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Mechanical properties of CFRP laminates manufactured from unidirectional prepregs using CSCNT-dispersed epoxy
Composites: Part A 38 (2007) 2121–2130 www.elsevier.com/locate/compositesa
Mechanical properties of CFRP laminates manufactured from unidirectional prepregs using CSCNT-dispersed epoxy Tomohiro Yokozeki
a,*
, Yutaka Iwahori b, Shin Ishiwata c, Kiyoshi Enomoto
d
a
Department of Aeronautics and Astronautics, University of Tokyo, 7-3-1 Hongo, Bunkyo-Ku, Tokyo 113-8656, Japan Advanced Composite Technology Center, Institute of Aerospace Technology, Japan Aerospace Exploration Agency, 6-13-1 Osawa, Mitaka, Tokyo 181-0015, Japan c GSI Creos Corporation, 1-12 Minami-Watarida, Kawasaki-ku, Kawasaki, Kanagawa 210-0855, Japan R&D Institute of Metals and Composites for Future Industries, 3-25-2 Toranomon, Minato-ku, Tokyo 105-0001, Japan b
d
Received 22 February 2007; received in revised form 8 May 2007; accepted 3 July 2007
Abstract This study presents the experimental results of the mechanical properties of three-phase CFRP laminates consisting of traditional carbon fibers and epoxy matrix modified using cup-stacked carbon nanotubes (CSCNTs) in comparison to those of CFRP laminates without CSCNTs. The prepreg system of carbon fibers impregnated with CSCNT-dispersed epoxy is developed, and successful fabrication of three-phase CFRP laminates is achieved using an autoclave. Basic mechanical properties of unidirectional laminates (stiffness, strength, fracture toughness, etc.) are summarized. Next, quasi-isotropic laminates are subjected to tension, compression, flexural, and compression after impact (CAI) tests. Improvement of stiffness and strength and no adverse effects on mechanical properties due to CSCNT dispersion are experimentally verified. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: A. Polymer matrix composites; A. Laminates; B. Strength; B. Fracture toughness; Cup-stacked carbon nanotube
1. Introduction Extensive attention has been paid to nanofibers and nanoparticles as the superior reinforcements of engineering polymers. Many nano-fillers (carbon nanotubes, nanoclays, etc.) have been incorporated into the traditional polymers in order to enhance the mechanical properties as well as to add multifunctionality (e.g. thermal, electrical, and gas/liquid barrier properties) [1–7]. Carbon nanotubes (CNTs) have been reported to possess exceptional mechanical properties (e.g. extensional stiffness 1 Tpa for multiwalled CNTs, MWCNTs) [8] and considered to be the most promising class of nano-fillers. Cup-stacked carbon nanotube (CSCNT) is also considered to be a superior candidate as a polymer modifier [9]. *
Corresponding author. Tel.: +81 3 5841 7023; fax: +81 3 5841 6598. E-mail address: [email protected] (T. Yokozeki).
1359-835X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2007.07.002
Fig. 1a shows the schematic view of the CSCNT, CARBEREÒ manufactured by GSI Creos Corporation in Japan. Although this type of carbon nanotube is commonly named carbon nanofiber (CNF), the authors intend to use the term ‘‘CSCNT’’ in order to distinguish the cupstacked type nanotube from the multi-walled type carbon nanofiber in this article. CSCNT has novel structural characteristics such as a larger hollow core and a larger portion of open ends than other CNTs. Several layers of truncated conical graphene sheets are stacked and placed in relation to each other like metal bellows. The diameters of the basic type of carbon nanotubes, MWCNTs, and single-walled CNTs (SWCNTs), are reported to be typically 0.6–50 nm. On the other hand, the diameter range of the present CSCNT’s is 80–100 nm and their length could be up to 200 lm. The growth conditions of the CSCNT can be precisely controlled in a production method of chemical vapor deposition (CVD) with the use of a floating reactant
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Fig. 1. Cup-stacked carbon nanotube, CARBEREÒ: (a) schematic view of CARBEREÒ, (b) TEM image of stacking morphology of CSCNT.
method [9]. The stacking morphology of truncated conical graphene sheets exhibits an angle to the fiber axis, and almost every portion of the graphene sheet edges are exposed to the outside as indicated in Fig. 1b of a TEM image. This nano-structure of CSCNT is expected to contribute to increase in surface energy for van der Waals forces between CSCNT and polymer matrix, which suggests the advantage in the load transfer between CSCNT and polymer matrix. It should be noted that although CSCNTs have much lower strength than single-walled/ multi-walled CNTs, they are expected to exhibit excellent mechanical properties compared to the conventional carbon fibers [10]. Therefore, dispersion of CSCNTs into the polymers results in the improvement of the mechanical and electric properties of the polymers [11]. The incorporation of nano-fillers into the polymers is also expected to result in the improvement of the mechanical properties of three-phase nanocomposites consisting of traditional long fibers and nano-fillers-dispersed polymers. Although researches on two-phase nanocomposites consisting of nano-fillers and polymers have been extensively performed using many types of nano-fillers and polymers based on several dispersion techniques including surface treatment of nano-fillers, successful processing and characterization of three-phase nanocomposites have been rarely reported. Timmerman et al. [12] demonstrated that the incorporation of nanoclays resulted in the mitigation of microcracking in carbon fiber-reinforced plastics (CFRP) under cryogenic thermal cycling. Iwahori et al. [10] manufactured CSCNT-dispersed CFRP fabrics. They evaluated the mechanical properties of the CSCNT-dispersed CFRP and found an improvement in stiffness and strength (e.g. compressive strength) in two-phase and three-phase nanocomposite materials. Gojny et al. [13,14] successfully manufactured carbon black-dispersed and CNT-dispersed glass fiber-reinforced plastics (GFRP), and demonstrated the improvement in matrix-dominated properties (e.g. interlaminar shear strength) and electrical conductivity of GFRP laminates. Siddiqui et al. [15] investigated the mechanical properties of nanoclay-dispersed CFRP, and showed that the interlaminar fracture toughness of nanoclay-dispersed CFRP is higher than that of the conventional CFRP. Subramaniyan and Sun [16] reported that the compressive strength of unidirectional GFRP with nanoclays increased compared to the conventional GFRP. Thakre
et al. [17] manufactured CFRP fabrics with SWCNTs using the VARTM process, and showed a slight increase in interlaminar shear strength. Liao et al. [18] demonstrated that flexural stiffness and strength of GFRP can be improved by dispersing SWCNTs into resin. Yokozeki et al. [19] conducted tension tests of cross-ply CFRP laminates with CSCNTs, and retardation of matrix crack onset and accumulation was identified in the case of CFRP laminates with CSCNTs. Most of the researches concerning three-phase nanocomposites have been confined to only the characterization of specific properties and lack the overall experimental characterization of mechanical properties. It is necessary to evaluate the basic mechanical properties (e.g. stiffness and strength) and damage resistance characteristics of three-phase nanocomposite laminates for the enhancement of their applicability to the structural elements. The main goal of this research is to improve the matrixdominated mechanical properties of CFRP laminates by using nano-reinforcements. As the conventional CFRP laminates have enough tensile properties, exceptional strength improvement of the resin in tension is not necessary in our concept. Instead, the compressive properties and fracture resistance of CFRP laminates would be improved by using nano-fillers. Therefore, CSCNTs are selected as nano-fillers because of their merit in load transfer between CNT and polymer. In this study, basic mechanical properties and fracture properties of unidirectional three-phase nanocomposite laminates, which were fabricated using prepregs consisting of traditional carbon fibers and CSCNT-dispersed epoxy, are summarized in the following section. Next, quasi-isotropic laminates were fabricated and subjected to tension, compression, flexural, and compression after impact (CAI) tests. Comparative studies of each test results between CFRP with and without CSCNTs are provided. 2. Evaluation of mechanical properties of unidirectional laminates 2.1. Material system The CSCNTs used in this study were CARBERE (GSI Creos Corporation), synthesized by CVD using a floating reactant method [9]. For successful dispersion of CSCNTs into the polymer, CSCNTs were subjected to the dry mill
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using zirconia beads to control the lengths of CSCNTs. In this study, the nominal aspect ratio (AR) of CSCNTs was set to be 10, and a SEM image of the controlled CSCNTs is shown in Fig. 2a. The reason why this small aspect ratio was selected is to improve the quality of homogeneous dispersion of CSCNTs and avoid the voids or aggregates of CSCNTs in spite of less effectiveness for the stiffness improvement of resin. Actually, our preliminary study on the CSCNT-dispersion into epoxy matrix and the prepreg development indicated that use of CSCNT with AR = 10 contributes to the improvement in the quality of dispersion and the moldability compared to use of longer CSCNTs (e.g. AR = 100). In this study, no surface functionalization was applied to the CSCNTs. Typical properties of the used CSCNT are listed in Table 1. The resins used were bisphenol-A based epoxy, EP827 (Japan Epoxy Resin Co. Ltd), and dicyandiamide was used as the curing agent. In order to disperse CSCNTs into the epoxy resin, two-step mixing procedures were employed; EP827 epoxy and CSCNTs were combined using the planetary mixer at 70 °C, and then, CSCNTs were dispersed using the wet mill with zirconia beads at 70 °C for
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Table 1 Typical properties of used CSCNT Outer diameter (nm) Inner diameter (nm) Nominal aspect ratio Density (g/cm2) Stiffness (GPa) Tensile strength (GPa)
80–100 50–60 10 2.1 1400 7.0
45 min. The blended CSCNT-dispersed epoxy was diluted with EP827, and the curing agent was added to the compounds. CSCNT-dispersed epoxy with weight fractions of CSCNTs to the compound of 5 wt% was prepared in addition to neat epoxy resin. A TEM image of the CSCNT-dispersed epoxy is shown in Fig. 2b, which indicates the well-dispersed CSCNTs in the resin. All these processes were conducted by GSI Creos Corporation. Unidirectional prepregs were developed using T700SC12K fibers and the above-mentioned epoxy filled with CSCNTs (0 wt% and 5 wt%) by GSI Creos Corporation, which are now commercially available. The prepreg fiber areal weight was set to 125 g/m2 and the nominal resin
Fig. 2. Microscopic observations: (a) SEM of CSCNT (AR = 10), (b) TEM of CSCNT-dispersed epoxy, and (c) SEM of three-phase nanocomposite CFRP (5 wt%-CSCNT).
