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Epoxy Composites with Voids
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Energy Procedia
Energy Procedia00 16(2011) (2012)000–000 1737 – 1743 Energy Procedia
www.elsevier.com/locate/procedia
2012 International Conference on Future Energy, Environment, and Materials
Tensile Strength of Hygrothermally Conditioned Carbon/Epoxy Composites with Voids Zhang A-ying1,2, Zhang Dong-xing1*, Li 1Di-hong1, Sun Tao1, Xiao Hai-ying1, Jia Jin 1
Harbin Institute of Technology, Harbin, China, 150001; 2. Harbin University, Harbin, China, 150086
Abstract The effects of voids on the longitude tensile strength and modulus of T300/914 laminates that exposed to room temperature, hygrothermal and drying environment was discussed in this paper. The experimental results reveal that the saturated moisture content and the rate of water uptake increase with porosity increasing from 0.33% to 1.5%, which proves that voids facilitate moisture absorption. The tensile strength of non-aged, aged and drying specimens were characterized and analyzed. Compared to non-aged specimens, a relatively small reduction in the tensile strength with the increasing porosity; the tensile modulus of aged specimens basically unchanged. For the same porosity, the tensile strength of drying specimens is higher than that of non-aged specimens for the same porosity. © 2011 Published by Elsevier B.V. Selection and/or peer-review under responsibility of International Materials Science Society.
© 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of [name organizer] Keywords-Carbon fiber; Laminates; Hygrothermal environment; Void content; Mechanical property
1.
Introduction
Carbon/epoxy composites are widely used in aerospace as an advanced aerospace materials [1,2]. Composite Structure must be considered for manufacturing-induced defects such as voids and delaminations that may have detrimental effects on the composites. Voids are among the most common manufacturing induced defects in polymeric composites [3,4], degrading their structural performance and reducing the mechanical strength of composite laminates [5-7]. In general, voids decrease the static strength and cause a greater susceptibility to water penetration and environmental conditions [5]. Costa [8] indicates that voids in polymer matrix composites may result in significant reductions in matrix-dominated mechanical properties such as interlaminar shear, and compressive and flexural strengths. And voids affect the same mechanical properties affected by environmental conditions. Cândido [9] also reports that moisture absorption may induce mechanical and physic-chemical changes
1876-6102 © 2011 Published by Elsevier B.V. Selection and/or peer-review under responsibility of International Materials Science Society. doi:10.1016/j.egypro.2012.01.269
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Zhang A-ying et al. / Energy Procedia 16 (2012) 1737 – 1743
Author name / Energy Procedia 00 (2011) 000–000
that toughen the polymeric matrix and/or deteriorate matrix/fiber interface by interface debonding or micro-cracking. Meziere [10] reports that humidity has an influence on the fiber/matrix interface as well as on the matrix properties. The behavior of any composite depends on the efficiency of the fiber/matrix interface and this can be reduced by the presence of water. Considerable efforts have been made to focus on the effects on void content on mechanical behaviors at room temperature and hygrothermal environment, few papers have been published on the role of voids on mechanical strength when laminates exposed to room temperature, hygrothermal and drying environment. The objective of this research was to investigate the effect of voids on the longitude tensile strength and modulus of T300/914 laminates which exposed to room temperature, hygrothermal and drying environment, respectively. The experiment was defined in order to put particular emphasis on the study of tensile properties degradation as a function of the void contents in different environment. 2.
