Multiphase Matrix Polymers for CarbonâFiber Composites

22 Multiphase Matrix Polymers for Carbon-Fiber Composites Willard D. Bascom , S-Y. Gweon , and D. Grande Downloaded by UNIV OF BATH on July 4, 2016 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/ba-1993-0233.ch022

1,‡

1

2

Materials Science and Engineering Department, University of Utah, Salt Lake City, UT 84112 Boeing Materials Technology, Boeing Commercial Airplanes, Seattle, WA 98124

1

2

The

effect of different matrix polymers on the impact resistance of

carbon-fiber-reinforced

polymers is reviewed. These polymers include

unmodified, tetrafunctional epoxies, elastomer-modified epoxies, epox ies plasticized with low levels of thermoplastics, thermoplastics, and thermoplastic-modified thermosets (ΤΜΤ).

The results of some recent

work on the impact resistance of TMT matrix composites is discussed. The

TMTs were of two types: thermoplastic particles interlayered

between plies and co-continuous interpenetrating network polymers. Suppression of interlaminar longitudinal cracking was found to corre late with improved resistance to impact damage.

STRUCTURAL APPLICATIONS O F C A R B O N - F I B E R - R E I N F O R C E D POLYMERS ( C F R P ) in aircraft, space vehicles, and automobiles can be seriously limited by low "damage resistance." This term generally refers to impact damage inflicted by low-velocity impacts ( ~ 100 m/s) such as runway debris striking an aircraft during takeoff or landing or a careless mechanic dropping a tool on a composite wing. This type of impact damage is especially serious because there is extensive internal damage but very litde surface damage, thus making inspection difficult, i f not impossible. Significant reduction of structural performance—"low damage tolerance"—can result. Work at the N A S A Langley Research Center (1) during the late 1970s and early 1980s showed that low-velocity impact damage to C F R P could cause a loss of up to * Deceased.

0065-2393/93/0233-0519$06.00/0 © 1993 American Chemical Society

Riew and Kinloch; Toughened Plastics I Advances in Chemistry; American Chemical Society: Washington, DC, 1993.

Downloaded by UNIV OF BATH on July 4, 2016 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/ba-1993-0233.ch022

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RUBBER-TOUGHENED PLASTICS

50% in compression strength before any surface damage was detectable (Figure 1). Low damage tolerance compromises the potential use of C F R P as weight-reducing metal substitutes on airframe structures. Consequently, considerable research has been and continues to be devoted to understanding the failure mechanisms associated with low-velocity impact and finding means of improving impact resistance. Early investigations demonstrated that impact damage primarily consisted of delamination between the carbon-fiber plies and transverse cracking through the plies. Furthermore, it was determined that this damage occurred because of the low fracture energy of the highly cross-finked epoxy matrix polymers. Figure 2 shows a section cut through the impact-damaged region of a low-fracture-energy matrix resin laminate (2). The extensive longitudinal and transverse cracking is clearly evident. Replacement of these epoxy polymers with higher fracture energy "tough" polymers significantly reduced the extent of impact damage with a corresponding improvement in postimpact compression strength. However, the tough matrix polymers available at that time were unsatisfactory for structural C F R P primarily because of their low compressive strength, especially under hot and wet conditions.

1

h

impact energy Figure 1. Schematic of the effect of low-level impact on the compressive strength of carbon-fiber composites with low-toughness matrix resins.



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Carbon-Fiber Composites

Figure 2. Photomicrograph of polished section through the impact-damage region of a CFRP having a brittle matrix. (Reproduced with permission from reference 2. Copyright 1986 Chapman and Hall.)

