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Effect of Fiber Strength on Notch Bending Fracture of Unidirectional Long Carbon Fiber–Reinforced Epoxy Composites
Effect of Fiber Strength on Notch Bending Fracture of Unidirectional Long Carbon Fiber–Reinforced Epoxy Composites
Yoshiyuki Tomita,* Toru Tamaki,* and Kojiro Morioka† *Department of Metallurgy and Materials Science, College of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai, Osaka 599-8531, Japan; †Technology Research Institute of Osaka Prefecture, 2-7-1 Ayumino, Izumi, Osaka, 594-1157, Japan The unidirectional long carbon fibers with 3.5 and 5.5GPa average tensile strength reinforced epoxy matrix composites (designated as 3.5 and 5.5 CF composites, respectively) were studied to determine the effect of fiber strength on notch bending fracture behavior of a CF composite. Three-point slow bend and instrumented Charpy impact tests were conducted. Two different V-notch subsize Charpy specimens with several different angles between the fibers and longitudinal direction of the specimen were used. Compared with the 3.5 CF composite with angles of 0 and 10 degrees, the 5.5 CF composite with the same angles exhibited higher fracture energies. However, slow bend and Charpy impact fracture energies of the composites decreased significantly at angles of 22.5 degrees and larger. Compared with the 3.5 CF composite, the 5.5 CF composite showed a greater anisotropy in fracture energy. The results are described and the fracture mechanism is briefly discussed on the basis of fractography. © Elsevier Science Inc., 1998
INTRODUCTION
design exploiting the higher strength and stiffness properties of the fiber reinforcement cannot be developed without a comprehensive understanding of the parameters controlling the fracture behavior of the composites; that is, the types of reinforcement, its volume fraction and mechanical properties, and the fiber–matrix adhesive strength [2–8]. A fundamental program has been initiated in the authors’ laboratory to obtain better comprehension of the parameters controlling the fracture behavior of the composites. In a previous paper, Tomita and Tempaku [9] investigated the effect of fiber strength on tensile fracture of unidirectional long carbon fiber–reinforced epoxy matrix composites. They demonstrated that, for the smooth tensile specimens of the 3.5 CF composite, a zigzag fracture occurred perpendicular to the loading direction and the
Carbon fiber–reinforced plastics possess attractive mechanical properties such as high specific stiffness and high strength in addition to a relatively high tolerance of environmental changes [1]. Furthermore, components made of the composites achieve a weight savings of the order of 20% compared with conventional constructions made of light metals; these savings are often accompanied by comparatively higher reliability. Therefore, composites are attractive for application in many engineering structures. The potential new markets for such composites are in commercial aircraft, transportation, machinery, marine, and public works industries. A requirement for many of the envisioned applications is the need to withstand structural loading under severely stressed conditions. However, a good 123 MATERIALS CHARACTERIZATION 41:123–135 (1998) © Elsevier Science Inc., 1998 655 Avenue of the Americas, New York, NY 10010
1044-5803/98/$19.00 PII S1044-5803(98)00026-6
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Table 1 Mechanical and Physical Properties of Carbon Fiber and Epoxy Resin Carbon fiber
Tensile fracture stress Young’s modulus Tensile fracture strain Density
T300
T800H
Epoxy resin No. 2500
3.5 (GPa) 230 (GPa) 1.5 (%) 1.76 (g/cm3)
5.5 (GPa) 294 (GPa) 1.9 (%) 1.81 (g/cm3)
54.9 (MPa) 3.72 (GPa) 1.7 (%) 1.25 (g/cm3)
fracture process involved a brittle fracture, or pull out, of the fibers. For the 5.5 CF composite, the fracture was approximately parallel to the loading direction, and a fiber– matrix interfacial fracture was observed. The authors also showed that the observed difference in tensile fracture stress and profile of the two composites resulted from the differences in tensile fracture strength, in strain of the reinforcing fiber, and in the fiber–matrix adhesive strength. A better comprehension of the initiation and propagation behavior of cracks formed by fiber breaks or matrix fracture, which may be produced by stress concentration under tensile loading as well as under compressive and shear loading, is important for good control of mechanical properties of these composite materials when used for structural components. In the present work, a study of the effect of fiber strength on notch bending fracture of unidirectional long carbon fibers with 3.5 and 5.5GPa average tensile strength reinforced epoxy matrix composites was conducted. The aim of this work was to clarify
the initiation and propagation behavior of cracks in the composite.
