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epoxy laminated composites in the hydrostatic pressure condition
Materials Science and Engineering A 419 (2006) 209–213
Fracture behavior of seawater-absorbed carbon/epoxy laminated composites in the hydrostatic pressure condition K.Y. Rhee a,∗ , S.R. Ha a , S.J. Park b , H.J. Kim c , D.H. Jung c a
Center for Media Transport System, School of Mechanical and Industrial System Engineering, Kyunghee University, Yongin 449-701, Republic of Korea b Advanced Materials Division, Korea Research Institute of Chemical Technology, Taejon 305-600, Republic of Korea c Ocean Development System Laboratory, Korea Research Institute of Ships and Ocean Engineering, Taejon 305-600, Republic of Korea Received in revised form 16 December 2005; accepted 16 December 2005
Abstract It was shown in the previous study that the fracture toughness of dried carbon/epoxy composites increased 38% as the applied hydrostatic pressure increased from 0.1 to 200 MPa. However, no research has been conducted on the fracture behavior of seawater-absorbed carbon/epoxy composites in a hydrostatic pressure environment. This work investigates the compressive fracture behavior of seawater-absorbed carbon/epoxy composites subjected to various hydrostatic pressures. Compressive fracture tests were performed under four hydrostatic pressure levels, 0.1 (atmospheric pressure), 100, 200, and 270 MPa. The compliance and fracture loads were determined from load–displacement curves as a function of hydrostatic pressure. Fracture toughness was determined from the elastic work factor approach. The results showed that compliance decreases but fracture load increases as the applied hydrostatic pressure increases. Fracture toughness increased with increasing pressure. Specifically, fracture toughness increased from 2.66 to 4.44 kJ/m2 , a 67% increase, as hydrostatic pressure increased from 0.1 to 270 MPa. Optical microscope examination of the fractured surface showed that local delaminations and microcracks are suppressed with increasing hydrostatic pressure. SEM examination showed that seawater-absorbed specimen shows a lot more epoxy fracture than the dried specimen. © 2006 Published by Elsevier B.V. Keywords: Carbon/epoxy composite; Hydrostatic pressure; Work factor approach; Delamination; Fracture toughness
1. Introduction Polymer matrix composites are used in a variety of structural components due to their stiffness and strength and their lightweight savings. Currently, polymeric composites are widely applied to build offshore oil structures such as floating platforms and tension leg platforms because they are more corrosionresistant than metals. However, it is known that polymeric composites absorb moisture and exhibit some degree of degradation due to this absorption factor. Accordingly, many studies have been conducted to investigate the effect of moisture absorption on the mechanical behavior of glass/epoxy and carbon/epoxy composites, focusing on tensile, compressive, and interlaminar shear [1–5]. It is known from their research results that moisture absorption causes plasticization and swelling of the epoxy, weakening the interfacial strength between fibers and epoxy,
∗
Corresponding author. Tel.: +82 31 201 2565; fax: +82 31 202 6693. E-mail address: [email protected] (K.Y. Rhee).
0921-5093/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.msea.2005.12.025
which results in the reduction of tensile and shear strength. However, the effect of moisture absorption on fracture behavior is different depending on the studies conducted. Sloan and Seymour [6] reported that the mode I fracture resistance, GIR of carbon/epoxy specimens exposed to seawater, was more than twice than that of dry specimens. However, Chou [7] reported that the GIR of moisture-absorbed carbon/epoxy composites was less than that of dry cases. The effect of hydrostatic pressure on the mechanical properties of polymer matrix composites was also studied by many researchers [8–13]. These studies focused on the tensile and compressive behaviors glass/epoxy and carbon/epoxy composites in hydrostatic pressure environment. It is generally accepted that the tensile and compressive strengths of polymeric composites increase as the applied hydrostatic pressure increases. Recently, the hydrostatic pressure effect on the compressive fracture behavior of carbon/epoxy composites was investigated by Rhee [14]. His result showed that fracture toughness increased about 38% as hydrostatic pressure increased from 0.1 to 200 MPa.