Experimental
2.1 Fabrication of Composite The specimens were prepared from Hexcel T300/914 woven carbon bre/epoxy matrix prepreg with approximately bre volume fraction of 58%. The specimens were cut from the laminates with the size of 270mm×300mm×4.5mm. All laminates were layed-up at [(±45)4/(0,90)/(±45)2]S. The composite laminate was hot pressed at 135 °C for 0.5 h and post-cured at 180 °C for 2.5 h in an autoclave. Finally, they were ramped to 45 °C to produce the final composite laminates. In addition, all the steps were conducted in vacuum bag of 0.06 MPa. The carbon/epoxy laminates with three different porosities levels ranging from 0.33 to 1.5% were produced by adopting three different magnitudes of autoclave pressures. Three conditions were considered in this work: (a) ambient condition: the specimens were exposed to room temperature, defined as non-aged specimen; (b) hygrothermal condition: the specimens were dried in an oven at 70 °C until their weights were stabilized and then immersed in water bath at the temperature of 70 °C until moisture saturation, defined as aged specimen; and (c) drying condition: the saturated specimens were dried in an oven at the temperature of 70 °C until reaching equilibrium, defined as drying specimen. 2.2 Hygrothermal Test Water absorbed specimens were prepared according to the following procedure. Prior to absorption experiment, all specimens were dried in a heating oven at 70 oC until their weights became stable. Specimens were immersed in a water bath at 70 oC for 4 weeks until reaching the limit of saturation. Water contents in the specimen were measured by the difference of weight throughout the immersion test. Specimen was removed from water bath once, and again brought back to the water bath after measuring weight. Specimens need to be cooled back to room temperature before weighting. The changes in weight and size of the specimens were measured as a function of immersion time. The water gain percentage, Mi was determined from the equation (1), (1) Mi=(Gi-G0)/ G0×100% Where Gi is the wet weight of specimen (g), and G0 is the dry weight of specimen (g). In drying test, the saturated specimens were dried in a heating oven at 70 oC until there was no change in their weights. The specimens were then removed from the oven and were cooled down to room
Zhang A-ying et al. / Energy Procedia 16 (2012) 1737 – 1743
Author name / Energy Procedia 00 (2011) 000–000
temperature for further testing. The drying specimens were experimented to determine if some properties can regain their original state by removing the absorbed moisture after cyclic wetting and drying process. 2.3 Mechanical Property Test The effects of hygrothermal and drying conditions on the tensile strength and modulus in laminates with three different porosities levels ranged from 0.33% to 1.5% were investigated. Five rectangular specimens had the dimensions of 230 mm×25 mm×4.5 mm were tested to determine the average tensile strength. The tensile tests were performed in an Instron 5582 mechanical test machine, and the loading rate is 2mm/min. 3.
Results and discussion
3.1 Moisture Absorption The moisture absorption behavior is shown in Figure. 1, where the percent weight gain is plotted as a function of the immersion time for aged specimens with different void contents ranging 0.33 to 1.5%. Moisture content increases linearly with the immersion time at first and then reach a pseudo equilibrium state. The solid lines are obtained by tting the initial stage of the moisture absorption. As shown in Figure. 1 the moisture content increases with the increasing of the immersion time.
Figure 1. Moisture absorption curves of different porosity
Table 1 shows that saturated moisture content are affected by both the immersion time and the porosity. For both the tensile specimens, the saturated moisture content increase with the increasing of the immersion time and porosity. This means that the increasing porosity led to an acceleration of moisture. Table 1
Saturated moisture content with different porosity Specimen Tensile test
Porosity /% 0.33 0.71 1.5
Immersion time /h 672 672 720
Saturated moisture content/g 1.345 1.396 1.505
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Zhang A-ying et al. / Energy Procedia 16 (2012) 1737 – 1743
Author name / Energy Procedia 00 (2011) 000–000
The water absorption curves of both materials obeyed Fick's law in the linear region before reaching to saturation level as depicted in Figure 1. Through the experimental results, it is noted that the voids significantly affect the characteristics of the diffusion rate and equilibrium water absorption content (Mm) as listed in Table 2. As expected, there is an increasing rate of water absorption characteristics for both tensile and compressive specimens with the increasing of porosity. According to Fick's law, diffusion coefficient, D is determined from the equation (2), (2) D=π(h/4Mm)2k2 Where h is the thickness of specimen (mm), Mm is the equilibrium moisture absorption content (g), and k is the slope at the initial stage of the moisture absorption curve. As listed in Table 2, D and k increase with the increasing of porosity, and is affected by the dimension of specimen. It is noted that the rate of water uptake increase with the porosity, which prove voids facilitate moisture absorption and might also provide paths by which water move into composites. Table 2
3.2
Diffusion coefficient with different porosity Specimen
Dimension /mm×mm
Porosity /%
Mm
k
D /mm2/h
Tensile test
230×25
0.33 0.71 1.5
1.241 1.25 1.294
0.04956 0.05736 0.06083
0.0063 0.0084 0.0088
/g
Tensile Property Test
As shown in Figure 2, the tensile strength of non-aged specimens decreased 0.8% with the porosity increasing from 0.33% to 1.5%. Compared to non-aged specimens, the tensile strength of aged specimens decreased 6.5%, 2.5% and 1.2% with porosity of 0.33%, 0.71% and 1.5%, respectively. The presence of moisture within polymeric composites often degrades their physical and mechanical properties [9,11]. Figure 2(a) and (b) shows that the curve of tensile strength and tensile modulus of aged specimens appeared to be ups and downs, and eventually present a downward trend. The experimental results demonstrate that moisture may indeed either increase or decrease the tensile strength [9]. Moisture may degrade the fiber/matrix interface and/or toughen the polymeric matrix [8,11,12]. Therefore, the effect of moisture on the tensile strength depends on the dominant failure mechanism. a)
Zhang A-ying et al. / Energy Procedia 16 (2012) 1737 – 1743
Author name / Energy Procedia 00 (2011) 000–000
b)
Figure 2. Tensile property curves of aged specimens with different porosity. a)Tensile strength. b)Tensile modulus.