EXPERIMENTAL PROCEDURE The materials used in this investigation were two unidirectional long carbon fiber (with 3.5 and 5.5GPa in average tensile fracture stress, respectively)–reinforced epoxy matrix composites (designated as 3.5 and 5.5 CF composites, respectively). The epoxy resin (no. 2500) and polyacrylonitrile carbon fibers (T300 and T800H) were supplied by the Toray Corporation of Japan. For the 3.5 CF composite, the 3.5mm-thick plates were manufactured by using a hotpressing method: laminates (16 ply) of 0.205mm prepregs (preimpregnated) consisted of long carbon fibers (6–7mm in diameter) and epoxy resin. These materials were consolidated under a pressure of 680kPa at a temperature of 403K for 7.2ks. For the 5.5 CF composite, the 3.0mm-thick plates also were manufactured by using a hot-pressing method; laminates (19 ply) of
Table 2 Slow Bend and Charpy Impact Fracture Energies of 3.5 and 5.5 CF Composites Angle (degrees) Specimens
3.5 CF Slow bend energy (J/cm2) 3 104 Charpy impact energy (J/cm2) 3 104 5.5 CF Slow bend energy (J/cm2) 3 104 Charpy impact energy (J/cm2) 3 104
0
10
22.5
30
45
67.5
90
16.07 19.96
17.18 20.06
7.86 7.94
0.83 3.14
0.13 2.02
0.10 3.02
0.10 2.98
25.75 19.96
23.75 20.06
7.11 7.94
0.75 3.14
0.17 2.02
0.17 3.02
0.12 2.98
Notch Bending Fracture of Carbon Composites
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FIG. 1. Effect of angle between fiber and longitudinal direction of specimen on slow bend fracture energy of 3.5 and 5.5 CF composites.
0.180mm prepregs consisted of long carbon fibers (6–7mm in diameter) and epoxy resin and were consolidated together or manufactured under a pressure of 1000kPa at atemperature of 403K for 7.2ks. The volume fraction of the fibers in the two com-
posites was 0.6. The mechanical and physical properties of the carbon fibersand epoxy resin are given in Table 1. The test specimens with angles 0.0, 10.0, 22.5, 30.0, 45.0, 67.5, and 90.0 degrees between the fiber and specimen were machined from the plates.
FIG. 2. Effect of angle between fiber and longitudinal direction of specimen on Charpy impact fracture energy of 3.5 and 5.5 CF composites.
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Three-point slow bend and Charpy impact tests were performed by using the subsize Charpy V-notch specimens (3.0 or 3.5mm thickness) to evaluate their toughness. The slow bend test (span length of 40mm) was made by using an Instron machine at a crosshead speed of 0.01mm/s at ambient temperature (293K). Charpy specimens were broken by using an instrumented Charpy impact machine with a hammer velocity of 3.5m/s (for dynamic value) calibrated to the 49J capacity at 293K. Fractographs were taken of the fresh fracture surfaces from the slow bend and Charpy impact specimens by using a scanning electron microscope.
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RESULTS AND DISCUSSION EFFECT OF FIBER STRENGTH ON FRACTURE ENERGY Table 2 gives the slow bend and Charpy impact fracture energy of the 3.5 and 5.5 CF composites. Figures 1 and 2 show the slow bend and Charpy impact fracture energies of the 3.5 and 5.5 CF composites plotted versus the angles between the fibers and the longitudinal direction of the specimen, respectively. Compared with the 3.5 CF composite with angles of 0 and 10 degrees, the 5.5 CF composite with the same angles had a higher fracture energy for both the
FIG. 3. Macroscopic fracture appearances of 3.5 CF composite after (a) slow bend and (b) Charpy impact tests. Angles between fiber and longitudinal direction of specimen are 0.0, 10.0, 22.5, 30.0, 45.0, 67.5, and 90.0 degrees.