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K.Y. Rhee et al. / Materials Science and Engineering A 419 (2006) 209–213
To date no study has been conducted to evaluate the combined effect of moisture absorption and hydrostatic pressure on the fracture behavior of polymeric composites. A study on this combined effect is essential to expand the use of polymeric composites to manufacture submersible structures; these structures undergo moisture absorption and hydrostatic pressure in a deepsea environment. The main focus of this study has been to investigate the combined effects of seawater absorption and hydrostatic pressure on the fracture behavior of carbon/epoxy composites. For this purpose, fracture tests were performed using seawater-absorbed carbon/epoxy composite, whose seawater absorption capability was at a maximum absorption level, in a hydrostatic pressure environment. Compliance, fracture load and fracture toughness were determined as a function of applied hydrostatic pressure. After the fracture tests were performed, optical microscope examination was made on the fracture surfaces to determine the fracture mechanism of carbon/epoxy composites in a deep-sea environment. 2. Experimental 2.1. Material and specimen preparation Carbon/epoxy composites plates were fabricated using a continuous filament non-woven carbon/epoxy prepregnated (prepreg) tape (USN 150B, SK Chemical, Korea). The prepreg tape has a 64% fiber content by volume and the nominal cured thickness was approximately 0.125 mm. The prepreg tapes were cut and stacked as [0o ]88 to give a final plate thickness of 10.5 mm. A kapton film (thickness: 13 m) was inserted at the midplane (between 44th and 45th plies) to produce the initial crack. The length of the initial crack was 8 mm. The stacked plates were cured in an autoclave under 5 kg/cm2 of pressure at 126 ◦ C for 2 h. Then, the cured plates were fabricated as dog bone specimens where the sample length was 32 mm. The shape and dimension of test sample are shown in Fig. 1. The test samples were kept in sterile-filtered seawater (SIGMA, S-9148) over 13 months in order to produce fully seawater-absorbed specimens. It was shown in the previous study that the absorption of seawater for the carbon/epoxy composites used in this study were fully saturated at 6 months [15].
Fig. 1. Schematic diagram of dog bone test sample.
2.2. Fracture test The compressive fracture tests of fully seawater-absorbed carbon/epoxy composites were conducted under normal atmospheric pressure and under three levels of hydrostatic pressure conditions using a high pressure tension–compression apparatus which can produce 700 MPa of hydrostatic pressure. The apparatus consists of two thick-walled cylindrical pressure vessels, one for testing and the other for pressure compensation. These two vessels are interconnected by a high pressure tubing so that a constant pressure can be maintained in the test chamber during the compression test. Dow Corning 200 silicon oil was the hydraulic fluid used as the pressure medium. Displacement of the loading piston was sensed by a linear variable differential transducer (LVDT) and load was measured with a full-bridge strain gauge inside the wall of the loading piston. Electrical signals from these two transducers were fed into two signal conditioners (Daytronics, Models 3130 and 3170), then sent to an IBM PC and analyzed. The working mechanism of the testing apparatus was well described by Carlsson [16]. The hydrostatic pressures applied were 100, 200, and 270 MPa. Fracture tests were performed applying a constant strain rate of 0.05%/s at atmospheric pressure and hydrostatic pressure conditions. At least three tests were carried out at each pressure level in order to assure the repeatable nature and reliability of test results. 3. Results and discussion The effect of hydrostatic pressure on the fracture behavior of seawater-absorbed carbon/epoxy composites can be determined by comparing compliance, fracture load, and fracture toughness of atmospheric pressure condition with those of hydrostatic pressure conditions. Compliance and the fracture load can be determined from the load–displacement curve. The typical load–displacement curves at atmospheric pressure and various hydrostatic pressures are shown in Fig. 2; which shows that fracture behavior is significantly affected by hydrostatic pressure. It can also be seen in the figure that for each pressure level; load
Fig. 2. Typical load–displacement curves of seawater-absorbed carbon/epoxy composites at atmospheric pressure and various hydrostatic pressures.
K.Y. Rhee et al. / Materials Science and Engineering A 419 (2006) 209–213
Fig. 3. Variation of compliance of seawater-absorbed carbon/epoxy composites as a function of applied hydrostatic pressure.
