- Email: [email protected]
Interlaminar properties of carbon fiber composites with halloysite nanotube-toughened epoxy matrix
Composites Science and Technology 71 (2011) 717–723
Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Interlaminar properties of carbon fiber composites with halloysite nanotube-toughened epoxy matrix Yueping Ye a, Haibin Chen a, Jingshen Wu a,⇑, Chi Ming Chan b,c,⇑ a
Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Hong Kong, China Department of Chemical and Biomolecular Engineering, The Hong Kong University of Science and Technology, Hong Kong, China c Division of Environment, The Hong Kong University of Science and Technology, Hong Kong, China b
a r t i c l e
i n f o
Article history: Received 22 August 2010 Received in revised form 14 January 2011 Accepted 25 January 2011 Available online 1 February 2011 Keywords: Halloysite nanotube A. Nanocomposites A. Hybrid composites B. Fracture toughness
a b s t r a c t Halloysite nanotubes (HNTs), which are geometrically similar to multi-walled carbon nanotubes, can improve the impact strength of epoxy substantially, according to our previous work [1]. Using a HNTtoughened epoxy as the matrix, a set of hybrid composites was prepared with carbon fiber-woven fabrics. The interlaminar properties of the composites were investigated by a short-beam shear test, a doublecantilever-beam test and an end-notched flexure test. The results showed that the addition of HNTs to the composites improved the interlaminar shear strength and the fracture resistance under Mode I and Mode II loadings greatly. The morphological study of the hybrid composites revealed that HNTs were non-uniformly dispersed in the epoxy matrix, forming a unique microstructure with a large number of HNT-rich composite particles enveloped by a continuous epoxy-rich phase. A study of the fracture mechanism uncovered the important role of this special morphology during the fracturing of the hybrid composites. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Carbon fiber-reinforced epoxy (EP/CF) composites have been widely used in many areas, including aerospace, automobile, marine, military, etc., due to their unique properties, such as high strength, high modulus and lightweight. However, the strength in the through-thickness direction is a limiting design factor in conventional composites because there are no fibers oriented in the thickness direction to sustain transverse loads. The low strength in the through-thickness direction generally leads to interlaminar failures, such as delamination. To improve the interlaminar fracture toughness, a considerable amount of research has been conducted. Examples of such attempts include the use of Z-pins to connect the laminates, and extending the fibers through the thickness direction by weaving, knitting, braiding or stitching [2–6]. However, these techniques are labor-intensive and require special fabrication processes, which greatly increase the manufacturing cost [5]. Moreover, the complex combination of materials makes it difficult to accurately predict the in-plane mechanical properties, i.e., the tensile, compressive and flexural properties, for a particular stitched composite. For instance, stitching may improve the in-plane proper⇑ Corresponding authors. Address: Department of Chemical and Biomolecular Engineering, The Hong Kong University of Science and Technology, Hong Kong, China. Tel.: +852 23587125; fax: +852 2358 0054 (C.M. Chan), tel.: +852 23587200; fax: +852 23581543 (J. Wu). E-mail addresses: [email protected] (J. Wu), [email protected] (C.M. Chan). 0266-3538/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2011.01.018
ties, or it may leave the properties un-changed; in some instances it may even seriously decrease the in-plane properties [2]. With improvements in nanocomposite technology, many researchers attempted to improve the interlaminar properties of fiber-reinforced composites using nanofillers [7–12]. The incorporation of alumina nanofillers into EP/CF composites resulted in both higher interlaminar shear strength (ILSS) and fracture toughness [12]. Gojny et al. [7] found that the addition of 0.3 wt% doublewalled carbon nanotubes (CNTs) to fiber-reinforced epoxy composites increased the ILSS by 20%. By growing aligned CNTs on the surface of SiC fibers, the Mode I (GIc) and Mode II (GIIc) interlaminar fracture toughness of the 3D composites were improved by 348% and 54%, respectively [8]. Siddiqui et al. [10] also increased the value of GIc by 60% by adding 3 wt% organoclay to an epoxy. All these studies have shown just how promising the applications of nanofillers are in fiber-reinforced composites. Moreover, the introduction of nanofillers does not increase the weight of the components manufactured with EP/CF composites. Halloysite is a fine clay mineral consisting of tubular particles with a multi-layered wall structure. In recent years, there have been great interests in the application of HNTs in polymer materials [13– 15]. Our previous work [1,16] has shown that HNTs, as low-cost nanotubes, can improve the mechanical properties of an epoxy significantly. The addition of 2.3 wt% HNTs to the epoxy increased its Charpy impact strength by about four times with slight improvements in the flexural modulus and strength. The underlying toughening mechanisms were identified as massive bridging, pull-
718
Y. Ye et al. / Composites Science and Technology 71 (2011) 717–723
Fig. 1. SEM micrographs of EP/HNT/CF composites with 3 wt% HNTs.
out, and breaking of nanotubes, micro-cracking, as well as maincrack deflection. Considering that the HNT crack-bridging capability and the low damage resistance of conventional EP/CF composites are largely a result of the propagation of internal defects (e.g., micro-cracks) under external loadings, we used the EP/HNT nanocomposite as the matrix in the fabrication of the CF composite in the present study. We anticipated that the EP/HNT/CF hybrid composites would benefit from the high impact toughness due to the presence of the HNTs, leading to a new class of CF composites. The microstructure of the EP/HNT/CF hybrid composites was examined using a scanning electron microscope (SEM). The mechanical properties and the failure mechanisms of the hybrid composites were studied.
Fig. 2. TEM micrograph showing the HNT-rich region.
