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Use of rubber modification technique to improve fracture-resistance of hoop wound composites
Materials and Design 30 (2009) 1976–1984
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Use of rubber modification technique to improve fracture-resistance of hoop wound composites M. Abadyan a, V. Khademi a, R. Bagheri b,*, H. Haddadpour a, M.A. Kouchakzadeh a, M. Farsadi c a
Aerospace Engineering Department, Sharif University of Technology, Azadi Avenue, P.O. Box 11165-8639, Tehran, Iran Polymeric Materials Research Group (PMRG), Materials Science and Engineering Department, Sharif University of Technology, Azadi Avenue, P.O. Box 11165-9466, Tehran, Iran c Aerospace Engineering Department, University of Imam Hosein, Babaei Avenue, Tehran, Iran b
a r t i c l e
i n f o
Article history: Received 29 April 2008 Accepted 3 September 2008 Available online 8 September 2008 Keywords: Rubber toughening (C) Epoxy resin (A) Filament winding (C)
a b s t r a c t Toughness improvement of an epoxy resin and respective hoop wound composite were investigated systematically using amine-terminated butadiene acrylonitrile (ATBN) liquid rubber. Rubber modification improves fracture toughness of epoxy resin with slight reduction in the glass transition temperature (Tg), flexural and compressive properties of resin. Impact resistance of composite is improved by rubber modification similar to modified resin. Interlaminar shear strength (ILSS), compressive modulus and strength, and flexural strength of composite decreased slightly with rubber modification. To interpret the data, the void content of composite samples was determined and the damaged surfaces of fractured samples were investigated. The findings explained the performance of rubber modified specimens in comparison with the unmodified one. In addition, a simple formulation was developed to predict the impact strength of modified composite samples. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Epoxy resins are widely utilized for many industrial high performance applications such as glass reinforced epoxy (GRE) structures. However, GRE has one major disadvantage. It is brittle and exhibits low impact and fracture strength due to low toughness of the epoxy matrix. The lower impact strength of GRE limits its structural applications compared to metals and polymer alloys, especially for fabrication of thin walled structures, automotive application, etc. Various methods of toughening have been proposed to modify GRE. These include toughening of epoxy matrix, fiber hybridization, and coating of fibers with appropriate polymers and specific geometric design of fibers [1]. Rubber modification, i.e. addition of rubber particulate phase to a glassy polymer matrix, is one of the approaches for improving toughness of hoop wound GREs [2,3]. Hoop winding angle is frequently used for producing pipes, the outer layer of metal-composite cylinders, etc. Because of the hoop angle of fibers, cracks can easily propagate in the plane perpendicular to the tube axis and the fibers cannot efficiently prevent matrix cracking in comparison with that of helical wound tubes. Therefore, one may conclude that other methods of toughness improvement could not be as effective as matrix toughening for hoop wound GREs. Liquid rubbers such as carboxyl-terminated butadiene acrylonitrile rubber (CTBN) and amineterminated butadiene acrylonitrile rubber (ATBN), have a high po* Corresponding author. Tel.: +98 21 6616 5207; fax: +98 21 6600 5717. E-mail address: [email protected] (R. Bagheri). 0261-3069/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2008.09.001
tential as modifiers for filament winding without viscosity change or filtration drawbacks [4] which are critical issues in this process. While rubber modification of unreinforced resins and flat laminated composites have been investigated for several decades, rubber modification of filament wound composites, especially hoop wound, has been studied by only few researchers. Ophir et al. [5] investigated a modified epoxy composition for filament winding of pressure vessels. Their product showed an improved toughness with minimal sacrifice of its thermal and mechanical properties. Zhang et al. [2] used CTBN-modified resin to produce glass/epoxy wound tubes. They found that the weepage stress in hoop loading of pipes was higher for unmodified as compared with those of modified pipes. However, the strain at weepage was higher for the pipes based on toughened epoxy [2]. Sanjana and Testa [3] used an instrumented impact test and reported that the toughness improvement of hoop wound epoxy tubes is affected by rate, temperature, and fiber orientation as well as modifier type. Their study showed negligible improvement in impact strength of composites which was the result of low toughenability of the resin used [3]. Further investigation is required to complete previous research and explain some ambiguities in toughening of hoop wound composites. For example, the fracture mechanisms of hoop composites were not investigated in those studies. In addition, there was no acceptable correlation between impact strength of modified composite and that of modified resin. Therefore, the goal of the current investigation is to study the role of rubber modification in performance of hoop wound tubes in a more systematic fashion. ATBN rubber modifier is used to modify the epoxy matrix and the
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corresponding wound composites. The fracture behavior of specimens is also investigated via microscopic techniques.
rate of 10 °C/min. The reported data are the average of at least six tests.
2. Experimantal
2.3.2. Composite evaluation The density and fiber volume fraction of composite specimens were measured according to ASTM D792 and ASTM D3171, respectively and ASTM D2734 test method was used to determine void content of composite samples. Transverse compressive properties of hoop wound composite (compressive properties in the direction perpendicular to the fibers direction) were determined according to ASTM D5449 using tubular specimens. The tubes had dimensions of 100 mm inner diameter 2 mm thick 140 mm long. The flexural tests were performed according to ASTM D790 using three-point loading specimens with dimensions of 62 12.7 4.2 mm3. ASTM D2344 guideline was followed to measure the ILSS of composite. ILSS test samples were cut from a 73 mm inner diameter tube with 6.35 mm thickness and width. The samples had span to thickness ratio of 5 according to standard guideline. The glass transition temperatures of the composite were determined by a differential scanning calorimeter (DSC). The samples were heated from 60 to 130 °C at the heating rate of 10 °C/ min. For hoop wound composites, through thickness cracks that propagate perpendicular to winding axis are more critical and prone to interlaminar cracks which propagate between the hoop layers during impact. To study impact resistance of composites against through thickness matrix failure, special samples were made. Both notched and unnotched impact tests were conducted. Impact specimens were cut from 90 mm inner diameter composite cylinders according to Fig. 1a. In the case of notched specimens, notched were made parallel to the fibers orientation (Fig. 1a). Notch geometry, length and thickness of the specimen were simi-
2.1. Materials The model system used in this study is based on a diglycidyle ether of bisphenol A epoxy with an epoxy equivalent weight of 170 g/eq (Araldite LY564) from Hauntsman and a cycloaliphatic polyamine hardener (HY2962), from Vantico. The modifiers used are ATBN copolymer (Hycar 1300 16) with 16 wt% of acrylonitrile, from Novion. Reinforcement used was a glass rowing with 2400 tex from Vetrotex. 2.2. Sample preparation The stoichiometric ratio of the curing agent and resin were mixed and degassed at room temperature for about 20 min. The solution was then cast into a 5 mm-thick glass mould. The cast material was cured for 4 h at 90 °C in a circulating air oven. The same curing schedule was employed for all toughened epoxies, as well. The modifier content was varied up to 15 phr. Epoxy/rubber mixtures were mixed at room temperature under vacuum for 20 min. to obtain a uniform solution before adding the curing agent. Table 1 presents the epoxy formulations used. All formulations made were then reinforced with glass fiber to wind cylindrical composite tubes. A hoop angle, in which the fibers had an angle close to 90° with respect to the tube axis, was employed. They were cured at the same temperature that was used for resin samples. 2.3. Characterization techniques 2.3.1. Resin evaluation Compressive yield strength and Young’s modulus of unreinforced resin were determined according to ASTM D695 using a screw-driven computer controlled Hunsfield testing frame. Coupons were prepared by machining the cast sheet to give specimen dimensions of 6 6 12 mm3. Flexural tests were performed according to ASTM D790 using tetragonal-shaped specimens of 3.2 12.7 125 mm3. Plane strain fracture toughness, KIC, was determined using single-edge-notch specimens tested in threepoint-bending (3PB) geometry. Specimens with 5 mm thickness were used for this experiment. The ASTM D5045 guideline was followed to measure KIC. Pre-cracks were introduced at the bottom of 2 mm deep notches by hammering a razor blade which was chilled in liquid nitrogen. Izod impact test was accomplished according to ASTM D256 using an 1 J hammer energy. Samples were prepared and machined to the standard shape (62 12.7 4.2 mm3). Specimens were tested using. The glass transition temperatures of the resin were determined by a differential scanning calorimeter (DSC). The samples were heated from 60 to 130 °C at the heating Table 1 Formulations made in this study Sample code
Resin (phr)
Hardener (phr)
Modifier type
Modifier content (phr)
Reinforced with glass fiber
NEAT NEATC A5 A5C A10 A10C A15 A15C
100 100 100 100 100 100 100 100
25 25 25 25 25 25 25 25
– – ATBN ATBN ATBN ATBN ATBN ATBN
0 0 5 5 10 10 15 15
No Yes No Yes No Yes No Yes
Fig. 1. Schematic diagrams of: (a) geometries, (b) constraints and the load conditions, used for impact assessment of filament wound tubes.