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content including CSCNTs was 35 wt% (the weight percentages of CSCNT in the final three-phase nanocomposites were 0 and 1.8, respectively). The nominal ply thickness was 0.12 mm. Unidirectional [0]16 and [0]36 laminates were stacked and fabricated using an autoclave. The stacked prepregs were subjected to a pressure of 490 kPa and to a curing temperature of 130 °C for the duration of two hours. The resulting volume fractions of the carbon fiber were 60% for all panels. A typical SEM image of the fabricated three-phase nanocomposite CFRP is presented in Fig. 2c, which shows the good quality of the prepared specimens. In this study, CFRP laminates using epoxy with 0 wt% and 5 wt% CSCNTs are referred to as 0 wt%-laminates and 5 wt%-laminates, respectively. 2.2. Experimental procedure In order to obtain the basic mechanical properties of three-phase nanocomposite laminates, tension tests and thermal expansion measurements were conducted using 0° and 90° specimens which were cut from the [0]16 panels. In addition, mode-I and mode-II interlaminar fracture toughness was evaluated using the [0]36 specimens. Tension tests of 0° and 90° specimens were carried out using a hydraulic-driven testing machine (8802, Instron Co. Ltd.) at a crosshead speed of 1 mm/min at room temperature (25 °C) according to ASTM D 3039 and JIS K 7073 except specimen configuration. Specimens with 200 mm length and 15 mm width were cut from the panels for 0° tension tests, while specimens with 170 mm length and 15 mm width were used for 90° tension tests. Backto-back strain gauges were attached to the specimens, and Young’s moduli were evaluated from the stress–strain data between 0.1% and 0.3% strains. Five specimens were used for each tension test. In order to measure the coefficients of thermal expansion (CTE), 0° and 90° specimens with 15 mm length and 5 mm width were cut from the panels. Their longitudinal expansions were measured by laser interferometry (LIX1, ULVAC-RIKO Inc.) at a temperature change rate of 3 °C/min. CTEs were calculated by linear regression using the expansion data as a function of temperature between 0 °C and 40 °C. Two specimens were used for each thermal expansion test. Double cantilever beam (DCB) tests and end-notched flexure (ENF) tests were performed at room temperature using [0]36 specimens with 200 mm length and 12.7 mm width according to the JIS K 7086 standard in order to evaluate mode-I and mode-II interlaminar fracture toughness of 0 wt%- and 5 wt%-laminates. In DCB tests, the mode-I fracture toughnesses were measured as a function of the crack growth, which was visually observed from specimen sides, using the experimental compliance method. In ENF tests, three-point bending with the span length of 100 mm was applied to the specimens, and the mode-II interlaminar fracture toughness was evaluated using the equation based on beam theory in reference to JIS
K 7086. Three specimens were used for DCB and ENF tests. 2.3. Results and discussion The Young’s moduli and strength obtained from tension tests are summarized in Table 2. The average values are presented and the coefficients of variance are shown in parentheses. Because the 0° specimens failed due to splitting rather than to fiber breakage, strengths of 0° specimens are not listed in Table 2. This is due to the use of relatively thick specimens (2 mm) for the tension tests of unidirectional 0° laminates. During 0° tension tests, transverse tensile stresses were induced near the grips due to constraint of Poisson deformation at the grips, which resulted in splitting failure. When thin specimens are used as the standard test method indicated, 0° tension strength could be measured. In the case of 90° tension tests, specimens exhibited tension failure in the gauge section. Some specimens failed near the grips, and other specimens were broken in the central region. Both 0 wt%- and 5 wt%-laminates exhibited similar failure modes. The above-mentioned result suggests that the incorporation of CSCNT into CFRP resulted in a slight improvement of stiffness and strength in the 90° direction. This study investigates the mechanical properties of the three-phase nanocomposites with a high volume fraction of traditional carbon fiber (60%), and thus, the contribution of CSCNT to the increase in stiffness and strength is considered to be small even in the 90° direction. Therefore, the variation of stiffness and strength of the three-phase nanocomposites due to CSCNT-dispersion is very small. The average CTEs are also presented in Table 2. The longitudinal CTEs (0° direction) are comparable between 0 wt%-laminates and 5 wt%-laminates, whereas the transverse CTEs (90° direction) of 5 wt%-laminates are lower than those of 0 wt%-laminates. It is considered that the longitudinal CTEs variation due to CSCNT-dispersion is negligible because the carbon fibers are very stiff, whereas transverse CTEs are slightly influenced by CSCNT. Kim et al. [6] found that the water diffusivity and CTE of epoxy/organoclay nanocomposites decreased with an increase in clay content, and this behavior was highly dependent on the types of the organoclays. Timmerman et al. [12] showed that the transverse CTE of three-phase Table 2 Summary of mechanical properties of unidirectional laminates
0° Stiffness (GPa) 90° Stiffness (GPa) 90° Strength (MPa) 0° CTE (10 6/°C) 90° CTE (10 6/°C) Mode-I toughness (kJ/m2) Mode-II toughness (kJ/m2)
0 wt%
5 wt%
131 (2.0) 8.61 (0.02) 51.2 (2.8) 0.1 20.7 0.086 (0.007) 0.605 (0.128)
129 (2.0) 9.11 (0.01) 57.9 (1.7) 0.1 20.3 0.170 (0.023) 0.786 (0.047)
Values in parentheses are standard deviations.
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nanocomposites was affected by the dispersion of nanoclays although the longitudinal CTE was not. The tendency of the decrease of CTEs coincides with the results of Refs. [6,12], although this study investigates three-phase nanocomposites using CSCNT. It is expected that a slight decrease in residual thermal strains can be achieved by the dispersion of CSCNT into epoxy resin [19].