2. Experimental work 2.1. Fabrication of composites The EP/HNT/CF hybrid composites were prepared with carbon fiber-woven fabrics and a HNT-filled epoxy by the hand lay-up process. Plain woven carbon fibers (TI3101 supplied by Taiwan Electrical Insulators Co.) with a unit weight of 200 g/m2 were used as the major reinforcement. The materials and processing conditions for the EP/HNT nanocomposite solution were the same as those reported previously [1]. The nanocomposite was prepared using halloysite nanotubes (supplied by Imerys Tableware New Zealand Limited), and EPON Resin 828 (Bisphenol A, supplied by Resolution Performance Products) with a curing agent, 4, 40 -methylene dianiline (MDA, Aldrich), at a 100/27 weight ratio. A certain amount of halloysite nanotubes was first dispersed in acetone and
Fig. 3. Interlaminar shear strength of EP/CF composites with different HNT contents.
Y. Ye et al. / Composites Science and Technology 71 (2011) 717–723
mechanically stirred for half an hour at room temperature. The mixture was then introduced into the epoxy resin and stirred for another 2 h at 75 °C. After degassing to remove the remaining acetone, MDA was added with gentle mixing. The HNT-filled epoxy was then brushed onto twelve plies of carbon fiber fabrics by the hand lay-up process. A 15-lm thick Teflon film was inserted into the middle plane of the laminates as the initial crack for the Mode I and Mode II interlaminar fracture toughness tests. The laminates were finally stacked on an aluminum mold and cured in a hot pressing machine (Technical Machine Products Corp.). They were pre-cured at 80 °C for 2 h and post-cured at 160 °C for another 4 h. A low pressure of 0.3 MPa was applied throughout the whole curing process to maintain a laminate thickness of 3.2 ± 0.2 mm. The carbon fiber volume fraction of the composites was 29 ± 1 vol%, determined by the combustion of the cured laminates according to ASTM D3171. The halloysite contents varied between 1–5 wt% based on the nanocomposite matrix. 2.2. Material characterization The morphology of EP/HNT/CF hybrid composites was studied with a SEM (JEOL JSM-6700F). The short-beam shear (SBS) test
719
(ASTM D2344) was employed to characterize the apparent interlaminar shear strength of the composites on a universal testing machine (UTM, MTS Sintech 10/D). The SBS specimen of 20.0 6.4 3.2 mm3 was placed on two cylindrical supports of 3 mm in diameter and bent by a cylindrical head of 6 mm in diameter at the centre of the specimen. The tests were conducted with a crosshead rate of 1 mm/min and a span/thickness ratio of 4. More than eight specimens for each composite system were tested. To investigate the reinforcing effects of HNTs on ILSS, the cross-sectional area of the damaged samples was examined using an optical microscope and SEM. The opening Mode-I interlaminar fracture toughness (GIc) of the composites was determined using the double-cantilever-beam (DCB) test according to ASTM D5528. Teflon film was inserted into the mid-plane of the laminates as the starter crack in the composite preparation process and the initial crack length (a0) was 35 mm. Specimens with a length of 125 mm and a width (W) of 20 mm were cut from the molded sheets, to the end of which a pair of aluminum blocks was bonded using an adhesive. To monitor the delamination length, the DCB specimen edges were polished and fine lines at intervals of 1–2 mm were marked along the length direction. The experiment was performed with a crosshead speed
Fig. 4. Typical failure modes in a damaged SBS sample with 3 wt% HNTs: (a) the entire view of the damaged zone; (b) trans-ply cracking through a HNT-rich particle; (c) the enlarged area of A in Fig. 3b showing nanotube bridging in the HNT-rich region; (d) matrix micro-cracking; (e) the enlarged area of B in Fig. 3d showing nanotube bridging in the epoxy matrix; (f) the enlarged area of C in Fig. 3d showing a damaged HNT-rich region.
720
Y. Ye et al. / Composites Science and Technology 71 (2011) 717–723
of 2 mm/min on a universal testing machine (UTM). The load, P, and the corresponding displacement, d, were recorded as a function of the crack length (a). Taking into account the rotation at the delamination front, GIc was calculated according to the modified-beam theory as follows:
3Pd GIc ¼ 2Wða þ jDjÞ
ð1Þ
where D was determined experimentally by generating a leastsquare fit of the cube root of compliance, C1/3, against the crack length. The compliance, C, is the ratio of the displacement to the applied load, d/P, for a specific crack length. The Mode II interlaminar fracture toughness (GIIc) was evaluated using the end-notched flexure (ENF) tests on a hydraulic UTM (MTS 858 Mini Bionix) at a loading rate of 1 mm/min. The specimen length and width (W) were 120 and 20 mm, respectively. The specimen was tested on a three-point bending fixture with a half-span length (L) of 50 mm. The overall Teflon film length was 35 mm and the initial crack length (a0) was designed to be around 25 mm. For the calculation of GIIc, several methods have been proposed [17], including the analytical compliance method, the directbeam theory and the corrected-beam theory [18]. Tanaka et al. found that the toughness values calculated by different methods were not much different [19]. The correct-beam theory was applied and the value of GIIc was determined by
GIIc ¼
9a20 P2m K 4mWL3 H
ð2Þ
where m is the slope of the initial straight-line portion of the load– deflection curve; Pm is the maximum load; K is a correction factor
Fig. 5. The load–displacement curves (a) and R curves (b) for the composites with different halloysite contents in DCB tests.
for the moment arm; and H is a correction factor for the compliance. K and H can be determined by
K ¼ 1 0:6099ð/=LÞ2
ð3Þ
and
H ¼ 1 þ 0:3766ð/=LÞ2
ð4Þ
where u is the displacement at maximum load. For both DCB and ENF tests, at least five measurements were made under a given set of conditions and the fracture surface of the broken specimens after each test was examined using SEM. In addition, before the mechanical tests, each sample was annealed at 180 °C (10 °C above Tg of the epoxy matrix) for 2 h to eliminate any residual stress introduced during the mechanical cutting. 3. Results and discussion 3.1. Morphology The microstructure of EP/HNT/CF hybrid composites was examined by SEM and the micrographs are shown in Fig. 1. HNTs were non-uniformly dispersed in the epoxy matrix. Similar to what was observed in the EP/HNT nanocomposites [1], some HNTs were randomly dispersed in the matrix with large inter-tube distances, while others were dispersed in the epoxy with much shorter inter-tube distances, resulting in the formation of many HNT-rich regions. Although at the first glance the HNT-rich regions appeared to be the clusters of HNTs, a closer examination of these regions with a TEM revealed that the spaces among the HNTs were actually filled by epoxy, as shown in Fig. 2. The HNT-rich regions can be regarded as the rigid composite particles with high HNT content, which play an important role in the toughening of epoxy [1].