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Table 2 Compressive and flexural properties of resin
3. Results and discussion
Sample
Compressive strength (MPa)
Compressive modulus (GPa)
Flexural strength (MPa)
3.1. Resin tests
NEAT A5 A10 A15
103 97 90 82
3.15 3.00 2.90 2.60
129 118 111 101
3.1.1. Compressive and flexural properties Table 2 shows the compressive and flexural properties of epoxy resin as a function of rubber content. As expected, the compressive strength and modulus gradually decrease with increasing rubber content (Table 2). 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 yield strength [6]. Reduction of the macroscopic yield stress leads to promote the crack tip blunting and produce larger plastic zone around the crack tip which contributes to fracture toughness improvement [7]. Similar reduction is observed in flexural strength. Flexural strength gradually decreases with increasing in rubber content (Table 2). 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 [8].
lar to that mentioned in ASTM D256 for Izod test. Width of specimen is such that the area of the cross-section of composite specimen is equal to that of resin specimen. Similar experiment was conducted by Sanjana et al. [3] to assess composites. Impact tests were run on an impact testing machine with 1 J hammer energy. Samples were supported in a way similar to that mentioned in ASTM D256 for Izod test. The impact direction was coaxial with the symmetric axis of sample cross-section according to Fig. 1b. For each mechanical test, six specimens were used and average of six values is reported. 2.3.3. Microscopy evaluation Fracture surfaces of the SEN-3PB and impact specimens were examined using stereo optical microscope and scanning electron microscope (SEM). SEM samples were coated with a thin layer of gold before examination to protect the fracture surfaces from beam damage and also to prevent charge build up. Also, a reflected optical microscope was incorporated to provide more information from the fracture surface of specimens. Table 3 Fracture toughness of resin Sample
KIC (MPa m0.5)
NEAT A5 A10 A15
0.6 1.8 2.2 2.5
3.1.2. Fracture toughness The results of fracture toughness measurements are listed in Table 3. As seen in this table, modifier increases fracture toughness. Based on the results shown in Table 3 and previous studies, it can be expected that composites made using rubber modified epoxy show improved interlaminar fracture toughness [9–13]. Fig. 2 contains the optical micrographs showing the damage zone of SEN-3PB specimens. As seen, plastic deformations at the plastic zone of modified resins are accompanied by stress whitening. The size of stress whitening increases with increasing the rubber content; such that in A15 specimen, it expands to the whole surface in front of the crack tip (Fig. 2d). The difference in the damage zone sizes seen in Fig. 3 is in agreement with fracture toughness data reported in Table 3. Fig. 3 illustrates fracture surfaces of neat and A10 specimens. The flat and almost featureless image seen in Fig. 3a indicates
Fig. 2. Optical micrographs taken from the fracture surface of specimens: (a) NEAT, (b)A5, (c) A10, (d) A15.
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Fig. 3. SEM micrographs taken from the fracture surface of specimens: (a) NEAT, (b) A10, (c) A10 at higher magnification.
Fig. 4. SEM micrographs taken from fracture surface of impact test samples: (a) NEAT, (b) A10, (c) A10 at higher magnification.
the typical brittle fracture of neat epoxies. On the other hand, the rough fracture surface of A10 specimen seen in Fig. 3b shows the significant amount of plastic deformation occurred in this material prior to fracture. Same figure seen at higher magnification, Fig. 4c, shows good dispersion of rubber particles in the matrix and occurrence of cavitation in these particles at the crack tip. Cavitation is one of the most important mechanisms in rubber modification. Bagheri and Pearson [14,15] investigated toughening mechanisms in rubber toughened epoxies and showed that cavitation is a pre-
requisite for massive shear deformation of the matrix. The latter is known as the main source of energy absorption in fracture of rubber toughened epoxies [14–17]. 3.1.3. Impact test The results of impact energy absorption (impact strength) versus ATBN content for notched test specimens are reported in Table 4. Normalized impact strength is defined as the ratio of impact strength of the sample to that of the unmodified sample. Similar
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3.2. Composite tests
Table 4 Impact energy absorption (impact strength) of resin Sample
Impact strength (J/m2)
Normalized impact strength
NEAT A5 A10 A15
3500 ± 350 5650 ± 300 6600 ± 500 7350 ± 450
1.00 ± 0.10 1.61 ± 0.08 1.89 ± 0.14 2.09 ± 0.13
Table 5 Glass transition temperature of resin Sample
Tg (°C)
NEAT A5 A10 A15
106.9 104.4 104.6 104.2
Fig. 5. Measured void content as a function of modifier content.
to fracture test results, modifier increases impact strength (Table 4). Impact fracture surfaces of neat and A10 specimens are observed in Fig. 4a and b respectively. The rough fracture surface of the A10 specimen compared to that of the neat epoxy seen in this figure agrees well with impact strength measurements seen in Table 4. Fig. 4c shows the fracture surface of A10 specimen at higher magnification. However comparison between Fig. 3c and Fig. 4c reveals less cavitation and void grows on the damaged surface of the impact test specimen. Decrease in cavitation and void growth corresponds to decrease in plastic deformation and energy absorption. It is the result of higher loading rate of impact test in comparison with fracture toughness test [16,17]. Previous research [18] showed that at high loading rate, toughening mechanisms such as cavitation and void growth are suppressed. 3.1.4. Glass transition temperature (Tg) evaluation The variations of glass transition temperature are demonstrated in Table 5. A reduction in glass transition temperature can be seen in samples containing rubber modifiers. As seen from Table 5, increase in modifier content leads to a slight decrease in Tg. The reduction in glass transition temperature of epoxy by addition of liquid rubber can be attributed to lack of complete precipitation of the rubber molecules from the matrix [19]. Please note that dissolved rubber molecules can plasticize the epoxy matrix and thus, reduce its Tg. In addition to reduction of the glass transition temperature, incomplete precipitation of rubbery phase may also modestly increase toughness and decrease compressive and flexural strength of modified resin [19].
