- Email: [email protected]
The Ni-plated carbon fiber as a tracer for observation of the fiber orientation in the carbon fiber reinforced plastic with X-ray CT
Composites Part B 76 (2015) 38e43
Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
The Ni-plated carbon fiber as a tracer for observation of the fiber orientation in the carbon fiber reinforced plastic with X-ray CT Azusa Nagura a, *, Kazuaki Okamoto a, Kiyoharu Itoh a, Yusuke Imai b, Daisuke Shimamoto b, Yuji Hotta b a
Nagoya Municipal Industrial Research Institute, 3-4-41, Rokuban, Atsuta-ku, Nagoya 456-0058, Japan National Institute of Advanced Industrial Science and Technology (AIST), Advanced Manufacturing Research Institute, 2266-98 Anagahora, Shimoshidami, Moriyama-ku, Nagoya 463-8560, Japan
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 December 2014 Received in revised form 4 February 2015 Accepted 6 February 2015 Available online 18 February 2015
Microfocus X-ray computed tomography (XCT) is effective to evaluate the 3-dimensional (3D) structure (position and orientation) of the fillers in the composites. However, the observation of the carbon fiber (CF) in resin by XCT is not easy due to the small difference between X-ray attenuation coefficient of CF and that of resin. We conducted XCT scanning of carbon fiber reinforced plastics (CFRPs) containing Niplated CF. The fiber orientation can be clearly observed in CT images due to a large X-ray attenuation coefficient of Ni-plating. Ni-plated CF essentially showed the same fiber orientation as that of normal CF. We also carried out the 3D characterization of the fiber orientation in an injection-molded dumbbell specimen with weld line. The fibers around the weld line are oriented perpendicular to the longitudinal direction of the specimen. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Micro X-ray computed tomography (XCT) A. Carbon fiber D. Non-destructive testing E. Injection molding
1. Introduction In recent years, carbon fiber reinforced plastic (CFRP) have been demanded in various fields like aeronautic, automotive, energy, and medical industries [1]. Especially CFRP utilizing thermoplastic as matrix, carbon fiber reinforced thermoplastic (CFRTP), is attracting increasing attention due to its high recyclability [2]. Moreover, CFRTP is promising from the economical view point since it needs relatively short cycle time [3]. In this sense, injection molding of CFRTP is desirable, because the cycle time of injection molding is normally up to several minutes. However, injection molding of CFRTP still has difficulties to overcome. Since carbon fiber (CF) has high aspect ratio, the properties of CFRTP are anisotropic [4]. For example, when CF oriented in a same direction, the strength of the composite is high at direction parallel to fiber axis but low at direction perpendicular to fiber axis. Therefore a facile method to observe the fiber in the composite is crucial.
* Corresponding author. Tel.: þ81 52 654 9950. E-mail addresses: [email protected] (A. Nagura), okamoto. [email protected] (K. Okamoto), [email protected] (K. Itoh), [email protected] (Y. Imai), [email protected] (D. Shimamoto), [email protected] (Y. Hotta). http://dx.doi.org/10.1016/j.compositesb.2015.02.009 1359-8368/© 2015 Elsevier Ltd. All rights reserved.
Microfocus X-ray computed tomography (XCT) is effective to evaluate the 3-dimensional (3D) structure (orientation and position) of the fillers in the composites because one can gain image data in a relatively short time without destructing the sample. Therefore XCT is frequently utilized to observe the glass fiber in composites [5e7]. However, the observation of the CF by XCT is not easy. The contrast of the XCT image is proportional to differences in X-ray attenuation coefficients between the fiber and the resin [8,9]. Because the attenuation coefficient of the material is mainly determined by its elemental composition and density [10], the Xray attenuation coefficient of the CF is close to that of the resin. For this reason, it is hard to gain the contrast between CF and resin in the XCT image. Recently, several groups have succeeded in observing the CF in the CFRP by XCT. Djukic and co-workers evaluated 3D structure of the CF fabric in thermosetting resin. They coated the CF tows with gold, copper, and iodine [8,11,12]. Since these materials have much higher attenuation coefficients than resin [10], the contrast between carbon tow and matrix was enhanced. However, they have not observed the 3D structure of dispersed thin filaments, but bunch of CF which is composed of thousands of filaments. Nishikawa and co-workers directly observed the dispersed carbon filament in CFRTP by developing the high contrast XCT [13,14]. Their XCT utilizes low-energy X-ray effectively therefore obtains the
A. Nagura et al. / Composites Part B 76 (2015) 38e43
contrast of the materials which are composed of light elements. Kastner and co-workers also directly observed the dispersed filament as well as carbon filament in the CF tow. They utilized nanofocus X-ray computed tomography which can gain higherresolution image than microfocus XCT [15,16]. Moreover, Cosmi and co-workers observed short carbon fiber in injection molded sample with synchrotron radiation computed tomography (SRCT) [17]. However, in order to clearly observe the CF in the matrix with these systems, the scanning range is up to as small as several cubic millimeters so far. In this work, we investigated the possibility of Ni-plated CF as a tracer to observe the fiber orientation in injection-molded CFRTP specimens. Ni-plating was utilized because it is commonly used due to its high wear resistance and the X-ray absorption coefficient is enough larger than that of carbon [10]. We first prepared the samples containing different content of Ni-plated CF and observed the fibers by XCT, and then, quantitatively compared the fiber orientation of Ni-plated CF with that of CF. Finally we discuss the fiber orientation around the weld line. 2. Experimental 2.1. Materials
39