The reduction in net section with the increasing void content has a detrimental effect on strength. Whilst for the aged specimens, the tensile strength with the porosity of 1.5% is higher than porosity of 0.33% as shown in Figure 2(a). The water absorbed by the epoxy laminates, in general, causes reversible plasticization of the matrix and lowers its glass transition temperature [13]. In this case, the plasticizer effect of water in the composite structures is superior to the degradation effect. Compared to non-aged specimens, the tensile modulus of aged specimens basically unchanged as shown in Figure 2(b). The tensile modulus test is principally driven by the behavior of the carbon fiber which is basically non-hygroscopic. The specimens after tensile test were studied by scanning electron microscope and regions of fracture are shown in Figure 3 for both non-aged and aged specimens. Water absorption and their resulting effects contribute to the loss of compatibilization between fibers and matrix, which results in debonding and weakening of the interface adhesion [14].The behavior of laminates depends on the efficiency of the fiber/matrix interface and this can be reduced by the presence of water [10]. Figure 3(b) shows that the cracks of the fiber/matrix interface are more severe than that of non-aged specimens as shown in Figure 3(a). a)
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Zhang A-ying et al. / Energy Procedia 16 (2012) 1737 – 1743
Author name / Energy Procedia 00 (2011) 000–000
b)
Figure 3. SEM of the non-aged and aged specimens after tensile test. a)Non-aged specimens. b)Aged specimens.
Figure 4 shows that the tensile strength of aged specimens is lower than that of non-aged specimens for the same porosity. The general trend of the tensile strength of drying specimens decreased with the porosity increasing from 0.33% to 1.5%. And the tensile strength of drying specimens is higher than that of non-aged specimens for the same porosity. This may be concluded that drying condition lead to the increase of the degree of cure, which eliminate some of the residual stress and improve the adhesion of fiber/matrix interface.
Figure 4. Tensile strength in different conditions of different porosity
4.
Conclusions
⑴ For both the tensile specimens, the saturated moisture content and the rate of water uptake increase with porosity increasing from 0.33% to 1.5%, which proves voids facilitate moisture absorption.
Zhang A-ying et al. / Energy Procedia 16 (2012) 1737 – 1743
Author name / Energy Procedia 00 (2011) 000–000
⑵The tensile strength of non-aged specimens decreased 0.8% with the porosity increasing from 0.33% to 1.5%. The curve of tensile strength and tensile modulus of aged specimens appeared to be ups and downs, and eventually present a downward trend. Compared to non-aged specimens, the tensile strength of aged specimens decreased 6.5%, 2.5% and 1.2% with porosity of 0.33%, 0.71% and 1.5%, respectively; the tensile modulus of aged specimens basically unchanged. The tensile strength of aged specimens is lower than that of non-aged specimens for the same porosity. And the tensile strength of drying specimens is higher than that of non-aged specimens for the same porosity. Acknowledgement The authors are gratefully to Harbin Aircraft Industry Group for supporting this work. References [1] Atadero R. A., Karbhari V. M.. Calibration of resistance factors for reliability based design of externally-bonded FRP composites[J]. Composites Part B, 2008, 39(4) : 665-679. [2] Marouani S., Curtil L., Hamelin P.. Composites realized by hand lay-up process in a civil engineering environment: initial properties and durability[J]. Materials and structures, 2008, 41(5) : 831-851. [3] Puglia P. D., Sheikh M. A, Hayhurstb D. R. Classification