- Email: [email protected]
Effect of rubber modification on fracture toughness properties of glass reinforced hot cured epoxy composites
Materials and Design 47 (2013) 16–20
Contents lists available at SciVerse ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Technical Report
Effect of rubber modification on fracture toughness properties of glass reinforced hot cured epoxy composites M.R. Dadfar ⇑, F. Ghadami School of Metallurgy and Materials, College of Engineering, University of Tehran, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 15 October 2012 Accepted 14 December 2012 Available online 25 December 2012
a b s t r a c t In this study, carboxyl-terminated butadiene acrylonitrile (CTBN) added to epoxy resin to improve the fracture property of resin and glass reinforced epoxy (GRE) composites. Tensile strength and Young’s modulus of unreinforced resin with/without modifier were examined. Plane strain fracture toughness (KIC) was calculated using single-edge-notch specimens that tested in three point bending (3PB) geometry. Results indicated that fracture toughness improve through increasing rubber modifier content. Fracture surfaces of the 3PB specimens were observed by using optical microscope and scanning electron microscope. Tensile properties perpendicular and parallel to the fibers direction were tested, respectively. GRE composite interlaminar fracture toughness increased with increasing rubber modifier in composite. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Hot cured epoxy resins are widely used because of their long term curing time with adequate strength. However the major problem of these types of resins is brittle fracture behavior. Epoxies are the mostly studied thermoset materials and have a very wide range of industrial applications [1]. They are extensively used as matrix materials in glass reinforced epoxy (GRE) composite materials and as adhesives in a multitude of applications i.e., aerospace, defense, automotive, marine and sporting goods due to their high specific stiffness and strength. These materials provide high durability, design flexibility and light weight which make them attractive materials in these applications [2]. Their use has broadened due to an increased understanding of fracture and toughening processes and as a consequence, most epoxy resins are toughened in some form or another. Epoxy resins are usually modified in one of three ways: the addition of hard particles such as glass bead and metastable zirconia; the addition of elastomeric materials or by the addition of thermoplastics [3]. The mechanical properties profile of epoxy matrices can be influenced, for example, by modifying the molecular architecture and structure, i.e. by increasing the crosslink density to generate high stiffness and strength. Highly crosslinked cured epoxy matrices, however, often produce a highly undesirable property: they are relatively brittle, having poor resistance to crack initiation and growth because plastic deformation is constrained. Modifiers less rigid than the polymer matrix may serve as excellent tougheners in matrices which show ductility to some degree. Rubber modifier, for example, can induce the formation of microvoids ⇑ Corresponding author. Tel.: +98 9125536259. E-mail address: [email protected] (M.R. Dadfar). 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.12.035
which is subsequently accompanied by the activation of yielding processes due to the reduction of the local yield stress, i.e. the plastic resistance of the material. In this case, a substantial amount of energy is dissipated within the plastic zone near the crack tip [4]. Liquid rubbers such as carboxyl-terminated butadiene acrylonitrile rubber (CTBN) have a high potential as modifiers for GRE composites without viscosity change [5,6]. When this system is cured, the epoxy polymerizes and the CTBN react with epoxy to form a copolymer. With increase in molecular weight, the soluble reactive liquid CTBN rubber phase separates from epoxy because of decrease in rubber/epoxy compatability. The elastomeric phase forms small discrete particles, typically in the micrometer range, that are dispersed in and bonded to the epoxy matrix. Although the morphology of rubber in toughened epoxy systems is mostly spherical, the average particle size and distribution can vary greatly depending on the cure reactions, cure cycle and concentration of rubber in the epoxy system [7]. Abadyan et al. investigated Rubber modification of hoop filament wound epoxy composites by using amineterminated butadiene acrylonitrile (ATBN) and carboxyl-terminated butadiene acrylonitrile (CTBN) oligomers [8]. He and his cooperators also investigated two different hybrid modified epoxies. In one system, epoxy has been modified by amine-terminated butadiene acrylonitrile (ATBN) and hollow glass spheres as fine and coarse modifiers, respectively. The other hybrid epoxy has been modified by the combination of ATBN and recycled Tire particles. The results of fracture toughness measurement of blends revealed synergistic toughening for both hybrid systems in some formulations [9]. Therefore, this study is aimed to investigate the influence of CTBN addition on the mechanical properties and fracture toughness of hot cured epoxy resin and GRE composites.