3.2.1. Assessment of void content To study the effect of resin modification on toughness improvement of final composite, as mentioned before, same resin formulations were used to wind proper cylindrical composite tubes. In order to estimate the void content, the density of composite specimens were determined using Archimedes technique. Glass volume fraction of samples was 69% which was determined using ash test. Different porosity levels ranging from 0.9% to 2.2% were found in composite samples. Void content of samples are plotted in Fig. 5 as a function of rubber content. Increase in void content is the result of increasing matrix viscosity due to introduction of the rubbery phase. It is noteworthy that ATBN is a viscous reactive oligomer and thus, increase the molecular weight of the epoxy resin upon mixing with resin. Therefore, increase in viscosity due to increase in molecular weight is expected. In many composite applications such as aerospace structures, the high void content levels are not tolerable due to unexpected reduction in mechanical properties [20]. Previous researchers [20] reported that the tensile, flexural and interlaminar shear strength fall approximately by 7–10% per percent void. Judd and Wright [21] reported that regardless of the utilized materials, voids induce reduction in different mechanical properties including ILSS, tensile and flexural strength and modulus and impact resistance. Olivero et al. [22] reported that doubling the void content from 0.35% to 0.72% by volume resulted in a 15% decrease in ultimate tensile strength and 14% decrease in stiffness for composites reinforced with randomly-oriented reinforcement at 21% fiber volume fraction. Hence, one could expect that mechanical performance of modified composite be corrupted by voids. 3.2.2. Compression and flexural test Table 6 presents the transverse compressive data and flexural strength of composite tubes. The compressive property of final composite in the perpendicular direction to fibers (transverse) is more affected by rubber modification than the direction parallel to fibers. As seen, flexural strength and transverse compressive properties of hoop wound specimens gradually decrease with increasing rubber content similar to that of unreinforced resin. These can be attributed to (i) weakening effect of rubber modifiers in the matrix and (ii) increase in void content due to viscosity increasing effect of rubbery phase (Fig. 5). However flexural strength is less affected by rubber modification in comparison with transverse compressive properties due to higher contribution of fibers in load bearing. 3.2.3. ILSS test Table 7 presents interlaminar shear data obtained from circumferentially wound samples which show some reduction in the interlaminar shear strength (ILSS) with increasing in rubber content. Rubber modification of resin influences ILSS of modified composite as reported by other researchers [3,9,10]. ILSS decreases slightly with ATBN content. Interlaminar shear strength (ILSS) is an important parameter in predicting the low velocity impact damage initiation of laminated
Table 6 Transverse compressive properties and flexural strength of composite tubes Sample
Transverse compressive strength (MPa)
Transverse compressive modulus (GPa)
Flexural strength (MPa)
NEATC A5C A10C A15C
108 101 95 89
9.4 9.0 8.8 8.0
521 519 518 514
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M. Abadyan et al. / Materials and Design 30 (2009) 1976–1984 Table 7 Interlaminar shear strength data of composites specimens Sample
ILSS (MPa)
NEATC A5C A10C A15C
48.5 ± 3 48 ± 2.5 45 ± 2 43 ± 2.5
or wound composites [23,24]. For predicting delamination initiation and matrix cracking, some failure criteria such as Hashin, Tsai-Wu, Choi, etc. are developed which use strength approach [25]. Strength approach is based on comparing the impact induced stress with the ultimate strength of material and ILSS is one of the major terms in all related formulas. The higher ILSS leads to the lower interlaminar matrix cracking and delamination initiation during low velocity impact [23]. One of the most important factors that explains the reduction in ILSS in rubber modified composites is their higher void content. Ghiorse [26] claimed, for carbon/epoxy composites, that each 1% increase in void content induced a 10% reduction in ILSS. Similarly, Goodwin et al. [27] reported a 7% reduction in ILSS per 1% increase in voidage up to 10% in the resin transfer mould (RTM) composite containing 57% satin preform. Fig. 5 shows that A10C samples have higher void content in comparison with unmodified composite which leads to reduction in ILSS in rubber modified samples. Fig. 6 shows the typical load–displacement curves for NEATC, A5C, A10C and A15C samples in ILSS test. The loading curves drop after a peak load due to crack propagation between the layers. After load drops, crack is arrested and load rises again. As seen, both the peak load and the amount of load drop after the peak, reduces with increase in rubber content. Rubber modification increases mode II fracture toughness [9–13] which suppresses crack propagation at higher load levels and produces the less load dropping. This trend compensates the lower ILSS drawback of modified composite during damage. 3.2.4. Impact strength To study the effect of resin modification on toughness improvement of final composite against through thickness cracks, impact samples from wound cylindrical composite tubes were made according to Fig. 1 geometry. The results of impact strength versus rubber content for notched and unnotched test specimens are reported in Table 8. It is interesting that the normalized test data show lower scattering for composite samples (Table 8) compared to that of unreinforced samples (Table 4). All samples fractured through thickness and increase in modifier content led to increased impact strength (Table 8). An earlier investigation in which similar type of impact test was used to evaluate impact strength of rubber modified filament wound composites, revealed no significant improvement in impact strength of the hoop composite [3]. This can be attributed to the fact that the epoxy resin used in that particular investigation was not ductile enough to be toughened by rubber modification. This is why no improvement in impact strength of the composite was observed. Since composite is wound in hoop angle, the contribution of fibers in absorbing the impact energy is not influenced effectively by resin modification compared to helical wound composites. Also considering the high volume fraction of fibers in the composite, rubber content of modified composite is only about 31% of that of the corresponding unreinforced material. Therefore, it is predictable that normalized impact strength of composite is lower than that of the respective resin. In addition, Table 8 shows that normalized impact strength of unnotched specimens are still higher than those of the notched ones. Therefore one may conclude that rubber modification improves crack initiation energy of the composites
Fig. 6. Load vs. displacement of ILSS sample test.
Table 8 Impact strength of composite tubes Sample
Notched (J/m2)
(normalized)
Unnotched (J/m2)
(normalized)
NEATC A5C A10C A15C
1.00 ± 0.05 1.39 ± 0.02 1.46 ± 0.02 1.62 ± 0.01
1.00 ± 0.04 1.2 ± 0.06 1.27 ± 0.02 1.33 ± 0.02
7050 ± 350 9800 ± 200 10300 ± 150 11400 ± 100
6300 ± 250 7600 ± 400 8000 ± 100 8400 ± 100
more than that of their crack propagation energy. It’s in accordance with previous investigations [3]. Introduction of the notch almost eliminates crack initiation energy, thus reduces the beneficial influence of rubber modification on impact resistance of the composites (Table 8). 3.2.4.1. Predicting impact strength by rule of mixtures. The interesting point in comparison of Tables 4 and 8 is that while modified resin samples have an impact strength of 61–109% higher than their neat epoxy matrix (Table 4), modified composite samples have 20– 33% higher impact strength than their unmodified composite (Table 8). This discrepancy might be rationalized via rule of mixtures that considers the reduction of resin content in the composite samples. Rule of mixtures is a simple criterion that neglects the interactions between different phases. This approach could be rationalized via this fact that the fracture surfaces studied did not show any evidence of fiber-matrix interactive mechanisms such as fiber pull-out or bridging. In this section, rule of mixtures is employed to estimate the impact strength of final composite. For assessing the ability of rule of mixtures to predict the impact strength of filament wound samples, ecm is defined as energy absorption per unit area of modified composite samples which is computed by rule of mixtures. It is assumed that for test samples, the total impact energy absorbed by composite is the summation of (i) the energy absorbed by resin and (ii) the energy absorbed by fibers individually. By this assumption, the first term depends on modifier content of matrix (similar to unreinforced resin). The second term, then, depends on fiber content and shows the contribution of fibers in energy absorption. Considering the same winding pattern, reinforcement volume fraction and geometry of specimens, for a composite sample which has no modifier, the energy absorbed per unit area due to fibers, ef, can be evaluated as:
ef ¼ ðecu eru V r Þ=V f ;
ð1Þ
where Vr and Vf are volume fraction of resin and fiber in composite, and ecu and eru are unmodified composite strength (absorbed en-
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Table 9 ecm for notched composite samples Sample
ecm* (J/m2)
Difference with experiment (%)
A5C A10C A15C
7150 7450 7700
6 7 8
*
ples. By this assumption, the total impact energy absorption of modified composite specimens could be computed by rule of mixture:
ecm ¼ ef V f þ erm V r ;
where erm is impact strength of modified resin. Substituting Eq. (1) and (2) in Eq. (3) yields:
Evaluated with mean values.
ecm ¼ ecu þ ðerm eru Þ V r : Table 10 Glass transition temperature of composite Sample
Tg (°C)
NEATC A5C A10C A15C
107.5 107.1 107.2 107
ergy per unit area) and unmodified resin strength respectively. By neglecting the void content of resin, it could be assumed that:
V r þ V f 1:
ð3Þ
ð2Þ
In order to use the rule of mixture, it is assumed that under same geometry, winding pattern and volume fraction of reinforcement, ef, remains the same for both modified and unmodified sam-
ð4Þ
For both notched and unnotched samples, ecm is reported in Table 9. As seen ecm is slightly lower than experimental values, however it is an acceptable approximation of experimental data. 3.2.5. Glass transition temperature (Tg) evaluation The variations of glass transition temperature are reported in Table 10. The data shows that an increase in modifier content leads to a slight decrease in Tg of composite samples. However, the 0.5 °C decrease in Tg value is too small to influence the properties of the composite. Comparison between Tables 5 and 10 reveals that composite specimens show less reduction in Tg by addition of rubbery phase compared to resin specimens. According to what mentioned in Section 3.1.4, one may conclude that more complete precipitation of the rubbery phase has occurred in composite
Fig. 7. Reflected optical micrographs taken from fracture surface of: (a) NEATC, (b) A10C specimens.