and quantification of initial porosity in a CMC laminate[J]. Composites Part A, 2004, 35(2), 223-230. [4] Birt E. A., Smith R. A.. A review of NDE methods for porosity measurement in fibre-reinforced polymer composites[J]. Non-Destructive Testing and Condition Monitoring, 2004, 46(11), 681-686. [5] Costa M. L., Almeida S. F. M., Rezende M. C.. The influence of porosity on the interlaminar shear strength of carbon epoxy and carbon bismaleimide fabric laminates[J]. Composites Science and Technology, 2001, 61(14), 2101-2108. [6] Wisnom M. R., Reynolds T., et al. Reduction in interlaminar shear strength by discrete and distributed voids[J]. Composites Science and Technology, 1996, 56(1), 93-101. [7] Chambers A. R., Earl J. S., Squires C. A., et al. The effect of voids on the flexural fatigue performance of unidirectional carbon fibre composites developed for wind turbine applications[J]. International Journal of Fatigue, 2006, 28(10), 1389-1398. [8] Costa M. L., Rezende M. C., Almeida S. F. M.. Strength of hygrothermally conditioned polymer composites with voids[J]. Journal of Composite Materials, 2005, 39(21), 1943-1961. [9] Cândido G. M., Costa M. L., Rezende M. C., et al. Hygrothermal effects on quasi isotropic carbon epoxy laminates with machined and molded edges[J]. Composites Part B, 2008, 39(3); 490-496. [10] Meziere Y., Bunsell A. R., Favry Y., et al. Large strain cyclic fatigue testing of unidirectional carbon fibre reinforced epoxy resin[J]. Composites Part A, 2005, 36(12), 1627-1636. [11] Costa M. L., Rezende M. C., Sergio F.. Effect of Void Content on the Moisture Absorption in Polymeric Composites[J]. Polymer-Plastics Technology and Engineering, 2006,45(6), 691-698. [12] Cândido G. M., Rezende M. C , Almeida S. F. M.. Hygrothermal effects on the tensile strength of carbon epoxy laminates with molded edges[J]. Materials Research, 2000, 3(2), 11-17. [13] Wolff, E.G.. Moisture Effects on Polymer Matrix Composites[J]. SAMPE Journal, 1993, 29(3), 11-19. [14] Kim H. J., Seo D. W.. Effect of water absorption fatigue on mechanical properties of sisal textile-reinforced composites[J]. International Journal of Fatigue, 2006, 28(10), 1307-1314.
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Energy Procedia
Energy Procedia00 16(2011) (2012)000–000 1737 – 1743 Energy Procedia
www.elsevier.com/locate/procedia
2012 International Conference on Future Energy, Environment, and Materials
Tensile Strength of Hygrothermally Conditioned Carbon/Epoxy Composites with Voids Zhang A-ying1,2, Zhang Dong-xing1*, Li 1Di-hong1, Sun Tao1, Xiao Hai-ying1, Jia Jin 1
Harbin Institute of Technology, Harbin, China, 150001; 2. Harbin University, Harbin, China, 150086
Abstract The effects of voids on the longitude tensile strength and modulus of T300/914 laminates that exposed to room temperature, hygrothermal and drying environment was discussed in this paper. The experimental results reveal that the saturated moisture content and the rate of water uptake increase with porosity increasing from 0.33% to 1.5%, which proves that voids facilitate moisture absorption. The tensile strength of non-aged, aged and drying specimens were characterized and analyzed. Compared to non-aged specimens, a relatively small reduction in the tensile strength with the increasing porosity; the tensile modulus of aged specimens basically unchanged. For the same porosity, the tensile strength of drying specimens is higher than that of non-aged specimens for the same porosity. © 2011 Published by Elsevier B.V. Selection and/or peer-review under responsibility of International Materials Science Society.
© 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of [name organizer] Keywords-Carbon fiber; Laminates; Hygrothermal environment; Void content; Mechanical property
1.