0.3 0wt%-laminate
0.25
5wt%-laminate
GIc [kJ/m2]
0.2 0.15 0.1 0.05 0
0
20
40
60
80
crack growth [mm] Fig. 3. Comparison of interlaminar mode-I fracture toughnesses (Rcurves).
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The obtained mode-I interlaminar fracture toughnesses are plotted as a function of the crack growth (R-curves) as shown in Fig. 3. A slight increase in fracture toughness can be observed as crack length increases in the case of 5 wt%-laminates, whereas no increase was identified in the case of 0 wt%-laminates. A small amount of fiber bridging was visually observed during crack growth in 5 wt%laminates, which is considered to be the reason for the above-mentioned slight increase in fracture toughness. The measured fracture toughnesses between the crack growth length of 20 mm and 60 mm were averaged and summarized in Table 2. The calculated mode-II interlaminar fracture toughnesses were also presented in Table 2. It can be concluded that the both mode-I and mode-II interlaminar fracture toughnesses are clearly improved by CSCNT dispersion. It should be noted that the retardation of matrix crack onset and accumulation in cross-ply laminates with CSCNTs of the same material system was presented in Ref. [19]. The interlaminar fracture toughness improvement coincides with the trend in the case of matrix cracking. Fracture surface observation by SEM was conducted to investigate the mechanism of the enhancement of the fracture toughness in CSCNT-dispersed CFRP. Fig. 4 shows the comparison of fracture surfaces of the DCB specimens between 0 wt%-laminates and 5 wt%-laminates. The SEM images of the ENF specimens are also presented in Fig. 5. The fracture surfaces of 5 wt%-laminates are clearly
Fig. 4. SEM images of fracture surfaces of DCB specimens: (a) 0 wt%-laminate, (b) 5 wt%-laminate.
Fig. 5. SEM images of fracture surfaces of ENF specimens: (a) 0 wt%-laminate, (b) 5 wt%-laminate.
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rough compared to that of 0 wt%-laminates. This result indicates that the incorporation of CSCNT into the conventional CFRP creates fracture surface increase due to crack deflection, which may cause the enhancement of interlaminar fracture toughness. From the results of unidirectional laminates, 5 wt%-laminates have superior mechanical properties compared to 0 wt%-laminates. No adverse effects on the mechanical properties of unidirectional laminates due to CSCNT-dispersion was confirmed in this study.
Table 3 Summary of specimen configuration of quasi-isotropic laminates
3. Evaluation of mechanical properties of quasi-isotropic laminates
Instron Co. Ltd.) at a crosshead speed of 1 mm/min according to ASTM D 3039. Back-to-back strain gauges were attached to the specimens, and Young’s moduli were evaluated from the stress–strain data between 0.1% and 0.3% strains. Five specimens were used for each tension test. Compression tests of the non-hole specimens were performed using an originally developed testing device (NAL-NHC-II method) [20]. Specimens without end tabs were clamped in steel plates, see Fig. 6a, and the ends of the specimens were compressed with the support of steel cylinders as shown in Fig. 6b. The length of the gauge section of the present method was 10 mm. Compressive loadings were applied using a mechanical-driven testing machine (4482, Instron Co. Ltd.) at a crosshead speed of 1 mm/min. Back-to-back strain gauges were attached to the specimens for the measurement of the longitudinal strains.
3.1. Experimental procedure In the case of quasi-isotropic laminates, 10 wt%-laminates (CFRP laminates using epoxy with 10 wt% CSCNTs) were also fabricated in addition to 0 wt%- and 5 wt%-laminates based on the procedure described in Section 2.1. The stacking sequence was [0/90/45/-45]3S. The nominal laminate thickness was 3 mm. Specimens for tension, compression (non-hole), bending, and CAI tests were cut from the fabricated panels. Specimen configuration is summarized in Table 3. Five specimens were used for tension, compression, and bending tests, and two specimens were used for CAI test. All tests were conducted at room temperature. Tension tests of quasi-isotropic laminates were carried out using the hydraulic-driven testing machine (8802,
Length (mm) Width (mm) Specimen number Method
Tension
Compression
Bending
CAI
250 25 5
80 15 5
110 15 5
150 100 2
ASTM D 3039
NAL-II [20]
JIS K7074
SACMA SRM-2
Fig. 6. Apparatus of compression test (NAL-NHC-II method): (a) specimens clamped between steel plates, (b) specimens compressed with the support of steel cylinders.
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Three-point bending tests were conducted in reference to JIS K 7074 standard. The span length was set to be 80 mm. The radii of the loading pin and the supporting pins were 5 mm and 2 mm, respectively. The bending load was applied to the specimens under displacement control at a crosshead speed of 2 mm/min. Flexural stiffness and strength were determined using the experimental loaddeflection curve based on beam theory. CAI tests of the three quasi-isotropic laminates were performed in reference to the SACMA method [21]. The specimens were subjected to impact test, ultrasonic inspection, and compressive test in series. The impact test was performed using a weight-drop type machine (Dynatup 9250, Instron) with a hemispherical impacter (diameter: 15.9 mm). The impact energy was set to be 6.67 J/mm according to the standard [21]. Damage inspection of the post-impact specimens was performed using an ultrasonic flaw detector (Gnes Corporation) with a 5 MHz probe. Prior to compressive tests, back-to-back strain gauges were attached to the specimen surfaces far from the damaged area. Compressive loadings were applied using a mechanical-driven machine (1182, Instron) at a crosshead speed of 1 mm/min. The loading and supporting fixtures were prepared in accordance with the standard [21].
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were used in this study, tension strength of CFRP laminates is expected to be controlled by traditional fiber properties. Therefore, it is considered that three CFRP laminates exhibited similar tensile strengths. During the compression tests, back-to-back strain gauges indicated no evidence of global buckling (i.e. almost identical values were obtained from both gauges). No visible damage (e.g., matrix cracking, delamination) was apparent until laminate failure. Fractured specimens revealed failure modes comprising fiber fracture, matrix cracking, and delaminations in the gauge sections. All specimens exhibited the above-mentioned failure mode. It has been recognized that improvement in resin stiffness results in an increase in the compressive strength of unidirectional fiber composites [16] because the resin restrains fiber buckling. Therefore, it is reasonable that the compressive strength and bending strength can be improved by CSCNT-dispersion. However, the bending strength of the 10 wt%-laminate is lower than that of the 5 wt%-laminate, although the compressive strength exhibits the opposite trend. In this study, [0/90/45/-45]3S laminates were used for three point bending. The failure of
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3.2. Results and discussion
7
3.2.1. Tension, compression, and bending tests The obtained stiffnesses and strengths are summarized in Table 4, in which the average values and standard deviations are included. This table indicates that the stiffnesses slightly increase in conjunction with increase in CSCNT content. The tensile strengths are almost independent of CSCNT content, while compressive and bending strengths tend to increase by incorporation of CSCNT. It was demonstrated that compressive and bending strengths of CFRP laminates can be improved by CSCNT-dispersion into resin. In the tension tests, all specimens exhibited the accumulation of ply-level damages and delaminations (cracking sounds were clearly audible) followed by the overall failure due to fiber breakage. No difference in the failure mode was recognized among three CFRP laminates. Because quasiisotropic laminates containing 0°, 90°, and ±45° layers
6
0wt%
Table 4 Summary of experimental results of tension, compression and bending tests 0 wt%
5 wt%
10 wt%
Tension
Stiffness (GPa) Strength (MPa)
46.5 (0.6) 848 (18.2)
47.9 (0.7) 844 (5.3)
48.3 (1.7) 850 (40.0)
Compression
Stiffness (GPa) Strength (MPa)
42.9 (1.6) 488 (16.6)
43.2 (1.5) 501 (25.8)
45.1 (1.5) 539 (26.8)
Bending
Stiffness (GPa) Strength (Mpa)
53.0 (1.4) 875 (17.7)
55.1 (1.8) 912 (31.8)
55.8 (0.9) 888 (17.4)
Values in parentheses are standard deviations.
Load [kN]
5wt% 10wt%
5 4 3 2 1 0
0
1
2
3 4 Time [ms]
5
6
7
Fig. 7. Comparison of time histories of the applied load during impact test.