Fig. 8. SEM photos of surface of fiber and neighborhood resin: (a) NEATC, (b) A10C, (c) A10C at higher magnification.
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absorption in matrix during impact. Higher fiber/matrix interfacial strength of modified resin produces the better impact performance of modified resin in comparison with unmodified one. 4. Conclusions
Fig. 9. SEM micrographs of surface of composite impact test samples: (a) NEATC, (b) A10C.
specimens compared to unreinforced samples. This might be attributed to the presence of fiber sizing that could influence the kinetics of phase separation of epoxy-rubber mixture [28]. In addition, fiber surface/polymer interactions could influence the Tg of polymer; presence of the rigid surface of glass fiber may decrease the mobility of the polymer chains in the vicinity of fibers [19]. 3.2.6. Fractography In order to better understand the cross-sectional morphology and quality of the A10C and NEATC samples were investigated. Fig. 7 illustrates the optical micrographs of the cross-section of the hoop wound tube. The marks indicate void regions between fibers which have more concentration in A10C samples. These regions are vulnerable to stress due to stress concentration and consequent poor strength compared to the regions with no porosity. This structural difference may account for the different behaviors of the two materials in the compressive, flexural and interlaminar shear test; i.e., A10C sample shows more porosity with corresponding lower mechanical properties compared to the NEATC sample. Fig. 8 shows SEM micrographs of the fracture surface of notched impact test samples. Hackling type morphology is observed for unmodified matrix remaining between the fibers (Fig. 8a), and in the case of modified resin (Fig. 8b), cavitated rubber particles are observed between fibers with no hackling in resin surface (Fig. 8c). Fig. 9 shows some typical fibers on the fracture surface of composite samples. This figure illustrates that the fiber in unmodified specimen (Fig. 9a), contains less remaining resin on its surface in comparison with rubber modified samples (Fig. 9b). Therefore, it confirms that rubber modified resin has more adhesion to fiber surface than unmodified resin. Jang and Yang [9] studied the mechanical properties of polybenoxazine composites modified with two kinds of rubber modifiers providing both strong and weak matrix/fiber interfaces. The results of their investigation revealed that the modified composite with the good matrix/fiber adhesion possessed 20% higher flexural and interlaminar shear strengths compared to the composite having weak interface. Some researchers mentioned that in layered composites, if fiber/matrix interfacial strength is not strong enough, development of plastic zone will be limited due to the failure of the fiber/matrix interface [9,29]. Cracks which propagate between fiber and matrix prevent introduction of sufficient plastic deformation and energy
In this paper a conventional rubber modifiers were employed as toughening agents in epoxy system and respective hoop filament wound composites. Mode I fracture toughness and impact strength of resin increased by rubber modification, without significant reduction in other mechanical properties such as flexural and compressive properties. However, impact strength of composites increase with rubber modification. The normalized impact strength of unnotched specimens was higher than that of notched samples. Due to the notch sensitivity of this material, rubber modification has a greater influence in the unnotched impact strength than the notched impact strength. In addition, a simple formulation based on rule of mixtures was developed to predict the impact resistance of modified composite samples. SEM fractography illustrates that rubber modified composite has better fiber/matrix adhesion than unmodified composite which could effectively increase composites toughness. However rubber modification increases the void content in the final composite. Reduction in flexural, compressive and interlaminar shear strengths by addition of rubber modifier were observed which can be attributed to the increase of the void content as well as the weakening effect of soft rubber particles in the modified formulations. References [1] Riew CK, Kinloch AJ. Preformed particles as tougheners for adhesives and matrices. Adv Chem Ser 1996;225:112–68. [2] Zhang H, Berglund LA, Ericson M. Rubber-toughening of glass fiber epoxy filament wound composites. Polym Eng Sci 1991;31(14):1057–63. [3] Sanjana ZN, Testa JH. Toughened epoxy resins for filament winding. In: Proceedings of the 30th National SAMPE symposium and exhibition, Advancing Technology in Materials and Processes, vol. 30. CA Covina, March 1985. p. 1221–30. [4] Kim J, Mai YW. High strength, high fracture toughness fiber composites with interface control – a review. Comp Sci Tech 1991;41:333–78. [5] Ophir Z, Buchman A, Flashner F, Liran I, Simons H, Dodiuk H. Modified epoxy formulation for improving the fracture-resistance of filament-wound pressure vessels. J Adhesion Sci Tech 1995;9(2):159–75. [6] Arends CB. Polymer toughening. USA New York: Marcel Dekker Inc; 1996. [7] Pearson RA, Sue HJ, Yee AF. Toughening of plastics advances in modeling and experiments. USA Washington DC: ACS Symposium series; 2000. [8] Hertzberg RW. Deformation and fracture mechanics of engineering materials, USA. John Wiley and Sons Inc; 1996. [9] Jang J, Yang H. Toughness improvement of carbon-fiber/polybenzoxazine composites by rubber modification. Comp Sci Tech 2000;60:457–63. [10] Gilbert EN, Hayes BS, Seferis JC. Interlayer toughened unidirectional carbon prepreg systems: effect of preformed particle morphology. Comp Part A 2003;34(3):245–52. [11] Chen SF, Jang BZ. Fracture behavior of interleaved fiber-resin composites. Comp Sci Tech 1991;41(1):77–97. [12] Kim KY, Ye L. Interlaminar fracture toughness of CF/PEI composites at elevated temperatures: roles of matrix toughness and fiber/matrix adhesion. Comp: Part A 2004;35:477–87. [13] DeCarli M, Kozielski K, Tian W, Varley R. Toughening of a carbon fibre reinforced epoxy anhydride composite using an epoxy terminated hyperbranched modifier. Comp Sci Tech 2005;65:2156–66. [14] Bagheri R, Pearson RA. Role of particle cavitation in rubber-toughened epoxies: 1 Microvoid toughening. Polymer 1996;37(20):4529–38. [15] Bagheri R, Pearson RA. Role of particle cavitation in rubber-toughened epoxies: 2 Inter-particle distance. Polymer 2000;41:269–76. [16] Bagheri R, Pearson RA. Role of blend morphology in rubber-toughened polymers. J Mater Sci 1996;31:3945–54. [17] Bagheri R, Pearson RA. Interfacial studies in CTBN-modified epoxies. J Appl Polym Sci 1995;58:427–37. [18] Du J, Thouless MD, Yee AF. Effect of rate on crack growth in a rubber-modified epoxy. Acta Mater 2000;48:3581–92. [19] Bagheri R. Rubber toughened epoxies. Ph.D. dissertation, Lehigh University, 1995. [20] Liu L, Zhang BM, Wang DF, Wu ZJ. Effects of cure cycles on void content and mechanical properties of composite laminates. Comp Struct 2006;73:303–9.
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[21] Judd NCW, Wright WW. Voids and their effects on the mechanical properties of composites – an appraisal. SAMPE J 1978;14(1):10–4. [22] Olivero KA, Barraza HJ, O_Rear EA, Altan MC. Effect of injection rate and postfill cure pressure on resin transfer molded disks. J Comp Mater 2002;36(16):2011–28. [23] Abrate S. Impact on laminated composite materials. Appl Mech Rev 1991;44(4):155–90. [24] Schoepper GA, Abrate S. Delamination threshold loads for low velocity impact on composite laminates. Comp Part A 2000;31:903–15. [25] Moura MFS, Marques AT. Prediction of low velocity impact damage in carbon– epoxy laminates. Comp Part A 2002;33:361–8.