Introduction
Carbon/epoxy composites are widely used in aerospace as an advanced aerospace materials [1,2]. Composite Structure must be considered for manufacturing-induced defects such as voids and delaminations that may have detrimental effects on the composites. Voids are among the most common manufacturing induced defects in polymeric composites [3,4], degrading their structural performance and reducing the mechanical strength of composite laminates [5-7]. In general, voids decrease the static strength and cause a greater susceptibility to water penetration and environmental conditions [5]. Costa [8] indicates that voids in polymer matrix composites may result in significant reductions in matrix-dominated mechanical properties such as interlaminar shear, and compressive and flexural strengths. And voids affect the same mechanical properties affected by environmental conditions. Cândido [9] also reports that moisture absorption may induce mechanical and physic-chemical changes
1876-6102 © 2011 Published by Elsevier B.V. Selection and/or peer-review under responsibility of International Materials Science Society. doi:10.1016/j.egypro.2012.01.269
1738
Zhang A-ying et al. / Energy Procedia 16 (2012) 1737 – 1743
Author name / Energy Procedia 00 (2011) 000–000
that toughen the polymeric matrix and/or deteriorate matrix/fiber interface by interface debonding or micro-cracking. Meziere [10] reports that humidity has an influence on the fiber/matrix interface as well as on the matrix properties. The behavior of any composite depends on the efficiency of the fiber/matrix interface and this can be reduced by the presence of water. Considerable efforts have been made to focus on the effects on void content on mechanical behaviors at room temperature and hygrothermal environment, few papers have been published on the role of voids on mechanical strength when laminates exposed to room temperature, hygrothermal and drying environment. The objective of this research was to investigate the effect of voids on the longitude tensile strength and modulus of T300/914 laminates which exposed to room temperature, hygrothermal and drying environment, respectively. The experiment was defined in order to put particular emphasis on the study of tensile properties degradation as a function of the void contents in different environment. 2.
Experimental
2.1 Fabrication of Composite The specimens were prepared from Hexcel T300/914 woven carbon bre/epoxy matrix prepreg with approximately bre volume fraction of 58%. The specimens were cut from the laminates with the size of 270mm×300mm×4.5mm. All laminates were layed-up at [(±45)4/(0,90)/(±45)2]S. The composite laminate was hot pressed at 135 °C for 0.5 h and post-cured at 180 °C for 2.5 h in an autoclave. Finally, they were ramped to 45 °C to produce the final composite laminates. In addition, all the steps were conducted in vacuum bag of 0.06 MPa. The carbon/epoxy laminates with three different porosities levels ranging from 0.33 to 1.5% were produced by adopting three different magnitudes of autoclave pressures. Three conditions were considered in this work: (a) ambient condition: the specimens were exposed to room temperature, defined as non-aged specimen; (b) hygrothermal condition: the specimens were dried in an oven at 70 °C until their weights were stabilized and then immersed in water bath at the temperature of 70 °C until moisture saturation, defined as aged specimen; and (c) drying condition: the saturated specimens were dried in an oven at the temperature of 70 °C until reaching equilibrium, defined as drying specimen. 2.2 Hygrothermal Test Water absorbed specimens were prepared according to the following procedure. Prior to absorption experiment, all specimens were dried in a heating oven at 70 oC until their weights became stable. Specimens were immersed in a water bath at 70 oC for 4 weeks until reaching the limit of saturation. Water contents in the specimen were measured by the difference of weight throughout the immersion test. Specimen was removed from water bath once, and again brought back to the water bath after measuring weight. Specimens need to be cooled back to room temperature before weighting. The changes in weight and size of the specimens were measured as a function of immersion time. The water gain percentage, Mi was determined from the equation (1), (1) Mi=(Gi-G0)/ G0×100% Where Gi is the wet weight of specimen (g), and G0 is the dry weight of specimen (g). In drying test, the saturated specimens were dried in a heating oven at 70 oC until there was no change in their weights. The specimens were then removed from the oven and were cooled down to room
Zhang A-ying et al. / Energy Procedia 16 (2012) 1737 – 1743
Author name / Energy Procedia 00 (2011) 000–000
temperature for further testing. The drying specimens were experimented to determine if some properties can regain their original state by removing the absorbed moisture after cyclic wetting and drying process. 2.3 Mechanical Property Test The effects of hygrothermal and drying conditions on the tensile strength and modulus in laminates with three different porosities levels ranged from 0.33% to 1.5% were investigated. Five rectangular specimens had the dimensions of 230 mm×25 mm×4.5 mm were tested to determine the average tensile strength. The tensile tests were performed in an Instron 5582 mechanical test machine, and the loading rate is 2mm/min. 3.
Results and discussion
3.1 Moisture Absorption The moisture absorption behavior is shown in Figure. 1, where the percent weight gain is plotted as a function of the immersion time for aged specimens with different void contents ranging 0.33 to 1.5%. Moisture content increases linearly with the immersion time at first and then reach a pseudo equilibrium state. The solid lines are obtained by tting the initial stage of the moisture absorption. As shown in Figure. 1 the moisture content increases with the increasing of the immersion time.