M.R. Dadfar, F. Ghadami / Materials and Design 47 (2013) 16–20
17
2. Experimental details 2.1. Materials Three main constituents used as resin system and GRE composite matrix, were Diglycidyl ether of bisphenol A (DGEBA) epoxy resin (LY556), anhydride curing agent (Aradur 917) and imidazole accelerator (DY070) from Huntsman. The modifier used was Hypro™ CTBN liquid rubber that was a carboxyl terminated butadiene acrylonitrile copolymer. Reinforcement used for GRE composite was unidirectional E-glass fibers provided by Interglass Inc., Germany. Fig. 2. Fracture toughness of unmodified and modified epoxy resins.
2.2. Specimen preparation The stoichiometric ratio of the curing agent, epoxy resin and rubber modifier were mixed and degassed about 20 min at room
Table 1 Formulations used for resin and composite specimens. Sample code
Resin (phr)
Hardener (phr)
Accelerator (phr)
Rubber modifier content (CTBN) (phr)
Reinforced with glass fiber
LY-Neat LY-5CTBN LY-10CTBN LY-15CTBN GRE-Neat GRE-5CTBN GRE-10CTBN GRE-15CTBN
100 100 100 100 100 100 100 100
90 90 90 90 90 90 90 90
1 1 1 1 1 1 1 1
– 5 10 15 – 5 10 15
No No No No Yes Yes Yes Yes
Fig. 1. Opening Mode I interlaminar fracture toughness testing method.
Table 2 Tensile property of unmodified and modified epoxy resins. Specimen
Tensile strength (MPa)
Young’s modulus (MPa)
LY-Neat LY-5CTBN LY-10CTBN LY-15CTBN
71 65 58 53
2980 2680 2350 2050
Fig. 3. Transition optical micrographs taken from the crack tip: (a) LY-5CTBN, (b) LY-10CTBN, (c) LY-15CTBN.
18
M.R. Dadfar, F. Ghadami / Materials and Design 47 (2013) 16–20
Fig. 4. Scanning electron micrographs taken from the fracture surface of resin specimens: (a) LY-5CTBN, (b) LY-10CTBN, (c) LY-15CTBN, (d) LY-15CTBN at higher magnification (A) formation cavities zone and (B) plastic deformation.
temperature. The solution was then cast into a 5 mm-thick mould. The cast material was cured for 4 h at 80 °C and then post cured for 8 h at 160 °C in a circulating air oven. The same curing schedule was employed for neat epoxy, as well. Table 1 presents the formulations used for resin and GRE composite specimens. All formulations were made then reinforced with hand layup glass fiber process to form composite sheets. They were cured at the same temperature cycle which was used for resin specimens. 2.3. Characterization 2.3.1. Resin characterization Tensile strength and Young’s modulus of resin with/without modifier were determined according to ASTM: D638-10. Tensile specimens were prepared by machining the cast sheet. Plane strain fracture toughness of resin (KIC) was calculated using ASTM: D5045-99(2007) single-edge-notch specimens tested in 3PB geometry. Pre-cracks in specimens were introduced at the bottom of 2 mm deep notches by hammering a razor blade. Fracture surfaces of the 3PB specimens were observed using optical microscope and field emission scanning electron microscope (FESEM). 2.3.2. Composite characterization Tensile properties perpendicular and parallel to the fibers direction of GRE composite specimens were examined according to ASTM: D3039-08. The Mode I interlaminar fracture toughness tests of unidirectional fiber-reinforced polymer matrix composites were performed according to ASTM: D5528-01(2007) (Fig. 1) using specimens with dimensions of 25 5 200 mm3. Opening Mode I interlaminar fracture toughness, GIC, was calculated from the load–deflection curve at the point of deviation from linearity (NL) calculation method. This calculation assumes that the delamination starts to grow from the insert in the interior of the specimen at this point. In order to better understanding of the crosssectional morphology and fracture surfaces, specimens were investigated with field emission scanning electron microscope (FESEM).