[26] Ghiorse SR. Effect of void content on the mechanical properties of carbon/ epoxy laminates. SAMPE Qtly Jan 1993:54–9. [27] Goodwin AA, Howe CA, Paton RJ. The role of voids in reducing the interlaminar shear strength in RTM laminates. Proceedings of ICCm-11, vol. IV. Australian Composite Structures Society; 1997. p. 11–9. [28] Mezzenga R, Boogh L, Manson JE. A review of dendritic hyperbranched polymer as modifiers in epoxy composites. Comp Sci Technol 2001;61:787–95. [29] Parker DS, Yee AF. Factors influencing the mode I interlaminar fracture toughness of a rubber toughened thermoplastic matrix composite. J Thermoplastic Comp Mater 1989;2:2–18.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Use of rubber modification technique to improve fracture-resistance of hoop wound composites M. Abadyan a, V. Khademi a, R. Bagheri b,*, H. Haddadpour a, M.A. Kouchakzadeh a, M. Farsadi c a
Aerospace Engineering Department, Sharif University of Technology, Azadi Avenue, P.O. Box 11165-8639, Tehran, Iran Polymeric Materials Research Group (PMRG), Materials Science and Engineering Department, Sharif University of Technology, Azadi Avenue, P.O. Box 11165-9466, Tehran, Iran c Aerospace Engineering Department, University of Imam Hosein, Babaei Avenue, Tehran, Iran b
a r t i c l e
i n f o
Article history: Received 29 April 2008 Accepted 3 September 2008 Available online 8 September 2008 Keywords: Rubber toughening (C) Epoxy resin (A) Filament winding (C)
a b s t r a c t Toughness improvement of an epoxy resin and respective hoop wound composite were investigated systematically using amine-terminated butadiene acrylonitrile (ATBN) liquid rubber. Rubber modification improves fracture toughness of epoxy resin with slight reduction in the glass transition temperature (Tg), flexural and compressive properties of resin. Impact resistance of composite is improved by rubber modification similar to modified resin. Interlaminar shear strength (ILSS), compressive modulus and strength, and flexural strength of composite decreased slightly with rubber modification. To interpret the data, the void content of composite samples was determined and the damaged surfaces of fractured samples were investigated. The findings explained the performance of rubber modified specimens in comparison with the unmodified one. In addition, a simple formulation was developed to predict the impact strength of modified composite samples. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Epoxy resins are widely utilized for many industrial high performance applications such as glass reinforced epoxy (GRE) structures. However, GRE has one major disadvantage. It is brittle and exhibits low impact and fracture strength due to low toughness of the epoxy matrix. The lower impact strength of GRE limits its structural applications compared to metals and polymer alloys, especially for fabrication of thin walled structures, automotive application, etc. Various methods of toughening have been proposed to modify GRE. These include toughening of epoxy matrix, fiber hybridization, and coating of fibers with appropriate polymers and specific geometric design of fibers [1]. Rubber modification, i.e. addition of rubber particulate phase to a glassy polymer matrix, is one of the approaches for improving toughness of hoop wound GREs [2,3]. Hoop winding angle is frequently used for producing pipes, the outer layer of metal-composite cylinders, etc. Because of the hoop angle of fibers, cracks can easily propagate in the plane perpendicular to the tube axis and the fibers cannot efficiently prevent matrix cracking in comparison with that of helical wound tubes. Therefore, one may conclude that other methods of toughness improvement could not be as effective as matrix toughening for hoop wound GREs. Liquid rubbers such as carboxyl-terminated butadiene acrylonitrile rubber (CTBN) and amineterminated butadiene acrylonitrile rubber (ATBN), have a high po* Corresponding author. Tel.: +98 21 6616 5207; fax: +98 21 6600 5717. E-mail address: [email protected] (R. Bagheri). 0261-3069/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2008.09.001
tential as modifiers for filament winding without viscosity change or filtration drawbacks [4] which are critical issues in this process. While rubber modification of unreinforced resins and flat laminated composites have been investigated for several decades, rubber modification of filament wound composites, especially hoop wound, has been studied by only few researchers. Ophir et al. [5] investigated a modified epoxy composition for filament winding of pressure vessels. Their product showed an improved toughness with minimal sacrifice of its thermal and mechanical properties. Zhang et al. [2] used CTBN-modified resin to produce glass/epoxy wound tubes. They found that the weepage stress in hoop loading of pipes was higher for unmodified as compared with those of modified pipes. However, the strain at weepage was higher for the pipes based on toughened epoxy [2]. Sanjana and Testa [3] used an instrumented impact test and reported that the toughness improvement of hoop wound epoxy tubes is affected by rate, temperature, and fiber orientation as well as modifier type. Their study showed negligible improvement in impact strength of composites which was the result of low toughenability of the resin used [3]. Further investigation is required to complete previous research and explain some ambiguities in toughening of hoop wound composites. For example, the fracture mechanisms of hoop composites were not investigated in those studies. In addition, there was no acceptable correlation between impact strength of modified composite and that of modified resin. Therefore, the goal of the current investigation is to study the role of rubber modification in performance of hoop wound tubes in a more systematic fashion. ATBN rubber modifier is used to modify the epoxy matrix and the
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corresponding wound composites. The fracture behavior of specimens is also investigated via microscopic techniques.
rate of 10 °C/min. The reported data are the average of at least six tests.
2. Experimantal
2.3.2. Composite evaluation The density and fiber volume fraction of composite specimens were measured according to ASTM D792 and ASTM D3171, respectively and ASTM D2734 test method was used to determine void content of composite samples. Transverse compressive properties of hoop wound composite (compressive properties in the direction perpendicular to the fibers direction) were determined according to ASTM D5449 using tubular specimens. The tubes had dimensions of 100 mm inner diameter 2 mm thick 140 mm long. The flexural tests were performed according to ASTM D790 using three-point loading specimens with dimensions of 62 12.7 4.2 mm3. ASTM D2344 guideline was followed to measure the ILSS of composite. ILSS test samples were cut from a 73 mm inner diameter tube with 6.35 mm thickness and width. The samples had span to thickness ratio of 5 according to standard guideline. The glass transition temperatures of the composite were determined by a differential scanning calorimeter (DSC). The samples were heated from 60 to 130 °C at the heating rate of 10 °C/ min. For hoop wound composites, through thickness cracks that propagate perpendicular to winding axis are more critical and prone to interlaminar cracks which propagate between the hoop layers during impact. To study impact resistance of composites against through thickness matrix failure, special samples were made. Both notched and unnotched impact tests were conducted. Impact specimens were cut from 90 mm inner diameter composite cylinders according to Fig. 1a. In the case of notched specimens, notched were made parallel to the fibers orientation (Fig. 1a). Notch geometry, length and thickness of the specimen were simi-
2.1. Materials The model system used in this study is based on a diglycidyle ether of bisphenol A epoxy with an epoxy equivalent weight of 170 g/eq (Araldite LY564) from Hauntsman and a cycloaliphatic polyamine hardener (HY2962), from Vantico. The modifiers used are ATBN copolymer (Hycar 1300 16) with 16 wt% of acrylonitrile, from Novion. Reinforcement used was a glass rowing with 2400 tex from Vetrotex. 2.2. Sample preparation The stoichiometric ratio of the curing agent and resin were mixed and degassed at room temperature for about 20 min. The solution was then cast into a 5 mm-thick glass mould. The cast material was cured for 4 h at 90 °C in a circulating air oven. The same curing schedule was employed for all toughened epoxies, as well. The modifier content was varied up to 15 phr. Epoxy/rubber mixtures were mixed at room temperature under vacuum for 20 min. to obtain a uniform solution before adding the curing agent. Table 1 presents the epoxy formulations used. All formulations made were then reinforced with glass fiber to wind cylindrical composite tubes. A hoop angle, in which the fibers had an angle close to 90° with respect to the tube axis, was employed. They were cured at the same temperature that was used for resin samples. 2.3. Characterization techniques 2.3.1. Resin evaluation Compressive yield strength and Young’s modulus of unreinforced resin were determined according to ASTM D695 using a screw-driven computer controlled Hunsfield testing frame. Coupons were prepared by machining the cast sheet to give specimen dimensions of 6 6 12 mm3. Flexural tests were performed according to ASTM D790 using tetragonal-shaped specimens of 3.2 12.7 125 mm3. Plane strain fracture toughness, KIC, was determined using single-edge-notch specimens tested in threepoint-bending (3PB) geometry. Specimens with 5 mm thickness were used for this experiment. The ASTM D5045 guideline was followed to measure KIC. Pre-cracks were introduced at the bottom of 2 mm deep notches by hammering a razor blade which was chilled in liquid nitrogen. Izod impact test was accomplished according to ASTM D256 using an 1 J hammer energy. Samples were prepared and machined to the standard shape (62 12.7 4.2 mm3). Specimens were tested using. The glass transition temperatures of the resin were determined by a differential scanning calorimeter (DSC). The samples were heated from 60 to 130 °C at the heating Table 1 Formulations made in this study Sample code
Resin (phr)
Hardener (phr)
Modifier type
Modifier content (phr)
Reinforced with glass fiber
NEAT NEATC A5 A5C A10 A10C A15 A15C
100 100 100 100 100 100 100 100
25 25 25 25 25 25 25 25
– – ATBN ATBN ATBN ATBN ATBN ATBN
0 0 5 5 10 10 15 15
No Yes No Yes No Yes No Yes
Fig. 1. Schematic diagrams of: (a) geometries, (b) constraints and the load conditions, used for impact assessment of filament wound tubes.