Figure 1. Moisture absorption curves of different porosity
Table 1 shows that saturated moisture content are affected by both the immersion time and the porosity. For both the tensile specimens, the saturated moisture content increase with the increasing of the immersion time and porosity. This means that the increasing porosity led to an acceleration of moisture. Table 1
Saturated moisture content with different porosity Specimen Tensile test
Porosity /% 0.33 0.71 1.5
Immersion time /h 672 672 720
Saturated moisture content/g 1.345 1.396 1.505
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Zhang A-ying et al. / Energy Procedia 16 (2012) 1737 – 1743
Author name / Energy Procedia 00 (2011) 000–000
The water absorption curves of both materials obeyed Fick's law in the linear region before reaching to saturation level as depicted in Figure 1. Through the experimental results, it is noted that the voids significantly affect the characteristics of the diffusion rate and equilibrium water absorption content (Mm) as listed in Table 2. As expected, there is an increasing rate of water absorption characteristics for both tensile and compressive specimens with the increasing of porosity. According to Fick's law, diffusion coefficient, D is determined from the equation (2), (2) D=π(h/4Mm)2k2 Where h is the thickness of specimen (mm), Mm is the equilibrium moisture absorption content (g), and k is the slope at the initial stage of the moisture absorption curve. As listed in Table 2, D and k increase with the increasing of porosity, and is affected by the dimension of specimen. It is noted that the rate of water uptake increase with the porosity, which prove voids facilitate moisture absorption and might also provide paths by which water move into composites. Table 2
3.2
Diffusion coefficient with different porosity Specimen
Dimension /mm×mm
Porosity /%
Mm
k
D /mm2/h
Tensile test
230×25
0.33 0.71 1.5
1.241 1.25 1.294
0.04956 0.05736 0.06083
0.0063 0.0084 0.0088
/g
Tensile Property Test
As shown in Figure 2, the tensile strength of non-aged specimens decreased 0.8% with the porosity increasing from 0.33% to 1.5%. Compared to non-aged specimens, the tensile strength of aged specimens decreased 6.5%, 2.5% and 1.2% with porosity of 0.33%, 0.71% and 1.5%, respectively. The presence of moisture within polymeric composites often degrades their physical and mechanical properties [9,11]. Figure 2(a) and (b) shows that the curve of tensile strength and tensile modulus of aged specimens appeared to be ups and downs, and eventually present a downward trend. The experimental results demonstrate that moisture may indeed either increase or decrease the tensile strength [9]. Moisture may degrade the fiber/matrix interface and/or toughen the polymeric matrix [8,11,12]. Therefore, the effect of moisture on the tensile strength depends on the dominant failure mechanism. a)
Zhang A-ying et al. / Energy Procedia 16 (2012) 1737 – 1743
Author name / Energy Procedia 00 (2011) 000–000
b)
Figure 2. Tensile property curves of aged specimens with different porosity. a)Tensile strength. b)Tensile modulus.
The reduction in net section with the increasing void content has a detrimental effect on strength. Whilst for the aged specimens, the tensile strength with the porosity of 1.5% is higher than porosity of 0.33% as shown in Figure 2(a). The water absorbed by the epoxy laminates, in general, causes reversible plasticization of the matrix and lowers its glass transition temperature [13]. In this case, the plasticizer effect of water in the composite structures is superior to the degradation effect. Compared to non-aged specimens, the tensile modulus of aged specimens basically unchanged as shown in Figure 2(b). The tensile modulus test is principally driven by the behavior of the carbon fiber which is basically non-hygroscopic. The specimens after tensile test were studied by scanning electron microscope and regions of fracture are shown in Figure 3 for both non-aged and aged specimens. Water absorption and their resulting effects contribute to the loss of compatibilization between fibers and matrix, which results in debonding and weakening of the interface adhesion [14].The behavior of laminates depends on the efficiency of the fiber/matrix interface and this can be reduced by the presence of water [10]. Figure 3(b) shows that the cracks of the fiber/matrix interface are more severe than that of non-aged specimens as shown in Figure 3(a). a)
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Zhang A-ying et al. / Energy Procedia 16 (2012) 1737 – 1743
Author name / Energy Procedia 00 (2011) 000–000
b)
Figure 3. SEM of the non-aged and aged specimens after tensile test. a)Non-aged specimens. b)Aged specimens.
