided S -2 fiberglass-epoxy composites Compressive strength Braid angle, degree Tensile strength In-plane shear Hoop Long Hoop Long MPa ksi MPa ksi MPa ksi MPa ksi MPa ksi 1 320 1 92 21 3 700 1 02 220 32 55 8 89 125 0 1 82 83 12 380 55 100 14 75 11 86.75 1030 149 … … 330 48 … … … … 82. 50 730 106 … … 27 5 40 … … … … 78 The... pattern AS-4, AS-4, Celion, AS-4, 12K Celion, T300, 30K T300, Eight 3K 1 × 1 6K 1 × 1 6K 1 × 1 1 × 1 12K 1 × 1 1×1 harness satin fabric 68 68 56 68 68 68 65 Vf, % 736.8 841.4 857.7 1067 .2 121 9 .8 655.6 517.1 (75.000) Tensile ( 122 .0) ( 124 .4) (154.790) (176.910) (96.530) strength, MPa (106.8) (ksi) 83.5 119.3 87.8 114.7 113.1 97.8 (14 .2) 73.8 (10.7) Elastic (17.3) ( 12. 7) (16.6) (16.4) modulus, GPa ( 12. 1) (106... 114.8 126 .0 71.4 121 . 4 71.4 … 69.0 (10.000) Short-beam (16.6) (18 .2) (10.3) (17.600) (10.350) shear, MPa (ksi) 1.051 0.968 0.980 0.874 0.875 0.045 Poisson's ratio 0.945 885.3 739.8 … 1063.3 … 813.5 689.5 Flexural (107.3) (154 .21 0 ) (117.990) (100.000) strength, MPa ( 128 .4) (ksi) 84.5 95 .2 … 1385 .2 … 77.5 (11 .2) 65.5 (9.5) Flexural (13.8) (20 .1) modulus, GPa ( 12. 3) (106 psi) ±15° ±15° ±13° ±17.5° 20 ° 0°... damage than did the laminated composites under drop weight impact test In the study of three- dimensional braid commingled Celion 3KPEEK thermoplastic composites, it was found, as shown in Fig 14, that the compression-after- impact-strength of the three-dimensional composites was less sensitive than for the state- of-the-art, unidirectional tape laid-up graphite- PEEK composites The most drastic difference,... 405.7 (68.9) Tensile strength, (33 .2) (140.8) ( 52. 7) MPa (ksi) 97.8 (14 .2) 50.5 (7.3) 126 .4 (18.3) 76.4 117.4 (17.0) 82. 4 ( 12. 0) Elastic modulus, (11.1) GPa (106 psi) … 179.5 … 22 6.4 … 385.4 (55.9) Compressive (26 .0 ( 32. 8) strength, MPa (ksi) … 38.7 (5.6) … 56.6 (8 .2) … 80.8 (11.7) Compressive 6 modulus, GPa (10 psi) 813.5 465 .2 647 .2 (93.9) 508.1 816.0 (118.3) 6 32. 7 (91.8) Flexural strength, (118.0)... laminated carbon-PEEK composites Properties of Three-Dimensional Braid Composite I-Beams To illustrate the design flexibility and the structural properties of the three-dimensional braid net-shape composite, a study was carried out by S.S Yau, T.W Chu, and F.K Ko on three-dimensional braided E-glass-polyester I-beams (Ref 33) It was demonstrated that mechanical properties of the net-shape composites can... braided graphite-epoxy composites ELC EHT Braid angle, degree Vf, % ELT νLHT νLHC νHLT 6 6 6 GPa 10 psi GPa 10 psi GPa 10 psi 33.8 61.4 8.9 62. 7 9.1 6.8 0.98 0.56 0.64 0.044 45 29 .3 49.0 7.1 49.6 7 .2 15 .2 2 .20 0.43 0.45 0.088 63 56.3 … … 52. 4 7.6 43.6 6. 32 … 0.13 0.110 80 Vf, fiber volume; E, modulus of elasticity; ν, Poisson's ratio In another study by D Brookstein and T Tsiang (Ref 28 ), it was demonstrated,... the Mechanics of Composites Review, Oct 1983, Air Force Materials Laboratory 31 F Ko and D Hartman, Impact Behavior of 2- D and 3-D Glass/Epoxy Composites, SAMPE J., July/Aug 1986, p 26 29 32 F.K Ko, H Chu, and E Ying, Damage Tolerance of 3-D Braided Intermingled Carbon/ PEEK Composites, Advanced Composites: The Latest Developments, Proceedings of the Second Conference on Advanced Composites, ASM International,... the Mechanics of Composites Review, Oct 1983, Air Force Materials Laboratory 31 F Ko and D Hartman, Impact Behavior of 2- D and 3-D Glass/Epoxy Composites, SAMPE J., July/Aug 1986, p 26 29 32 F.K Ko, H Chu, and E Ying, Damage Tolerance of 3-D Braided Intermingled Carbon/ PEEK Composites, Advanced Composites: The Latest Developments, Proceedings of the Second Conference on Advanced Composites, ASM International, . 1.63 15.1 5.94 2. 7 1.1 S-glass 1.8 0 .26 0.76 0.11 1.99 9 .2 3.6 3.9 1.5 E-glass 0. 82 0. 12 0. 52 0.075 1.99 4 .2 1.7 2. 7 1.1 Aramid 1.5 0 .22 0.39 0.057 1.36 10.9 4 .29 2. 9 1.1 Aluminum. high modulus 0 .20 0. 029 0.0 52 0.0075 1.63 1 .25 0.49 0.33 0.13 S-glass 0.055 0.0080 0.0 025 0.0036 1.99 0 .28 0.11 0.13 0.051 E-glass 0.041 0.0059 0.018 0.0 026 1.99 0 .21 0.083 0.09 0.035. Graphite (1000– 12, 000 filaments/tow) 300– 120 0 150–600 5–10 20 0–390 Fiberglass (24 50– 12, 240 filaments/tow) 490 24 00 24 5– 120 0 4–13 160–510 Aramid (800– 320 0 filaments/tow) 20 00–7850 980–3900