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Table 2 Compressive and flexural properties of resin
3. Results and discussion
Sample
Compressive strength (MPa)
Compressive modulus (GPa)
Flexural strength (MPa)
3.1. Resin tests
NEAT A5 A10 A15
103 97 90 82
3.15 3.00 2.90 2.60
129 118 111 101
3.1.1. Compressive and flexural properties Table 2 shows the compressive and flexural properties of epoxy resin as a function of rubber content. As expected, the compressive strength and modulus gradually decrease with increasing rubber content (Table 2). 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 yield strength [6]. Reduction of the macroscopic yield stress leads to promote the crack tip blunting and produce larger plastic zone around the crack tip which contributes to fracture toughness improvement [7]. Similar reduction is observed in flexural strength. Flexural strength gradually decreases with increasing in rubber content (Table 2). 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 [8].
lar to that mentioned in ASTM D256 for Izod test. Width of specimen is such that the area of the cross-section of composite specimen is equal to that of resin specimen. Similar experiment was conducted by Sanjana et al. [3] to assess composites. Impact tests were run on an impact testing machine with 1 J hammer energy. Samples were supported in a way similar to that mentioned in ASTM D256 for Izod test. The impact direction was coaxial with the symmetric axis of sample cross-section according to Fig. 1b. For each mechanical test, six specimens were used and average of six values is reported. 2.3.3. Microscopy evaluation Fracture surfaces of the SEN-3PB and impact specimens were examined using stereo optical microscope and scanning electron microscope (SEM). SEM samples were coated with a thin layer of gold before examination to protect the fracture surfaces from beam damage and also to prevent charge build up. Also, a reflected optical microscope was incorporated to provide more information from the fracture surface of specimens. Table 3 Fracture toughness of resin Sample
KIC (MPa m0.5)
NEAT A5 A10 A15
0.6 1.8 2.2 2.5
3.1.2. Fracture toughness The results of fracture toughness measurements are listed in Table 3. As seen in this table, modifier increases fracture toughness. Based on the results shown in Table 3 and previous studies, it can be expected that composites made using rubber modified epoxy show improved interlaminar fracture toughness [9–13]. Fig. 2 contains the optical micrographs showing the damage zone of SEN-3PB specimens. As seen, plastic deformations at the plastic zone of modified resins are accompanied by stress whitening. The size of stress whitening increases with increasing the rubber content; such that in A15 specimen, it expands to the whole surface in front of the crack tip (Fig. 2d). The difference in the damage zone sizes seen in Fig. 3 is in agreement with fracture toughness data reported in Table 3. Fig. 3 illustrates fracture surfaces of neat and A10 specimens. The flat and almost featureless image seen in Fig. 3a indicates
Fig. 2. Optical micrographs taken from the fracture surface of specimens: (a) NEAT, (b)A5, (c) A10, (d) A15.
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Fig. 3. SEM micrographs taken from the fracture surface of specimens: (a) NEAT, (b) A10, (c) A10 at higher magnification.
Fig. 4. SEM micrographs taken from fracture surface of impact test samples: (a) NEAT, (b) A10, (c) A10 at higher magnification.
the typical brittle fracture of neat epoxies. On the other hand, the rough fracture surface of A10 specimen seen in Fig. 3b shows the significant amount of plastic deformation occurred in this material prior to fracture. Same figure seen at higher magnification, Fig. 4c, shows good dispersion of rubber particles in the matrix and occurrence of cavitation in these particles at the crack tip. Cavitation is one of the most important mechanisms in rubber modification. Bagheri and Pearson [14,15] investigated toughening mechanisms in rubber toughened epoxies and showed that cavitation is a pre-
requisite for massive shear deformation of the matrix. The latter is known as the main source of energy absorption in fracture of rubber toughened epoxies [14–17]. 3.1.3. Impact test The results of impact energy absorption (impact strength) versus ATBN content for notched test specimens are reported in Table 4. Normalized impact strength is defined as the ratio of impact strength of the sample to that of the unmodified sample. Similar
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3.2. Composite tests
Table 4 Impact energy absorption (impact strength) of resin Sample
Impact strength (J/m2)
Normalized impact strength
NEAT A5 A10 A15
3500 ± 350 5650 ± 300 6600 ± 500 7350 ± 450
1.00 ± 0.10 1.61 ± 0.08 1.89 ± 0.14 2.09 ± 0.13
Table 5 Glass transition temperature of resin Sample
Tg (°C)
NEAT A5 A10 A15
106.9 104.4 104.6 104.2
Fig. 5. Measured void content as a function of modifier content.
to fracture test results, modifier increases impact strength (Table 4). Impact fracture surfaces of neat and A10 specimens are observed in Fig. 4a and b respectively. The rough fracture surface of the A10 specimen compared to that of the neat epoxy seen in this figure agrees well with impact strength measurements seen in Table 4. Fig. 4c shows the fracture surface of A10 specimen at higher magnification. However comparison between Fig. 3c and Fig. 4c reveals less cavitation and void grows on the damaged surface of the impact test specimen. Decrease in cavitation and void growth corresponds to decrease in plastic deformation and energy absorption. It is the result of higher loading rate of impact test in comparison with fracture toughness test [16,17]. Previous research [18] showed that at high loading rate, toughening mechanisms such as cavitation and void growth are suppressed. 3.1.4. Glass transition temperature (Tg) evaluation The variations of glass transition temperature are demonstrated in Table 5. A reduction in glass transition temperature can be seen in samples containing rubber modifiers. As seen from Table 5, increase in modifier content leads to a slight decrease in Tg. The reduction in glass transition temperature of epoxy by addition of liquid rubber can be attributed to lack of complete precipitation of the rubber molecules from the matrix [19]. Please note that dissolved rubber molecules can plasticize the epoxy matrix and thus, reduce its Tg. In addition to reduction of the glass transition temperature, incomplete precipitation of rubbery phase may also modestly increase toughness and decrease compressive and flexural strength of modified resin [19].