Table 5 Summary of specimen configuration of quasi-isotropic laminates Specimen
Impact peak load (kN)
Delamination area (mm2)
Delamination width (mm)
CAI strength (MPa)
0 wt%-01 0 wt%-02 0 wt%-average
7.47 7.31 7.39
802 821 812
30.3 31.0 30.7
178 171 175
5 wt%-01 5 wt%-02 5 wt%-average
7.05 7.23 7.14
781 795 788
29.6 29.9 29.8
182 170 176
10 wt%-01 10 wt%-02 10 wt%-average
7.24 7.30 7.27
752 847 800
25.7 31.1 28.4
195 182 188
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the top surface layer (0° layer) beneath the loading nose was the first observed damage mode during the three point bending. The influence of stress concentration near the loading nose is significant in the bending strength (even when a cushion pad is used). As discussed in ref [19], an
improvement in mechanical properties of a unidirectional 5 wt%-laminate was more effective than that of a unidirectional laminate with a higher content of CSCNT. There exist more clusters of CSCNT in CFRP laminates with more than 5 wt% CSCNT, and they may have imperfect
Fig. 8. Comparison of C-scan images of the impacted specimens: (a) 0 wt%-laminate, (b) 5 wt%-laminate, (c) 10 wt%-laminate.
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interfaces and serve as flaws. Thus, the 0° layer in 10 wt%laminate is considered to be susceptible to failure due to stress concentration. The manufacturing process used in this study might not be suitable for 10 wt%-laminates. Further investigations are necessary for the reproducibility of the results and the quality improvement of the specimens.
among the three laminates. Nevertheless, the trend of CAI strength increase (or no degradation in CAI strength) of CSCNT-dispersed CFRP laminates was demonstrated in this study.
3.2.2. CAI test Typical time histories of the applied load during transverse impact are summarized in Fig. 7. All load-time curves exhibit peak loads at about 3 ms, and the recorded peak loads are shown in Table 5. It is concluded that 0 wt%-, 5 wt%-, and 10 wt%-laminates exhibit almost identical time histories, while peak loads of 5 wt%- and 10 wt%-laminates are slightly lower than those of 0 wt%-laminates. Ultrasonic images of the impacted specimens of the three laminates are compared in Fig. 8. Almost circular delaminations can be observed in all laminates. Projected delamination areas were measured from C-scan images and summarized in Table 5. Because CAI strengths were measured using the impacted specimens by applying a compressive load in the 0° direction, delamination width is also considered to be a parameter that controls the CAI strength [22]. The effective delamination widths were calculated (see Table 5) from the B-scan images in the width direction that indicate the projected cross-sectional views of the impacted specimens in reference to Ishikawa et al. [22]. 5 wt%- and 10 wt%-laminates have slightly smaller delamination areas and widths than 0 wt%-laminates. The compressive strengths of the impacted specimens are also summarized in Table 5. An increase (8%) in CAI strength can be observed in 10 wt%-laminates compared to 0 wt%-laminates. This trend (8% increase) coincides with the case of non-hole compressive strength (i.e. about 10% increase in non-hole compressive strength of 10 wt%-laminates compared to 0 wt%-laminate). The effective delamination widths of 10 wt%-laminates were smaller than those of 0 wt%-laminates, which might result in a higher CAI strength of 10 wt%-laminates. However, 0 wt%and 5 wt%-laminates exhibited similar CAI strengths, and only two specimens were subjected to CAI test for each laminate. Further investigations on CAI strength are necessary to clearly conclude the effect of CSCNT dispersion on CAI strength of CFRP laminates. As described in Section 2.3, mode-I and mode-II interlaminar fracture toughnesses of CFRP were improved by CSCNT-dispersion. Therefore, improvement in impact resistance and CAI strength was expected in the cases of 5 wt%- and 10 wt%-laminates. However, the difference of delamination size and CAI strength among the three laminates was relatively small compared to the expectation. Quasi-isotropic laminates with 3 mm thickness were used for CAI test in this study due to availability, and 3 mm thickness was relatively thin so that impact energy was not absorbed mainly as damage accumulation but as elastic deformation of the specimens. Thus, the damage behaviors of the specimens are not considered to be quite different
Three-phase nanocomposite laminates were fabricated using the developed prepregs consisting of traditional carbon fibers and CSCNT-dispersed epoxy, and were subjected to evaluation for comparative study of several mechanical properties between CFRP with (5 wt%) and without CSCNTs. Investigation of mechanical properties of unidirectional laminates (Vf = 60%) suggested that the incorporation of CSCNT into CFRP resulted in:
4. Conclusions
Slight improvement of stiffness and strength in the 90° direction. Slight decrease of CTE in the 90° direction. Almost no influence on properties in 0° direction. Clear improvement of mode-I and mode-II interlaminar fracture toughnesses. SEM observation indicated that an increase in fracture surface area due to crack deflection in CSCNT-dispersed epoxy was the main contribution to fracture toughness improvement. Quasi-isotropic laminates (with 0, 5, and 10 wt% of CSCNT in epoxy) were also fabricated and subjected to tension, non-hole compression, bending and CAI tests. The experimental results suggested that incorporation of CSCNT into CFRP resulted in: Slight increase in stiffness in tension, compression and bending tests. No influence in tension strength. Slight improvement of compression and bending strength. Slight improvement or no degradation of CAI strength. Based on the evaluation test results, improvement of several properties and no adverse effects on mechanical properties due to CSCNT-dispersion were experimentally verified. Overall, successful improvement in the weak points of CFRP laminates could be achieved by using CSCNTs. Further investigations for more improvement in thermomechanical properties and optimization of CSCNT content and manufacturing process are now in progress. Acknowledgements This research was conducted as a part of the project, ‘‘Problem Investigation for Application Expansion of Composite for Airframe Structures’’, under contract from RIMCOF (R& D Institute of Metals and Composites for Future Industries), supported by the NEDO (New Energy
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and Industrial Technology Development Organization), Japan. The authors would like to acknowledge Motoi Fujishima and Kozue Nakane for their contribution in the evaluation tests. References [1] Schadler LS. Polymer-based and polymer-filled nanocomposites. In: Nanocomposite Science and Technology. Weinheim: Wiley-VCH; 2003. p. 77–153. [2] Kojima Y, Usuki A, Kawasaki M, Okada A, Fukushima Y, Kurauchi T, et al. Mechanical properties of nylon 6-clay hybrid. J Mater Res 1993;8:1185–9. [3] Gojny FH, Wichmann MHG, Fiedler B, Schulte K. Influence of different carbon nanotubes on the mechanical properties of epoxy matrix composites – a comparative study. Compos Sci Technol 2005;65:2300–13. [4] Delozier DM, Watson KA, Smith JG, Connell JW. Preparation and characterization of space durable polymer nanocomposite films. Compos Sci Technol 2005;65:749–55. [5] Liu W, Hoa SV, Pugh M. Fracture toughness and water uptake of high-performance epoxy/nanoclay nanocomposites. Compos Sci Technol 2005;65:2364–73. [6] Kim JK, Hu C, Woo RSC, Sham M-L. Moisture barrier characteristics of organoclay-epoxy nanocomposites. Compos Sci Technol 2005;65:805–13. [7] Ogasawara T, Ishida Y, Ishikawa T, Aoki T, Ogura T. Helium gas permeability of montmorillonite/epoxy nanocomposites. Compos A 2006;37:2236–40. [8] Thostenson ET, Reng Z, Chou T-W. Advances in the science and technology of carbon nanotubes and their composites: a review. Compos Sci Technol 2001;61:1899–912. [9] Endo M, Kim YA, Hayashi T Fukai Y, Oshida K, Terrones M, Yanagisawa T, et al. Structural characterization of cup-stacked-type nanofibers with an entirely hollow core. Appl Phys Letters 2002;80:1267–9. [10] Iwahori Y, Ishiwata S, Sumizawa T, Ishikawa T. Mechanical properties improvements in two-phase and three-phase composites using carbon nano-fiber dispersed resin. Compos A 2005;36:1430–9.