3. Results and discussion 3.1. Resin results 3.1.1. Tensile properties Rubber modifiers are generally known to affect mechanical properties depending on their compatibility with the matrix, particle size, shape and content, as well as the intrinsic strength of the rubbery phase [10]. Table 2 shows the tensile properties of neat epoxy resin and different content of rubber. As expected, the tensile strength and modulus gradually decreased with increasing rubber content. This is due to the fact that the strength and modulus of rubber is much lower than that of the epoxy matrix. In addition, low modulus rubber particles act as stress concentrators and decrease the tensile strength [11]. 3.1.2. Fracture toughness The results of fracture toughness measurements are presented in Fig. 2. According to this figure, by increasing CTBN modifier content, fracture toughness (KIC) improved gently. Reduction of the macroscopic yield stress leads to produce larger plastic zone around the crack tip which contributes to fracture toughness improvement [12]. Fig. 3 shows the optical microscope images of crack tip in front it for modifier specimens. In this figure, deformation of specimen in front of crack tip can be observed. As seen, plastic deformations at the plastic zone of modified resins are accompanied by deformation zone (Fig. 3a–c). The size of deformation zone increases with increasing the rubber content; such that in LY-15CTBN specimen, it expands to the surface in front of the crack tip (Fig. 3c). In this case, the dominant toughening mechanism is shear yielding, cavitation and the concomitant plastic deformation ahead of the crack tip. 3.1.3. Resin fractography Fractographic observations were performed using small areas cut from 3PB specimen surfaces. Fig. 4 shows the whitening region
M.R. Dadfar, F. Ghadami / Materials and Design 47 (2013) 16–20 Table 3 Transverse and longitudinal tension properties of GRE composite. Specimen
E22 (GPa)
rt (MPa)
E11 (GPa)
rL (MPa)
GRE-Neat GRE-5CTBN GRE-10CTBN GRE-15CTBN
11 10.8 10.2 9.7
64 67 70 72
28.4 24.3 19.7 17.8
584 553 500 489
19
related to the increase in void content due to rubbery phase addition and needs further investigation. However transverse strength (rt) is less affected by rubber modification but it increased with increasing CTBN content in GRE composites. It can be explained that interface between fibers and matrix is responsible for transverse strength in composites. Note that rubbery phase improved the resin–fibers adhesion in interface [17] and so crack propagates within the toughened matrix instead of matrix–fiber interface. Therefore, transverse tensile strength of modified GRE composite has been increased [18]. 3.2.2. Interlaminar fracture toughness test Based on the results shown in Fig. 5 that presents interlaminar fracture toughness (GIC) data obtained from GRE composite specimens and other research studies [18–22], it can be expected that composites made using rubber modified epoxy improve interlaminar fracture toughness. Interlaminar fracture toughness is an important parameter in predicting the required load to separating two layers from each other. Results showed that addition of CTBN leads to the better adherence between resin matrix and E-glass fibers and therefore higher interlaminar matrix crack propagation resistance and delamination initiation.
Fig. 5. Interlaminar fracture toughness of unmodified and modified GRE composites.
that implies of plastic deformation and spherical shapes that indicate holes and cavities [13,14]. The latter is known as the main source of energy absorption in fracture of rubber toughened epoxies [15,16]. From the fracture surface of modified epoxy, it can be concluded that the amount of cavitations rises with increasing of CTBN modifier that confirms fracture toughness improvement with increasing of CTBN content in epoxy resin. 3.2. Composite tests 3.2.1. Tensile test Table 3 presents tensile properties of GRE composites in parallel and perpendicular to fibers direction. As seen, transverse and longitudinal modulus (E22 and E11) properties of composite specimens gradually decreased with increasing rubber content similar to that of unreinforced resin. These can be attributed to the fact that the modulus of rubber is much lower than that of the epoxy matrix. According to this table, longitudinal strength (rL) of modified GRE composites is lower than unmodified one. This might be
3.2.3. Composite fractography Fig. 6 illustrates the field emission scanning electron microscope micrographs of the cross-section of the unmodified and modified GRE composites. This figure indicated that the fiber in unmodified specimen (Fig. 6a), contains less remaining resin on its surface in comparison with rubber modified specimens (Fig. 6b). Therefore, it confirms that rubber modified resin has more adhesion to fiber surface than unmodified resin. From this figure, the marks indicate cavitation regions between fibers. Jang and Yang [18] studied the mechanical properties of polybenoxazine composites modified with two types of liquid rubber modifiers providing both strong and weak interfaces. A fiber pullout failure mechanism was observed in the particular case of the unmodified GRE composite with those observed by Erden et al. [23]. These features suggest poor interfacial bonding between the glass fiber and the unmodified epoxy matrix. 4. Conclusions For the rubber-modified epoxy resin, we examined the tensile strength and Young’s modulus of unreinforced resin with/without
Fig. 6. FESEM micrographs of interlaminar fracture toughness GRE composite samples: (a) GRE-Neat, (b) GRE-10CTBN.