Modified Epoxies and Thermopfostics One route to increasing the fracture energy of epoxy and other thermosetting polymers is to incorporate a dispersed elastomeric phase. This technology has been successfully used to formulate high-peel-strength adhesives (3). Liquid elastomers, notably the carboxyl-terminated polybutadiene-acrylonitrile (CTBN), are added to the liquid epoxy. During cure an elastomer-epoxy copolymer phase separates to form micrometer-size inclusions. Numerous investigations of CTBN-modified epoxies (3-6) revealed that the dispersed phase increased the epoxy fracture energy from ~ 100 J / m to as much as 3500 J / m (6). These investigations found that the toughening mechanisms 2

2


522


Table I. Effect of Elastomer Modifiers on the Neat-Epoxy Fracture Energy and Composite Interlaminar Fracture Energy Material


Unmodified epoxy neat resin (Hexcel 205) Elastomer modified neat resin (Hexcel F-185) 205/glass cloth F-185/glass cloth 205/graphite cloth F-185/graphite cloth

Fracture Energy (kj/m ) 2

0.27 5.1 1.0 4.4 0.60 4.6

involved dilation of the inclusions and shear yielding of the epoxy matrix. The effectiveness of a C T B N elastomer on the fracture energy of an unreinforced difunctional epoxy and on the interlaminar fracture energy of glass- and carbon-fiber-reinforced composites is shown in Table I. The neat-resin toughness was increased by nearly a factor of 20 X . The interlaminar fracture energies of the glass- and carbon-fiber-reinforced composites were increased by ~ 4 X and ~ 8 X , respectively. The reduced elastomer effectiveness in the composites compared to neat resin was attributed to the fibers constraining the development of the crack-tip damage zone (7). Yee (8) demonstrated that for highly cross-linked epoxies the toughening action of the elastomers is considerably reduced because the shear-yield strength of the epoxies is too high to allow the dilation and shear-yielding deformations required for high toughness. In structural adhesives, the epoxies are difunctional with relatively low shear-yield strengths. However, for C F R P the matrix is typically a highly cross-linked tetrafunctional epoxy in which elastomer inclusions have very little effect on toughness. Consequently, elastomer-modified epoxies are not extensively applied in high-performance structural composites where a highly cross-linked and high-modulus matrix is critical. There have been efforts to modify the base epoxies by the addition of thermoplastic modifiers such as polyether sulfone, polycarbonate, and polyphenylene oxide. Because these thermoplastic additives have high glass-transition temperatures and high moduli, the high modulus of the tetrafunctional epoxy is not sacrificed and the toughness and, thus, the delamination resistance are enhanced. The effectiveness of this approach was marginal because of the low solubility (1-5%) of the thermoplastics in epoxy formulations. The increase in interlaminar fracture energy in C F R P is at best a factor of 2, which does not translate into an acceptable increase in impact resistance. In 3

In the past mode-I interlaminar fracture energy was used widely as a measure of impact-damage resistance. There is increasing evidence that mode-II interlaminar fracture energy is a better impact-resistance index. Nonetheless, mode-I interlaminar fracture energies and mode-I fracture energies of neat resins have proved useful in the development of damage tolerant composites.

3



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some instances it was possible to induce phase separation of the thermoplastic (9), but the resulting high viscosity and the slow thermoplastic dissolution and dispersion created serious practical processing problems. Another approach to improved impact-damage resistance is to replace thermosetting matrix materials with thermoplastics that are inherently tough. Previous experience with thermoplastics such as polysulfone were discouraging because of their tendency to creep under low stresses and their poor resistance to attack by organic solvents. However, currently available engineering thermoplastics, notably, polyetheretherketone ( P E E K ) , the polyamide-imides (Torlon), and polyphenylene sulfide (PPS), have low creep characteristics and are solvent resistant. Composites of these thermoplastic matrix polymers reinforced with carbon-fiber offer considerably higher damage resistance than the thermoplastic-modified epoxies (Table II). However, the high-temperature and high-pressure processing conditions required for thermoplastic matrix composites are totally different from the processing conditions for thermosetting polymers, and this difference would necessitate a major investment in new equipment. In addition, some of the thermoplastic polymers develop a degree of crystalhnity during processing that affects their mechanical properties and is very dependent on heating and cooling rates. Consequendy, for large structures (e.g., an airplane wing) the properties in a thin section may not be the same as in a thicker section because of different thermal histories. Capital investment and difficult processing conditions have deterred the use of thermoplastics as matrices i n C F R P , at least for large structures. Thermoplastics well may be the matrix of choice for the production of small components at high production rates using conventional and relatively inexpensive compression-molding techniques.