Notch Bending Fracture of Carbon Composites
slow bend and the Charpy impact tests. However, slow bend and Charpy impact fracture energies of both composites decreased significantly at angles of 22.5 degrees and larger. Compared with the 3.5 CF composite, the 5.5 CF composite exhibited a greater anisotropy in fracture energy. MACROSCOPIC FRACTURE MORPHOLOGY Figures 3 and 4 show the macroscopic fracture appearances of the 3.5 and 5.5 CF composites, respectively, after the slow bend and Charpy impact tests. The specimens
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with angles of 0 and 10 degrees fractured in a direction approximately parallel to the loading direction. However, the specimens with angles at 22.5 degrees and larger fractured along the fiber direction. This feature was common for the slow bend and Charpy impact specimens. Thus, the fracture behavior of the two composites falls into two categories: (1) fractures approximately perpendicular to the loading direction observed for the specimens with angles of 0 and 10 degrees; and (2) fractures approximately parallel to the fiber direction observed for the specimens with angles at 22.5 degrees and larger.
FIG. 4. Macroscopic fracture appearances of 5.5 CF composite after (a) slow bend and (b) Charpy impact tests. Angles between fiber and longitudinal direction of specimen are: 0.0, 10.0, 22.5, 30.0, 45.0, 67.5, and 90.0 degrees.
FIG. 5. Scanning electron micographs of fracture surfaces of 3.5 CF composite with angle of 0 degrees after slow bend test: (a) overall fracture surface (b–e) enlargements of indicated areas of (a).
128 Y. Tomita et al.
FIG. 6. Scanning electron micrographs of fracture surfaces of 3.5 CF composite with angle of 0 degrees after Charpy impact test: (a) overall fracture surface; (b–e) enlargements of indicated areas of (a).
Notch Bending Fracture of Carbon Composites 129
FIG. 7. Scanning electron micrographs of fracture surfaces of 5.5 CF composite with angle of 0 degrees after slow bend test: (a) overall fracture surface; (b–e) enlargement of indicated areas of (a).
130 Y. Tomita et al.
FIG. 8. Scanning electron micrographs of fracture surfaces of 5.5 CF composite with angle of 0 degrees after Charpy impact test: (a) overall fracture surface; (b–e) enlargements of indicated areas of (a).
Notch Bending Fracture of Carbon Composites 131
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FIG. 9. Scanning electron micrographs of cracks initiated on V-notch side for (a) 3.5 and (b) 5.5 CF composites. Large arrows indicate cracks.
EFFECT OF FIBER STRENGTH ON FRACTURE BEHAVIOR OF SPECIMENS WITH ANGLES OF 0 AND 10 DEGREES To clarify the fracture behavior of the specimens with angles of 0 and 10 degrees, fractographs were taken of the fracture surfaces from the slow bend and Charpy
impact specimens. The typical fracture surfaces are shown in Figs. 5 through 8. The results are summarized as follows. First, despite the different types of composites and loading, the crack extension process consisted of (1) a fiber pullout [see (a) to (c) in Figs. 5 to 8] and (2) the brittle fracture of
FIG. 10. Scanning electron micrographs of (a) cracks initiated at compressive side and (b) crack propagation in 3.5 CF composite. Arrows indicate cracks.
Notch Bending Fracture of Carbon Composites
both the fibers and the epoxy matrix [see (d) and (e) in Figs. 5 to 8]. Second, compared with the 3.5 CF composite, however, the regions of brittle fracture of the 5.5 CF composite were larger than those of the 3.5 CF composite. This feature was common for the slow bend and Charpy impact specimens. To clarify the observed fracture behavior of the 3.5 and 5.5 CF composites with 0 and 10 degrees, their crack initiation and propagation were observed in the slow bend specimens. This examination revealed that (1) the crack parallel to the specimen length is first initiated under the V-notch near the maximum load (Fig. 9); (2) a main crack, however, is initiated by fiber breaks at a compressive side [Fig. 10(a)]; and (3) the main crack grows up through fractures of fibers and matrix toward he V-notch side until eventually it links up with a pullout of the fibers produced under the notch [Fig. 10(b)]. From these results, the fracture model schematically shown in Fig. 11 is postulated for the composites. When the load is applied to the specimen, the crack parallel to the specimen length is first initiated by tensile stress under the V-notch [Fig. 11(a,b)]. However, the main crack is formed by fiber breaks at a compressive side [Fig. 11(c)]. This formation could be due to the facts that (1) the notch effect may be minimized because the crack parallel to the specimen length initiates by shear stress under the V-notch and (2) fiber breaks occur by the development of delamination during compressive loading. The main crack then grows through fractures of both the fibers and the epoxy matrix toward the notch side until eventually it links up with the pullout of the fibers produced by tensile stress [Fig. 11(d,e)]. As a result, the resistance to the crack growth may be the major source of observed fracture energy. Thus, higher fracture energy observed for the 5.5 CF composite can be attributed to the higher resistance to fracture of the fibers compared with the 3.5 CF composite. Why does the area of brittle fractures through fibers and matrix become large as
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the fiber strength increases? This may be due to the fact that the elastic stored energy, which is released by the brittle fracture of the fibers, becomes large as the fiber strength increases; that is, when the elastic stored energy is released, the crack propagates in a brittle manner toward the epoxy matrix. The
FIG. 11. Schematic representation of fracture model of specimens with angles of 0 and 10 degrees.