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pressure. As shown in the figure, fracture load was affected by hydrostatic pressure but the response of the fracture load to hydrostatic pressure was opposite of compliance. In other words, fracture load increased with increasing hydrostatic pressure. The fracture load increased approximately 35% as the applied hydrostatic pressure increased from 0.1 to 270 MPa. The primary reason fracture load increased with increasing hydrostatic pressure was shown to be that hydrostatic pressure acts as a compressive normal stress along the surfaces of specimen. This compressive normal stress leads to a suppression of debonding at the interface and crack propagation occurs. Therefore, fracture load increased as the applied hydrostatic pressure increased. In general, fracture toughness of fiber-reinforced composites is determined experimentally from the compliance method. However, it requires four to five specimens of different crack lengths. Particularly, for the fracture tests in the hydrostatic pressure condition, the method requires additional efforts to determine fracture toughness because it takes a long time to stabilize pressure. Therefore, fracture toughness was determined from the work factor approach in this study. The merit of applying the work factor approach is that fracture toughness can be determined from a single fracture test. Fracture toughness, Gc was determined from the work factor approach as follows [17]:
increases almost linearly with displacement and the non-linear portion is relatively small up to the fracture point. Compliance was determined by measuring the inverse slope of the linear region through the origin in the load–displacement curve because compliance was defined as displacement divided by applied load. Fig. 3 shows the variation of compliance as a function of applied hydrostatic pressure. Two facts are evident based on this figure. One is that compliance was affected by hydrostatic pressure and the other is that compliance decreased with increasing pressure. The values of average compliance at atmospheric pressure (0.1 MPa) and 270 MPa pressure levels are 8.91 × 10−8 and 8.08 × 10−8 m/N, respectively. That is, average compliance decreased about 10% as the applied hydrostatic pressure was increased from 0.1 to 270 MPa. In order to investigate the effect of hydrostatic pressure on the fracture load of seawater-absorbed carbon/epoxy composites, the fracture load was determined as a function of hydrostatic pressure. In this study, the fracture load for a given pressure level was determined as the maximum load in the corresponding load–displacement curve because fracture behavior displayed linear elastic properties irrespective of the pressure level. Fig. 4 shows the variation of fracture load as a function of hydrostatic
where a is a crack length and L is a gauge length. Fig. 5 shows the variation of fracture toughness of seawater-absorbed carbon/epoxy composites as a function of hydrostatic pressure. As shown in the figure, fracture toughness is affected by the hydrostatic pressure and increased as hydrostatic pressure increased.
Fig. 4. Variation of fracture load of seawater-absorbed carbon/epoxy composites as a function of hydrostatic pressure.
Fig. 5. Variation of fracture toughness of seawater-absorbed carbon/epoxy composites as a function of hydrostatic pressure.
Gc =
ηel Ac Bb
(1)
where b represents the remaining ligament, B the specimen width, Ac the area under the load–displacement record up to fracture and ηel is the elastic work factor. It was shown in the previous study that ηel is not affected by the applied hydrostatic pressure and stacking sequence [18]. The ηel form of the specimen used in this study was determined as follows: ηel = 0.83 − 0.91
a L
(2)
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The averaged values of fracture toughness at atmospheric pressure (0.1 MPa), 100, 200, and 270 MPa are 2.66, 3.18, 3.91, and 4.44 kJ/m2 , that is, fracture toughness increased 67% when the applied hydrostatic pressure increased from 0.1 to 270 MPa. The increased rate of fracture toughness with hydrostatic pressure for seawater-absorbed carbon/epoxy composites was larger than that of regular (dry) carbon/epoxy composites. Previous studies showed that fracture toughness of dry carbon/epoxy composites increased about 38% when the applied hydrostatic pressure increases from 0.1 to 200 MPa [14]. However, the fracture toughness of seawater-absorbed specimen is lower than that of dry specimen at the same hydrostatic pressure. This agrees with the result of Chou’s work [7]. The decrease of fracture toughness occurs because the wetting between fibers and the epoxy resin becomes poor as the specimen absorbs seawater. It can be inferred from Eq. (1) that the increase of fracture toughness with increasing pressure occurs due to the increase of fracture load because compliance decreased as the hydrostatic pressure increased. The phenomenon of fracture toughness increase with increasing pressure was also related to epoxy behavior in the hydrostatic pressure condition. It was shown in the previous study that epoxy material exhibited more ductile behavior as hydrostatic pressure increased [10,19]. For example, the compressive-stress–strain curve of carbon/epoxy composites showed linear elastic behavior at atmospheric pressure but exhibited curvature toward the end of the curve as the hydrostatic pressure increased. After fracture tests, the fractured specimens were examined to investigate the fracture modes of seawater-absorbed carbon/epoxy laminated composites at various atmospheric and hydrostatic pressure conditions. Fig. 6 shows the typical surface morphology of specimens fractured at normal atmospheric pressure and 270 MPa hydrostatic pressure condition. As shown in the photographs, fracture occurred at one end of specimen, and no cracks or damage took place on the other end of specimen for each pressure level. At normal atmospheric pressure, fracture occurred by a combination of various compressive fracture processes such as fiber/matrix debonding, kinking of fibers, and fiber breakage. But final fracture occurred mainly due to the propagation of initial delamination. At the same time, local delaminations occurred at multiple sites on one end of the specimens and propagated along the length of the specimen. Then, shear fracture at about 30◦ along the fiber direction intersected the longitudinal propagation of delamination.