Multiphase Thermosetting Polymers A recently developed class of matrix polymers offers high toughness and can be processed using conventional methods already in place for thermosetting Table II. Polymer and Interlaminar Fracture Energies of a High-Modulus Epoxy and Typical Thermoplastics Fracture Energy (Unreinforced Polymer) Interlaminar Fracture Energy (kj/m ) (kj/m )

Polymer Tetrafunctional epoxy Polysulfone^ Polyetherimide^ Polyetheretherketone

a

0

a

c

2

2

0.159 2.50 3.30 4.80

0.262 1.200 0.935 2.10-2.70

W. D. Bascom, unpublished data. Reference 7. Reference 17.


524



Figure 3. Schematic of interleaving thermoplastic powder between preimpregnated plies. (Adapted from Toray Industries Inc., European Patent Application 87311364.1, December 23, 1987.) polymers. Similar to the elastomer-modified epoxy resins, enhanced toughness in the new polymers is achieved by incorporating a dispersed phase within the epoxy matrix. Also, like the CTBN-modified epoxies, the dispersed phase does not significantly affect the high modulus of the epoxy matrix or its heat distortion temperature, which are critical to the performance of structural C F R P . Thermoplastic-modified thermosetting ( T M T ) polymers involve the incorporation of thermoplastic domains within the thermosetting matrix. This morphology can be accomplished by distributing thermoplastic polymer particles between layers of epoxy-impregnated carbon-fiber. Particle diameters range in size from a few micrometers to submicrometers. One method of "interleaving" polymer particles is shown in Figure 3. A second way to develop a T M T polymer is by chemically dispersing the thermoplastic into the epoxy to form a co-continuous interpenetrating network morphology. The detailed chemistry of commercial T M T s is proprietary information. Further details regarding the T M T resins can be found in reviews by Recker et al. (10) and Kubel (II).

Damage Resistance of TMT Matrix Composites The impact resistance of three commercial TMT-matrix C F R P materials was studied using a repetitive impact technique in which an 8 X 5-cm laminate panel is end-supported as shown in Figure 4 and repetitively impacted with increasing impact energy. The panel stiffness was measured at each impact event. After various levels of cumulative impact energy, the panels were examined using ultrasonic C-scan to locate the damaged region. The laminate was cut around the damaged zone and then potted in a clear epoxy. Sections, 1.9 mm thick, were sequentially cut and polished. Each section was examined using fight microscopy to determine the extent and type of damage. Maps were then generated to show the extent of damage through the laminate. Details of this technique are given in reference 12.


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The change in stiffness for a 32-ply 0°-90° fiber orientation panel of IM6/3501-6 is shown in Figure 5. The 3501-6 matrix is a brittle, amine-cured tetrafunctional epoxy. From C-scans and microscopy of the sections through the damaged zone it was determined that the first sharp decline in stiffness at

Figure 4. Fixture used to support composite samples for répétitive impact testing.

6000

200

Cumulative Impact Energy (J) Figure 5. Stiffness vs. cumulative impact energy for IM6/3501-6, (0-90) . The open and filled points correspond to two different methods of calculating laminate stiffness (see reference 12). 8S



526


~ 25-J cumulative impact energy corresponds to the damage extending through the laminate thickness. The second drop in stiffness at ~ 150 J corresponds to the damage extending to the edges of the laminate. A damage map taken after 110-J cumulative impact energy is shown in Figure 6. Similar impact tests were done on 32-ply 0°-90° fiber orientation coupons of AS4/APC-2 (APC-2 is a product of ICI Ltd, United Kingdom, that is based on P E E K ) . The change in stiffness with increasing cumulative impact energy is shown in Figure 7. Unlike the IM6/3501-6, there is a gradual loss in stiffness with continued impact loading and the retention of stiffness of the P E E K matrix laminate was higher than for the 3501-6 matrix laminate. A damage map for the A S 4 / A P C - 2 material after a cumulative impact energy of 115 J is presented in Figure 8. Note that the extent of damage is much less than for the 3501-6 material (Figure 6) after a similar impact loading. These differences are consistent with the higher damage resistance of P E E K composites compared to 3501-6 composites. Repetitive impact tests were conducted on panels of T800/3900 (Hexcel Corp., Dublin C A ) , IM7/977-1, and IM7/977-2 (ICI Fiberite, Tempe, AZ).