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higher the fiber strength, the larger are the areas propagated in a brittle manner. EFFECT OF FIBER STRENGTH ON FRACTURE BEHAVIOR OF SPECIMENS WITH ANGLES AT 22.5 DEGREES AND LARGER To clarify the fracture behavior of the specimens with angles at 22.5 degrees and larger, fractographs were taken of fracture surfaces from the slow bend and Charpy impact specimens. The typical fracture surfaces are shown in Fig. 12. The scanning electron microscopic observations revealed that both composites showed fiber–matrix interface fracture independent of the angles. This feature was common for the slow bend and Charpy impact specimens. It could be due to the following reason: when bend stress (s) is applied to the specimens
Y. Tomita et al.
with angles at 22.5 degrees and larger, a shear (t) or transverse tensile stress(sT) will be produced in the fiber–matrix interfaces, as schematically shown in Fig. 13. When the angle is low, the shear stress is produced in the fiber–matrix interfaces and transverse tensile stress acts the angle increases. Thus, the composites show a transition from fiber-dominant fracture to fiber–matrix interface dominated fracture as the angle is increased. However, compared with the 3.5 CF composite, the 5.5 CF composite exhibited a greater anisotropy in fracture energy. This greater anisotropy could be due to the fact that fiber–matrix adhesive strength of the 5.5 CF composite is smaller than that of the 3.5 CF composite. At present, however, there are insufficient data to clarify the mechanism, but it is hoped that a solution to the problem will be found from ongoing fundamental studies.
FIG. 12. Scanning electron micrographs of fracture surfaces of composites with angles of 45 degrees after slow bend and Charpy impact tests, respectively: (a, b) 3.5 CF composite after slow bend and Charpy impact tests, respectively; (c, d) 5.5 CF composite after slow bend and Charpy impact tests, respectively.
Notch Bending Fracture of Carbon Composites
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6. The anisotropy in the fracture energy for both composites resulted from a transition from fiber-dominated fracture to fiber–matrix interface-dominated fracture as the angle between the specimen long axis and the fiber increased.
FIG. 13. Schematic representation of fracture stress that is loaded in fiber–matrix interfaces.
CONCLUSIONS 1. The 5.5 CF composite with angles of 0 and 10 degrees between the fiber and longitudinal direction of the specimen had high slow bend and Charpy impact fracture energies compared with the 3.5 CF composite with the same angles. 2. For the two composites with angles of 0 and 10 degrees, the main crack proceeded to final fracture by the formation of cracks at fiber breaks at the compressive side, their growth through brittle fracture of fibers and epoxy matrix, and their linking up with the pullout of the fibers produced by tensile stress under the V-notch. 3. The higher fracture energy observed for the 5.5 CF composite can therefore be attributed to the higher resistance to fracture of the fibers compared with the 3.5 CF composite. 4. Slow and Charpy impact fracture energies of both composites decreased significantly at angles of 22.5 degrees and larger. 5. Compared with the 3.5 CF composite, the 5.5 CF composite exhibited a greater anisotropy in fracture energy.