[11] Choi Y-K, Gotoh Y, Sugimoto K, Song SM, Yanagisawa T, Endo M. Processing and characterization of epoxy nanocomposites reinforced by cup-stacked carbon nanotubes. Polymer 2005;46:11489–98. [12] Timmerman JF, Hayes BS, Seferis JC. Nanoclay reinforcement effects on the cryogenic microcracking of carbon fiber/epoxy composites. Compos Sci Technol 2002;62:1249–58. [13] Gojny FH, Wichmann MHG, Fiedler B, Schulte K. Influence of nano-reinforcement on the mechanical and electrical properties of conventional fibre reinforced composites. Compos A 2005;36: 1525–35. [14] Wichmann MHG, Sumfleth J, Gojny FH, Quaresimin M, Fiedler B, Schulte K. Glass-fibre-reinforced composites with enhanced mechanical and electrical properties – benefits and limitations of a nanoparticle modified matrix. Eng Fract Mech 2006;73:2346–59. [15] Siddiqui NA, Woo RSC, Kim J-K, Leung CRY, Munir A. Mode I interlaminar fracture behavior and mechanical properties of CFRPs with nanoclay-filled epoxy matrix. Compos A 2007;38:449–60. [16] Subramaniyan AK, Sun CT. Enhancing compressive strength of unidirectional polymeric composites using nanoclay. Compos A 2006;37:2257–68. [17] Thakre PR, Lagoudas DC, Zhu J, Barrera EV, Gates TS. Processing and characterization of epoxy/SWCNT/Woven fabric composites. AIAA Paper 2006-1857. [18] Liao Y-H, Liang Z, Park Y-B, Wang B, Zhang C. Fabrication and characterization of carbon nanotube/glass fiber-reinforced multiscale composites. AIAA Paper 2006-1858. [19] Yokozeki T, Iwahori Y, Ishiwata S. Matrix cracking behaviors in carbon fiber/epoxy laminates filled with cup-stacked carbon nanotubes (CSCNTs). Compos A 2007;38:917–24. [20] Ogasawara T, Ishikawa T. Evaluation of standard test methods for compressive strengths of graphite/epoxy composite laminates Part-II. In: Proceedings of 46th JSASS/JSME structures conference, Hachinohe, Japan; 2004, p. 146–8 (in Japanese). [21] SACMA Recommended Method 2R-94. The Suppliers of Advanced Composite Materials Association, 1994. [22] Ishikawa T, Aoki Y, Suemasu H. Pursuit of mechanical behavior in compression after impact (CAI) and open hole compression (OHC). In: Proceedings of 15th international conference on composite materials (ICCM-15), Durban, South Africa; 2005, CD-ROM.
Mechanical properties of CFRP laminates manufactured from unidirectional prepregs using CSCNT-dispersed epoxy Tomohiro Yokozeki
a,*
, Yutaka Iwahori b, Shin Ishiwata c, Kiyoshi Enomoto
d
a
Department of Aeronautics and Astronautics, University of Tokyo, 7-3-1 Hongo, Bunkyo-Ku, Tokyo 113-8656, Japan Advanced Composite Technology Center, Institute of Aerospace Technology, Japan Aerospace Exploration Agency, 6-13-1 Osawa, Mitaka, Tokyo 181-0015, Japan c GSI Creos Corporation, 1-12 Minami-Watarida, Kawasaki-ku, Kawasaki, Kanagawa 210-0855, Japan R&D Institute of Metals and Composites for Future Industries, 3-25-2 Toranomon, Minato-ku, Tokyo 105-0001, Japan b
d
Received 22 February 2007; received in revised form 8 May 2007; accepted 3 July 2007
Abstract This study presents the experimental results of the mechanical properties of three-phase CFRP laminates consisting of traditional carbon fibers and epoxy matrix modified using cup-stacked carbon nanotubes (CSCNTs) in comparison to those of CFRP laminates without CSCNTs. The prepreg system of carbon fibers impregnated with CSCNT-dispersed epoxy is developed, and successful fabrication of three-phase CFRP laminates is achieved using an autoclave. Basic mechanical properties of unidirectional laminates (stiffness, strength, fracture toughness, etc.) are summarized. Next, quasi-isotropic laminates are subjected to tension, compression, flexural, and compression after impact (CAI) tests. Improvement of stiffness and strength and no adverse effects on mechanical properties due to CSCNT dispersion are experimentally verified. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: A. Polymer matrix composites; A. Laminates; B. Strength; B. Fracture toughness; Cup-stacked carbon nanotube
1. Introduction Extensive attention has been paid to nanofibers and nanoparticles as the superior reinforcements of engineering polymers. Many nano-fillers (carbon nanotubes, nanoclays, etc.) have been incorporated into the traditional polymers in order to enhance the mechanical properties as well as to add multifunctionality (e.g. thermal, electrical, and gas/liquid barrier properties) [1–7]. Carbon nanotubes (CNTs) have been reported to possess exceptional mechanical properties (e.g. extensional stiffness 1 Tpa for multiwalled CNTs, MWCNTs) [8] and considered to be the most promising class of nano-fillers. Cup-stacked carbon nanotube (CSCNT) is also considered to be a superior candidate as a polymer modifier [9]. *
Corresponding author. Tel.: +81 3 5841 7023; fax: +81 3 5841 6598. E-mail address: [email protected] (T. Yokozeki).
1359-835X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2007.07.002
Fig. 1a shows the schematic view of the CSCNT, CARBEREÒ manufactured by GSI Creos Corporation in Japan. Although this type of carbon nanotube is commonly named carbon nanofiber (CNF), the authors intend to use the term ‘‘CSCNT’’ in order to distinguish the cupstacked type nanotube from the multi-walled type carbon nanofiber in this article. CSCNT has novel structural characteristics such as a larger hollow core and a larger portion of open ends than other CNTs. Several layers of truncated conical graphene sheets are stacked and placed in relation to each other like metal bellows. The diameters of the basic type of carbon nanotubes, MWCNTs, and single-walled CNTs (SWCNTs), are reported to be typically 0.6–50 nm. On the other hand, the diameter range of the present CSCNT’s is 80–100 nm and their length could be up to 200 lm. The growth conditions of the CSCNT can be precisely controlled in a production method of chemical vapor deposition (CVD) with the use of a floating reactant
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Fig. 1. Cup-stacked carbon nanotube, CARBEREÒ: (a) schematic view of CARBEREÒ, (b) TEM image of stacking morphology of CSCNT.