Figure 6. Damage map of IM6/3501-6, (0-90) , after a cumulative impact energy of 110]. 8s



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The 3900 matrix is a particulate interleaf T M T and the 977-1 and 977-2 matrices are co-continuous interpenetrating-network T M T s . In all cases the panels were 32-ply 0°-90° fiber orientation and were cured in an autoclave using the manufacturers' suggested cure cycle. The changes in stiffness with increasing cumulative impact energy for the laminate materials are presented in Figures 9-11. Damage maps are presented in Figures 12-14. The T800/3900 exhibited only a slight loss in stiffness out to a cumulative impact energy of nearly 250 J. The loss in stiffness by the IM7/977-1 was similar to that of the A S 4 / A P C - 2 (Figure 7). The behavior of the IM7/977-2 was similar to that of the IM7/3501-6 except that the second drop-off in stiffness occurred after 200-J cumulative impact energy for the 977-2 matrix compared to 150 J for the 3501-6. Based on earlier work (12), this difference suggests the 977-2 has a higher resistance to longitudinal delamination than the 3501-6. At this point in time, we do not have any explanation for the initial increase in panel stiffness, which is especially noticeable for the IM7/977-1 (Figure 10). The damage map for the T800/3900 (Figure 12) was taken after a cumulative impact energy of 225 J. The damage has extended through the laminate thickness. However, this extent of damage is usually realized at much lower impact energies; at about 50 J for IM7/3501-6 and 150 J for

0

100

200

300

400

Cumulative Impact Energy (J) Figure 7. Stiffness vs. cumulative impact energy for AS4/APC-2, (0-90) . The open and filled points correspond to two different methods of calcuhting laminate stiffness (see reference 12). 8s



528


Figure 8. Damage map of AS4/APC-2, (0-90) , after a cumulative impact energy of 115]. 8s

AS4/APC-2. Damage maps for 977-1 and 977-2 are presented in Figures 13 and 14. After 150-J cumulative impact energy, the 977-1 showed only limited damage, whereas the damage of the 977-2 extended through the laminate thickness. Comparison of the damage maps for the 977-1 with that of 3501-6 (Figure 6) indicates much less lateral damage for the 977-1 material, which confirms the earlier suggestion that the 977-1 laminate is more resistant to longitudinal delamination than the 3501-6 laminate. Photographs of polished sections through the damaged region of a T800/3900 coupon are shown in Figure 15. Because of the differential polishing rate of the continuous matrix phase compared to the interleaf particles, the particles can be seen in the interplay regions. Note that transverse cracks are blunted by the particles, thereby preventing the development of interlaminar, longitudinal cracking. This blunting mechanism is shown schematically in Figure 16. Efforts to observe the dispersed phase in the 977-1 and 977-2 laminate have not been successful either by simple polishing or by acid-permanganate etching.


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5000

4000 H

Ε Ε

3000 Η

0)


(Λ Q) C

2000

CO

1000 Η

100

200

300

Cumulative Impact Energy (J) Figure 9. Stiffness vs. cumulative impact energy for Τ800/3900, (0-90) . 8s

7000

6000 H

5000 H

Ε Ε 4000 H

Φ

c CO

3000 Η

2000

1000

H

0

100

200

300

Cumulative Impact Energy, J Figure 10. Stiffness vs. cumulative impact energy for IM7/977-1, (0-90) . 8s


530

RUBBER-TOUGHENED PLASTICS 5000

4000

Ε Ε Ζ

3000

(/)


w Φ c

2000

CO

1000 H

300

Cumulative Impact Energy (J) Figure 11. Stiffness vs. cumulative impact energy for IM7/977-2, (0-90) . 8s