Figure 4 shows that the tensile strength of aged specimens is lower than that of non-aged specimens for the same porosity. The general trend of the tensile strength of drying specimens decreased with the porosity increasing from 0.33% to 1.5%. And the tensile strength of drying specimens is higher than that of non-aged specimens for the same porosity. This may be concluded that drying condition lead to the increase of the degree of cure, which eliminate some of the residual stress and improve the adhesion of fiber/matrix interface.
Figure 4. Tensile strength in different conditions of different porosity
4.
Conclusions
⑴ For both the tensile specimens, the saturated moisture content and the rate of water uptake increase with porosity increasing from 0.33% to 1.5%, which proves voids facilitate moisture absorption.
Zhang A-ying et al. / Energy Procedia 16 (2012) 1737 – 1743
Author name / Energy Procedia 00 (2011) 000–000
⑵The tensile strength of non-aged specimens decreased 0.8% with the porosity increasing from 0.33% to 1.5%. The curve of tensile strength and tensile modulus of aged specimens appeared to be ups and downs, and eventually present a downward trend. Compared to non-aged specimens, the tensile strength of aged specimens decreased 6.5%, 2.5% and 1.2% with porosity of 0.33%, 0.71% and 1.5%, respectively; the tensile modulus of aged specimens basically unchanged. The tensile strength of aged specimens is lower than that of non-aged specimens for the same porosity. And the tensile strength of drying specimens is higher than that of non-aged specimens for the same porosity. Acknowledgement The authors are gratefully to Harbin Aircraft Industry Group for supporting this work. References [1] Atadero R. A., Karbhari V. M.. Calibration of resistance factors for reliability based design of externally-bonded FRP composites[J]. Composites Part B, 2008, 39(4) : 665-679. [2] Marouani S., Curtil L., Hamelin P.. Composites realized by hand lay-up process in a civil engineering environment: initial properties and durability[J]. Materials and structures, 2008, 41(5) : 831-851. [3] Puglia P. D., Sheikh M. A, Hayhurstb D. R. Classification and quantification of initial porosity in a CMC laminate[J]. Composites Part A, 2004, 35(2), 223-230. [4] Birt E. A., Smith R. A.. A review of NDE methods for porosity measurement in fibre-reinforced polymer composites[J]. Non-Destructive Testing and Condition Monitoring, 2004, 46(11), 681-686. [5] Costa M. L., Almeida S. F. M., Rezende M. C.. The influence of porosity on the interlaminar shear strength of carbon epoxy and carbon bismaleimide fabric laminates[J]. Composites Science and Technology, 2001, 61(14), 2101-2108. [6] Wisnom M. R., Reynolds T., et al. Reduction in interlaminar shear strength by discrete and distributed voids[J]. Composites Science and Technology, 1996, 56(1), 93-101. [7] Chambers A. R., Earl J. S., Squires C. A., et al. The effect of voids on the flexural fatigue performance of unidirectional carbon fibre composites developed for wind turbine applications[J]. International Journal of Fatigue, 2006, 28(10), 1389-1398. [8] Costa M. L., Rezende M. C., Almeida S. F. M.. Strength of hygrothermally conditioned polymer composites with voids[J]. Journal of Composite Materials, 2005, 39(21), 1943-1961. [9] Cândido G. M., Costa M. L., Rezende M. C., et al. Hygrothermal effects on quasi isotropic carbon epoxy laminates with machined and molded edges[J]. Composites Part B, 2008, 39(3); 490-496. [10] Meziere Y., Bunsell A. R., Favry Y., et al. Large strain cyclic fatigue testing of unidirectional carbon fibre reinforced epoxy resin[J]. Composites Part A, 2005, 36(12), 1627-1636. [11] Costa M. L., Rezende M. C., Sergio F.. Effect of Void Content on the Moisture Absorption in Polymeric Composites[J]. Polymer-Plastics Technology and Engineering, 2006,45(6), 691-698. [12] Cândido G. M., Rezende M. C , Almeida S. F. M.. Hygrothermal effects on the tensile strength of carbon epoxy laminates with molded edges[J]. Materials Research, 2000, 3(2), 11-17. [13] Wolff, E.G.. Moisture Effects on Polymer Matrix Composites[J]. SAMPE Journal, 1993, 29(3), 11-19. [14] Kim H. J., Seo D. W.. Effect of water absorption fatigue on mechanical properties of sisal textile-reinforced composites[J]. International Journal of Fatigue, 2006, 28(10), 1307-1314.
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