References 1. ASM Staff Report: Sea duty for composites. Adv. Mater. Process. 142:16–20 (1992). 2. A. Kelly: Interface effects and the work of fracture of a fibrous composite. Proc. R. Soc. Lond. A319:95– 116 (1970). 3. P. D. Ewins and R. P. Potter: Some observations of nature of fiber reinforced plastics and the implications for structural design. Philos. Trans. R. Soc. Lond. A294:507–517 (1980). 4. A. C. Moloney, H. H. Kausch, and H. R. Stieger: The fracture of particulate-filled epoxide resins. J. Mater. Sci. 18:208–216 (1983). 5. J. Spanoudakis and R. J. Young: Crack propagation in a glass particle-filled epoxide resins 1: effect of particle volume fraction and size. J. Mater. Sci. 19:473–486 (1984). 6. J. Spanoudakis and R. J. Young: Crack propagation in a glass particle-filled epoxiside resins 2: effect of particle-matrix adhesion. J. Mater. Sci. 19: 487–496 (1984). 7. A. C. Moloneg, H. H. Kausch, T. Kaiser, and H. Bear: Parameters determining the strength and toughness of particulate filled epoxside resins. J. Mater. Sci. 22:381–393 (1987). 8. J. Cook and J. E. Gordon: A mechanism for the control of crack propagation in all-brittle systems. Proc. R. Soc. Lond. A 282:508–520 (1964). 9. Y. Tomita and M. Tempaku: Effect of fiber strength on tensile fracture of unidirectional long carbon fiber-reinforced epoxy matrix composites. Mater. Char. 38:91–96 (1997). Received July 1997; accepted January 1998.
Yoshiyuki Tomita,* Toru Tamaki,* and Kojiro Morioka† *Department of Metallurgy and Materials Science, College of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai, Osaka 599-8531, Japan; †Technology Research Institute of Osaka Prefecture, 2-7-1 Ayumino, Izumi, Osaka, 594-1157, Japan The unidirectional long carbon fibers with 3.5 and 5.5GPa average tensile strength reinforced epoxy matrix composites (designated as 3.5 and 5.5 CF composites, respectively) were studied to determine the effect of fiber strength on notch bending fracture behavior of a CF composite. Three-point slow bend and instrumented Charpy impact tests were conducted. Two different V-notch subsize Charpy specimens with several different angles between the fibers and longitudinal direction of the specimen were used. Compared with the 3.5 CF composite with angles of 0 and 10 degrees, the 5.5 CF composite with the same angles exhibited higher fracture energies. However, slow bend and Charpy impact fracture energies of the composites decreased significantly at angles of 22.5 degrees and larger. Compared with the 3.5 CF composite, the 5.5 CF composite showed a greater anisotropy in fracture energy. The results are described and the fracture mechanism is briefly discussed on the basis of fractography. © Elsevier Science Inc., 1998
INTRODUCTION
design exploiting the higher strength and stiffness properties of the fiber reinforcement cannot be developed without a comprehensive understanding of the parameters controlling the fracture behavior of the composites; that is, the types of reinforcement, its volume fraction and mechanical properties, and the fiber–matrix adhesive strength [2–8]. A fundamental program has been initiated in the authors’ laboratory to obtain better comprehension of the parameters controlling the fracture behavior of the composites. In a previous paper, Tomita and Tempaku [9] investigated the effect of fiber strength on tensile fracture of unidirectional long carbon fiber–reinforced epoxy matrix composites. They demonstrated that, for the smooth tensile specimens of the 3.5 CF composite, a zigzag fracture occurred perpendicular to the loading direction and the
Carbon fiber–reinforced plastics possess attractive mechanical properties such as high specific stiffness and high strength in addition to a relatively high tolerance of environmental changes [1]. Furthermore, components made of the composites achieve a weight savings of the order of 20% compared with conventional constructions made of light metals; these savings are often accompanied by comparatively higher reliability. Therefore, composites are attractive for application in many engineering structures. The potential new markets for such composites are in commercial aircraft, transportation, machinery, marine, and public works industries. A requirement for many of the envisioned applications is the need to withstand structural loading under severely stressed conditions. However, a good 123 MATERIALS CHARACTERIZATION 41:123–135 (1998) © Elsevier Science Inc., 1998 655 Avenue of the Americas, New York, NY 10010
1044-5803/98/$19.00 PII S1044-5803(98)00026-6
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Table 1 Mechanical and Physical Properties of Carbon Fiber and Epoxy Resin Carbon fiber
Tensile fracture stress Young’s modulus Tensile fracture strain Density
T300
T800H
Epoxy resin No. 2500
3.5 (GPa) 230 (GPa) 1.5 (%) 1.76 (g/cm3)
5.5 (GPa) 294 (GPa) 1.9 (%) 1.81 (g/cm3)
54.9 (MPa) 3.72 (GPa) 1.7 (%) 1.25 (g/cm3)
fracture process involved a brittle fracture, or pull out, of the fibers. For the 5.5 CF composite, the fracture was approximately parallel to the loading direction, and a fiber– matrix interfacial fracture was observed. The authors also showed that the observed difference in tensile fracture stress and profile of the two composites resulted from the differences in tensile fracture strength, in strain of the reinforcing fiber, and in the fiber–matrix adhesive strength. A better comprehension of the initiation and propagation behavior of cracks formed by fiber breaks or matrix fracture, which may be produced by stress concentration under tensile loading as well as under compressive and shear loading, is important for good control of mechanical properties of these composite materials when used for structural components. In the present work, a study of the effect of fiber strength on notch bending fracture of unidirectional long carbon fibers with 3.5 and 5.5GPa average tensile strength reinforced epoxy matrix composites was conducted. The aim of this work was to clarify
the initiation and propagation behavior of cracks in the composite.