molded using hand-operated injection molding machine (Custom Scientific Instruments, Inc., mini-maxmolder CS-183mmx) with a kneading temperature of 230 C. Fig. 1 shows the photograph of the samples. 2.3.2. Preparation of samples 6e7 We prepared the samples containing the Ni-plated CF (sample 6) and normal CF (sample 7) to compare the fiber orientation. The composition ratio of sample 6 and 7 is also summarized in Table 1. These samples were molded by electric injection molding machine (Sumitomo heavy industries, SE18S) in order to strictly unify the molding condition. The fiber and polypropylene were kneaded at 220 C by a laboratory rotating disk extruder (Custom Scientific Instruments, Inc., MAX mixing extruder CS-194). The resulting compound was molded with injection molding machine with following conditions: barrel temperature ¼ 220 C, mold temperature ¼ 40 C, injection speed ¼ 50 mm/s, screw speed ¼ 70 rpm, back pressure ¼ 4.0 MPa, holding pressure ¼ 50 MPa 8 s Fig. 2 shows the photograph of sample 6, which is the dumbbell-shaped specimen with weld line at the center. 2.4. Observation by X-ray CT
PAN-based carbon fiber (T700SC-12000, Toray Industries, Inc.) was washed with deionized water and dried in the air before use. Alkaline type electroless Ni plating solution (electroless nickel A and B, Okuno Chemical Industries, Co. Ltd.,), plating catalyzer (Catalyst C-7, Okuno Chemical Industries, Co. Ltd.,), hydrochloric acid (Wako Pure Chemical Industries, Ltd.,), polystyrene (PSJpolystyrene HF77, PS Japan Corporation), polypropylene (NOVATEC PP MH4, Japan Polypropylene Corporation) were used as received. 2.2. Preparation of the Ni-plated CF The Ni-plated CF was prepared by electroless plating as following procedures. First, the CF was soaked in 5% aqueous solution of plating catalyst, and then dipped in 10% hydrochloric acid. Then, the CF was immersed in a Ni-plating bath (20% aqueous solution of electroless Ni A and B) at 35 C for 60 min. After each step, the fiber was rinsed by deionized water. Finally, the resulting Niplated CF was dried in the air for more than 24 h. After dried, the Ni-plated CF was observed with scanning electron microscopy (SEM) (Hitachi field emission SEM Se4300S, Hitachi HighTechnologies Corporation). 2.3. Preparation of the samples 2.3.1. Preparation of samples 1e5 We prepared five samples containing different content of the Niplated CF (0, 0.3, 1, 3, and 10 wt%). Normal CF was also contained so that each sample contained the same content of fibers. The composition ratio is summarized in Table 1. These samples were
Two XCT scanning systems were utilized for observation of the composites. One is a general-purpose microfocus XCT system (TOSCANER-32252mhd, Toshiba IT & Control Systems Corporation). This system was used for the samples 1-5. All the scannings were performed at a tube voltage of 50 kV and a tube current of 200 mA. The samples were rotated with the total angle of 360 and the angular increment of 0.036 with the cumulative number of 8. The resolution of CT image was 13.2 mm. The scanning range is indicated in Fig. 1. We also scanned with various magnifications for sample 5 by changing the ratio of X-ray focusesample distance to X-ray focusedetector distance. The resolutions of the scanning were changed from 13.2 to 21.9 and 36.5 mm. The corresponding scanning ranges were 13.5 13.5 7.5, 22.4 22.4 15.8, and 37.4 37.4 21.0 mm, respectively. The other system used for sample 6 and 7 is similar to that of Nishikawa's high contrast XCT (FLEX-M345, BEAMSENSE Co., Ltd.), which can directly gain the contrast between normal CF and resin. The tube voltage and tube current were 70 kV and 130 mA for sample 6 and 40 kV and 100 mA for sample 7. Each sample was rotated with the total angle of 180 and the angular increment of 0.25 . The scanning position is indicated in Fig. 3. The image processing was conducted with 3Dview (RMR Systems Ltd.). The fiber orientation analysis was performed with image analysis software, ExFact Analysis for Fiber (Nihon Visual Science, Inc.). Since the size of whole data was too large to be analyzed, four parts of the data were cut out and used for the analysis. The positions of these regions are shown in Fig. 3. The
Table 1 Composition ratios of the samples. Sample
Ni-plated CF(wt%)
CF(wt%)
Resin(wt%)
1 2 3 4 5 6 7
e 0.3 1 3 10 1 e
10 9.7 9 7 e 4 5
90 90 90 90 90 95 95
Fig. 1. The photograph of the samples 1-5 and their dimension. The squares indicate the scanning range.
40
A. Nagura et al. / Composites Part B 76 (2015) 38e43
3. Results and discussion 3.1. Ni plating of CF Fig. 4 shows SEM images of the Ni-plated CF. It was confirmed that almost whole surface of the fiber was covered by the Ni coating as shown in Fig. 4(a). Magnified view in Fig. 4(b) at the location where Ni coating accidentally flaked off gives us the estimate of Ni coating thickness to be 100e150 nm. 3.2. Content of the Ni-plated CF for XCT observation The XCT images of samples 1e5 are shown in Fig. 5. Fibrous objects were clearly observed in the samples 2e5, while no fiber could be identified in the sample 1 containing only normal CF. This result indicates that the Ni-plating successfully enhances the contrast between the fiber and the matrix. Uniform distribution of the Ni-plated CF in Fig. 5(b)e(e) suggests that the Ni plating on CF has little effect on the dispersion behavior of CF in the matrix. Individual Ni-plated CF was easily distinguished in the XCT images, while the identification of individual CF becomes more difficult due to overlapping at higher contents. Therefore, it is considered that the preferable content of Ni-plated CF is up to 10 wt%. 3.3. Maximum resolution
Fig. 2. The photograph of the sample 6 and its dimension.
regions (a) and (b) are nearby weld line and regions (c) and (d) are 2.75 mm away from the weld line. (a) and (c) are at the surface of the sample and (b) and (d) are about 1.4 mm deeper from the surface. It is known that the fiber around the weld line shows characteristic orientation [18]. In addition, the fiber orientation around surface and that inside are generally different [19]. Therefore we chose the four regions above in order to compare the fiber behavior at different conditions. The dimension of each region is 364 364 364 mm. Finally, we observed the fiber orientation around the weld line in sample 6 with a general-purpose microfocus XCT system. The tube voltage, tube current, total angle, angular increment, cumulative number, and resolution were 50 kV, 210 mA, 360 , 0.036 , 8, and 13.7 mm, respectively. The image processing was performed with image analysis software VG Studio MAX 2.1 (Volume Graphics GmbH).