method [9]. The stacking morphology of truncated conical graphene sheets exhibits an angle to the fiber axis, and almost every portion of the graphene sheet edges are exposed to the outside as indicated in Fig. 1b of a TEM image. This nano-structure of CSCNT is expected to contribute to increase in surface energy for van der Waals forces between CSCNT and polymer matrix, which suggests the advantage in the load transfer between CSCNT and polymer matrix. It should be noted that although CSCNTs have much lower strength than single-walled/ multi-walled CNTs, they are expected to exhibit excellent mechanical properties compared to the conventional carbon fibers [10]. Therefore, dispersion of CSCNTs into the polymers results in the improvement of the mechanical and electric properties of the polymers [11]. The incorporation of nano-fillers into the polymers is also expected to result in the improvement of the mechanical properties of three-phase nanocomposites consisting of traditional long fibers and nano-fillers-dispersed polymers. Although researches on two-phase nanocomposites consisting of nano-fillers and polymers have been extensively performed using many types of nano-fillers and polymers based on several dispersion techniques including surface treatment of nano-fillers, successful processing and characterization of three-phase nanocomposites have been rarely reported. Timmerman et al. [12] demonstrated that the incorporation of nanoclays resulted in the mitigation of microcracking in carbon fiber-reinforced plastics (CFRP) under cryogenic thermal cycling. Iwahori et al. [10] manufactured CSCNT-dispersed CFRP fabrics. They evaluated the mechanical properties of the CSCNT-dispersed CFRP and found an improvement in stiffness and strength (e.g. compressive strength) in two-phase and three-phase nanocomposite materials. Gojny et al. [13,14] successfully manufactured carbon black-dispersed and CNT-dispersed glass fiber-reinforced plastics (GFRP), and demonstrated the improvement in matrix-dominated properties (e.g. interlaminar shear strength) and electrical conductivity of GFRP laminates. Siddiqui et al. [15] investigated the mechanical properties of nanoclay-dispersed CFRP, and showed that the interlaminar fracture toughness of nanoclay-dispersed CFRP is higher than that of the conventional CFRP. Subramaniyan and Sun [16] reported that the compressive strength of unidirectional GFRP with nanoclays increased compared to the conventional GFRP. Thakre
et al. [17] manufactured CFRP fabrics with SWCNTs using the VARTM process, and showed a slight increase in interlaminar shear strength. Liao et al. [18] demonstrated that flexural stiffness and strength of GFRP can be improved by dispersing SWCNTs into resin. Yokozeki et al. [19] conducted tension tests of cross-ply CFRP laminates with CSCNTs, and retardation of matrix crack onset and accumulation was identified in the case of CFRP laminates with CSCNTs. Most of the researches concerning three-phase nanocomposites have been confined to only the characterization of specific properties and lack the overall experimental characterization of mechanical properties. It is necessary to evaluate the basic mechanical properties (e.g. stiffness and strength) and damage resistance characteristics of three-phase nanocomposite laminates for the enhancement of their applicability to the structural elements. The main goal of this research is to improve the matrixdominated mechanical properties of CFRP laminates by using nano-reinforcements. As the conventional CFRP laminates have enough tensile properties, exceptional strength improvement of the resin in tension is not necessary in our concept. Instead, the compressive properties and fracture resistance of CFRP laminates would be improved by using nano-fillers. Therefore, CSCNTs are selected as nano-fillers because of their merit in load transfer between CNT and polymer. In this study, basic mechanical properties and fracture properties of unidirectional three-phase nanocomposite laminates, which were fabricated using prepregs consisting of traditional carbon fibers and CSCNT-dispersed epoxy, are summarized in the following section. Next, quasi-isotropic laminates were fabricated and subjected to tension, compression, flexural, and compression after impact (CAI) tests. Comparative studies of each test results between CFRP with and without CSCNTs are provided. 2. Evaluation of mechanical properties of unidirectional laminates 2.1. Material system The CSCNTs used in this study were CARBERE (GSI Creos Corporation), synthesized by CVD using a floating reactant method [9]. For successful dispersion of CSCNTs into the polymer, CSCNTs were subjected to the dry mill
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using zirconia beads to control the lengths of CSCNTs. In this study, the nominal aspect ratio (AR) of CSCNTs was set to be 10, and a SEM image of the controlled CSCNTs is shown in Fig. 2a. The reason why this small aspect ratio was selected is to improve the quality of homogeneous dispersion of CSCNTs and avoid the voids or aggregates of CSCNTs in spite of less effectiveness for the stiffness improvement of resin. Actually, our preliminary study on the CSCNT-dispersion into epoxy matrix and the prepreg development indicated that use of CSCNT with AR = 10 contributes to the improvement in the quality of dispersion and the moldability compared to use of longer CSCNTs (e.g. AR = 100). In this study, no surface functionalization was applied to the CSCNTs. Typical properties of the used CSCNT are listed in Table 1. The resins used were bisphenol-A based epoxy, EP827 (Japan Epoxy Resin Co. Ltd), and dicyandiamide was used as the curing agent. In order to disperse CSCNTs into the epoxy resin, two-step mixing procedures were employed; EP827 epoxy and CSCNTs were combined using the planetary mixer at 70 °C, and then, CSCNTs were dispersed using the wet mill with zirconia beads at 70 °C for
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Table 1 Typical properties of used CSCNT Outer diameter (nm) Inner diameter (nm) Nominal aspect ratio Density (g/cm2) Stiffness (GPa) Tensile strength (GPa)
80–100 50–60 10 2.1 1400 7.0
45 min. The blended CSCNT-dispersed epoxy was diluted with EP827, and the curing agent was added to the compounds. CSCNT-dispersed epoxy with weight fractions of CSCNTs to the compound of 5 wt% was prepared in addition to neat epoxy resin. A TEM image of the CSCNT-dispersed epoxy is shown in Fig. 2b, which indicates the well-dispersed CSCNTs in the resin. All these processes were conducted by GSI Creos Corporation. Unidirectional prepregs were developed using T700SC12K fibers and the above-mentioned epoxy filled with CSCNTs (0 wt% and 5 wt%) by GSI Creos Corporation, which are now commercially available. The prepreg fiber areal weight was set to 125 g/m2 and the nominal resin
Fig. 2. Microscopic observations: (a) SEM of CSCNT (AR = 10), (b) TEM of CSCNT-dispersed epoxy, and (c) SEM of three-phase nanocomposite CFRP (5 wt%-CSCNT).
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content including CSCNTs was 35 wt% (the weight percentages of CSCNT in the final three-phase nanocomposites were 0 and 1.8, respectively). The nominal ply thickness was 0.12 mm. Unidirectional [0]16 and [0]36 laminates were stacked and fabricated using an autoclave. The stacked prepregs were subjected to a pressure of 490 kPa and to a curing temperature of 130 °C for the duration of two hours. The resulting volume fractions of the carbon fiber were 60% for all panels. A typical SEM image of the fabricated three-phase nanocomposite CFRP is presented in Fig. 2c, which shows the good quality of the prepared specimens. In this study, CFRP laminates using epoxy with 0 wt% and 5 wt% CSCNTs are referred to as 0 wt%-laminates and 5 wt%-laminates, respectively. 2.2. Experimental procedure In order to obtain the basic mechanical properties of three-phase nanocomposite laminates, tension tests and thermal expansion measurements were conducted using 0° and 90° specimens which were cut from the [0]16 panels. In addition, mode-I and mode-II interlaminar fracture toughness was evaluated using the [0]36 specimens. Tension tests of 0° and 90° specimens were carried out using a hydraulic-driven testing machine (8802, Instron Co. Ltd.) at a crosshead speed of 1 mm/min at room temperature (25 °C) according to ASTM D 3039 and JIS K 7073 except specimen configuration. Specimens with 200 mm length and 15 mm width were cut from the panels for 0° tension tests, while specimens with 170 mm length and 15 mm width were used for 90° tension tests. Backto-back strain gauges were attached to the specimens, and Young’s moduli were evaluated from the stress–strain data between 0.1% and 0.3% strains. Five specimens were used for each tension test. In order to measure the coefficients of thermal expansion (CTE), 0° and 90° specimens with 15 mm length and 5 mm width were cut from the panels. Their longitudinal expansions were measured by laser interferometry (LIX1, ULVAC-RIKO Inc.) at a temperature change rate of 3 °C/min. CTEs were calculated by linear regression using the expansion data as a function of temperature between 0 °C and 40 °C. Two specimens were used for each thermal expansion test. Double cantilever beam (DCB) tests and end-notched flexure (ENF) tests were performed at room temperature using [0]36 specimens with 200 mm length and 12.7 mm width according to the JIS K 7086 standard in order to evaluate mode-I and mode-II interlaminar fracture toughness of 0 wt%- and 5 wt%-laminates. In DCB tests, the mode-I fracture toughnesses were measured as a function of the crack growth, which was visually observed from specimen sides, using the experimental compliance method. In ENF tests, three-point bending with the span length of 100 mm was applied to the specimens, and the mode-II interlaminar fracture toughness was evaluated using the equation based on beam theory in reference to JIS
K 7086. Three specimens were used for DCB and ENF tests. 2.3. Results and discussion The Young’s moduli and strength obtained from tension tests are summarized in Table 2. The average values are presented and the coefficients of variance are shown in parentheses. Because the 0° specimens failed due to splitting rather than to fiber breakage, strengths of 0° specimens are not listed in Table 2. This is due to the use of relatively thick specimens (2 mm) for the tension tests of unidirectional 0° laminates. During 0° tension tests, transverse tensile stresses were induced near the grips due to constraint of Poisson deformation at the grips, which resulted in splitting failure. When thin specimens are used as the standard test method indicated, 0° tension strength could be measured. In the case of 90° tension tests, specimens exhibited tension failure in the gauge section. Some specimens failed near the grips, and other specimens were broken in the central region. Both 0 wt%- and 5 wt%-laminates exhibited similar failure modes. The above-mentioned result suggests that the incorporation of CSCNT into CFRP resulted in a slight improvement of stiffness and strength in the 90° direction. This study investigates the mechanical properties of the three-phase nanocomposites with a high volume fraction of traditional carbon fiber (60%), and thus, the contribution of CSCNT to the increase in stiffness and strength is considered to be small even in the 90° direction. Therefore, the variation of stiffness and strength of the three-phase nanocomposites due to CSCNT-dispersion is very small. The average CTEs are also presented in Table 2. The longitudinal CTEs (0° direction) are comparable between 0 wt%-laminates and 5 wt%-laminates, whereas the transverse CTEs (90° direction) of 5 wt%-laminates are lower than those of 0 wt%-laminates. It is considered that the longitudinal CTEs variation due to CSCNT-dispersion is negligible because the carbon fibers are very stiff, whereas transverse CTEs are slightly influenced by CSCNT. Kim et al. [6] found that the water diffusivity and CTE of epoxy/organoclay nanocomposites decreased with an increase in clay content, and this behavior was highly dependent on the types of the organoclays. Timmerman et al. [12] showed that the transverse CTE of three-phase Table 2 Summary of mechanical properties of unidirectional laminates
0° Stiffness (GPa) 90° Stiffness (GPa) 90° Strength (MPa) 0° CTE (10 6/°C) 90° CTE (10 6/°C) Mode-I toughness (kJ/m2) Mode-II toughness (kJ/m2)
0 wt%
5 wt%
131 (2.0) 8.61 (0.02) 51.2 (2.8) 0.1 20.7 0.086 (0.007) 0.605 (0.128)
129 (2.0) 9.11 (0.01) 57.9 (1.7) 0.1 20.3 0.170 (0.023) 0.786 (0.047)
Values in parentheses are standard deviations.