Fig. 6. Overview of the fracture surface of the EP/CF composite after the DCB test (crack propagation direction: from top to the bottom).
Y. Ye et al. / Composites Science and Technology 71 (2011) 717–723
3.2. Interlaminar shear strength The interlaminar shear strength of the EP/HNT/CF composites with different amounts of HNTs is illustrated in Fig. 3. An increase in the HNT concentration increased the ILSS of the hybrid composites. The improvement on ILSS reached 25% with 5 wt% HNT. A similar enhancement was obtained by Wichmann et al. [11] using CNTs, which are much more expensive. Hussain et al. [12] proposed that the addition of nano-sized fillers to an epoxy matrix caused higher thermal residual stresses on the surface of the fibers, which increased the fiber–matrix interfacial bonding, leading to the improved ILSS. To investigate the reinforcing effects of HNTs on the ILSS of the EP/HNT/CF hybrid composites, the damaged areas of the composite samples after the SBS tests were investigated and are shown in Fig. 4a–f. As shown in Fig. 4a, cracks were found propagating along the fiber–matrix interface or through the carbon fiber plies, representing a typical interlaminar shear failure. The holes were the result of the pull-out of the fiber bundles caused by grinding and ultrasonication during the sample preparation process. Fig. 4b shows a trans-ply crack passing through a HNT-rich particle (labelled as ‘A’). During the trans-laminar crack propagation process, the cracks extended by breaking the HNT-rich particles. A detailed investiga-
721
tion of the damaged HNT-rich regions revealed that micro-cracks, which were generated in the HNT-rich regions, were stabilized by HNT bridging (cf., Fig. 4c). The formation and stabilization of micro-cracks turned the HNT-rich particles into damaged zones, which can absorb a substantial amount of energy and stop or slow down the crack propagation, making the system tougher and stronger. Numerous micro-cracks were created in the epoxy matrix as illustrated in Fig. 4d, but the growth of the micro-cracks was arrested by HNT bridging (cf., Fig. 4e) or prevented by the HNT-rich particles (cf., Fig. 4f). Each of the above mechanisms contributed to the enhancement of ILSS. 3.3. Mode-I interlaminar fracture toughness Typical load–displacement curves obtained from the DCB tests of the EP/HNT/CF hybrid composites are shown in Fig. 5a. For all the specimens, the load increased linearly with the increase in the displacement to the maximum load at which point the crack initiated, followed by a gradual decrease as the crack further propagated. The crack growth resistance curves (R curves) of GIc were compared among the EP/CF composites with different HNT contents, as shown in Fig. 5b. The initiation value of Mode-I interlaminar fracture toughness was determined using the load and
Fig. 7. Fracture surfaces of the DCB specimens: (a and b) EP/CF composite without HNT; (c)–(f) EP/HNT/CF hybrid composite with 3 wt% HNT.
722
Y. Ye et al. / Composites Science and Technology 71 (2011) 717–723
displacement measured when the load reached a maximum value. From the R curves, it can be seen that the initiation GIc values of the composites with 3–5 wt% HNTs were nearly twice that of EP/CF
Fig. 8. Mode II interlaminar fracture toughness of composites with different halloysite loadings.
composite without HNTs. As the delamination grew from the Teflon insert, a resistance-type fracture behavior developed where the propagation GIc first increased monotonically and then stabilized with further delamination growth. Both the initiation and propagation GIc values increased in general with increasing HNT concentration. These observations are consistent with a report on the EP/CF composites with nanoclay [10]. Microscopic studies of the fracture surfaces showed that the crack propagated following an undulating pattern of the yarns for all the composites, i.e., the warp and weft yarn fibers of a single ply were exposed with islands of matrix among them, as shown in Fig. 6. Such a pattern was a result of the multiple-crack fronts of the delamination at the woven-fabric laminate interface due to different yarn orientations [20]. A further examination of the fracture surface revealed that the composite made from the neat epoxy (cf., Fig. 7a and b) exhibited remarkable river stripes of brittle fracture in the matrix, including islands of matrix (cf., Fig. 7a) and matrix among CFs (cf., Fig. 7b). Debonding between the carbon fibers and the epoxy matrix due to weak interfacial bonding is also clearly observable in Fig. 7b. The hybrid composites containing HNTs presented a much rougher matrix surface than those with the neat epoxy, as a result of the crack bifurcation and pinning, as displayed in Fig. 7c–f. Fig. 7c shows that the HNT-rich particles impeded the
Fig. 9. Fracture surfaces of the ENF specimens: (a) EP/CF composite without HNT, (b)–(f) EP/HNT/CF hybrid composites. (The long arrows indicate the crack-propagation direction.)