3.2.1. Assessment of void content To study the effect of resin modification on toughness improvement of final composite, as mentioned before, same resin formulations were used to wind proper cylindrical composite tubes. In order to estimate the void content, the density of composite specimens were determined using Archimedes technique. Glass volume fraction of samples was 69% which was determined using ash test. Different porosity levels ranging from 0.9% to 2.2% were found in composite samples. Void content of samples are plotted in Fig. 5 as a function of rubber content. Increase in void content is the result of increasing matrix viscosity due to introduction of the rubbery phase. It is noteworthy that ATBN is a viscous reactive oligomer and thus, increase the molecular weight of the epoxy resin upon mixing with resin. Therefore, increase in viscosity due to increase in molecular weight is expected. In many composite applications such as aerospace structures, the high void content levels are not tolerable due to unexpected reduction in mechanical properties [20]. Previous researchers [20] reported that the tensile, flexural and interlaminar shear strength fall approximately by 7–10% per percent void. Judd and Wright [21] reported that regardless of the utilized materials, voids induce reduction in different mechanical properties including ILSS, tensile and flexural strength and modulus and impact resistance. Olivero et al. [22] reported that doubling the void content from 0.35% to 0.72% by volume resulted in a 15% decrease in ultimate tensile strength and 14% decrease in stiffness for composites reinforced with randomly-oriented reinforcement at 21% fiber volume fraction. Hence, one could expect that mechanical performance of modified composite be corrupted by voids. 3.2.2. Compression and flexural test Table 6 presents the transverse compressive data and flexural strength of composite tubes. The compressive property of final composite in the perpendicular direction to fibers (transverse) is more affected by rubber modification than the direction parallel to fibers. As seen, flexural strength and transverse compressive properties of hoop wound specimens gradually decrease with increasing rubber content similar to that of unreinforced resin. These can be attributed to (i) weakening effect of rubber modifiers in the matrix and (ii) increase in void content due to viscosity increasing effect of rubbery phase (Fig. 5). However flexural strength is less affected by rubber modification in comparison with transverse compressive properties due to higher contribution of fibers in load bearing. 3.2.3. ILSS test Table 7 presents interlaminar shear data obtained from circumferentially wound samples which show some reduction in the interlaminar shear strength (ILSS) with increasing in rubber content. Rubber modification of resin influences ILSS of modified composite as reported by other researchers [3,9,10]. ILSS decreases slightly with ATBN content. Interlaminar shear strength (ILSS) is an important parameter in predicting the low velocity impact damage initiation of laminated
Table 6 Transverse compressive properties and flexural strength of composite tubes Sample
Transverse compressive strength (MPa)
Transverse compressive modulus (GPa)
Flexural strength (MPa)
NEATC A5C A10C A15C
108 101 95 89
9.4 9.0 8.8 8.0
521 519 518 514
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M. Abadyan et al. / Materials and Design 30 (2009) 1976–1984 Table 7 Interlaminar shear strength data of composites specimens Sample
ILSS (MPa)
NEATC A5C A10C A15C
48.5 ± 3 48 ± 2.5 45 ± 2 43 ± 2.5
or wound composites [23,24]. For predicting delamination initiation and matrix cracking, some failure criteria such as Hashin, Tsai-Wu, Choi, etc. are developed which use strength approach [25]. Strength approach is based on comparing the impact induced stress with the ultimate strength of material and ILSS is one of the major terms in all related formulas. The higher ILSS leads to the lower interlaminar matrix cracking and delamination initiation during low velocity impact [23]. One of the most important factors that explains the reduction in ILSS in rubber modified composites is their higher void content. Ghiorse [26] claimed, for carbon/epoxy composites, that each 1% increase in void content induced a 10% reduction in ILSS. Similarly, Goodwin et al. [27] reported a 7% reduction in ILSS per 1% increase in voidage up to 10% in the resin transfer mould (RTM) composite containing 57% satin preform. Fig. 5 shows that A10C samples have higher void content in comparison with unmodified composite which leads to reduction in ILSS in rubber modified samples. Fig. 6 shows the typical load–displacement curves for NEATC, A5C, A10C and A15C samples in ILSS test. The loading curves drop after a peak load due to crack propagation between the layers. After load drops, crack is arrested and load rises again. As seen, both the peak load and the amount of load drop after the peak, reduces with increase in rubber content. Rubber modification increases mode II fracture toughness [9–13] which suppresses crack propagation at higher load levels and produces the less load dropping. This trend compensates the lower ILSS drawback of modified composite during damage. 3.2.4. Impact strength To study the effect of resin modification on toughness improvement of final composite against through thickness cracks, impact samples from wound cylindrical composite tubes were made according to Fig. 1 geometry. The results of impact strength versus rubber content for notched and unnotched test specimens are reported in Table 8. It is interesting that the normalized test data show lower scattering for composite samples (Table 8) compared to that of unreinforced samples (Table 4). All samples fractured through thickness and increase in modifier content led to increased impact strength (Table 8). An earlier investigation in which similar type of impact test was used to evaluate impact strength of rubber modified filament wound composites, revealed no significant improvement in impact strength of the hoop composite [3]. This can be attributed to the fact that the epoxy resin used in that particular investigation was not ductile enough to be toughened by rubber modification. This is why no improvement in impact strength of the composite was observed. Since composite is wound in hoop angle, the contribution of fibers in absorbing the impact energy is not influenced effectively by resin modification compared to helical wound composites. Also considering the high volume fraction of fibers in the composite, rubber content of modified composite is only about 31% of that of the corresponding unreinforced material. Therefore, it is predictable that normalized impact strength of composite is lower than that of the respective resin. In addition, Table 8 shows that normalized impact strength of unnotched specimens are still higher than those of the notched ones. Therefore one may conclude that rubber modification improves crack initiation energy of the composites
Fig. 6. Load vs. displacement of ILSS sample test.
Table 8 Impact strength of composite tubes Sample
Notched (J/m2)
(normalized)
Unnotched (J/m2)
(normalized)
NEATC A5C A10C A15C
1.00 ± 0.05 1.39 ± 0.02 1.46 ± 0.02 1.62 ± 0.01
1.00 ± 0.04 1.2 ± 0.06 1.27 ± 0.02 1.33 ± 0.02
7050 ± 350 9800 ± 200 10300 ± 150 11400 ± 100
6300 ± 250 7600 ± 400 8000 ± 100 8400 ± 100
more than that of their crack propagation energy. It’s in accordance with previous investigations [3]. Introduction of the notch almost eliminates crack initiation energy, thus reduces the beneficial influence of rubber modification on impact resistance of the composites (Table 8). 3.2.4.1. Predicting impact strength by rule of mixtures. The interesting point in comparison of Tables 4 and 8 is that while modified resin samples have an impact strength of 61–109% higher than their neat epoxy matrix (Table 4), modified composite samples have 20– 33% higher impact strength than their unmodified composite (Table 8). This discrepancy might be rationalized via rule of mixtures that considers the reduction of resin content in the composite samples. Rule of mixtures is a simple criterion that neglects the interactions between different phases. This approach could be rationalized via this fact that the fracture surfaces studied did not show any evidence of fiber-matrix interactive mechanisms such as fiber pull-out or bridging. In this section, rule of mixtures is employed to estimate the impact strength of final composite. For assessing the ability of rule of mixtures to predict the impact strength of filament wound samples, ecm is defined as energy absorption per unit area of modified composite samples which is computed by rule of mixtures. It is assumed that for test samples, the total impact energy absorbed by composite is the summation of (i) the energy absorbed by resin and (ii) the energy absorbed by fibers individually. By this assumption, the first term depends on modifier content of matrix (similar to unreinforced resin). The second term, then, depends on fiber content and shows the contribution of fibers in energy absorption. Considering the same winding pattern, reinforcement volume fraction and geometry of specimens, for a composite sample which has no modifier, the energy absorbed per unit area due to fibers, ef, can be evaluated as:
ef ¼ ðecu eru V r Þ=V f ;
ð1Þ
where Vr and Vf are volume fraction of resin and fiber in composite, and ecu and eru are unmodified composite strength (absorbed en-
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Table 9 ecm for notched composite samples Sample
ecm* (J/m2)
Difference with experiment (%)
A5C A10C A15C
7150 7450 7700
6 7 8
*
ples. By this assumption, the total impact energy absorption of modified composite specimens could be computed by rule of mixture:
ecm ¼ ef V f þ erm V r ;
where erm is impact strength of modified resin. Substituting Eq. (1) and (2) in Eq. (3) yields:
Evaluated with mean values.