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M.R. Dadfar, F. Ghadami / Materials and Design 47 (2013) 16–20
modifier. Plane strain fracture toughness (KIC) was calculated using single-edge-notch specimens that tested in three point bending (3PB) geometry. Fracture surfaces of the 3PB specimens were observed by using optical microscope and scanning electron microscope. The following conclusions were obtained. In this paper a liquid rubber modifier (CTBN) was employed as toughening agents in hot cured epoxy system. Fracture toughness of modified resin increased by CTBN content, by cavitation mechanism with graduate reduction in Young’s modulus. Longitudinal tensile strength of composites decreased with increasing the amount of CTBN. Reduction in longitudinal tensile strength and Young’s modulus by addition of rubber modifier can be attributed to the increase of the void content. Transverse tensile strength of modified GRE composite specimens increased with increasing CTBN content due to improvement of fiber–matrix interfacial adhesion. Interlaminar fracture toughness of modified GRE composite specimens increased by CTBN addition due to better fiber– matrix interfacial adhesion. FESEM fractography of CTBN modified GRE composite specimens illustrated cavitations in resin. From cavitations presence in composite because of CTBN addition and this fact that rubber modified GRE composites has better fiber–matrix adhesion in interface it can been concluded that addition of CTBN, increase the interlaminar fracture toughness.
Acknowledgments The authors would like to thank Dr. R. Bagheri at Sharif University of Technology, Polymer Laboratory in Materials Science and Engineering Department for dispose his equipment and his cooperation during this work. References [1] Bagheri R, Marouf BT, Pearson RA. Rubber-toughened epoxies: a critical review. Polym Rev 2009;49(3):201–25. [2] Bozkurt E, Kaya E, Tanoglu M. Mechanical and thermal behavior of non-crimp glass fiber reinforced layered clay/epoxy nanocomposites. Compos Sci Technol 2007;67:3394–403.
[3] Lowe A, Hyok Kwon O, Wing Mai Y. Fatigue and fracture behavior of novel rubber modified epoxy resins. Polymer 1996;37(4):565–72. [4] Wetzel B, Rosso P, Haupert F, Friedrich K. Epoxy nanocomposites – fracture and toughening mechanisms. Eng Fract Mech 2006;73:2375–98. [5] Abadyan M, Khademi V, Bagheri R, Haddadpour H, Kouchakzadeh MA, Farsadi M. Use of rubber modification technique to improve fracture-resistance of hoop wound composites. Mater Des 2009;30:1976–84. [6] Gabr MH, Abd Elrahman M, Okubo K, Fujii T. A study on mechanical properties of bacterial cellulose/epoxy reinforced by plain woven carbon fiber modified with liquid rubber. Composites: Part A 2010;41:1263–71. [7] Balakrishnan S, Start PR, Raghavan D, Hudson SD. The influence of clay and elastomer concentration on the morphology and fracture energy of preformed acrylic rubber dispersed clay filled epoxy nanocomposites. Polymer 2005;46:11255–62. [8] Abadyan M, Bagheri R, Haddadpour H, Motamedi P. Investigation of the fracture resistance in hoop wound composites modified with two different reactive oligomers. Mater Des 2009;30:3048–55. [9] Abadyan M, Bagheri R, Kouchakzadeh MA, Hosseini Kordkheili SA. Exploring the tensile strain energy absorption of hybrid modified epoxies containing soft particles. Mater Des 2011;32:2900–8. [10] Kinloch AJ. Damage zone around crack tip and fracture toughness of rubbermodified epoxy resin under mixed-mode conditions. Polymer 1983;24: 1341–54. [11] Arends CB. Polymer toughening. New York (USA): Marcel Dekker Inc.; 1996. [12] Pearson RA, Sue HJ, Yee AF. Toughening of plastics advances in modeling and experiments. Washington DC (USA): ACS Symposium series; 2000. [13] Bagheri R, Pearson RA. Role of particle cavitation in rubber-toughened epoxies: 1 Microvoid toughening. Polymer 1996;37(20):4529–38. [14] Bagheri R, Pearson RA. Role of particle cavitation in rubber-toughened epoxies: 2 inter-particle distance. Polymer 2000;41:269–76. [15] Bagheri R, Pearson RA. Role