Fig. 6. Typical surface morphology of seawater-absorbed carbon/epoxy specimens fractured at normal atmospheric pressure (a) and 270 MPa hydrostatic pressure condition (b).
At 270 MPa, hydrostatic pressure condition, primary fracture mode was also a propagation of initial delamination. Debonding between the fiber and matrix, fiber breakage, and microcracks occurred similar to the results of normal atmospheric pressure. However, shearing fractures cutting through the plies occurred along the about 45◦ angle with respect to the loading direction. Particularly, numerous microcracks and local delaminations that could occur in the portion of cross-sectional area at the pressure level of 0.1 MPa were suppressed. This indicates that hydrostatic pressure which works as a compressive normal stress makes it more difficult for delaminations and microcracks to be initiated and propagated, leading to higher fracture toughness. The fracture surface of seawater-absorbed specimen was examined using SEM and was compared with that of dried specimen to observe the fracture mechanism of seawater-absorbed carbon/epoxy laminate composites in the hydrostatic pressure condition. Fig. 7 compares pictures of the dried specimen (a)
Fig. 7. SEM micrographs of fracture surfaces for the dried specimen (a), and the seawater-absorbed specimen (b) under the same hydrostatic pressure of 270 MPa.
K.Y. Rhee et al. / Materials Science and Engineering A 419 (2006) 209–213
and the seawater-absorbed specimen (b) under the same hydrostatic pressure of 270 MPa. Both specimens show lots of epoxy attachment and plastic deformation due to the hydrostatic pressure of 270 MPa. However, seawater-absorbed specimen shows a lot more epoxy fracture. This can be understood as a phenomenon that occurs due to the degradation of the epoxy resin due to seawater absorption according to increased retting time, degradation of the interface between the fiber and resin, and degradation of each ply interface. General seawater absorption swelling mechanisms of the carbon/epoxy laminate composite are the following: first, the increased free volume between the molecules due to seawater penetration into small pores; second, increased free volume due to seawater penetration into the interface between the carbon fiber and the epoxy resin as well as the delamination interface between layers. The absorbed seawater plasticizes the epoxy, causes cracks by creating volume expansion, and changes the inside stress condition. Therefore, the toughness of seawater-absorbed carbon/epoxy composites decreases due to the degradation of the epoxy and interface through these damaging processes. 4. Summary In this study, compressive fracture tests were performed using seawater-absorbed carbon/epoxy composites under various hydrostatic pressures ranging from 0.1 to 270 MPa to investigate the fracture characteristics of polymer matrix composites in a deep-sea environment. Fracture toughness was determined from the work factor approach and plotted as a function of applied hydrostatic pressure. The followings conclusions were drawn from this study: (1) Fracture characteristics of seawater-absorbed carbon/epoxy composites are significantly influenced by hydrostatic pres-
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sure. Compliance decreases but fracture load and fracture toughness increase with increasing hydrostatic pressure. (2) Fracture modes in the hydrostatic pressure condition are similar to those in a normal atmospheric pressure condition. However, local delaminations and microcracks are suppressed by hydrostatic pressure. The angle of the shear fracture plane increases with increasing hydrostatic pressure. Acknowledgement This work was supported by Ministry of Maritime Affairs and Fisheries as a part of the Deep Ocean Water Application Research Program. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
W.C. Tucker, R. Brown, J. Compos. Mater. 23 (1988) 787. K. Ogi, N. Takeda, Compos. Mater. 31 (1997) 530. S.C. Soutis, D. Turkmen, J. Compos. Mater. 31 (1997) 832. T.S. Grant, W.L. Bradley, J. Compos. Mater. 29 (1995) 852. M. Todo, T. Nakamura, K. Takahashi, J. Compos. Mater. 34 (2000) 630. F.E. Sloan, R.J. Seymour, J. Compos. Mater. 26 (1992) 2655. I. Chou, Compo. Mater. 7 (1998) 377. C.W. Weaver, J.G. Williams, J. Mater. Sci. 10 (1975) 1323. A.S. Wronski, T.V. Parry, J. Mater. Sci. 17 (1982) 3656. E.S. Shin, K.D. Pae, J. Compos. Mater. 26 (1992) 462. K.D. Pae, K.Y. Rhee, Compos. Sci. Technol. 53 (1995) 281. K.Y. Rhee, K.D. Pae, J. Compos. Mater. 29 (1995) 1295. P.A. Zinoviev, S.V. Tsvetkov, G.G. Kulish, R.W. Van den Berg, L. Van Schepdael, Compos. Sci. Technol. 61 (2001) 1151. K.Y. Rhee, J. Compos. Mater. 34 (2000) 599. K.Y. Rhee, S.M. Lee, S.J. Park, Mater. Sci. Eng. 384 (2004) 308. K.S. Carlson, MS thesis Rutgers University. J.R. Rice, P.C. Paris, J.G. Merkle, ASTM STP 536 (1973) 231. K.Y. Rhee, J.H. Lee, S.J. Park, Mater. Sci. Eng. 349 (2003) 218. E.S. Shin, K.D. Pae, J. Compos. Mater. 26 (1992) 828.