Y. Ye et al. / Composites Science and Technology 71 (2011) 717–723
crack growth by crack deflection or bowing out. A higher resolution SEM micrograph, as shown in Fig. 7d, indicates that many HNTs were pulled out or debonded from the matrix, which may have contributed to the improvement in GIc. In addition, the HNTs dispersed in the matrix among CFs were also pulled out, as shown in Fig. 7e and f. From these results, we can conclude that the main crack was bridged by thousands of HNTs in the crack opening process and these HNTs were pulled out upon further crack propagation. 3.4. Mode II interlaminar fracture toughness Fig. 8 shows a plot of the interlaminar fracture toughness of the fiber composites as a function of the halloysite content. The composites with the EP/HNT matrix have higher fracture toughness than that of the neat epoxy-based fiber composite. Adding only 1 wt% HNT, the GIIc increased by 24%. When the HNT content was increased to 2 wt%, the GIIc increased to 37%. It is well known that the fracture toughness of fiber-reinforced polymer composites arises mainly from the energy-dissipating events, such as fiber– matrix debonding, fiber pull-out and bridging, as well as the fracturing of the matrix and fibers [21]. The improvement in GIIc for the EP/HNT/CF hybrid composites can be attributed mainly to the increased toughness of the epoxy matrix due to the presence of the HNTs. Fig. 9 shows the fracture surfaces of the EP/CF composites with and without HNTs after the ENF tests. The composite made from the neat epoxy (cf., Fig. 9a) showed sheared hackle markings in the matrix, which is characteristic of brittle fracture for the epoxy. Moreover, the cracks (indicated by the short arrows in Fig. 9a) propagated along the fiber–matrix interface, reflecting weak fiber–matrix interfacial adhesion. For the hybrid composite samples containing HNTs, the cracks propagated through the breaking of the fibers but not along the fiber–matrix interface, as shown by the short arrows in Fig. 9b and c, suggesting strong adhesion between the fibers and the matrix. Fig. 9c also shows that the HNTrich particles at the locations near the carbon fibers were damaged by nanotube pull-out and the formation of micro-cracks. The debonding between an EP/HNT composite particle and the epoxy matrix is shown in Fig. 9d. Because the HNT-rich particles were tough and strong, they worked like microfillers in the fracture process. The crack bowing phenomenon can be clearly observed in the samples with HNT-rich particles as shown in Fig. 9e. According to the crack-bowing theory, a higher toughness is achieved because the advancing crack is pinned by the particles, causing the crack front to bow out between particles, resulting in longer crack lengths. From the SEM micrograph shown in Fig. 9f, we can see that several nanotubes bridged a micro-crack of about 200 nm. Due to the nanotube bridging, the micro-cracks were arrested and prevented from developing into larger and more harmful cracks. Clearly, there are two main factors contributing to the GIIc enhancement. The increased toughness of the epoxy matrix due to the incorporation of HNTs is the most important factor. Stronger interfacial adhesion between the carbon fibers and the epoxy matrix due to an unclear mechanism brought about by the nanotubes is another contributing factor to the higher values of GIIc.
723
4. Conclusion Halloysite nanotubes have been used as nano-fillers in the carbon fiber-reinforced epoxy laminates. The morphological study of the EP/HNT/CF hybrid composites revealed that the HNTs were non-uniformly dispersed in the epoxy matrix, forming a unique microstructure with a large number of HNT-rich composite particles enveloped by a continuous epoxy-rich phase. The effects of the HNTs on the mechanical properties of the composites were investigated. A 25% enhancement in the interlaminar shear strength was obtained with 5 wt% HNTs. Both the initiation and propagation GIc values of the EP/CF composites increased with increasing HNT content; particularly, the GIc values almost doubled with a 3–5 wt% clay loading. Moreover, GIIc was improved by 37% after introducing 2 wt% HNTs into the composites. A study of the fracture mechanism demonstrated that the EP/HNT composite particles acted as typical microfillers during the fracturing of the hybrid composites and impeded the development of crack growth. The pull-out and bridging of the HNTs in the damaged HNT-rich particles contributed positively to the interlaminar properties. Meanwhile, the HNTs in the epoxy-rich regions stabilized the micro-cracks of the matrix by nanotube bridging. Acknowledgements The work was financially supported by the Nanotechnology Concentration Program of HKUST and the Hong Kong Research Grants Council (Grant Numbers 621306 and 610907). The technical support from the Center for Engineering Materials and Reliability (CEMAR), the Advanced Engineering Materials Facility (AEMF) and the Materials Characterization and Preparation Facility (MCPF) of HKUST is highly appreciated. References [1] Ye YP, Chen HB, Wu JS, Ye L. Polymer 2007;48(21):6426–33. [2] Mouritz AP, Leong KH, Herszberg I. Compos Pt A 1997;28(12):979–91. [3] Mouritz AP, Bannister MK, Falzon PJ, Leong KH. Compos Pt A 1999;30(12): 1445–61. [4] Rugg KL, Cox BN, Massabò R. Compos Pt A 2002;33(2):177–90. [5] Partridge IK, Cartié DDR. Compos Pt A 2005;36(1):55–64. [6] Zhang X, Hounslow L, Grassi M. Compos Sci Technol 2006;66(15):2785–94. [7] Gojny FH, Wichmann MHG, Fiedler B, Bauhofer W, Schulte K. Compos Pt A 2005;36(11):1525–35. [8] Veedu VP, Cao A, Li X, Ma K, Soldano C, Ajayan PM, et al. Nature Mater 2006;5(6):457–62. [9] Fan Z, Santare MH, Advani SG. Compos Pt A 2008;39(4):540–54. [10] Siddiqui NA, Woo RSC, Kim JK, Leung CCK, Munir A. Compos Pt A 2007;38(2): 449–60. [11] Wichmann MHG, Sumfleth J, Gojny FH, Quaresimin M, Fiedler B, Schulte K. Eng Fract Mech 2006;73(16):2346–59. [12] Hussain M, Nakahira A, Niihara K. Mater Lett 1996;26(3):185–91. [13] Ning N-Y, Yin Q-J, Luo F, Zhang Q, Du R, Fu Q. Polymer 2007;48(25):7374–84. [14] Du M, Guo B, Lei Y, Liu M, Jia D. Polymer 2008;49(22):4871–6. [15] Liu M, Guo B, Du M, Chen F, Jia D. Polymer 2009;50(13):3022–30. [16] Deng S, Zhang J, Ye L, Wu JS. Polymer 2008;49(23):5119–27. [17] Deng S, Ye L, Mai Y-W. Compos Sci Technol 1999;59(11):1725–34. [18] Carlsson LA, Gillespie JW, Pipes RB. J Compos Mater 1986;20(6):594–604. [19] Tanaka K, Kageyama K, Hojo M. Composites 1995;26(4):257–67. [20] Kim JK, Sham ML. Compos Sci Technol 2000;60(5):745–61. [21] Wu JS, Yu DM, Chan C-M, Kim J-K, Mai Y-W. J Appl Polym Sci 2000;76(7): 1000–10.
Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Interlaminar properties of carbon fiber composites with halloysite nanotube-toughened epoxy matrix Yueping Ye a, Haibin Chen a, Jingshen Wu a,⇑, Chi Ming Chan b,c,⇑ a
Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Hong Kong, China Department of Chemical and Biomolecular Engineering, The Hong Kong University of Science and Technology, Hong Kong, China c Division of Environment, The Hong Kong University of Science and Technology, Hong Kong, China b
a r t i c l e
i n f o
Article history: Received 22 August 2010 Received in revised form 14 January 2011 Accepted 25 January 2011 Available online 1 February 2011 Keywords: Halloysite nanotube A. Nanocomposites A. Hybrid composites B. Fracture toughness
a b s t r a c t Halloysite nanotubes (HNTs), which are geometrically similar to multi-walled carbon nanotubes, can improve the impact strength of epoxy substantially, according to our previous work [1]. Using a HNTtoughened epoxy as the matrix, a set of hybrid composites was prepared with carbon fiber-woven fabrics. The interlaminar properties of the composites were investigated by a short-beam shear test, a doublecantilever-beam test and an end-notched flexure test. The results showed that the addition of HNTs to the composites improved the interlaminar shear strength and the fracture resistance under Mode I and Mode II loadings greatly. The morphological study of the hybrid composites revealed that HNTs were non-uniformly dispersed in the epoxy matrix, forming a unique microstructure with a large number of HNT-rich composite particles enveloped by a continuous epoxy-rich phase. A study of the fracture mechanism uncovered the important role of this special morphology during the fracturing of the hybrid composites. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Carbon fiber-reinforced epoxy (EP/CF) composites have been widely used in many areas, including aerospace, automobile, marine, military, etc., due to their unique properties, such as high strength, high modulus and lightweight. However, the strength in the through-thickness direction is a limiting design factor in conventional composites because there are no fibers oriented in the thickness direction to sustain transverse loads. The low strength in the through-thickness direction generally leads to interlaminar failures, such as delamination. To improve the interlaminar fracture toughness, a considerable amount of research has been conducted. Examples of such attempts include the use of Z-pins to connect the laminates, and extending the fibers through the thickness direction by weaving, knitting, braiding or stitching [2–6]. However, these techniques are labor-intensive and require special fabrication processes, which greatly increase the manufacturing cost [5]. Moreover, the complex combination of materials makes it difficult to accurately predict the in-plane mechanical properties, i.e., the tensile, compressive and flexural properties, for a particular stitched composite. For instance, stitching may improve the in-plane proper⇑ Corresponding authors. Address: Department of Chemical and Biomolecular Engineering, The Hong Kong University of Science and Technology, Hong Kong, China. Tel.: +852 23587125; fax: +852 2358 0054 (C.M. Chan), tel.: +852 23587200; fax: +852 23581543 (J. Wu). E-mail addresses: [email protected] (J. Wu), [email protected] (C.M. Chan). 0266-3538/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2011.01.018
ties, or it may leave the properties un-changed; in some instances it may even seriously decrease the in-plane properties [2]. With improvements in nanocomposite technology, many researchers attempted to improve the interlaminar properties of fiber-reinforced composites using nanofillers [7–12]. The incorporation of alumina nanofillers into EP/CF composites resulted in both higher interlaminar shear strength (ILSS) and fracture toughness [12]. Gojny et al. [7] found that the addition of 0.3 wt% doublewalled carbon nanotubes (CNTs) to fiber-reinforced epoxy composites increased the ILSS by 20%. By growing aligned CNTs on the surface of SiC fibers, the Mode I (GIc) and Mode II (GIIc) interlaminar fracture toughness of the 3D composites were improved by 348% and 54%, respectively [8]. Siddiqui et al. [10] also increased the value of GIc by 60% by adding 3 wt% organoclay to an epoxy. All these studies have shown just how promising the applications of nanofillers are in fiber-reinforced composites. Moreover, the introduction of nanofillers does not increase the weight of the components manufactured with EP/CF composites. Halloysite is a fine clay mineral consisting of tubular particles with a multi-layered wall structure. In recent years, there have been great interests in the application of HNTs in polymer materials [13– 15]. Our previous work [1,16] has shown that HNTs, as low-cost nanotubes, can improve the mechanical properties of an epoxy significantly. The addition of 2.3 wt% HNTs to the epoxy increased its Charpy impact strength by about four times with slight improvements in the flexural modulus and strength. The underlying toughening mechanisms were identified as massive bridging, pull-
718
Y. Ye et al. / Composites Science and Technology 71 (2011) 717–723
Fig. 1. SEM micrographs of EP/HNT/CF composites with 3 wt% HNTs.
out, and breaking of nanotubes, micro-cracking, as well as maincrack deflection. Considering that the HNT crack-bridging capability and the low damage resistance of conventional EP/CF composites are largely a result of the propagation of internal defects (e.g., micro-cracks) under external loadings, we used the EP/HNT nanocomposite as the matrix in the fabrication of the CF composite in the present study. We anticipated that the EP/HNT/CF hybrid composites would benefit from the high impact toughness due to the presence of the HNTs, leading to a new class of CF composites. The microstructure of the EP/HNT/CF hybrid composites was examined using a scanning electron microscope (SEM). The mechanical properties and the failure mechanisms of the hybrid composites were studied.
Fig. 2. TEM micrograph showing the HNT-rich region.
2. Experimental work 2.1. Fabrication of composites The EP/HNT/CF hybrid composites were prepared with carbon fiber-woven fabrics and a HNT-filled epoxy by the hand lay-up process. Plain woven carbon fibers (TI3101 supplied by Taiwan Electrical Insulators Co.) with a unit weight of 200 g/m2 were used as the major reinforcement. The materials and processing conditions for the EP/HNT nanocomposite solution were the same as those reported previously [1]. The nanocomposite was prepared using halloysite nanotubes (supplied by Imerys Tableware New Zealand Limited), and EPON Resin 828 (Bisphenol A, supplied by Resolution Performance Products) with a curing agent, 4, 40 -methylene dianiline (MDA, Aldrich), at a 100/27 weight ratio. A certain amount of halloysite nanotubes was first dispersed in acetone and
Fig. 3. Interlaminar shear strength of EP/CF composites with different HNT contents.