Fig. 3. The scanning range of the samples 6 and 7. Dark gray cubics (a)e(d) indicate the positions of fiber orientation analysis.
Fig. 6 shows the XCT images of sample 5 with various scanning resolutions. Since the pixel count of the image sensor was fixed at 1024 1024 pixels, the higher spatial resolution result in the narrower scanning range (ex: when resolution ¼ 13.2 mm, scanning range is 13.2 mm 1024 pixels ¼ 13.5 mm). It is generally hard to
Fig. 4. SEM images of the Ni-plated carbon fiber at (a) 500 and (b) 100,000 magnifications. The square in (a) indicates the observation range of (b).
A. Nagura et al. / Composites Part B 76 (2015) 38e43
41
Fig. 5. XCT images of the samples. (a) sample 1, (b) sample 2, (c) sample 3, (d) sample 4, and (e) sample 5. A tube voltage and a tube current are 50 kV and 200 mA.
recognize the exact shape of the thin object like CF by XCT when the resolution of the image is lower than the diameter of the object (~7e10 mm, in this case). However, the residual length of the CF in the composites is expected to be more than several hundreds of micrometers. Therefore it would be feasible to identify the individual Ni-plated CF as an array of voxels if there is enough contrast between the voxel containing Ni-plated CF and that of matrix resin. It is experimentally demonstrated in Fig. 6(a) and (b) where the corresponding spatial resolutions are 13.2 and 21.9 mm, respectively. It was possible to recognize the Ni-plated CF in these images, which allows us to evaluate the fiber orientation in the composites. When the resolution was much lower value of 36.5 mm as shown in Fig. 6(c), it was difficult to identify the fibers due to the lower contrast. 3.4. Comparison of the fiber orientation In order to show the ability of Ni-plated CF as a tracer for CF orientation in CFRTP, we compared fiber orientation in detail by observing the samples 6 and 7 using the high contrast XCT. Both the samples contain the same contents of fiber (5 wt%). A part of fiber in the sample 6 is Ni-plated CF, while all the fibers in the sample 7 is the normal CF. Fig. 7 shows the observation results of the samples 6 and 7. In Fig. 7(a), the observation condition was optimized to visualize the Ni-plated CF in the sample 6. Under this condition, the
Fig. 6. XCT images of the sample 5 with various resolution: (a) 13.2 mm, (b) 21.9 mm, (c) 36.5 mm.
contrast between the normal CF and the matrix resin was too low to be identified separately. When the condition was optimized for normal CF in the sample 7, we could visualize the CF as shown in Fig. 7(b) and (c). However, this condition was not applicable to the sample 6 because of too large attenuation by Ni-plated CF. Therefore it was impossible to visualize Ni-plated CF and normal CF at the same time. In Fig. 7, both Ni-plated CF and normal CF are essentially oriented to the longitudinal direction, but the fibers nearby weld line are oriented to perpendicularly. The fiber orientation of both Niplated CF and normal CF is qualitatively the same. It is noteworthy that similar fiber orientation is also observed in glass-fiber reinforced plastic (GFRP) [20]. This result indicates that the major factor determining the fiber orientation is the effect from the flowing resin rather than the character of the fiber itself. We next conducted quantitative fiber orientation analyses at four regions. The results of fiber angle distribution analysis are summarized in Fig. 8. The polar angle q is the angle between the fiber axis and width direction of the sample and the azimuthal angle f is the angle between the fiber axis and plane of the sample. The correlation coefficients of the orientation angles were found to
Fig. 7. XCT observation of the sample 6 and 7 (The scanning ranges are shown in Fig. 3). (a) 3-dimensional image of sample 6 (containing Ni-plated CF), (b) 3dimensional image of sample 7 (containing normal CF), and (c) XCT image of sample 7. The tube voltage and tube current were 70 kV and 130 mA for sample 6 and 40 kV and 100 mA for sample 7.
42
A. Nagura et al. / Composites Part B 76 (2015) 38e43
Fig. 8. Comparison between the fiber orientation of tracer (gray) and that of carbon fiber (black).
be 0.99-0.59, indicating a strong correlation between the orientation of the normal CF and Ni-plated CF. This result shows that Niplated CF can be used as the tracer for the CF orientation. 3.5. Visualization of weld region by Ni-plated CF tracer Fig. 8 also shows the detailed fiber orientation in each region. In the regions (a) and (b), the probability of both tracer and CF has maximum around f ¼ 90 , indicating that the fibers are oriented perpendicular to the longitudinal axis. In addition, the probability increases as q angle increases. This means that the fibers tend to be oriented perpendicular to the plane of the sample. In the region (c), fibers show similar behavior in q angle as regions (a) and (b), but probability does not correlate with f angles. The fiber orientation is parallel to the side face of the sample but at random in the plane. In
the region (d), the probability has maximum at q ¼ 60 e75 and f angle has maximum at f ¼ 0 e30 . These values indicate that the fiber orientation is mainly parallel to the longitudinal axis but to some extent oriented to the out-of-plane of the sample. As we confirmed that Ni-plated CF can be utilized as the tracer of CF orientation, we also conducted CT scanning of the sample 6 by general-purpose microfocus XCT system. The observation range was 14.0 14.0 5.0 mm. The results are shown in Fig. 9. Fig. 9(a) contains the cross sections perpendicular to the longitudinal direction and Fig. 9(b) contains the cross sections parallel to the longitudinal direction and perpendicular to the plane of the sample. In Fig. 9(a), linear-shaped fiber could be seen in the cross sections around weld line whereas spot-shaped fiber was observed in other cross sections. This result suggests that the fibers around weld line are oriented to perpendicular to the longitudinal
Fig. 9. 3-dimensional image of the fiber around the weld line of the sample 6. (a) cross sections perpendicular to the flow direction (b) cross sections parallel to the flow direction.