Fracture behavior of seawater-absorbed carbon/epoxy laminated composites in the hydrostatic pressure condition K.Y. Rhee a,∗ , S.R. Ha a , S.J. Park b , H.J. Kim c , D.H. Jung c a
Center for Media Transport System, School of Mechanical and Industrial System Engineering, Kyunghee University, Yongin 449-701, Republic of Korea b Advanced Materials Division, Korea Research Institute of Chemical Technology, Taejon 305-600, Republic of Korea c Ocean Development System Laboratory, Korea Research Institute of Ships and Ocean Engineering, Taejon 305-600, Republic of Korea Received in revised form 16 December 2005; accepted 16 December 2005
Abstract It was shown in the previous study that the fracture toughness of dried carbon/epoxy composites increased 38% as the applied hydrostatic pressure increased from 0.1 to 200 MPa. However, no research has been conducted on the fracture behavior of seawater-absorbed carbon/epoxy composites in a hydrostatic pressure environment. This work investigates the compressive fracture behavior of seawater-absorbed carbon/epoxy composites subjected to various hydrostatic pressures. Compressive fracture tests were performed under four hydrostatic pressure levels, 0.1 (atmospheric pressure), 100, 200, and 270 MPa. The compliance and fracture loads were determined from load–displacement curves as a function of hydrostatic pressure. Fracture toughness was determined from the elastic work factor approach. The results showed that compliance decreases but fracture load increases as the applied hydrostatic pressure increases. Fracture toughness increased with increasing pressure. Specifically, fracture toughness increased from 2.66 to 4.44 kJ/m2 , a 67% increase, as hydrostatic pressure increased from 0.1 to 270 MPa. Optical microscope examination of the fractured surface showed that local delaminations and microcracks are suppressed with increasing hydrostatic pressure. SEM examination showed that seawater-absorbed specimen shows a lot more epoxy fracture than the dried specimen. © 2006 Published by Elsevier B.V. Keywords: Carbon/epoxy composite; Hydrostatic pressure; Work factor approach; Delamination; Fracture toughness
1. Introduction Polymer matrix composites are used in a variety of structural components due to their stiffness and strength and their lightweight savings. Currently, polymeric composites are widely applied to build offshore oil structures such as floating platforms and tension leg platforms because they are more corrosionresistant than metals. However, it is known that polymeric composites absorb moisture and exhibit some degree of degradation due to this absorption factor. Accordingly, many studies have been conducted to investigate the effect of moisture absorption on the mechanical behavior of glass/epoxy and carbon/epoxy composites, focusing on tensile, compressive, and interlaminar shear [1–5]. It is known from their research results that moisture absorption causes plasticization and swelling of the epoxy, weakening the interfacial strength between fibers and epoxy,
∗
Corresponding author. Tel.: +82 31 201 2565; fax: +82 31 202 6693. E-mail address: [email protected] (K.Y. Rhee).
0921-5093/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.msea.2005.12.025
which results in the reduction of tensile and shear strength. However, the effect of moisture absorption on fracture behavior is different depending on the studies conducted. Sloan and Seymour [6] reported that the mode I fracture resistance, GIR of carbon/epoxy specimens exposed to seawater, was more than twice than that of dry specimens. However, Chou [7] reported that the GIR of moisture-absorbed carbon/epoxy composites was less than that of dry cases. The effect of hydrostatic pressure on the mechanical properties of polymer matrix composites was also studied by many researchers [8–13]. These studies focused on the tensile and compressive behaviors glass/epoxy and carbon/epoxy composites in hydrostatic pressure environment. It is generally accepted that the tensile and compressive strengths of polymeric composites increase as the applied hydrostatic pressure increases. Recently, the hydrostatic pressure effect on the compressive fracture behavior of carbon/epoxy composites was investigated by Rhee [14]. His result showed that fracture toughness increased about 38% as hydrostatic pressure increased from 0.1 to 200 MPa.