Y. Ye et al. / Composites Science and Technology 71 (2011) 717–723
mechanically stirred for half an hour at room temperature. The mixture was then introduced into the epoxy resin and stirred for another 2 h at 75 °C. After degassing to remove the remaining acetone, MDA was added with gentle mixing. The HNT-filled epoxy was then brushed onto twelve plies of carbon fiber fabrics by the hand lay-up process. A 15-lm thick Teflon film was inserted into the middle plane of the laminates as the initial crack for the Mode I and Mode II interlaminar fracture toughness tests. The laminates were finally stacked on an aluminum mold and cured in a hot pressing machine (Technical Machine Products Corp.). They were pre-cured at 80 °C for 2 h and post-cured at 160 °C for another 4 h. A low pressure of 0.3 MPa was applied throughout the whole curing process to maintain a laminate thickness of 3.2 ± 0.2 mm. The carbon fiber volume fraction of the composites was 29 ± 1 vol%, determined by the combustion of the cured laminates according to ASTM D3171. The halloysite contents varied between 1–5 wt% based on the nanocomposite matrix. 2.2. Material characterization The morphology of EP/HNT/CF hybrid composites was studied with a SEM (JEOL JSM-6700F). The short-beam shear (SBS) test
719
(ASTM D2344) was employed to characterize the apparent interlaminar shear strength of the composites on a universal testing machine (UTM, MTS Sintech 10/D). The SBS specimen of 20.0 6.4 3.2 mm3 was placed on two cylindrical supports of 3 mm in diameter and bent by a cylindrical head of 6 mm in diameter at the centre of the specimen. The tests were conducted with a crosshead rate of 1 mm/min and a span/thickness ratio of 4. More than eight specimens for each composite system were tested. To investigate the reinforcing effects of HNTs on ILSS, the cross-sectional area of the damaged samples was examined using an optical microscope and SEM. The opening Mode-I interlaminar fracture toughness (GIc) of the composites was determined using the double-cantilever-beam (DCB) test according to ASTM D5528. Teflon film was inserted into the mid-plane of the laminates as the starter crack in the composite preparation process and the initial crack length (a0) was 35 mm. Specimens with a length of 125 mm and a width (W) of 20 mm were cut from the molded sheets, to the end of which a pair of aluminum blocks was bonded using an adhesive. To monitor the delamination length, the DCB specimen edges were polished and fine lines at intervals of 1–2 mm were marked along the length direction. The experiment was performed with a crosshead speed
Fig. 4. Typical failure modes in a damaged SBS sample with 3 wt% HNTs: (a) the entire view of the damaged zone; (b) trans-ply cracking through a HNT-rich particle; (c) the enlarged area of A in Fig. 3b showing nanotube bridging in the HNT-rich region; (d) matrix micro-cracking; (e) the enlarged area of B in Fig. 3d showing nanotube bridging in the epoxy matrix; (f) the enlarged area of C in Fig. 3d showing a damaged HNT-rich region.
720
Y. Ye et al. / Composites Science and Technology 71 (2011) 717–723
of 2 mm/min on a universal testing machine (UTM). The load, P, and the corresponding displacement, d, were recorded as a function of the crack length (a). Taking into account the rotation at the delamination front, GIc was calculated according to the modified-beam theory as follows:
3Pd GIc ¼ 2Wða þ jDjÞ
ð1Þ
where D was determined experimentally by generating a leastsquare fit of the cube root of compliance, C1/3, against the crack length. The compliance, C, is the ratio of the displacement to the applied load, d/P, for a specific crack length. The Mode II interlaminar fracture toughness (GIIc) was evaluated using the end-notched flexure (ENF) tests on a hydraulic UTM (MTS 858 Mini Bionix) at a loading rate of 1 mm/min. The specimen length and width (W) were 120 and 20 mm, respectively. The specimen was tested on a three-point bending fixture with a half-span length (L) of 50 mm. The overall Teflon film length was 35 mm and the initial crack length (a0) was designed to be around 25 mm. For the calculation of GIIc, several methods have been proposed [17], including the analytical compliance method, the directbeam theory and the corrected-beam theory [18]. Tanaka et al. found that the toughness values calculated by different methods were not much different [19]. The correct-beam theory was applied and the value of GIIc was determined by
GIIc ¼
9a20 P2m K 4mWL3 H
ð2Þ
where m is the slope of the initial straight-line portion of the load– deflection curve; Pm is the maximum load; K is a correction factor
Fig. 5. The load–displacement curves (a) and R curves (b) for the composites with different halloysite contents in DCB tests.
for the moment arm; and H is a correction factor for the compliance. K and H can be determined by
K ¼ 1 0:6099ð/=LÞ2
ð3Þ
and
H ¼ 1 þ 0:3766ð/=LÞ2
ð4Þ
where u is the displacement at maximum load. For both DCB and ENF tests, at least five measurements were made under a given set of conditions and the fracture surface of the broken specimens after each test was examined using SEM. In addition, before the mechanical tests, each sample was annealed at 180 °C (10 °C above Tg of the epoxy matrix) for 2 h to eliminate any residual stress introduced during the mechanical cutting. 3. Results and discussion 3.1. Morphology The microstructure of EP/HNT/CF hybrid composites was examined by SEM and the micrographs are shown in Fig. 1. HNTs were non-uniformly dispersed in the epoxy matrix. Similar to what was observed in the EP/HNT nanocomposites [1], some HNTs were randomly dispersed in the matrix with large inter-tube distances, while others were dispersed in the epoxy with much shorter inter-tube distances, resulting in the formation of many HNT-rich regions. Although at the first glance the HNT-rich regions appeared to be the clusters of HNTs, a closer examination of these regions with a TEM revealed that the spaces among the HNTs were actually filled by epoxy, as shown in Fig. 2. The HNT-rich regions can be regarded as the rigid composite particles with high HNT content, which play an important role in the toughening of epoxy [1].