Based on these results, the damage resistance of the three T M T compos ites in the repetitive impact test rank as follows: T800/3900 > IM7/977-1 > IM7/977-2. This ranking appears to be related to the relative amount of transverse vs. longitudinal cracking in the damage zone. Polished sections of the damage regions were examined to determine the number of transverse cracks per millimeter of longitudinal cracking and the results are shown in Table III. It would appear that suppression of longitudinal cracking correlates with the impact-damage resistance ranking. 4

Test Methods for Damage Tolerance Both government (13) and industry (14) have standard tests for the impact tolerance of C F R P coupons. These tests measure the compression (strength) after impact (CAI) and the procedures specify the laminate configuration (ply lay-up and size), the test fixtures, the impacting procedures, and the postim pact loading procedures. Stated simply, the coupon is impacted and then tested for residual compression strength. This ranking relates only to impact-damage resistance. The relative merit of CFRP materials depends on a number of other factors that could outweigh differences in impact resistance.

4



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Figure 12. Damage map for T800/3900, (0-90) , after a cumulative impact energy of225 ]. 8s

The impact-damage resistance of the material influences the extent of damage that results from the impact event. Impact-damage tolerance of the coupon is determined by the material's compressive strength, the size and nature of the damage, and the structural response of the damaged panel during the compressive loading. Although the compression test portion of the typical C A I test measures the damage tolerance of the test coupon, the performance of materials in these tests is largely determined by their damage resistance during the impact portion of the test. Thus, the results obtained from C A I coupon tests reflect a combined result of impact-damage resistance and impact-damage tolerance. It is often helpful in practice to consider impact-damage resistance and impact-damage tolerance as two distinct topics. The C A I tests are costly in terms of time and materials. Consequently, simpler tests are frequently used to evaluate a matrix resin for improved C F R P impact resistance. The most common tests are for the fracture energy



532


Figure 13. Damage map for IM7/977-1, (0-90) , after a cumulative impact energy of 150 J. 8s

of the neat resin or for the mode-I interlaminar fracture energy. Hunston (7) showed that there is no linear correlation between resin and interlaminar fracture energies, and Kam and Walker (15) found that mode-I interlaminar fracture energies do not correlate with C A I test data. There is, however, increasing evidence that C A I data do correlate with mode-II interlaminar fracture energy. For example, the collection of data for a variety of matrix resins that is presented in Figure 17 indicates a strong correlation between C A I and mode-II delamination fracture energy ( G ) . To determine i f this correlation between C A I and G is universal and, if so, its physical meaning requires additional study. The repetitive impact with increasing impact energy test reported here is not a replacement for C A I testing. Nonetheless, it conserves materials, 5

I I c

I I c

In mode-I testing, the crack propagates in an opening or cleavage mode. In mode-II testing, the crack propagates by in-plane shear.

5



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Figure 14. Damage map for IM7/977-2, (0~90) , after a cumulative impact energy of 100 J. 8s

measures the onset of specific damage mechanisms, and allows relative ranking of impact-damage resistance.

Summary The relative merit of matrix polymers for damage tolerance as measured via C A I coupon tests is summarized in Table IV. Despite the low mode-I interlaminar fracture energies, the T M T matrix resins have C A I values in the same range as the thermoplastics. The TMT-based composite currently can be fabricated using existing conventional thermoset processing methods, which grants a temporary cost advantage over the thermoplastics. In the future, this cost differential is likely to be reduced as novel cost-effective methods for processing thermoplastics are developed (16). A l l other factors



W U ^0

M

S

G Ο

» oa M Sι3

50

H Figure 15. Scanning electron microscopy photomicrographs of a polished section through the damage zone of a O Ύ800/3900 hminate. Note the blunting of transverse cracks by the interply particles.



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Figure 16. Schematic of transverse crack blunting by interply particles.