of blend morphology in rubber-toughened polymers. J Mater Sci 1996;31:3945–54. [16] Bagheri R, Pearson RA. Interfacial studies in CTBN-modified epoxies. J Appl Polym Sci 1995;58:427–37. [17] Gabr MH, Abd Elrahman M, Okubo K, Fujii T. Interfacial adhesion improvement of plain woven carbon fiber reinforced epoxy filled with micro-fibrillated cellulose by addition liquid rubber. J Mater Sci 2010;45:3841–50. [18] Jang J, Yang H. Toughness improvement of carbon-fiber/polybenzoxazine composites by rubber modification. Compos Sci Technol 2000;60:457–63. [19] Gilbert EN, Hayes BS, Seferis JC. Interlayer toughened unidirectional carbon prepreg systems: effect of preformed particle morphology. Composites: Part A 2003;34(3):245–52. [20] Chen SF, Jang BZ. Fracture behavior of interleaved fiber–resin composites. Compos Sci Technol 1991;41(1):77–97. [21] Kim KY, Ye L. Interlaminar fracture toughness of CF/PEI composites at elevated temperatures: roles of matrix toughness and fiber/matrix adhesion. Composites: Part A 2004;35:477–87. [22] DeCarli M, Kozielski K, Tian W, Varley R. Toughening of a carbon fibre reinforced epoxy anhydride composite using an epoxy terminated hyperbranched modifier. Compos Sci Technol 2005;65:2156–66. [23] Erden S, Sever K, Seki Y, Sarikanat M. Enhancement of the mechanical properties of glass/polyester composites via matrix modification glass/ polyester composite siloxane matrix modification. Fibers Polym 2010;11(5):732–7.
Contents lists available at SciVerse ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Technical Report
Effect of rubber modification on fracture toughness properties of glass reinforced hot cured epoxy composites M.R. Dadfar ⇑, F. Ghadami School of Metallurgy and Materials, College of Engineering, University of Tehran, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 15 October 2012 Accepted 14 December 2012 Available online 25 December 2012
a b s t r a c t In this study, carboxyl-terminated butadiene acrylonitrile (CTBN) added to epoxy resin to improve the fracture property of resin and glass reinforced epoxy (GRE) composites. Tensile strength and Young’s modulus of unreinforced resin with/without modifier were examined. Plane strain fracture toughness (KIC) was calculated using single-edge-notch specimens that tested in three point bending (3PB) geometry. Results indicated that fracture toughness improve through increasing rubber modifier content. Fracture surfaces of the 3PB specimens were observed by using optical microscope and scanning electron microscope. Tensile properties perpendicular and parallel to the fibers direction were tested, respectively. GRE composite interlaminar fracture toughness increased with increasing rubber modifier in composite. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Hot cured epoxy resins are widely used because of their long term curing time with adequate strength. However the major problem of these types of resins is brittle fracture behavior. Epoxies are the mostly studied thermoset materials and have a very wide range of industrial applications [1]. They are extensively used as matrix materials in glass reinforced epoxy (GRE) composite materials and as adhesives in a multitude of applications i.e., aerospace, defense, automotive, marine and sporting goods due to their high specific stiffness and strength. These materials provide high durability, design flexibility and light weight which make them attractive materials in these applications [2]. Their use has broadened due to an increased understanding of fracture and toughening processes and as a consequence, most epoxy resins are toughened in some form or another. Epoxy resins are usually modified in one of three ways: the addition of hard particles such as glass bead and metastable zirconia; the addition of elastomeric materials or by the addition of thermoplastics [3]. The mechanical properties profile of epoxy matrices can be influenced, for example, by modifying the molecular architecture and structure, i.e. by increasing the crosslink density to generate high stiffness and strength. Highly crosslinked cured epoxy matrices, however, often produce a highly undesirable property: they are relatively brittle, having poor resistance to crack initiation and growth because plastic deformation is constrained. Modifiers less rigid than the polymer matrix may serve as excellent tougheners in matrices which show ductility to some degree. Rubber modifier, for example, can induce the formation of microvoids ⇑ Corresponding author. Tel.: +98 9125536259. E-mail address: [email protected] (M.R. Dadfar). 