Fig. 6. Overview of the fracture surface of the EP/CF composite after the DCB test (crack propagation direction: from top to the bottom).
Y. Ye et al. / Composites Science and Technology 71 (2011) 717–723
3.2. Interlaminar shear strength The interlaminar shear strength of the EP/HNT/CF composites with different amounts of HNTs is illustrated in Fig. 3. An increase in the HNT concentration increased the ILSS of the hybrid composites. The improvement on ILSS reached 25% with 5 wt% HNT. A similar enhancement was obtained by Wichmann et al. [11] using CNTs, which are much more expensive. Hussain et al. [12] proposed that the addition of nano-sized fillers to an epoxy matrix caused higher thermal residual stresses on the surface of the fibers, which increased the fiber–matrix interfacial bonding, leading to the improved ILSS. To investigate the reinforcing effects of HNTs on the ILSS of the EP/HNT/CF hybrid composites, the damaged areas of the composite samples after the SBS tests were investigated and are shown in Fig. 4a–f. As shown in Fig. 4a, cracks were found propagating along the fiber–matrix interface or through the carbon fiber plies, representing a typical interlaminar shear failure. The holes were the result of the pull-out of the fiber bundles caused by grinding and ultrasonication during the sample preparation process. Fig. 4b shows a trans-ply crack passing through a HNT-rich particle (labelled as ‘A’). During the trans-laminar crack propagation process, the cracks extended by breaking the HNT-rich particles. A detailed investiga-
721
tion of the damaged HNT-rich regions revealed that micro-cracks, which were generated in the HNT-rich regions, were stabilized by HNT bridging (cf., Fig. 4c). The formation and stabilization of micro-cracks turned the HNT-rich particles into damaged zones, which can absorb a substantial amount of energy and stop or slow down the crack propagation, making the system tougher and stronger. Numerous micro-cracks were created in the epoxy matrix as illustrated in Fig. 4d, but the growth of the micro-cracks was arrested by HNT bridging (cf., Fig. 4e) or prevented by the HNT-rich particles (cf., Fig. 4f). Each of the above mechanisms contributed to the enhancement of ILSS. 3.3. Mode-I interlaminar fracture toughness Typical load–displacement curves obtained from the DCB tests of the EP/HNT/CF hybrid composites are shown in Fig. 5a. For all the specimens, the load increased linearly with the increase in the displacement to the maximum load at which point the crack initiated, followed by a gradual decrease as the crack further propagated. The crack growth resistance curves (R curves) of GIc were compared among the EP/CF composites with different HNT contents, as shown in Fig. 5b. The initiation value of Mode-I interlaminar fracture toughness was determined using the load and
Fig. 7. Fracture surfaces of the DCB specimens: (a and b) EP/CF composite without HNT; (c)–(f) EP/HNT/CF hybrid composite with 3 wt% HNT.
722
Y. Ye et al. / Composites Science and Technology 71 (2011) 717–723
displacement measured when the load reached a maximum value. From the R curves, it can be seen that the initiation GIc values of the composites with 3–5 wt% HNTs were nearly twice that of EP/CF
Fig. 8. Mode II interlaminar fracture toughness of composites with different halloysite loadings.
composite without HNTs. As the delamination grew from the Teflon insert, a resistance-type fracture behavior developed where the propagation GIc first increased monotonically and then stabilized with further delamination growth. Both the initiation and propagation GIc values increased in general with increasing HNT concentration. These observations are consistent with a report on the EP/CF composites with nanoclay [10]. Microscopic studies of the fracture surfaces showed that the crack propagated following an undulating pattern of the yarns for all the composites, i.e., the warp and weft yarn fibers of a single ply were exposed with islands of matrix among them, as shown in Fig. 6. Such a pattern was a result of the multiple-crack fronts of the delamination at the woven-fabric laminate interface due to different yarn orientations [20]. A further examination of the fracture surface revealed that the composite made from the neat epoxy (cf., Fig. 7a and b) exhibited remarkable river stripes of brittle fracture in the matrix, including islands of matrix (cf., Fig. 7a) and matrix among CFs (cf., Fig. 7b). Debonding between the carbon fibers and the epoxy matrix due to weak interfacial bonding is also clearly observable in Fig. 7b. The hybrid composites containing HNTs presented a much rougher matrix surface than those with the neat epoxy, as a result of the crack bifurcation and pinning, as displayed in Fig. 7c–f. Fig. 7c shows that the HNT-rich particles impeded the
Fig. 9. Fracture surfaces of the ENF specimens: (a) EP/CF composite without HNT, (b)–(f) EP/HNT/CF hybrid composites. (The long arrows indicate the crack-propagation direction.)