ecm ¼ ecu þ ðerm eru Þ V r : Table 10 Glass transition temperature of composite Sample
Tg (°C)
NEATC A5C A10C A15C
107.5 107.1 107.2 107
ergy per unit area) and unmodified resin strength respectively. By neglecting the void content of resin, it could be assumed that:
V r þ V f 1:
ð3Þ
ð2Þ
In order to use the rule of mixture, it is assumed that under same geometry, winding pattern and volume fraction of reinforcement, ef, remains the same for both modified and unmodified sam-
ð4Þ
For both notched and unnotched samples, ecm is reported in Table 9. As seen ecm is slightly lower than experimental values, however it is an acceptable approximation of experimental data. 3.2.5. Glass transition temperature (Tg) evaluation The variations of glass transition temperature are reported in Table 10. The data shows that an increase in modifier content leads to a slight decrease in Tg of composite samples. However, the 0.5 °C decrease in Tg value is too small to influence the properties of the composite. Comparison between Tables 5 and 10 reveals that composite specimens show less reduction in Tg by addition of rubbery phase compared to resin specimens. According to what mentioned in Section 3.1.4, one may conclude that more complete precipitation of the rubbery phase has occurred in composite
Fig. 7. Reflected optical micrographs taken from fracture surface of: (a) NEATC, (b) A10C specimens.
Fig. 8. SEM photos of surface of fiber and neighborhood resin: (a) NEATC, (b) A10C, (c) A10C at higher magnification.
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absorption in matrix during impact. Higher fiber/matrix interfacial strength of modified resin produces the better impact performance of modified resin in comparison with unmodified one. 4. Conclusions
Fig. 9. SEM micrographs of surface of composite impact test samples: (a) NEATC, (b) A10C.
specimens compared to unreinforced samples. This might be attributed to the presence of fiber sizing that could influence the kinetics of phase separation of epoxy-rubber mixture [28]. In addition, fiber surface/polymer interactions could influence the Tg of polymer; presence of the rigid surface of glass fiber may decrease the mobility of the polymer chains in the vicinity of fibers [19]. 3.2.6. Fractography In order to better understand the cross-sectional morphology and quality of the A10C and NEATC samples were investigated. Fig. 7 illustrates the optical micrographs of the cross-section of the hoop wound tube. The marks indicate void regions between fibers which have more concentration in A10C samples. These regions are vulnerable to stress due to stress concentration and consequent poor strength compared to the regions with no porosity. This structural difference may account for the different behaviors of the two materials in the compressive, flexural and interlaminar shear test; i.e., A10C sample shows more porosity with corresponding lower mechanical properties compared to the NEATC sample. Fig. 8 shows SEM micrographs of the fracture surface of notched impact test samples. Hackling type morphology is observed for unmodified matrix remaining between the fibers (Fig. 8a), and in the case of modified resin (Fig. 8b), cavitated rubber particles are observed between fibers with no hackling in resin surface (Fig. 8c). Fig. 9 shows some typical fibers on the fracture surface of composite samples. This figure illustrates that the fiber in unmodified specimen (Fig. 9a), contains less remaining resin on its surface in comparison with rubber modified samples (Fig. 9b). Therefore, it confirms that rubber modified resin has more adhesion to fiber surface than unmodified resin. Jang and Yang [9] studied the mechanical properties of polybenoxazine composites modified with two kinds of rubber modifiers providing both strong and weak matrix/fiber interfaces. The results of their investigation revealed that the modified composite with the good matrix/fiber adhesion possessed 20% higher flexural and interlaminar shear strengths compared to the composite having weak interface. Some researchers mentioned that in layered composites, if fiber/matrix interfacial strength is not strong enough, development of plastic zone will be limited due to the failure of the fiber/matrix interface [9,29]. Cracks which propagate between fiber and matrix prevent introduction of sufficient plastic deformation and energy
In this paper a conventional rubber modifiers were employed as toughening agents in epoxy system and respective hoop filament wound composites. Mode I fracture toughness and impact strength of resin increased by rubber modification, without significant reduction in other mechanical properties such as flexural and compressive properties. However, impact strength of composites increase with rubber modification. The normalized impact strength of unnotched specimens was higher than that of notched samples. Due to the notch sensitivity of this material, rubber modification has a greater influence in the unnotched impact strength than the notched impact strength. In addition, a simple formulation based on rule of mixtures was developed to predict the impact resistance of modified composite samples. SEM fractography illustrates that rubber modified composite has better fiber/matrix adhesion than unmodified composite which could effectively increase composites toughness. However rubber modification increases the void content in the final composite. Reduction in flexural, compressive and interlaminar shear strengths by addition of rubber modifier were observed which can be attributed to the increase of the void content as well as the weakening effect of soft rubber particles in the modified formulations. References [1] Riew CK, Kinloch AJ. Preformed particles as tougheners for adhesives and matrices. Adv Chem Ser 1996;225:112–68. [2] Zhang H, Berglund LA, Ericson M. Rubber-toughening of glass fiber epoxy filament wound composites. Polym Eng Sci 1991;31(14):1057–63. [3] Sanjana ZN, Testa JH. Toughened epoxy resins for filament winding. In: Proceedings of the 30th National SAMPE symposium and exhibition, Advancing Technology in Materials and Processes, vol. 30. CA Covina, March 1985. p. 1221–30. [4] Kim J, Mai YW. High strength, high fracture toughness fiber composites with interface control – a review. Comp Sci Tech 1991;41:333–78. [5] Ophir Z, Buchman A, Flashner F, Liran I, Simons H, Dodiuk H. Modified epoxy formulation for improving the fracture-resistance of filament-wound pressure vessels. J Adhesion Sci Tech 1995;9(2):159–75. [6] Arends CB. Polymer toughening. USA New York: Marcel Dekker Inc; 1996. [7] Pearson RA, Sue HJ, Yee AF. Toughening of plastics advances in modeling and experiments. USA Washington DC: ACS Symposium series; 2000. [8] Hertzberg RW. Deformation and fracture mechanics of engineering materials, USA. John Wiley and Sons Inc; 1996. [9] Jang J, Yang H. Toughness improvement of carbon-fiber/polybenzoxazine composites by rubber modification. Comp Sci Tech 2000;60:457–63. [10] Gilbert EN, Hayes BS, Seferis JC. Interlayer toughened unidirectional carbon prepreg systems: effect of preformed particle morphology. Comp Part A 2003;34(3):245–52. [11] Chen SF, Jang BZ. Fracture behavior of interleaved fiber-resin composites. Comp Sci Tech 1991;41(1):77–97. [12] Kim KY, Ye L. Interlaminar fracture toughness of CF/PEI composites at elevated temperatures: roles of matrix toughness and fiber/matrix adhesion. Comp: Part A 2004;35:477–87. [13] DeCarli M, Kozielski K, Tian W, Varley R. Toughening of a carbon fibre reinforced epoxy anhydride composite using an epoxy terminated hyperbranched modifier. Comp Sci Tech 2005;65:2156–66. [14] Bagheri R, Pearson RA. Role of particle cavitation in rubber-toughened epoxies: 1 Microvoid toughening. Polymer 1996;37(20):4529–38. [15] Bagheri R, Pearson RA. Role of particle cavitation in rubber-toughened epoxies: 2 Inter-particle distance. Polymer 2000;41:269–76. [16] Bagheri R, Pearson RA. Role of blend morphology in rubber-toughened polymers. J Mater Sci 1996;31:3945–54. [17] Bagheri R, Pearson RA. Interfacial studies in CTBN-modified epoxies. J Appl Polym Sci 1995;58:427–37. [18] Du J, Thouless MD, Yee AF. Effect of rate on crack growth in a rubber-modified epoxy. Acta Mater 2000;48:3581–92. [19] Bagheri R. Rubber toughened epoxies. Ph.D. dissertation, Lehigh University, 1995. [20] Liu L, Zhang BM, Wang DF, Wu ZJ. Effects of cure cycles on void content and mechanical properties of composite laminates. Comp Struct 2006;73:303–9.
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