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.12.035
which is subsequently accompanied by the activation of yielding processes due to the reduction of the local yield stress, i.e. the plastic resistance of the material. In this case, a substantial amount of energy is dissipated within the plastic zone near the crack tip [4]. Liquid rubbers such as carboxyl-terminated butadiene acrylonitrile rubber (CTBN) have a high potential as modifiers for GRE composites without viscosity change [5,6]. When this system is cured, the epoxy polymerizes and the CTBN react with epoxy to form a copolymer. With increase in molecular weight, the soluble reactive liquid CTBN rubber phase separates from epoxy because of decrease in rubber/epoxy compatability. The elastomeric phase forms small discrete particles, typically in the micrometer range, that are dispersed in and bonded to the epoxy matrix. Although the morphology of rubber in toughened epoxy systems is mostly spherical, the average particle size and distribution can vary greatly depending on the cure reactions, cure cycle and concentration of rubber in the epoxy system [7]. Abadyan et al. investigated Rubber modification of hoop filament wound epoxy composites by using amineterminated butadiene acrylonitrile (ATBN) and carboxyl-terminated butadiene acrylonitrile (CTBN) oligomers [8]. He and his cooperators also investigated two different hybrid modified epoxies. In one system, epoxy has been modified by amine-terminated butadiene acrylonitrile (ATBN) and hollow glass spheres as fine and coarse modifiers, respectively. The other hybrid epoxy has been modified by the combination of ATBN and recycled Tire particles. The results of fracture toughness measurement of blends revealed synergistic toughening for both hybrid systems in some formulations [9]. Therefore, this study is aimed to investigate the influence of CTBN addition on the mechanical properties and fracture toughness of hot cured epoxy resin and GRE composites.
M.R. Dadfar, F. Ghadami / Materials and Design 47 (2013) 16–20
17
2. Experimental details 2.1. Materials Three main constituents used as resin system and GRE composite matrix, were Diglycidyl ether of bisphenol A (DGEBA) epoxy resin (LY556), anhydride curing agent (Aradur 917) and imidazole accelerator (DY070) from Huntsman. The modifier used was Hypro™ CTBN liquid rubber that was a carboxyl terminated butadiene acrylonitrile copolymer. Reinforcement used for GRE composite was unidirectional E-glass fibers provided by Interglass Inc., Germany. Fig. 2. Fracture toughness of unmodified and modified epoxy resins.
2.2. Specimen preparation The stoichiometric ratio of the curing agent, epoxy resin and rubber modifier were mixed and degassed about 20 min at room
Table 1 Formulations used for resin and composite specimens. Sample code
Resin (phr)
Hardener (phr)
Accelerator (phr)
Rubber modifier content (CTBN) (phr)
Reinforced with glass fiber
LY-Neat LY-5CTBN LY-10CTBN LY-15CTBN GRE-Neat GRE-5CTBN GRE-10CTBN GRE-15CTBN
100 100 100 100 100 100 100 100
90 90 90 90 90 90 90 90
1 1 1 1 1 1 1 1
– 5 10 15 – 5 10 15
No No No No Yes Yes Yes Yes
Fig. 1. Opening Mode I interlaminar fracture toughness testing method.
Table 2 Tensile property of unmodified and modified epoxy resins. Specimen
Tensile strength (MPa)
Young’s modulus (MPa)
LY-Neat LY-5CTBN LY-10CTBN LY-15CTBN
71 65 58 53
2980 2680 2350 2050
Fig. 3. Transition optical micrographs taken from the crack tip: (a) LY-5CTBN, (b) LY-10CTBN, (c) LY-15CTBN.