A. Nagura et al. / Composites Part B 76 (2015) 38e43
direction while other fibers are oriented parallel to the longitudinal direction. In Fig. 9(b), the fibers form half ellipse shapes around weld line. This fiber orientation is apparently due to the effect of fountain flow. It is noteworthy that we could gain the scanning data above with one scanning. The tracer realized such wide-ranged observation. 4. Conclusion In the present paper, we investigated the behavior of the Niplated CF in injection-molded CFRTP by XCT. The Ni-plated CF was clearly observed in XCT images due to its large X-ray attenuation coefficient although it was dispersed in matrix resin as not a tow but filaments. It generally showed the same fiber orientation as that of CF, indicating that it was useful as the tracer. Wide range observation of cubic centimeters was also successful. This result implicate that we can save the measurement time when observing a large product. 3D evaluation of fiber orientation in various resin flow is under study. References [1] Park SJ. Carbon fibers. Dordrecht: Springer; 2014. [2] Suzuki T, Takahashi J. Prediction of energy intensity of carbon fiber reinforced plastics for mass-produced passenger cars. In: Proceedings of the ninth Japan international SAMPE symposium. Tokyo, Japan, Nov-Dec; 2005. p. 14e9. [3] Suzuki K, Nagata K, Uzawa K, Matsuo T, Takahashi J. Research on the jointing method of CFRTP for structural applications. In: Proceedings of ICCM18conference. Jeju Island, South Korea, Aug; 2011. TH32e1. [4] Gowayed Y. Types of fiber and fiber arrangement in fiber-reinforced polymer (FRP) composites. In: Uddin N, editor. Developments in fiber-reinforced polymer (FRP) composites for civil engineering. Cambridge: Woodhead Publishing; 2013. p. 3e17. [5] Schell JSU, Renggli M, van Lenthe GH, Müller R, Ermanni P. Micro-computed tomography determination of glass fibre reinforced polymer meso-structure. Compos Sci Technol 2006;66(13):2016e22. € pplmayr T, Milosavljevic I, Aigner M, Hasslacher R, Plank B, Salaberger D, [6] Ko et al. Influence of fiber orientation and length distribution on the rheological characterization of glass-fiber-filled polypropylene. Polym Test 2013;32(3): 535e44.
43
[7] Arif MF, Meraghni F, Chemisky Y, Despringre N, Robert G. In situ damage mechanisms investigation of PA66/G30 composite: effect of relative humidity. Compos Part B-Eng 2014;58:487e95. [8] Djukic LP, Herszberg I, Walsh WR, Schoeppner GA, Prusty BG, Kelly DW. Contrast enhancement in visualization of woven composite tow architecture using a MicroCT scanner. Part 1: fabric coating and resin additives. Compos Part A-Appl S 2009;40(5):553e65. [9] Hosoya R, Yoshimura A, Koyanagi J, Yoneyama S. X-ray CT image-based measurement of fiber orientation distribution in CFRP laminates. In: Proceedings of the American society for composites 2014-twenty ninth technical conference. California, USA, Sep; 2014. p. 700e11. [10] Hubbell JH, Seltzer SM. Tables of X-Ray mass attenuation coefficients and mass energy-absorption coefficients from 1 keV to 20 MeV for elements Z ¼ 1 to 92 and 48 additional substances of dosimetric interest. http://www.nist. gov/pml/data/xraycoef/index.cfm (last accessed on Dec 8, 2014). [11] Djukic LP, Herszberg I, Walsh WR, Schoeppner GA, Prusty BG. Contrast enhancement in visualisation of woven composite architecture using a MicroCT Scanner. Part 2: tow and preform coatings. Compos Part A-Appl S 2009;40(5):1870e9. [12] Djukic LP, Pearce GM, Herszberg I, Bannister MK, Mollenhauer DH. Contrast enhancement of MicroCT scans to aid 3D modelling of carbon fibre fabric composites. Appl Compos Mater 2013;20(6):1215e30. [13] Nishikawa Y, Hatanaka Y, Iizuka S, Takahashi M. Survey of contrasts of polymers under a high-contrast X-ray computerized tomography. Polymer 2012;53(19):4287e92. [14] Nishikawa Y, Ohta N, Komasu Y, Takahashi M. Application of high-contrast Xray computerized tomography to carbon fiber/polymer composites. J Soc Mater Sci Jpn 2011;60(1):29e34. [15] Kastner J, Plank B, Salaberger D. High resolution X-ray computed tomography of fibre- and particle-filled polymers. In: Proceedings of 18th world conference on nondestructive testing. Durban, South Africa, Apr; 2012. p. 9e17. [16] Kastner J, Plank B, Reh A, Salaberger D, Heinzl C. Advanced X-ray tomographic methods for quantitative characterization of carbon fiber reinforced plymers. In: Proceedings of the 4th international symposium on NDT in aerospace. Augsburg, Germany, Nov; 2012. We.3.A.1. [17] Cosmi F, Bernasconi A, Sodini N. Phase contrast micro-tomography and morphological analysis of a short carbon fiber reinforced polyamide. Comp Sci Tech 2011;71(1):23e30. [18] Lim JK, Shoji T. Fiber orientation and weld strength of short-glass-fiber-filled polycarbonate. JSME Int J A-Mech M 1993;36(3):319e26. [19] Bay RS, Tucker III CL. Fiber orientation in simple injection moldings. Part II: experimental results. Polym Compos 1992;13(4):332e41. [20] Lee DJ, Kim MW, Kim SY, Lee SH, Youn JR. Three dimensional flow simulation and structural analysis on stiffness of fiber reinforced anisotropic parts. KoreaAust Reol J 2010;22(2):95e103.
Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
The Ni-plated carbon fiber as a tracer for observation of the fiber orientation in the carbon fiber reinforced plastic with X-ray CT Azusa Nagura a, *, Kazuaki Okamoto a, Kiyoharu Itoh a, Yusuke Imai b, Daisuke Shimamoto b, Yuji Hotta b a
Nagoya Municipal Industrial Research Institute, 3-4-41, Rokuban, Atsuta-ku, Nagoya 456-0058, Japan National Institute of Advanced Industrial Science and Technology (AIST), Advanced Manufacturing Research Institute, 2266-98 Anagahora, Shimoshidami, Moriyama-ku, Nagoya 463-8560, Japan
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 December 2014 Received in revised form 4 February 2015 Accepted 6 February 2015 Available online 18 February 2015
Microfocus X-ray computed tomography (XCT) is effective to evaluate the 3-dimensional (3D) structure (position and orientation) of the fillers in the composites. However, the observation of the carbon fiber (CF) in resin by XCT is not easy due to the small difference between X-ray attenuation coefficient of CF and that of resin. We conducted XCT scanning of carbon fiber reinforced plastics (CFRPs) containing Niplated CF. The fiber orientation can be clearly observed in CT images due to a large X-ray attenuation coefficient of Ni-plating. Ni-plated CF essentially showed the same fiber orientation as that of normal CF. We also carried out the 3D characterization of the fiber orientation in an injection-molded dumbbell specimen with weld line. The fibers around the weld line are oriented perpendicular to the longitudinal direction of the specimen. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Micro X-ray computed tomography (XCT) A. Carbon fiber D. Non-destructive testing E. Injection molding
1. Introduction In recent years, carbon fiber reinforced plastic (CFRP) have been demanded in various fields like aeronautic, automotive, energy, and medical industries [1]. Especially CFRP utilizing thermoplastic as matrix, carbon fiber reinforced thermoplastic (CFRTP), is attracting increasing attention due to its high recyclability [2]. Moreover, CFRTP is promising from the economical view point since it needs relatively short cycle time [3]. In this sense, injection molding of CFRTP is desirable, because the cycle time of injection molding is normally up to several minutes. However, injection molding of CFRTP still has difficulties to overcome. Since carbon fiber (CF) has high aspect ratio, the properties of CFRTP are anisotropic [4]. For example, when CF oriented in a same direction, the strength of the composite is high at direction parallel to fiber axis but low at direction perpendicular to fiber axis. Therefore a facile method to observe the fiber in the composite is crucial.
* Corresponding author. Tel.: þ81 52 654 9950. E-mail addresses: [email protected] (A. Nagura), okamoto. [email protected] (K. Okamoto), [email protected] (K. Itoh), [email protected] (Y. Imai), [email protected] (D. Shimamoto), [email protected] (Y. Hotta). http://dx.doi.org/10.1016/j.compositesb.2015.02.009 1359-8368/© 2015 Elsevier Ltd. All rights reserved.
Microfocus X-ray computed tomography (XCT) is effective to evaluate the 3-dimensional (3D) structure (orientation and position) of the fillers in the composites because one can gain image data in a relatively short time without destructing the sample. Therefore XCT is frequently utilized to observe the glass fiber in composites [5e7]. However, the observation of the CF by XCT is not easy. The contrast of the XCT image is proportional to differences in X-ray attenuation coefficients between the fiber and the resin [8,9]. Because the attenuation coefficient of the material is mainly determined by its elemental composition and density [10], the Xray attenuation coefficient of the CF is close to that of the resin. For this reason, it is hard to gain the contrast between CF and resin in the XCT image. Recently, several groups have succeeded in observing the CF in the CFRP by XCT. Djukic and co-workers evaluated 3D structure of the CF fabric in thermosetting resin. They coated the CF tows with gold, copper, and iodine [8,11,12]. Since these materials have much higher attenuation coefficients than resin [10], the contrast between carbon tow and matrix was enhanced. However, they have not observed the 3D structure of dispersed thin filaments, but bunch of CF which is composed of thousands of filaments. Nishikawa and co-workers directly observed the dispersed carbon filament in CFRTP by developing the high contrast XCT [13,14]. Their XCT utilizes low-energy X-ray effectively therefore obtains the
A. Nagura et al. / Composites Part B 76 (2015) 38e43
contrast of the materials which are composed of light elements. Kastner and co-workers also directly observed the dispersed filament as well as carbon filament in the CF tow. They utilized nanofocus X-ray computed tomography which can gain higherresolution image than microfocus XCT [15,16]. Moreover, Cosmi and co-workers observed short carbon fiber in injection molded sample with synchrotron radiation computed tomography (SRCT) [17]. However, in order to clearly observe the CF in the matrix with these systems, the scanning range is up to as small as several cubic millimeters so far. In this work, we investigated the possibility of Ni-plated CF as a tracer to observe the fiber orientation in injection-molded CFRTP specimens. Ni-plating was utilized because it is commonly used due to its high wear resistance and the X-ray absorption coefficient is enough larger than that of carbon [10]. We first prepared the samples containing different content of Ni-plated CF and observed the fibers by XCT, and then, quantitatively compared the fiber orientation of Ni-plated CF with that of CF. Finally we discuss the fiber orientation around the weld line. 2. Experimental 2.1. Materials
39