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K.Y. Rhee et al. / Materials Science and Engineering A 419 (2006) 209–213
To date no study has been conducted to evaluate the combined effect of moisture absorption and hydrostatic pressure on the fracture behavior of polymeric composites. A study on this combined effect is essential to expand the use of polymeric composites to manufacture submersible structures; these structures undergo moisture absorption and hydrostatic pressure in a deepsea environment. The main focus of this study has been to investigate the combined effects of seawater absorption and hydrostatic pressure on the fracture behavior of carbon/epoxy composites. For this purpose, fracture tests were performed using seawater-absorbed carbon/epoxy composite, whose seawater absorption capability was at a maximum absorption level, in a hydrostatic pressure environment. Compliance, fracture load and fracture toughness were determined as a function of applied hydrostatic pressure. After the fracture tests were performed, optical microscope examination was made on the fracture surfaces to determine the fracture mechanism of carbon/epoxy composites in a deep-sea environment. 2. Experimental 2.1. Material and specimen preparation Carbon/epoxy composites plates were fabricated using a continuous filament non-woven carbon/epoxy prepregnated (prepreg) tape (USN 150B, SK Chemical, Korea). The prepreg tape has a 64% fiber content by volume and the nominal cured thickness was approximately 0.125 mm. The prepreg tapes were cut and stacked as [0o ]88 to give a final plate thickness of 10.5 mm. A kapton film (thickness: 13 m) was inserted at the midplane (between 44th and 45th plies) to produce the initial crack. The length of the initial crack was 8 mm. The stacked plates were cured in an autoclave under 5 kg/cm2 of pressure at 126 ◦ C for 2 h. Then, the cured plates were fabricated as dog bone specimens where the sample length was 32 mm. The shape and dimension of test sample are shown in Fig. 1. The test samples were kept in sterile-filtered seawater (SIGMA, S-9148) over 13 months in order to produce fully seawater-absorbed specimens. It was shown in the previous study that the absorption of seawater for the carbon/epoxy composites used in this study were fully saturated at 6 months [15].
Fig. 1. Schematic diagram of dog bone test sample.
2.2. Fracture test The compressive fracture tests of fully seawater-absorbed carbon/epoxy composites were conducted under normal atmospheric pressure and under three levels of hydrostatic pressure conditions using a high pressure tension–compression apparatus which can produce 700 MPa of hydrostatic pressure. The apparatus consists of two thick-walled cylindrical pressure vessels, one for testing and the other for pressure compensation. These two vessels are interconnected by a high pressure tubing so that a constant pressure can be maintained in the test chamber during the compression test. Dow Corning 200 silicon oil was the hydraulic fluid used as the pressure medium. Displacement of the loading piston was sensed by a linear variable differential transducer (LVDT) and load was measured with a full-bridge strain gauge inside the wall of the loading piston. Electrical signals from these two transducers were fed into two signal conditioners (Daytronics, Models 3130 and 3170), then sent to an IBM PC and analyzed. The working mechanism of the testing apparatus was well described by Carlsson [16]. The hydrostatic pressures applied were 100, 200, and 270 MPa. Fracture tests were performed applying a constant strain rate of 0.05%/s at atmospheric pressure and hydrostatic pressure conditions. At least three tests were carried out at each pressure level in order to assure the repeatable nature and reliability of test results. 3. Results and discussion The effect of hydrostatic pressure on the fracture behavior of seawater-absorbed carbon/epoxy composites can be determined by comparing compliance, fracture load, and fracture toughness of atmospheric pressure condition with those of hydrostatic pressure conditions. Compliance and the fracture load can be determined from the load–displacement curve. The typical load–displacement curves at atmospheric pressure and various hydrostatic pressures are shown in Fig. 2; which shows that fracture behavior is significantly affected by hydrostatic pressure. It can also be seen in the figure that for each pressure level; load
Fig. 2. Typical load–displacement curves of seawater-absorbed carbon/epoxy composites at atmospheric pressure and various hydrostatic pressures.