18
M.R. Dadfar, F. Ghadami / Materials and Design 47 (2013) 16–20
Fig. 4. Scanning electron micrographs taken from the fracture surface of resin specimens: (a) LY-5CTBN, (b) LY-10CTBN, (c) LY-15CTBN, (d) LY-15CTBN at higher magnification (A) formation cavities zone and (B) plastic deformation.
temperature. The solution was then cast into a 5 mm-thick mould. The cast material was cured for 4 h at 80 °C and then post cured for 8 h at 160 °C in a circulating air oven. The same curing schedule was employed for neat epoxy, as well. Table 1 presents the formulations used for resin and GRE composite specimens. All formulations were made then reinforced with hand layup glass fiber process to form composite sheets. They were cured at the same temperature cycle which was used for resin specimens. 2.3. Characterization 2.3.1. Resin characterization Tensile strength and Young’s modulus of resin with/without modifier were determined according to ASTM: D638-10. Tensile specimens were prepared by machining the cast sheet. Plane strain fracture toughness of resin (KIC) was calculated using ASTM: D5045-99(2007) single-edge-notch specimens tested in 3PB geometry. Pre-cracks in specimens were introduced at the bottom of 2 mm deep notches by hammering a razor blade. Fracture surfaces of the 3PB specimens were observed using optical microscope and field emission scanning electron microscope (FESEM). 2.3.2. Composite characterization Tensile properties perpendicular and parallel to the fibers direction of GRE composite specimens were examined according to ASTM: D3039-08. The Mode I interlaminar fracture toughness tests of unidirectional fiber-reinforced polymer matrix composites were performed according to ASTM: D5528-01(2007) (Fig. 1) using specimens with dimensions of 25 5 200 mm3. Opening Mode I interlaminar fracture toughness, GIC, was calculated from the load–deflection curve at the point of deviation from linearity (NL) calculation method. This calculation assumes that the delamination starts to grow from the insert in the interior of the specimen at this point. In order to better understanding of the crosssectional morphology and fracture surfaces, specimens were investigated with field emission scanning electron microscope (FESEM).
3. Results and discussion 3.1. Resin results 3.1.1. Tensile properties Rubber modifiers are generally known to affect mechanical properties depending on their compatibility with the matrix, particle size, shape and content, as well as the intrinsic strength of the rubbery phase [10]. Table 2 shows the tensile properties of neat epoxy resin and different content of rubber. As expected, the tensile strength and modulus gradually decreased with increasing rubber content. This is due to the fact that the strength and modulus of rubber is much lower than that of the epoxy matrix. In addition, low modulus rubber particles act as stress concentrators and decrease the tensile strength [11]. 3.1.2. Fracture toughness The results of fracture toughness measurements are presented in Fig. 2. According to this figure, by increasing CTBN modifier content, fracture toughness (KIC) improved gently. Reduction of the macroscopic yield stress leads to produce larger plastic zone around the crack tip which contributes to fracture toughness improvement [12]. Fig. 3 shows the optical microscope images of crack tip in front it for modifier specimens. In this figure, deformation of specimen in front of crack tip can be observed. As seen, plastic deformations at the plastic zone of modified resins are accompanied by deformation zone (Fig. 3a–c). The size of deformation zone increases with increasing the rubber content; such that in LY-15CTBN specimen, it expands to the surface in front of the crack tip (Fig. 3c). In this case, the dominant toughening mechanism is shear yielding, cavitation and the concomitant plastic deformation ahead of the crack tip. 3.1.3. Resin fractography Fractographic observations were performed using small areas cut from 3PB specimen surfaces. Fig. 4 shows the whitening region
M.R. Dadfar, F. Ghadami / Materials and Design 47 (2013) 16–20 Table 3 Transverse and longitudinal tension properties of GRE composite. Specimen
E22 (GPa)
rt (MPa)
E11 (GPa)
rL (MPa)
GRE-Neat GRE-5CTBN GRE-10CTBN GRE-15CTBN
11 10.8 10.2 9.7
64 67 70 72
28.4 24.3 19.7 17.8
584 553 500 489
19
related to the increase in void content due to rubbery phase addition and needs further investigation. However transverse strength (rt) is less affected by rubber modification but it increased with increasing CTBN content in GRE composites. It can be explained that interface between fibers and matrix is responsible for transverse strength in composites. Note that rubbery phase improved the resin–fibers adhesion in interface [17] and so crack propagates within the toughened matrix instead of matrix–fiber interface. Therefore, transverse tensile strength of modified GRE composite has been increased [18]. 3.2.2. Interlaminar fracture toughness test Based on the results shown in Fig. 5 that presents interlaminar fracture toughness (GIC) data obtained from GRE composite specimens and other research studies [18–22], it can be expected that composites made using rubber modified epoxy improve interlaminar fracture toughness. Interlaminar fracture toughness is an important parameter in predicting the required load to separating two layers from each other. Results showed that addition of CTBN leads to the better adherence between resin matrix and E-glass fibers and therefore higher interlaminar matrix crack propagation resistance and delamination initiation.