molded using hand-operated injection molding machine (Custom Scientific Instruments, Inc., mini-maxmolder CS-183mmx) with a kneading temperature of 230 C. Fig. 1 shows the photograph of the samples. 2.3.2. Preparation of samples 6e7 We prepared the samples containing the Ni-plated CF (sample 6) and normal CF (sample 7) to compare the fiber orientation. The composition ratio of sample 6 and 7 is also summarized in Table 1. These samples were molded by electric injection molding machine (Sumitomo heavy industries, SE18S) in order to strictly unify the molding condition. The fiber and polypropylene were kneaded at 220 C by a laboratory rotating disk extruder (Custom Scientific Instruments, Inc., MAX mixing extruder CS-194). The resulting compound was molded with injection molding machine with following conditions: barrel temperature ¼ 220 C, mold temperature ¼ 40 C, injection speed ¼ 50 mm/s, screw speed ¼ 70 rpm, back pressure ¼ 4.0 MPa, holding pressure ¼ 50 MPa 8 s Fig. 2 shows the photograph of sample 6, which is the dumbbell-shaped specimen with weld line at the center. 2.4. Observation by X-ray CT
PAN-based carbon fiber (T700SC-12000, Toray Industries, Inc.) was washed with deionized water and dried in the air before use. Alkaline type electroless Ni plating solution (electroless nickel A and B, Okuno Chemical Industries, Co. Ltd.,), plating catalyzer (Catalyst C-7, Okuno Chemical Industries, Co. Ltd.,), hydrochloric acid (Wako Pure Chemical Industries, Ltd.,), polystyrene (PSJpolystyrene HF77, PS Japan Corporation), polypropylene (NOVATEC PP MH4, Japan Polypropylene Corporation) were used as received. 2.2. Preparation of the Ni-plated CF The Ni-plated CF was prepared by electroless plating as following procedures. First, the CF was soaked in 5% aqueous solution of plating catalyst, and then dipped in 10% hydrochloric acid. Then, the CF was immersed in a Ni-plating bath (20% aqueous solution of electroless Ni A and B) at 35 C for 60 min. After each step, the fiber was rinsed by deionized water. Finally, the resulting Niplated CF was dried in the air for more than 24 h. After dried, the Ni-plated CF was observed with scanning electron microscopy (SEM) (Hitachi field emission SEM Se4300S, Hitachi HighTechnologies Corporation). 2.3. Preparation of the samples 2.3.1. Preparation of samples 1e5 We prepared five samples containing different content of the Niplated CF (0, 0.3, 1, 3, and 10 wt%). Normal CF was also contained so that each sample contained the same content of fibers. The composition ratio is summarized in Table 1. These samples were
Two XCT scanning systems were utilized for observation of the composites. One is a general-purpose microfocus XCT system (TOSCANER-32252mhd, Toshiba IT & Control Systems Corporation). This system was used for the samples 1-5. All the scannings were performed at a tube voltage of 50 kV and a tube current of 200 mA. The samples were rotated with the total angle of 360 and the angular increment of 0.036 with the cumulative number of 8. The resolution of CT image was 13.2 mm. The scanning range is indicated in Fig. 1. We also scanned with various magnifications for sample 5 by changing the ratio of X-ray focusesample distance to X-ray focusedetector distance. The resolutions of the scanning were changed from 13.2 to 21.9 and 36.5 mm. The corresponding scanning ranges were 13.5 13.5 7.5, 22.4 22.4 15.8, and 37.4 37.4 21.0 mm, respectively. The other system used for sample 6 and 7 is similar to that of Nishikawa's high contrast XCT (FLEX-M345, BEAMSENSE Co., Ltd.), which can directly gain the contrast between normal CF and resin. The tube voltage and tube current were 70 kV and 130 mA for sample 6 and 40 kV and 100 mA for sample 7. Each sample was rotated with the total angle of 180 and the angular increment of 0.25 . The scanning position is indicated in Fig. 3. The image processing was conducted with 3Dview (RMR Systems Ltd.). The fiber orientation analysis was performed with image analysis software, ExFact Analysis for Fiber (Nihon Visual Science, Inc.). Since the size of whole data was too large to be analyzed, four parts of the data were cut out and used for the analysis. The positions of these regions are shown in Fig. 3. The
Table 1 Composition ratios of the samples. Sample
Ni-plated CF(wt%)
CF(wt%)
Resin(wt%)
1 2 3 4 5 6 7
e 0.3 1 3 10 1 e
10 9.7 9 7 e 4 5
90 90 90 90 90 95 95
Fig. 1. The photograph of the samples 1-5 and their dimension. The squares indicate the scanning range.
40
A. Nagura et al. / Composites Part B 76 (2015) 38e43
3. Results and discussion 3.1. Ni plating of CF Fig. 4 shows SEM images of the Ni-plated CF. It was confirmed that almost whole surface of the fiber was covered by the Ni coating as shown in Fig. 4(a). Magnified view in Fig. 4(b) at the location where Ni coating accidentally flaked off gives us the estimate of Ni coating thickness to be 100e150 nm. 3.2. Content of the Ni-plated CF for XCT observation The XCT images of samples 1e5 are shown in Fig. 5. Fibrous objects were clearly observed in the samples 2e5, while no fiber could be identified in the sample 1 containing only normal CF. This result indicates that the Ni-plating successfully enhances the contrast between the fiber and the matrix. Uniform distribution of the Ni-plated CF in Fig. 5(b)e(e) suggests that the Ni plating on CF has little effect on the dispersion behavior of CF in the matrix. Individual Ni-plated CF was easily distinguished in the XCT images, while the identification of individual CF becomes more difficult due to overlapping at higher contents. Therefore, it is considered that the preferable content of Ni-plated CF is up to 10 wt%. 3.3. Maximum resolution
Fig. 2. The photograph of the sample 6 and its dimension.
regions (a) and (b) are nearby weld line and regions (c) and (d) are 2.75 mm away from the weld line. (a) and (c) are at the surface of the sample and (b) and (d) are about 1.4 mm deeper from the surface. It is known that the fiber around the weld line shows characteristic orientation [18]. In addition, the fiber orientation around surface and that inside are generally different [19]. Therefore we chose the four regions above in order to compare the fiber behavior at different conditions. The dimension of each region is 364 364 364 mm. Finally, we observed the fiber orientation around the weld line in sample 6 with a general-purpose microfocus XCT system. The tube voltage, tube current, total angle, angular increment, cumulative number, and resolution were 50 kV, 210 mA, 360 , 0.036 , 8, and 13.7 mm, respectively. The image processing was performed with image analysis software VG Studio MAX 2.1 (Volume Graphics GmbH).