K.Y. Rhee et al. / Materials Science and Engineering A 419 (2006) 209–213
Fig. 3. Variation of compliance of seawater-absorbed carbon/epoxy composites as a function of applied hydrostatic pressure.
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pressure. As shown in the figure, fracture load was affected by hydrostatic pressure but the response of the fracture load to hydrostatic pressure was opposite of compliance. In other words, fracture load increased with increasing hydrostatic pressure. The fracture load increased approximately 35% as the applied hydrostatic pressure increased from 0.1 to 270 MPa. The primary reason fracture load increased with increasing hydrostatic pressure was shown to be that hydrostatic pressure acts as a compressive normal stress along the surfaces of specimen. This compressive normal stress leads to a suppression of debonding at the interface and crack propagation occurs. Therefore, fracture load increased as the applied hydrostatic pressure increased. In general, fracture toughness of fiber-reinforced composites is determined experimentally from the compliance method. However, it requires four to five specimens of different crack lengths. Particularly, for the fracture tests in the hydrostatic pressure condition, the method requires additional efforts to determine fracture toughness because it takes a long time to stabilize pressure. Therefore, fracture toughness was determined from the work factor approach in this study. The merit of applying the work factor approach is that fracture toughness can be determined from a single fracture test. Fracture toughness, Gc was determined from the work factor approach as follows [17]:
increases almost linearly with displacement and the non-linear portion is relatively small up to the fracture point. Compliance was determined by measuring the inverse slope of the linear region through the origin in the load–displacement curve because compliance was defined as displacement divided by applied load. Fig. 3 shows the variation of compliance as a function of applied hydrostatic pressure. Two facts are evident based on this figure. One is that compliance was affected by hydrostatic pressure and the other is that compliance decreased with increasing pressure. The values of average compliance at atmospheric pressure (0.1 MPa) and 270 MPa pressure levels are 8.91 × 10−8 and 8.08 × 10−8 m/N, respectively. That is, average compliance decreased about 10% as the applied hydrostatic pressure was increased from 0.1 to 270 MPa. In order to investigate the effect of hydrostatic pressure on the fracture load of seawater-absorbed carbon/epoxy composites, the fracture load was determined as a function of hydrostatic pressure. In this study, the fracture load for a given pressure level was determined as the maximum load in the corresponding load–displacement curve because fracture behavior displayed linear elastic properties irrespective of the pressure level. Fig. 4 shows the variation of fracture load as a function of hydrostatic
where a is a crack length and L is a gauge length. Fig. 5 shows the variation of fracture toughness of seawater-absorbed carbon/epoxy composites as a function of hydrostatic pressure. As shown in the figure, fracture toughness is affected by the hydrostatic pressure and increased as hydrostatic pressure increased.
Fig. 4. Variation of fracture load of seawater-absorbed carbon/epoxy composites as a function of hydrostatic pressure.
Fig. 5. Variation of fracture toughness of seawater-absorbed carbon/epoxy composites as a function of hydrostatic pressure.
Gc =
ηel Ac Bb
(1)
where b represents the remaining ligament, B the specimen width, Ac the area under the load–displacement record up to fracture and ηel is the elastic work factor. It was shown in the previous study that ηel is not affected by the applied hydrostatic pressure and stacking sequence [18]. The ηel form of the specimen used in this study was determined as follows: ηel = 0.83 − 0.91
a L
(2)
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The averaged values of fracture toughness at atmospheric pressure (0.1 MPa), 100, 200, and 270 MPa are 2.66, 3.18, 3.91, and 4.44 kJ/m2 , that is, fracture toughness increased 67% when the applied hydrostatic pressure increased from 0.1 to 270 MPa. The increased rate of fracture toughness with hydrostatic pressure for seawater-absorbed carbon/epoxy composites was larger than that of regular (dry) carbon/epoxy composites. Previous studies showed that fracture toughness of dry carbon/epoxy composites increased about 38% when the applied hydrostatic pressure increases from 0.1 to 200 MPa [14]. However, the fracture toughness of seawater-absorbed specimen is lower than that of dry specimen at the same hydrostatic pressure. This agrees with the result of Chou’s work [7]. The decrease of fracture toughness occurs because the wetting between fibers and the epoxy resin becomes poor as the specimen absorbs seawater. It can be inferred from Eq. (1) that the increase of fracture toughness with increasing pressure occurs due to the increase of fracture load because compliance decreased as the hydrostatic pressure increased. The phenomenon of fracture toughness increase with increasing pressure was also related to epoxy behavior in the hydrostatic pressure condition. It was shown in the previous study that epoxy material exhibited more ductile behavior as hydrostatic pressure increased [10,19]. For example, the compressive-stress–strain curve of carbon/epoxy composites showed linear elastic behavior at atmospheric pressure but exhibited curvature toward the end of the curve as the hydrostatic pressure increased. After fracture tests, the fractured specimens were examined to investigate the fracture modes of seawater-absorbed carbon/epoxy laminated composites at various atmospheric and hydrostatic pressure conditions. Fig. 6 shows the typical surface morphology of specimens fractured at normal atmospheric pressure and 270 MPa hydrostatic pressure condition. As shown in the photographs, fracture occurred at one end of specimen, and no cracks or damage took place on the other end of specimen for each pressure level. At normal atmospheric pressure, fracture occurred by a combination of various compressive fracture processes such as fiber/matrix debonding, kinking of fibers, and fiber breakage. But final fracture occurred mainly due to the propagation of initial delamination. At the same time, local delaminations occurred at multiple sites on one end of the specimens and propagated along the length of the specimen. Then, shear fracture at about 30◦ along the fiber direction intersected the longitudinal propagation of delamination.