Fig. 5. Interlaminar fracture toughness of unmodified and modified GRE composites.
that implies of plastic deformation and spherical shapes that indicate holes and cavities [13,14]. The latter is known as the main source of energy absorption in fracture of rubber toughened epoxies [15,16]. From the fracture surface of modified epoxy, it can be concluded that the amount of cavitations rises with increasing of CTBN modifier that confirms fracture toughness improvement with increasing of CTBN content in epoxy resin. 3.2. Composite tests 3.2.1. Tensile test Table 3 presents tensile properties of GRE composites in parallel and perpendicular to fibers direction. As seen, transverse and longitudinal modulus (E22 and E11) properties of composite specimens gradually decreased with increasing rubber content similar to that of unreinforced resin. These can be attributed to the fact that the modulus of rubber is much lower than that of the epoxy matrix. According to this table, longitudinal strength (rL) of modified GRE composites is lower than unmodified one. This might be
3.2.3. Composite fractography Fig. 6 illustrates the field emission scanning electron microscope micrographs of the cross-section of the unmodified and modified GRE composites. This figure indicated that the fiber in unmodified specimen (Fig. 6a), contains less remaining resin on its surface in comparison with rubber modified specimens (Fig. 6b). Therefore, it confirms that rubber modified resin has more adhesion to fiber surface than unmodified resin. From this figure, the marks indicate cavitation regions between fibers. Jang and Yang [18] studied the mechanical properties of polybenoxazine composites modified with two types of liquid rubber modifiers providing both strong and weak interfaces. A fiber pullout failure mechanism was observed in the particular case of the unmodified GRE composite with those observed by Erden et al. [23]. These features suggest poor interfacial bonding between the glass fiber and the unmodified epoxy matrix. 4. Conclusions For the rubber-modified epoxy resin, we examined the tensile strength and Young’s modulus of unreinforced resin with/without
Fig. 6. FESEM micrographs of interlaminar fracture toughness GRE composite samples: (a) GRE-Neat, (b) GRE-10CTBN.
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modifier. Plane strain fracture toughness (KIC) was calculated using single-edge-notch specimens that tested in three point bending (3PB) geometry. Fracture surfaces of the 3PB specimens were observed by using optical microscope and scanning electron microscope. The following conclusions were obtained. In this paper a liquid rubber modifier (CTBN) was employed as toughening agents in hot cured epoxy system. Fracture toughness of modified resin increased by CTBN content, by cavitation mechanism with graduate reduction in Young’s modulus. Longitudinal tensile strength of composites decreased with increasing the amount of CTBN. Reduction in longitudinal tensile strength and Young’s modulus by addition of rubber modifier can be attributed to the increase of the void content. Transverse tensile strength of modified GRE composite specimens increased with increasing CTBN content due to improvement of fiber–matrix interfacial adhesion. Interlaminar fracture toughness of modified GRE composite specimens increased by CTBN addition due to better fiber– matrix interfacial adhesion. FESEM fractography of CTBN modified GRE composite specimens illustrated cavitations in resin. From cavitations presence in composite because of CTBN addition and this fact that rubber modified GRE composites has better fiber–matrix adhesion in interface it can been concluded that addition of CTBN, increase the interlaminar fracture toughness.
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