EXPERIMENTAL PROCEDURE The materials used in this investigation were two unidirectional long carbon fiber (with 3.5 and 5.5GPa in average tensile fracture stress, respectively)–reinforced epoxy matrix composites (designated as 3.5 and 5.5 CF composites, respectively). The epoxy resin (no. 2500) and polyacrylonitrile carbon fibers (T300 and T800H) were supplied by the Toray Corporation of Japan. For the 3.5 CF composite, the 3.5mm-thick plates were manufactured by using a hotpressing method: laminates (16 ply) of 0.205mm prepregs (preimpregnated) consisted of long carbon fibers (6–7mm in diameter) and epoxy resin. These materials were consolidated under a pressure of 680kPa at a temperature of 403K for 7.2ks. For the 5.5 CF composite, the 3.0mm-thick plates also were manufactured by using a hot-pressing method; laminates (19 ply) of
Table 2 Slow Bend and Charpy Impact Fracture Energies of 3.5 and 5.5 CF Composites Angle (degrees) Specimens
3.5 CF Slow bend energy (J/cm2) 3 104 Charpy impact energy (J/cm2) 3 104 5.5 CF Slow bend energy (J/cm2) 3 104 Charpy impact energy (J/cm2) 3 104
0
10
22.5
30
45
67.5
90
16.07 19.96
17.18 20.06
7.86 7.94
0.83 3.14
0.13 2.02
0.10 3.02
0.10 2.98
25.75 19.96
23.75 20.06
7.11 7.94
0.75 3.14
0.17 2.02
0.17 3.02
0.12 2.98
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FIG. 1. Effect of angle between fiber and longitudinal direction of specimen on slow bend fracture energy of 3.5 and 5.5 CF composites.
0.180mm prepregs consisted of long carbon fibers (6–7mm in diameter) and epoxy resin and were consolidated together or manufactured under a pressure of 1000kPa at atemperature of 403K for 7.2ks. The volume fraction of the fibers in the two com-
posites was 0.6. The mechanical and physical properties of the carbon fibersand epoxy resin are given in Table 1. The test specimens with angles 0.0, 10.0, 22.5, 30.0, 45.0, 67.5, and 90.0 degrees between the fiber and specimen were machined from the plates.
FIG. 2. Effect of angle between fiber and longitudinal direction of specimen on Charpy impact fracture energy of 3.5 and 5.5 CF composites.
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Three-point slow bend and Charpy impact tests were performed by using the subsize Charpy V-notch specimens (3.0 or 3.5mm thickness) to evaluate their toughness. The slow bend test (span length of 40mm) was made by using an Instron machine at a crosshead speed of 0.01mm/s at ambient temperature (293K). Charpy specimens were broken by using an instrumented Charpy impact machine with a hammer velocity of 3.5m/s (for dynamic value) calibrated to the 49J capacity at 293K. Fractographs were taken of the fresh fracture surfaces from the slow bend and Charpy impact specimens by using a scanning electron microscope.
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RESULTS AND DISCUSSION EFFECT OF FIBER STRENGTH ON FRACTURE ENERGY Table 2 gives the slow bend and Charpy impact fracture energy of the 3.5 and 5.5 CF composites. Figures 1 and 2 show the slow bend and Charpy impact fracture energies of the 3.5 and 5.5 CF composites plotted versus the angles between the fibers and the longitudinal direction of the specimen, respectively. Compared with the 3.5 CF composite with angles of 0 and 10 degrees, the 5.5 CF composite with the same angles had a higher fracture energy for both the
FIG. 3. Macroscopic fracture appearances of 3.5 CF composite after (a) slow bend and (b) Charpy impact tests. Angles between fiber and longitudinal direction of specimen are 0.0, 10.0, 22.5, 30.0, 45.0, 67.5, and 90.0 degrees.