Table III. Ratio of Transverse Cracks to Longitudinal Cracks for Three TMT-Matrix Composites Transverse Cracks per Longitudinal Cracks (mm)

Material T800/3900 IM7/977-1 IM7/977-2

5.1 1.1 0.55

Table IV. Typical Fracture Energy and Compression after Impact Values Fracture Energy G , (kj/m ) Ic

Polymer Type Unmodified epoxy Modified epoxy CTBN-epoxy Thermoplastics TMT

Bulk 0.10-0.20 0.2- 0.5 2.0- 6.0 1.0- 3.0

2

Interlaminar 0.15 -0.25 0.20 -0.50 0.50 -2.5 0.75 -1.25 0.2-0.5

CAI (MPa) 140--170 170--200 200--400 200--400


536


Compression after impact

400 Ί

3001


£

200

100

Data on IM fibers

800

400

Laminate G

l l c

1200

[J/m ] 2

Figure 17. Correlation between compression after impact (CAI) and mode-II interlaminarfracture energy (G ). (Supplied by H. G. Keeker, BASF Structural Materials, Inc.) IIc

being equal, the cost of start-up equipment or equipment replacement for thermoset processing may give the edge to thermoplastics.

Acknowledgments Partial funding for this work was provided by National Aeronautics and Space Administration contract NAS-18889 under the direction of J. G . Davis and W . T. Freeman (NASA Langley Research Center) and from NASA grant N A G 1 - 7 0 5 directed by J. Crews (NASA Langley Research Center). W e thank Dr. H . G . Recker of B A S F Structural Materials, Inc. for providing an original copy of Figure 17.

References

1. Rhodes, M. D.; Williams, J. G.; Starnes, J. H. Effect of Low-Velocity Impact Damage on the Compressive Strength of Graphite-Epoxy Hat-Stiffened Panel National Aeronautics and Space Administration: Washington, DC, 1977; Techni cal Note D-8411.



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2. Bascom, W. D.; Boll, D. J.; Weidner, J. C.; Murri, W. J. J. Mater. Sci. 1986, 21, 2667. 3. Bolger, J. C. In Treatise on Adhesion and Adhesives; Patrick, R. L., Ed.; Dekker: New York, 1973; Vol. 2, p 1. 4. Sultan, J. N.; Laible, R. C.; McGarry, F. J. Appl. Polym. Symp. 1971, 16, 127. 5. Bascom, W. D.; Hunston, D. L. In Treatise on Adhesion and Adhesives; Patrick, R. L., Ed.; Dekker: New York, 1989; Vol. 6; p 123. 6. Bascom, W. D.; Cottington, R. L.; Jones, R. L.; Peyser, P. J. Appl. Polym. Sci. 1975, 19, 2545. 7. Hunston, D. L. Composites Tech. Rev. 1984, 4, 176. 8. Yee, A. F.; Pearson, R. A. Toughening Mechanism in Elastomer-Modified Epoxy Resins; National Aeronautics and Space Administration: Washington, D C , 1983 and 1984; Contractor Reports 3718 and 3852. 9. Bucknall, C. B. In Advanced Composites; Partridge, I. K., Ed.; Elsevier Applied Science Publishers: London, 1989; p 145. 10. Recker, H. G.; Alstadt, V.; Eberle, W.; Folda, T.; Gerth, D.; Heckmann, W.; Ittemann, P.; Tesch, H.; Wever, T. SAMPE J. 1990, 26, 73. 11. Kubel, E. J., Jr., Adv. Mater. Process. 1989, 136(2), 23. 12. Bascom, W. D. Fractography of Composite Delamination; National Aeronautics and Space Administration: Washington, DC, July 1990; Contractor Report 181965. 13. Standard Tests for Toughened Resin Composite; National Aeronautics and Space Administration: Washington, D C . 1983; NASA Reference Publication 1092; rev. 14. Specification BSS 7260 Type I; Boeing Commercial Airplane Co.: Seattle, WA. 15. Kam, C. Y.; Walker, J. V. Toughened Composites; Johnston, N. J., Ed.; American Society for Testing and Materials: Philadelphia, PA, 1985; ASTM Standard Technical Publication 937, p 9. 16. Silverman, E. M.; Forbes, W. C. SAMPE J. 1990, 26, 9. 17. Leach, D. C. In Advanced Composites; Partridge, I. K., Ed.; Elsevier Applied Science Publishers: London, 1989; p 43. RECEIVED

for review March 6, 1991 A C C E P T E D revised manuscript August 2,

1991.


Multiphase Matrix Polymers for CarbonâFiber Composites

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Multiphase Matrix Polymers for CarbonâFiber Composites