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nanocomposites was affected by the dispersion of nanoclays although the longitudinal CTE was not. The tendency of the decrease of CTEs coincides with the results of Refs. [6,12], although this study investigates three-phase nanocomposites using CSCNT. It is expected that a slight decrease in residual thermal strains can be achieved by the dispersion of CSCNT into epoxy resin [19].
0.3 0wt%-laminate
0.25
5wt%-laminate
GIc [kJ/m2]
0.2 0.15 0.1 0.05 0
0
20
40
60
80
crack growth [mm] Fig. 3. Comparison of interlaminar mode-I fracture toughnesses (Rcurves).
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The obtained mode-I interlaminar fracture toughnesses are plotted as a function of the crack growth (R-curves) as shown in Fig. 3. A slight increase in fracture toughness can be observed as crack length increases in the case of 5 wt%-laminates, whereas no increase was identified in the case of 0 wt%-laminates. A small amount of fiber bridging was visually observed during crack growth in 5 wt%laminates, which is considered to be the reason for the above-mentioned slight increase in fracture toughness. The measured fracture toughnesses between the crack growth length of 20 mm and 60 mm were averaged and summarized in Table 2. The calculated mode-II interlaminar fracture toughnesses were also presented in Table 2. It can be concluded that the both mode-I and mode-II interlaminar fracture toughnesses are clearly improved by CSCNT dispersion. It should be noted that the retardation of matrix crack onset and accumulation in cross-ply laminates with CSCNTs of the same material system was presented in Ref. [19]. The interlaminar fracture toughness improvement coincides with the trend in the case of matrix cracking. Fracture surface observation by SEM was conducted to investigate the mechanism of the enhancement of the fracture toughness in CSCNT-dispersed CFRP. Fig. 4 shows the comparison of fracture surfaces of the DCB specimens between 0 wt%-laminates and 5 wt%-laminates. The SEM images of the ENF specimens are also presented in Fig. 5. The fracture surfaces of 5 wt%-laminates are clearly
Fig. 4. SEM images of fracture surfaces of DCB specimens: (a) 0 wt%-laminate, (b) 5 wt%-laminate.
Fig. 5. SEM images of fracture surfaces of ENF specimens: (a) 0 wt%-laminate, (b) 5 wt%-laminate.
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rough compared to that of 0 wt%-laminates. This result indicates that the incorporation of CSCNT into the conventional CFRP creates fracture surface increase due to crack deflection, which may cause the enhancement of interlaminar fracture toughness. From the results of unidirectional laminates, 5 wt%-laminates have superior mechanical properties compared to 0 wt%-laminates. No adverse effects on the mechanical properties of unidirectional laminates due to CSCNT-dispersion was confirmed in this study.
Table 3 Summary of specimen configuration of quasi-isotropic laminates
3. Evaluation of mechanical properties of quasi-isotropic laminates
Instron Co. Ltd.) at a crosshead speed of 1 mm/min according to ASTM D 3039. Back-to-back strain gauges were attached to the specimens, and Young’s moduli were evaluated from the stress–strain data between 0.1% and 0.3% strains. Five specimens were used for each tension test. Compression tests of the non-hole specimens were performed using an originally developed testing device (NAL-NHC-II method) [20]. Specimens without end tabs were clamped in steel plates, see Fig. 6a, and the ends of the specimens were compressed with the support of steel cylinders as shown in Fig. 6b. The length of the gauge section of the present method was 10 mm. Compressive loadings were applied using a mechanical-driven testing machine (4482, Instron Co. Ltd.) at a crosshead speed of 1 mm/min. Back-to-back strain gauges were attached to the specimens for the measurement of the longitudinal strains.
3.1. Experimental procedure In the case of quasi-isotropic laminates, 10 wt%-laminates (CFRP laminates using epoxy with 10 wt% CSCNTs) were also fabricated in addition to 0 wt%- and 5 wt%-laminates based on the procedure described in Section 2.1. The stacking sequence was [0/90/45/-45]3S. The nominal laminate thickness was 3 mm. Specimens for tension, compression (non-hole), bending, and CAI tests were cut from the fabricated panels. Specimen configuration is summarized in Table 3. Five specimens were used for tension, compression, and bending tests, and two specimens were used for CAI test. All tests were conducted at room temperature. Tension tests of quasi-isotropic laminates were carried out using the hydraulic-driven testing machine (8802,
Length (mm) Width (mm) Specimen number Method
Tension
Compression
Bending
CAI
250 25 5
80 15 5
110 15 5
150 100 2
ASTM D 3039
NAL-II [20]
JIS K7074
SACMA SRM-2
Fig. 6. Apparatus of compression test (NAL-NHC-II method): (a) specimens clamped between steel plates, (b) specimens compressed with the support of steel cylinders.
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Three-point bending tests were conducted in reference to JIS K 7074 standard. The span length was set to be 80 mm. The radii of the loading pin and the supporting pins were 5 mm and 2 mm, respectively. The bending load was applied to the specimens under displacement control at a crosshead speed of 2 mm/min. Flexural stiffness and strength were determined using the experimental loaddeflection curve based on beam theory. CAI tests of the three quasi-isotropic laminates were performed in reference to the SACMA method [21]. The specimens were subjected to impact test, ultrasonic inspection, and compressive test in series. The impact test was performed using a weight-drop type machine (Dynatup 9250, Instron) with a hemispherical impacter (diameter: 15.9 mm). The impact energy was set to be 6.67 J/mm according to the standard [21]. Damage inspection of the post-impact specimens was performed using an ultrasonic flaw detector (Gnes Corporation) with a 5 MHz probe. Prior to compressive tests, back-to-back strain gauges were attached to the specimen surfaces far from the damaged area. Compressive loadings were applied using a mechanical-driven machine (1182, Instron) at a crosshead speed of 1 mm/min. The loading and supporting fixtures were prepared in accordance with the standard [21].