Notch Bending Fracture of Carbon Composites
slow bend and the Charpy impact tests. However, slow bend and Charpy impact fracture energies of both composites decreased significantly at angles of 22.5 degrees and larger. Compared with the 3.5 CF composite, the 5.5 CF composite exhibited a greater anisotropy in fracture energy. MACROSCOPIC FRACTURE MORPHOLOGY Figures 3 and 4 show the macroscopic fracture appearances of the 3.5 and 5.5 CF composites, respectively, after the slow bend and Charpy impact tests. The specimens
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with angles of 0 and 10 degrees fractured in a direction approximately parallel to the loading direction. However, the specimens with angles at 22.5 degrees and larger fractured along the fiber direction. This feature was common for the slow bend and Charpy impact specimens. Thus, the fracture behavior of the two composites falls into two categories: (1) fractures approximately perpendicular to the loading direction observed for the specimens with angles of 0 and 10 degrees; and (2) fractures approximately parallel to the fiber direction observed for the specimens with angles at 22.5 degrees and larger.
FIG. 4. Macroscopic fracture appearances of 5.5 CF composite after (a) slow bend and (b) Charpy impact tests. Angles between fiber and longitudinal direction of specimen are: 0.0, 10.0, 22.5, 30.0, 45.0, 67.5, and 90.0 degrees.
FIG. 5. Scanning electron micographs of fracture surfaces of 3.5 CF composite with angle of 0 degrees after slow bend test: (a) overall fracture surface (b–e) enlargements of indicated areas of (a).
128 Y. Tomita et al.
FIG. 6. Scanning electron micrographs of fracture surfaces of 3.5 CF composite with angle of 0 degrees after Charpy impact test: (a) overall fracture surface; (b–e) enlargements of indicated areas of (a).
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FIG. 7. Scanning electron micrographs of fracture surfaces of 5.5 CF composite with angle of 0 degrees after slow bend test: (a) overall fracture surface; (b–e) enlargement of indicated areas of (a).
130 Y. Tomita et al.
FIG. 8. Scanning electron micrographs of fracture surfaces of 5.5 CF composite with angle of 0 degrees after Charpy impact test: (a) overall fracture surface; (b–e) enlargements of indicated areas of (a).
Notch Bending Fracture of Carbon Composites 131
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FIG. 9. Scanning electron micrographs of cracks initiated on V-notch side for (a) 3.5 and (b) 5.5 CF composites. Large arrows indicate cracks.
EFFECT OF FIBER STRENGTH ON FRACTURE BEHAVIOR OF SPECIMENS WITH ANGLES OF 0 AND 10 DEGREES To clarify the fracture behavior of the specimens with angles of 0 and 10 degrees, fractographs were taken of the fracture surfaces from the slow bend and Charpy
impact specimens. The typical fracture surfaces are shown in Figs. 5 through 8. The results are summarized as follows. First, despite the different types of composites and loading, the crack extension process consisted of (1) a fiber pullout [see (a) to (c) in Figs. 5 to 8] and (2) the brittle fracture of
FIG. 10. Scanning electron micrographs of (a) cracks initiated at compressive side and (b) crack propagation in 3.5 CF composite. Arrows indicate cracks.