Fig. 3. The scanning range of the samples 6 and 7. Dark gray cubics (a)e(d) indicate the positions of fiber orientation analysis.
Fig. 6 shows the XCT images of sample 5 with various scanning resolutions. Since the pixel count of the image sensor was fixed at 1024 1024 pixels, the higher spatial resolution result in the narrower scanning range (ex: when resolution ¼ 13.2 mm, scanning range is 13.2 mm 1024 pixels ¼ 13.5 mm). It is generally hard to
Fig. 4. SEM images of the Ni-plated carbon fiber at (a) 500 and (b) 100,000 magnifications. The square in (a) indicates the observation range of (b).
A. Nagura et al. / Composites Part B 76 (2015) 38e43
41
Fig. 5. XCT images of the samples. (a) sample 1, (b) sample 2, (c) sample 3, (d) sample 4, and (e) sample 5. A tube voltage and a tube current are 50 kV and 200 mA.
recognize the exact shape of the thin object like CF by XCT when the resolution of the image is lower than the diameter of the object (~7e10 mm, in this case). However, the residual length of the CF in the composites is expected to be more than several hundreds of micrometers. Therefore it would be feasible to identify the individual Ni-plated CF as an array of voxels if there is enough contrast between the voxel containing Ni-plated CF and that of matrix resin. It is experimentally demonstrated in Fig. 6(a) and (b) where the corresponding spatial resolutions are 13.2 and 21.9 mm, respectively. It was possible to recognize the Ni-plated CF in these images, which allows us to evaluate the fiber orientation in the composites. When the resolution was much lower value of 36.5 mm as shown in Fig. 6(c), it was difficult to identify the fibers due to the lower contrast. 3.4. Comparison of the fiber orientation In order to show the ability of Ni-plated CF as a tracer for CF orientation in CFRTP, we compared fiber orientation in detail by observing the samples 6 and 7 using the high contrast XCT. Both the samples contain the same contents of fiber (5 wt%). A part of fiber in the sample 6 is Ni-plated CF, while all the fibers in the sample 7 is the normal CF. Fig. 7 shows the observation results of the samples 6 and 7. In Fig. 7(a), the observation condition was optimized to visualize the Ni-plated CF in the sample 6. Under this condition, the
Fig. 6. XCT images of the sample 5 with various resolution: (a) 13.2 mm, (b) 21.9 mm, (c) 36.5 mm.
contrast between the normal CF and the matrix resin was too low to be identified separately. When the condition was optimized for normal CF in the sample 7, we could visualize the CF as shown in Fig. 7(b) and (c). However, this condition was not applicable to the sample 6 because of too large attenuation by Ni-plated CF. Therefore it was impossible to visualize Ni-plated CF and normal CF at the same time. In Fig. 7, both Ni-plated CF and normal CF are essentially oriented to the longitudinal direction, but the fibers nearby weld line are oriented to perpendicularly. The fiber orientation of both Niplated CF and normal CF is qualitatively the same. It is noteworthy that similar fiber orientation is also observed in glass-fiber reinforced plastic (GFRP) [20]. This result indicates that the major factor determining the fiber orientation is the effect from the flowing resin rather than the character of the fiber itself. We next conducted quantitative fiber orientation analyses at four regions. The results of fiber angle distribution analysis are summarized in Fig. 8. The polar angle q is the angle between the fiber axis and width direction of the sample and the azimuthal angle f is the angle between the fiber axis and plane of the sample. The correlation coefficients of the orientation angles were found to
Fig. 7. XCT observation of the sample 6 and 7 (The scanning ranges are shown in Fig. 3). (a) 3-dimensional image of sample 6 (containing Ni-plated CF), (b) 3dimensional image of sample 7 (containing normal CF), and (c) XCT image of sample 7. The tube voltage and tube current were 70 kV and 130 mA for sample 6 and 40 kV and 100 mA for sample 7.
42
A. Nagura et al. / Composites Part B 76 (2015) 38e43
Fig. 8. Comparison between the fiber orientation of tracer (gray) and that of carbon fiber (black).
be 0.99-0.59, indicating a strong correlation between the orientation of the normal CF and Ni-plated CF. This result shows that Niplated CF can be used as the tracer for the CF orientation. 3.5. Visualization of weld region by Ni-plated CF tracer Fig. 8 also shows the detailed fiber orientation in each region. In the regions (a) and (b), the probability of both tracer and CF has maximum around f ¼ 90 , indicating that the fibers are oriented perpendicular to the longitudinal axis. In addition, the probability increases as q angle increases. This means that the fibers tend to be oriented perpendicular to the plane of the sample. In the region (c), fibers show similar behavior in q angle as regions (a) and (b), but probability does not correlate with f angles. The fiber orientation is parallel to the side face of the sample but at random in the plane. In
the region (d), the probability has maximum at q ¼ 60 e75 and f angle has maximum at f ¼ 0 e30 . These values indicate that the fiber orientation is mainly parallel to the longitudinal axis but to some extent oriented to the out-of-plane of the sample. As we confirmed that Ni-plated CF can be utilized as the tracer of CF orientation, we also conducted CT scanning of the sample 6 by general-purpose microfocus XCT system. The observation range was 14.0 14.0 5.0 mm. The results are shown in Fig. 9. Fig. 9(a) contains the cross sections perpendicular to the longitudinal direction and Fig. 9(b) contains the cross sections parallel to the longitudinal direction and perpendicular to the plane of the sample. In Fig. 9(a), linear-shaped fiber could be seen in the cross sections around weld line whereas spot-shaped fiber was observed in other cross sections. This result suggests that the fibers around weld line are oriented to perpendicular to the longitudinal
Fig. 9. 3-dimensional image of the fiber around the weld line of the sample 6. (a) cross sections perpendicular to the flow direction (b) cross sections parallel to the flow direction.