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were used in this study, tension strength of CFRP laminates is expected to be controlled by traditional fiber properties. Therefore, it is considered that three CFRP laminates exhibited similar tensile strengths. During the compression tests, back-to-back strain gauges indicated no evidence of global buckling (i.e. almost identical values were obtained from both gauges). No visible damage (e.g., matrix cracking, delamination) was apparent until laminate failure. Fractured specimens revealed failure modes comprising fiber fracture, matrix cracking, and delaminations in the gauge sections. All specimens exhibited the above-mentioned failure mode. It has been recognized that improvement in resin stiffness results in an increase in the compressive strength of unidirectional fiber composites [16] because the resin restrains fiber buckling. Therefore, it is reasonable that the compressive strength and bending strength can be improved by CSCNT-dispersion. However, the bending strength of the 10 wt%-laminate is lower than that of the 5 wt%-laminate, although the compressive strength exhibits the opposite trend. In this study, [0/90/45/-45]3S laminates were used for three point bending. The failure of
8
3.2. Results and discussion
7
3.2.1. Tension, compression, and bending tests The obtained stiffnesses and strengths are summarized in Table 4, in which the average values and standard deviations are included. This table indicates that the stiffnesses slightly increase in conjunction with increase in CSCNT content. The tensile strengths are almost independent of CSCNT content, while compressive and bending strengths tend to increase by incorporation of CSCNT. It was demonstrated that compressive and bending strengths of CFRP laminates can be improved by CSCNT-dispersion into resin. In the tension tests, all specimens exhibited the accumulation of ply-level damages and delaminations (cracking sounds were clearly audible) followed by the overall failure due to fiber breakage. No difference in the failure mode was recognized among three CFRP laminates. Because quasiisotropic laminates containing 0°, 90°, and ±45° layers
6
0wt%
Table 4 Summary of experimental results of tension, compression and bending tests 0 wt%
5 wt%
10 wt%
Tension
Stiffness (GPa) Strength (MPa)
46.5 (0.6) 848 (18.2)
47.9 (0.7) 844 (5.3)
48.3 (1.7) 850 (40.0)
Compression
Stiffness (GPa) Strength (MPa)
42.9 (1.6) 488 (16.6)
43.2 (1.5) 501 (25.8)
45.1 (1.5) 539 (26.8)
Bending
Stiffness (GPa) Strength (Mpa)
53.0 (1.4) 875 (17.7)
55.1 (1.8) 912 (31.8)
55.8 (0.9) 888 (17.4)
Values in parentheses are standard deviations.
Load [kN]
5wt% 10wt%
5 4 3 2 1 0
0
1
2
3 4 Time [ms]
5
6
7
Fig. 7. Comparison of time histories of the applied load during impact test.
Table 5 Summary of specimen configuration of quasi-isotropic laminates Specimen
Impact peak load (kN)
Delamination area (mm2)
Delamination width (mm)
CAI strength (MPa)
0 wt%-01 0 wt%-02 0 wt%-average
7.47 7.31 7.39
802 821 812
30.3 31.0 30.7
178 171 175
5 wt%-01 5 wt%-02 5 wt%-average
7.05 7.23 7.14
781 795 788
29.6 29.9 29.8
182 170 176
10 wt%-01 10 wt%-02 10 wt%-average
7.24 7.30 7.27
752 847 800
25.7 31.1 28.4
195 182 188
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the top surface layer (0° layer) beneath the loading nose was the first observed damage mode during the three point bending. The influence of stress concentration near the loading nose is significant in the bending strength (even when a cushion pad is used). As discussed in ref [19], an
improvement in mechanical properties of a unidirectional 5 wt%-laminate was more effective than that of a unidirectional laminate with a higher content of CSCNT. There exist more clusters of CSCNT in CFRP laminates with more than 5 wt% CSCNT, and they may have imperfect
Fig. 8. Comparison of C-scan images of the impacted specimens: (a) 0 wt%-laminate, (b) 5 wt%-laminate, (c) 10 wt%-laminate.
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interfaces and serve as flaws. Thus, the 0° layer in 10 wt%laminate is considered to be susceptible to failure due to stress concentration. The manufacturing process used in this study might not be suitable for 10 wt%-laminates. Further investigations are necessary for the reproducibility of the results and the quality improvement of the specimens.
among the three laminates. Nevertheless, the trend of CAI strength increase (or no degradation in CAI strength) of CSCNT-dispersed CFRP laminates was demonstrated in this study.
3.2.2. CAI test Typical time histories of the applied load during transverse impact are summarized in Fig. 7. All load-time curves exhibit peak loads at about 3 ms, and the recorded peak loads are shown in Table 5. It is concluded that 0 wt%-, 5 wt%-, and 10 wt%-laminates exhibit almost identical time histories, while peak loads of 5 wt%- and 10 wt%-laminates are slightly lower than those of 0 wt%-laminates. Ultrasonic images of the impacted specimens of the three laminates are compared in Fig. 8. Almost circular delaminations can be observed in all laminates. Projected delamination areas were measured from C-scan images and summarized in Table 5. Because CAI strengths were measured using the impacted specimens by applying a compressive load in the 0° direction, delamination width is also considered to be a parameter that controls the CAI strength [22]. The effective delamination widths were calculated (see Table 5) from the B-scan images in the width direction that indicate the projected cross-sectional views of the impacted specimens in reference to Ishikawa et al. [22]. 5 wt%- and 10 wt%-laminates have slightly smaller delamination areas and widths than 0 wt%-laminates. The compressive strengths of the impacted specimens are also summarized in Table 5. An increase (8%) in CAI strength can be observed in 10 wt%-laminates compared to 0 wt%-laminates. This trend (8% increase) coincides with the case of non-hole compressive strength (i.e. about 10% increase in non-hole compressive strength of 10 wt%-laminates compared to 0 wt%-laminate). The effective delamination widths of 10 wt%-laminates were smaller than those of 0 wt%-laminates, which might result in a higher CAI strength of 10 wt%-laminates. However, 0 wt%and 5 wt%-laminates exhibited similar CAI strengths, and only two specimens were subjected to CAI test for each laminate. Further investigations on CAI strength are necessary to clearly conclude the effect of CSCNT dispersion on CAI strength of CFRP laminates. As described in Section 2.3, mode-I and mode-II interlaminar fracture toughnesses of CFRP were improved by CSCNT-dispersion. Therefore, improvement in impact resistance and CAI strength was expected in the cases of 5 wt%- and 10 wt%-laminates. However, the difference of delamination size and CAI strength among the three laminates was relatively small compared to the expectation. Quasi-isotropic laminates with 3 mm thickness were used for CAI test in this study due to availability, and 3 mm thickness was relatively thin so that impact energy was not absorbed mainly as damage accumulation but as elastic deformation of the specimens. Thus, the damage behaviors of the specimens are not considered to be quite different
Three-phase nanocomposite laminates were fabricated using the developed prepregs consisting of traditional carbon fibers and CSCNT-dispersed epoxy, and were subjected to evaluation for comparative study of several mechanical properties between CFRP with (5 wt%) and without CSCNTs. Investigation of mechanical properties of unidirectional laminates (Vf = 60%) suggested that the incorporation of CSCNT into CFRP resulted in:
4. Conclusions
Slight improvement of stiffness and strength in the 90° direction. Slight decrease of CTE in the 90° direction. Almost no influence on properties in 0° direction. Clear improvement of mode-I and mode-II interlaminar fracture toughnesses. SEM observation indicated that an increase in fracture surface area due to crack deflection in CSCNT-dispersed epoxy was the main contribution to fracture toughness improvement. Quasi-isotropic laminates (with 0, 5, and 10 wt% of CSCNT in epoxy) were also fabricated and subjected to tension, non-hole compression, bending and CAI tests. The experimental results suggested that incorporation of CSCNT into CFRP resulted in: Slight increase in stiffness in tension, compression and bending tests. No influence in tension strength. Slight improvement of compression and bending strength. Slight improvement or no degradation of CAI strength. Based on the evaluation test results, improvement of several properties and no adverse effects on mechanical properties due to CSCNT-dispersion were experimentally verified. Overall, successful improvement in the weak points of CFRP laminates could be achieved by using CSCNTs. Further investigations for more improvement in thermomechanical properties and optimization of CSCNT content and manufacturing process are now in progress. Acknowledgements This research was conducted as a part of the project, ‘‘Problem Investigation for Application Expansion of Composite for Airframe Structures’’, under contract from RIMCOF (R& D Institute of Metals and Composites for Future Industries), supported by the NEDO (New Energy
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