Notch Bending Fracture of Carbon Composites
both the fibers and the epoxy matrix [see (d) and (e) in Figs. 5 to 8]. Second, compared with the 3.5 CF composite, however, the regions of brittle fracture of the 5.5 CF composite were larger than those of the 3.5 CF composite. This feature was common for the slow bend and Charpy impact specimens. To clarify the observed fracture behavior of the 3.5 and 5.5 CF composites with 0 and 10 degrees, their crack initiation and propagation were observed in the slow bend specimens. This examination revealed that (1) the crack parallel to the specimen length is first initiated under the V-notch near the maximum load (Fig. 9); (2) a main crack, however, is initiated by fiber breaks at a compressive side [Fig. 10(a)]; and (3) the main crack grows up through fractures of fibers and matrix toward he V-notch side until eventually it links up with a pullout of the fibers produced under the notch [Fig. 10(b)]. From these results, the fracture model schematically shown in Fig. 11 is postulated for the composites. When the load is applied to the specimen, the crack parallel to the specimen length is first initiated by tensile stress under the V-notch [Fig. 11(a,b)]. However, the main crack is formed by fiber breaks at a compressive side [Fig. 11(c)]. This formation could be due to the facts that (1) the notch effect may be minimized because the crack parallel to the specimen length initiates by shear stress under the V-notch and (2) fiber breaks occur by the development of delamination during compressive loading. The main crack then grows through fractures of both the fibers and the epoxy matrix toward the notch side until eventually it links up with the pullout of the fibers produced by tensile stress [Fig. 11(d,e)]. As a result, the resistance to the crack growth may be the major source of observed fracture energy. Thus, higher fracture energy observed for the 5.5 CF composite can be attributed to the higher resistance to fracture of the fibers compared with the 3.5 CF composite. Why does the area of brittle fractures through fibers and matrix become large as
133
the fiber strength increases? This may be due to the fact that the elastic stored energy, which is released by the brittle fracture of the fibers, becomes large as the fiber strength increases; that is, when the elastic stored energy is released, the crack propagates in a brittle manner toward the epoxy matrix. The
FIG. 11. Schematic representation of fracture model of specimens with angles of 0 and 10 degrees.
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higher the fiber strength, the larger are the areas propagated in a brittle manner. EFFECT OF FIBER STRENGTH ON FRACTURE BEHAVIOR OF SPECIMENS WITH ANGLES AT 22.5 DEGREES AND LARGER To clarify the fracture behavior of the specimens with angles at 22.5 degrees and larger, fractographs were taken of fracture surfaces from the slow bend and Charpy impact specimens. The typical fracture surfaces are shown in Fig. 12. The scanning electron microscopic observations revealed that both composites showed fiber–matrix interface fracture independent of the angles. This feature was common for the slow bend and Charpy impact specimens. It could be due to the following reason: when bend stress (s) is applied to the specimens
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with angles at 22.5 degrees and larger, a shear (t) or transverse tensile stress(sT) will be produced in the fiber–matrix interfaces, as schematically shown in Fig. 13. When the angle is low, the shear stress is produced in the fiber–matrix interfaces and transverse tensile stress acts the angle increases. Thus, the composites show a transition from fiber-dominant fracture to fiber–matrix interface dominated fracture as the angle is increased. However, compared with the 3.5 CF composite, the 5.5 CF composite exhibited a greater anisotropy in fracture energy. This greater anisotropy could be due to the fact that fiber–matrix adhesive strength of the 5.5 CF composite is smaller than that of the 3.5 CF composite. At present, however, there are insufficient data to clarify the mechanism, but it is hoped that a solution to the problem will be found from ongoing fundamental studies.
FIG. 12. Scanning electron micrographs of fracture surfaces of composites with angles of 45 degrees after slow bend and Charpy impact tests, respectively: (a, b) 3.5 CF composite after slow bend and Charpy impact tests, respectively; (c, d) 5.5 CF composite after slow bend and Charpy impact tests, respectively.
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6. The anisotropy in the fracture energy for both composites resulted from a transition from fiber-dominated fracture to fiber–matrix interface-dominated fracture as the angle between the specimen long axis and the fiber increased.
FIG. 13. Schematic representation of fracture stress that is loaded in fiber–matrix interfaces.
CONCLUSIONS 1. The 5.5 CF composite with angles of 0 and 10 degrees between the fiber and longitudinal direction of the specimen had high slow bend and Charpy impact fracture energies compared with the 3.5 CF composite with the same angles. 2. For the two composites with angles of 0 and 10 degrees, the main crack proceeded to final fracture by the formation of cracks at fiber breaks at the compressive side, their growth through brittle fracture of fibers and epoxy matrix, and their linking up with the pullout of the fibers produced by tensile stress under the V-notch. 3. The higher fracture energy observed for the 5.5 CF composite can therefore be attributed to the higher resistance to fracture of the fibers compared with the 3.5 CF composite. 4. Slow and Charpy impact fracture energies of both composites decreased significantly at angles of 22.5 degrees and larger. 5. Compared with the 3.5 CF composite, the 5.5 CF composite exhibited a greater anisotropy in fracture energy.
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