A. Nagura et al. / Composites Part B 76 (2015) 38e43
direction while other fibers are oriented parallel to the longitudinal direction. In Fig. 9(b), the fibers form half ellipse shapes around weld line. This fiber orientation is apparently due to the effect of fountain flow. It is noteworthy that we could gain the scanning data above with one scanning. The tracer realized such wide-ranged observation. 4. Conclusion In the present paper, we investigated the behavior of the Niplated CF in injection-molded CFRTP by XCT. The Ni-plated CF was clearly observed in XCT images due to its large X-ray attenuation coefficient although it was dispersed in matrix resin as not a tow but filaments. It generally showed the same fiber orientation as that of CF, indicating that it was useful as the tracer. Wide range observation of cubic centimeters was also successful. This result implicate that we can save the measurement time when observing a large product. 3D evaluation of fiber orientation in various resin flow is under study. References [1] Park SJ. Carbon fibers. Dordrecht: Springer; 2014. [2] Suzuki T, Takahashi J. Prediction of energy intensity of carbon fiber reinforced plastics for mass-produced passenger cars. In: Proceedings of the ninth Japan international SAMPE symposium. Tokyo, Japan, Nov-Dec; 2005. p. 14e9. [3] Suzuki K, Nagata K, Uzawa K, Matsuo T, Takahashi J. Research on the jointing method of CFRTP for structural applications. In: Proceedings of ICCM18conference. Jeju Island, South Korea, Aug; 2011. TH32e1. [4] Gowayed Y. Types of fiber and fiber arrangement in fiber-reinforced polymer (FRP) composites. In: Uddin N, editor. Developments in fiber-reinforced polymer (FRP) composites for civil engineering. Cambridge: Woodhead Publishing; 2013. p. 3e17. [5] Schell JSU, Renggli M, van Lenthe GH, Müller R, Ermanni P. Micro-computed tomography determination of glass fibre reinforced polymer meso-structure. Compos Sci Technol 2006;66(13):2016e22. € pplmayr T, Milosavljevic I, Aigner M, Hasslacher R, Plank B, Salaberger D, [6] Ko et al. Influence of fiber orientation and length distribution on the rheological characterization of glass-fiber-filled polypropylene. Polym Test 2013;32(3): 535e44.
43
[7] Arif MF, Meraghni F, Chemisky Y, Despringre N, Robert G. In situ damage mechanisms investigation of PA66/G30 composite: effect of relative humidity. Compos Part B-Eng 2014;58:487e95. [8] Djukic LP, Herszberg I, Walsh WR, Schoeppner GA, Prusty BG, Kelly DW. Contrast enhancement in visualization of woven composite tow architecture using a MicroCT scanner. Part 1: fabric coating and resin additives. Compos Part A-Appl S 2009;40(5):553e65. [9] Hosoya R, Yoshimura A, Koyanagi J, Yoneyama S. X-ray CT image-based measurement of fiber orientation distribution in CFRP laminates. In: Proceedings of the American society for composites 2014-twenty ninth technical conference. California, USA, Sep; 2014. p. 700e11. [10] Hubbell JH, Seltzer SM. Tables of X-Ray mass attenuation coefficients and mass energy-absorption coefficients from 1 keV to 20 MeV for elements Z ¼ 1 to 92 and 48 additional substances of dosimetric interest. http://www.nist. gov/pml/data/xraycoef/index.cfm (last accessed on Dec 8, 2014). [11] Djukic LP, Herszberg I, Walsh WR, Schoeppner GA, Prusty BG. Contrast enhancement in visualisation of woven composite architecture using a MicroCT Scanner. Part 2: tow and preform coatings. Compos Part A-Appl S 2009;40(5):1870e9. [12] Djukic LP, Pearce GM, Herszberg I, Bannister MK, Mollenhauer DH. Contrast enhancement of MicroCT scans to aid 3D modelling of carbon fibre fabric composites. Appl Compos Mater 2013;20(6):1215e30. [13] Nishikawa Y, Hatanaka Y, Iizuka S, Takahashi M. Survey of contrasts of polymers under a high-contrast X-ray computerized tomography. Polymer 2012;53(19):4287e92. [14] Nishikawa Y, Ohta N, Komasu Y, Takahashi M. Application of high-contrast Xray computerized tomography to carbon fiber/polymer composites. J Soc Mater Sci Jpn 2011;60(1):29e34. [15] Kastner J, Plank B, Salaberger D. High resolution X-ray computed tomography of fibre- and particle-filled polymers. In: Proceedings of 18th world conference on nondestructive testing. Durban, South Africa, Apr; 2012. p. 9e17. [16] Kastner J, Plank B, Reh A, Salaberger D, Heinzl C. Advanced X-ray tomographic methods for quantitative characterization of carbon fiber reinforced plymers. In: Proceedings of the 4th international symposium on NDT in aerospace. Augsburg, Germany, Nov; 2012. We.3.A.1. [17] Cosmi F, Bernasconi A, Sodini N. Phase contrast micro-tomography and morphological analysis of a short carbon fiber reinforced polyamide. Comp Sci Tech 2011;71(1):23e30. [18] Lim JK, Shoji T. Fiber orientation and weld strength of short-glass-fiber-filled polycarbonate. JSME Int J A-Mech M 1993;36(3):319e26. [19] Bay RS, Tucker III CL. Fiber orientation in simple injection moldings. Part II: experimental results. Polym Compos 1992;13(4):332e41. [20] Lee DJ, Kim MW, Kim SY, Lee SH, Youn JR. Three dimensional flow simulation and structural analysis on stiffness of fiber reinforced anisotropic parts. KoreaAust Reol J 2010;22(2):95e103.