Fig. 6. Typical surface morphology of seawater-absorbed carbon/epoxy specimens fractured at normal atmospheric pressure (a) and 270 MPa hydrostatic pressure condition (b).
At 270 MPa, hydrostatic pressure condition, primary fracture mode was also a propagation of initial delamination. Debonding between the fiber and matrix, fiber breakage, and microcracks occurred similar to the results of normal atmospheric pressure. However, shearing fractures cutting through the plies occurred along the about 45◦ angle with respect to the loading direction. Particularly, numerous microcracks and local delaminations that could occur in the portion of cross-sectional area at the pressure level of 0.1 MPa were suppressed. This indicates that hydrostatic pressure which works as a compressive normal stress makes it more difficult for delaminations and microcracks to be initiated and propagated, leading to higher fracture toughness. The fracture surface of seawater-absorbed specimen was examined using SEM and was compared with that of dried specimen to observe the fracture mechanism of seawater-absorbed carbon/epoxy laminate composites in the hydrostatic pressure condition. Fig. 7 compares pictures of the dried specimen (a)
Fig. 7. SEM micrographs of fracture surfaces for the dried specimen (a), and the seawater-absorbed specimen (b) under the same hydrostatic pressure of 270 MPa.
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and the seawater-absorbed specimen (b) under the same hydrostatic pressure of 270 MPa. Both specimens show lots of epoxy attachment and plastic deformation due to the hydrostatic pressure of 270 MPa. However, seawater-absorbed specimen shows a lot more epoxy fracture. This can be understood as a phenomenon that occurs due to the degradation of the epoxy resin due to seawater absorption according to increased retting time, degradation of the interface between the fiber and resin, and degradation of each ply interface. General seawater absorption swelling mechanisms of the carbon/epoxy laminate composite are the following: first, the increased free volume between the molecules due to seawater penetration into small pores; second, increased free volume due to seawater penetration into the interface between the carbon fiber and the epoxy resin as well as the delamination interface between layers. The absorbed seawater plasticizes the epoxy, causes cracks by creating volume expansion, and changes the inside stress condition. Therefore, the toughness of seawater-absorbed carbon/epoxy composites decreases due to the degradation of the epoxy and interface through these damaging processes. 4. Summary In this study, compressive fracture tests were performed using seawater-absorbed carbon/epoxy composites under various hydrostatic pressures ranging from 0.1 to 270 MPa to investigate the fracture characteristics of polymer matrix composites in a deep-sea environment. Fracture toughness was determined from the work factor approach and plotted as a function of applied hydrostatic pressure. The followings conclusions were drawn from this study: (1) Fracture characteristics of seawater-absorbed carbon/epoxy composites are significantly influenced by hydrostatic pres-
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sure. Compliance decreases but fracture load and fracture toughness increase with increasing hydrostatic pressure. (2) Fracture modes in the hydrostatic pressure condition are similar to those in a normal atmospheric pressure condition. However, local delaminations and microcracks are suppressed by hydrostatic pressure. The angle of the shear fracture plane increases with increasing hydrostatic pressure. Acknowledgement This work was supported by Ministry of Maritime Affairs and Fisheries as a part of the Deep Ocean Water Application Research Program. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
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