Y. Ye et al. / Composites Science and Technology 71 (2011) 717–723
crack growth by crack deflection or bowing out. A higher resolution SEM micrograph, as shown in Fig. 7d, indicates that many HNTs were pulled out or debonded from the matrix, which may have contributed to the improvement in GIc. In addition, the HNTs dispersed in the matrix among CFs were also pulled out, as shown in Fig. 7e and f. From these results, we can conclude that the main crack was bridged by thousands of HNTs in the crack opening process and these HNTs were pulled out upon further crack propagation. 3.4. Mode II interlaminar fracture toughness Fig. 8 shows a plot of the interlaminar fracture toughness of the fiber composites as a function of the halloysite content. The composites with the EP/HNT matrix have higher fracture toughness than that of the neat epoxy-based fiber composite. Adding only 1 wt% HNT, the GIIc increased by 24%. When the HNT content was increased to 2 wt%, the GIIc increased to 37%. It is well known that the fracture toughness of fiber-reinforced polymer composites arises mainly from the energy-dissipating events, such as fiber– matrix debonding, fiber pull-out and bridging, as well as the fracturing of the matrix and fibers [21]. The improvement in GIIc for the EP/HNT/CF hybrid composites can be attributed mainly to the increased toughness of the epoxy matrix due to the presence of the HNTs. Fig. 9 shows the fracture surfaces of the EP/CF composites with and without HNTs after the ENF tests. The composite made from the neat epoxy (cf., Fig. 9a) showed sheared hackle markings in the matrix, which is characteristic of brittle fracture for the epoxy. Moreover, the cracks (indicated by the short arrows in Fig. 9a) propagated along the fiber–matrix interface, reflecting weak fiber–matrix interfacial adhesion. For the hybrid composite samples containing HNTs, the cracks propagated through the breaking of the fibers but not along the fiber–matrix interface, as shown by the short arrows in Fig. 9b and c, suggesting strong adhesion between the fibers and the matrix. Fig. 9c also shows that the HNTrich particles at the locations near the carbon fibers were damaged by nanotube pull-out and the formation of micro-cracks. The debonding between an EP/HNT composite particle and the epoxy matrix is shown in Fig. 9d. Because the HNT-rich particles were tough and strong, they worked like microfillers in the fracture process. The crack bowing phenomenon can be clearly observed in the samples with HNT-rich particles as shown in Fig. 9e. According to the crack-bowing theory, a higher toughness is achieved because the advancing crack is pinned by the particles, causing the crack front to bow out between particles, resulting in longer crack lengths. From the SEM micrograph shown in Fig. 9f, we can see that several nanotubes bridged a micro-crack of about 200 nm. Due to the nanotube bridging, the micro-cracks were arrested and prevented from developing into larger and more harmful cracks. Clearly, there are two main factors contributing to the GIIc enhancement. The increased toughness of the epoxy matrix due to the incorporation of HNTs is the most important factor. Stronger interfacial adhesion between the carbon fibers and the epoxy matrix due to an unclear mechanism brought about by the nanotubes is another contributing factor to the higher values of GIIc.
723
4. Conclusion Halloysite nanotubes have been used as nano-fillers in the carbon fiber-reinforced epoxy laminates. The morphological study of the EP/HNT/CF hybrid composites revealed that the HNTs were non-uniformly dispersed in the epoxy matrix, forming a unique microstructure with a large number of HNT-rich composite particles enveloped by a continuous epoxy-rich phase. The effects of the HNTs on the mechanical properties of the composites were investigated. A 25% enhancement in the interlaminar shear strength was obtained with 5 wt% HNTs. Both the initiation and propagation GIc values of the EP/CF composites increased with increasing HNT content; particularly, the GIc values almost doubled with a 3–5 wt% clay loading. Moreover, GIIc was improved by 37% after introducing 2 wt% HNTs into the composites. A study of the fracture mechanism demonstrated that the EP/HNT composite particles acted as typical microfillers during the fracturing of the hybrid composites and impeded the development of crack growth. The pull-out and bridging of the HNTs in the damaged HNT-rich particles contributed positively to the interlaminar properties. Meanwhile, the HNTs in the epoxy-rich regions stabilized the micro-cracks of the matrix by nanotube bridging. Acknowledgements The work was financially supported by the Nanotechnology Concentration Program of HKUST and the Hong Kong Research Grants Council (Grant Numbers 621306 and 610907). The technical support from the Center for Engineering Materials and Reliability (CEMAR), the Advanced Engineering Materials Facility (AEMF) and the Materials Characterization and Preparation Facility (MCPF) of HKUST is highly appreciated. References [1] Ye YP, Chen HB, Wu JS, Ye L. Polymer 2007;48(21):6426–33. [2] Mouritz AP, Leong KH, Herszberg I. Compos Pt A 1997;28(12):979–91. [3] Mouritz AP, Bannister MK, Falzon PJ, Leong KH. Compos Pt A 1999;30(12): 1445–61. [4] Rugg KL, Cox BN, Massabò R. Compos Pt A 2002;33(2):177–90. [5] Partridge IK, Cartié DDR. Compos Pt A 2005;36(1):55–64. [6] Zhang X, Hounslow L, Grassi M. Compos Sci Technol 2006;66(15):2785–94. [7] Gojny FH, Wichmann MHG, Fiedler B, Bauhofer W, Schulte K. Compos Pt A 2005;36(11):1525–35. [8] Veedu VP, Cao A, Li X, Ma K, Soldano C, Ajayan PM, et al. Nature Mater 2006;5(6):457–62. [9] Fan Z, Santare MH, Advani SG. Compos Pt A 2008;39(4):540–54. [10] Siddiqui NA, Woo RSC, Kim JK, Leung CCK, Munir A. Compos Pt A 2007;38(2): 449–60. [11] Wichmann MHG, Sumfleth J, Gojny FH, Quaresimin M, Fiedler B, Schulte K. Eng Fract Mech 2006;73(16):2346–59. [12] Hussain M, Nakahira A, Niihara K. Mater Lett 1996;26(3):185–91. [13] Ning N-Y, Yin Q-J, Luo F, Zhang Q, Du R, Fu Q. Polymer 2007;48(25):7374–84. [14] Du M, Guo B, Lei Y, Liu M, Jia D. Polymer 2008;49(22):4871–6. [15] Liu M, Guo B, Du M, Chen F, Jia D. Polymer 2009;50(13):3022–30. [16] Deng S, Zhang J, Ye L, Wu JS. Polymer 2008;49(23):5119–27. [17] Deng S, Ye L, Mai Y-W. Compos Sci Technol 1999;59(11):1725–34. [18] Carlsson LA, Gillespie JW, Pipes RB. J Compos Mater 1986;20(6):594–604. [19] Tanaka K, Kageyama K, Hojo M. Composites 1995;26(4):257–67. [20] Kim JK, Sham ML. Compos Sci Technol 2000;60(5):745–61. [21] Wu JS, Yu DM, Chan C-M, Kim J-K, Mai Y-W. J Appl Polym Sci 2000;76(7): 1000–10.