Compressive failure of unidirectional hybrid fibre-reinforced epoxy composites containing carbon and silicon carbide fibres

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1 Sudarisman, I. J. Davies, and H. Hamada, Compressive failure of unidirectional hybrid fibre-reinforced epoxy composites containing carbon and silicon carbide fibres, Compos. Part A, 38(3) pp (2007). Compressive failure of unidirectional hybrid fibre-reinforced epoxy composites containing carbon and silicon carbide fibres Sudarisman*, I. J. Davies* a, and H. Hamada** *Department of Mechanical Engineering, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia **Advanced Fibro-Science, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto , Japan a Contact author: Dr. Ian J. Davies, Department of Mechanical Engineering, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia (Tel: ; Fax: ; I.Davies@curtin.edu.au) Abstract The compressive failure of unidirectional hybrid fibre-reinforced epoxy matrix composites containing carbon (C) and silicon carbide (SiC) fibres has been investigated. In contrast to the case of flexural testing previously investigated by the authors, no significant increase in compressive strength, elastic modulus, or work of fracture was noted for the case of composites containing a mixture of C and SiC fibres. The specific compressive strength and elastic modulus generally decreased with increasing SiC fibre content due to the higher density of these fibres. Failure modes of tested specimens were classified into two main groups, namely compressive shear and compressive crushing, with the presence of fibre kinking and longitudinal splitting being noted in both cases. Keywords: A. Hybrid; A. Laminates; A. Polymer-matrix composites (PMCs); B. Mechanical properties 1. Introduction Due to their superior strength- and stiffness-to-weight ratios and corrosion resistance, fibre-reinforced polymer (FRP) composites have found widespread use in various fields such as leisure and sport, construction and civil engineering, transportation and aerospace. From an engineering point of view, one important property of FRP composites is their compressive strength generally being lower compared to the respective tensile strength. For example, the ratios of compressive-to-tensile strength for unidirectional Kevlar 49/epoxy and S-2 glass/epoxy composites are approximately 0.15 and 0.49, respectively [1], whilst Ishikawa reported a value of 0.71 as being typical for carbon fibre-reinforced polymer (CFRP) composites (Carbolon Z-fibre/epoxy) with a fibre volume fraction, V f, of 0.65 [2]. In contrast to this, the value for FRP composites (Nicalon /epoxy) containing amorphous silicon carbide (SiC) fibres is close to unity [2]. Whilst the compressive-to-tensile strength ratio was greatest for the Nicalon -fibre/epoxy composite, the higher density of this system (2.00 g cm -3 ) when compared to other FRP composites (e.g., typically 1.60 g cm -3 for CFRP systems) would prove a disadvantage for many applications where specific strength is an important issue [3,4]. Despite this, the incorporation of higher density ceramic fibres within CFRP composites may lead to an overall increase in mechanical performance. For example, the present authors [5] found the flexural properties of eight-layer unidirectional hybrid carbon (T700S ) silicon carbide (Tyranno S1) epoxy (Epikote 828) composites to be considerably higher compared to those of the baseline CFRP Downloaded from:

2 composite with an increase of up to 22% in flexural strength being noted from the replacement of one C layer by SiC. Whilst the compressive behaviour of CFRP composites has been the subject of extensive investigation [e.g., 6, 7], there is a relative dearth of information available for the compressive properties of hybrid FRP composites [8-11]. In light of the successful results obtained previously for hybrid FRP composites under flexural loading [5], in this work the authors have investigated the compressive properties of hybrid FRP composites in the C/SiC/epoxy system as a function of fibre configuration. 2. Experimental procedure The present investigation utilised unidirectional FRP composites containing a mixture of C fibres (Type T700S, Toray Industries, Inc., Tokyo, Japan) and amorphous/nanocrystalline SiC-based fibres (Tyranno Si-Ti-C-O fibre, Type S1, Ube Industries Ltd., Ube City, Japan) embedded within an epoxy matrix (Epikote 828, Yuka Shell Epoxy Co. Ltd., Tokyo, Japan). Prepreg sheets were first produced by winding either C or SiC fibres through an acetone solution of resin and hardener onto a cylindrical mandrel and then heating for 20 minutes at 120 o C. After being cut into sections of approximate size mm, the prepreg sheets were stacked into an eight-layer sequence with a total of five stacking configurations being utilised in the present work (Fig. 1). Following this, the resulting plates were heated in vacuum (120 o C / 30 minutes) under a compressive stress of 2.5 MPa; the resulting composite plates having a nominal thickness of 2 mm. The value of V f for each composite was estimated using the constituent properties supplied by the manufacturers as shown in Table 1 [5,12] with V f being generally in the range 0.70~0.74. As expected, the specimen density was found to increase with SiC fibre content from 1.60 g cm -3 for the CFRP composite (C4 s ) to 2.09 g cm -3 for the SiC fibre-based FRP composite (S4 s ). Prior to testing, the composite plates were cut into specimens of approximate dimensions 150 mm 6.5 mm 2 mm. In order to prevent crushing of the specimens at the point of gripping, four aluminium tabs of 1 mm thickness were epoxy-glued onto the roughly sanded gripping areas of each specimen. Compressive testing was carried out in accordance with procedure A of the ASTM D3410 test standard [13] using a universal testing machine (Instron 4206; 10 kn loadcell) equipped with a Celanese compression fixture. All specimens were loaded until failure using a constant crosshead speed of 0.5 mm min -1. In addition to compressive stress, strain data was also collected for each composite configuration with strain gauges (3 mm gauge length) being carefully installed at the midspan of the test gauge lengths. Values of compressive modulus, E, were calculated at a loading condition of 0.1% strain with a minimum of five specimens being tested for each stacking configuration. The failure mode of each specimen was determined through visual inspection and optical microscopy. 3. Result and Discussion 3.1. Compressive strength The influence of SiC fibre content and stacking configuration on compressive strength properties has been presented in Figure 2(a) with the compressive strength of the CFRP specimens (1028 MPa) being consistent with values previously reported for the T700S/epoxy system [14]. The mean values of compressive strength (Figure 2(a)) were found to range between 933 MPa for the SC3 s specimens to 1333 MPa for the S4 s specimens with the higher compressive strength of the S4 s composite being attributed to the superior compressive strength of the SiC fibres (3.46 GPa [5]) in comparison with that of the carbon fibres (3.3 GPa [12]). The main result of this work is that the hybrid composites exhibited no significant increase in compressive strength compared to the baseline CFRP composite; this being in contrast to previous work [10] on a similar system which indicated an approximate 30% increase in compressive strength through the substitution of approximately 10% of C fibres by SiC fibres. The difference in hybrid composite behaviour was attributed to the test configuration (ASTM - 2 -

3 D695) and specimen preparation being different for the previous work [10], both of these variables having a significant influence on the compressive strength of FRP composites [15,16]. In addition, the present result was in contrast to earlier work by the authors [5] where a hybridization effect was clearly noted for the case of flexural strength. In fact, the compressive strength of specimens containing two SiC fibre layers, i.e., SC3 s (933 MPa), was below that of the CFRP composite (C4 s ), suggesting the substitution of a small amount of C fibres by SiC fibres to have a negative influence on compressive strength. Such a phenomenon may be due to the presence of stress concentrations [17] and/or in-plane shear stresses as a result of a discontinuity in mechanical properties at the fibre-matrix interface [18]. The C4 s, SC3 s, S2C2 s, and S3C s specimens exhibited global shear failure and occasional longitudinal splitting (Fig. 3(a)) which, upon closer inspection, also revealed the presence of kink banding at the fracture surface (Fig. 3(c)). The failure mode noted for these specimens was consistent with that generally observed in unidirectional CFRP composites [7]. In contrast to this, the S4 s specimens failed due to global crushing (Figure 3(b)) with fibre kinking also being apparent in several regions of the fracture surface (Fig. 3(d)). The simultaneous presence of kink banding and longitudinal splitting was similar to that previously observed for high fibre volume fraction glass fibre composites [7]. The identical failure mode of the specimens containing C fibres, together with the absence of any hybridization effect, suggested failure of the present hybrid specimens to be controlled by the presence of the (lower compressive strength) C fibres. Specific compressive strength data as a function of SiC fibre content has been presented in Figure 2(a). In terms of potential weight savings, the maximum specific compressive strength was achieved by the baseline CFRP specimens, C4 s (642.5 MPa/g cm -3 ), and slightly higher compared to that of the S4 s composite (637.8 MPa/g cm -3 ), despite the much higher compressive strength of the latter. This result was due to the significantly higher density of the SiC fibre (2.35 g cm -3 ) when compared to that of the C fibre (1.80 g cm -3 ), leading to lower specific compressive strength values in line with the increase in SiC fibre content Compressive modulus The influence of SiC fibre content on compressive modulus properties has been presented in Figure 2(b) together with the trends predicted from simple rule of mixture analysis. The compressive modulus was found to be maximum for the C4 s specimens (177.6 GPa) and gradually decreased with increasing SiC fibre content to reach GPa for the S4 s specimens, with the experimental data closely following the predicted trend. One point of interest in Figure 2(b) is that, compared to the compressive strength data shown in Figure 2(a), the elastic modulus decreased with partial or total substitution of SiC fibre for C fibre. This result was attributed to the nature of isostrain loading with the SiC fibres bearing a larger portion of the load compared to the C fibres, since the latter possesses a much higher compressive elastic modulus Stress-strain behaviour Stress-strain curves for the composite specimens were generally linear in nature apart from several of the C4 S specimens which exhibited global Euler buckling. Overall, the mean failure strain ranged from 0.77% for the S2C2 s specimens to 1.1% for the S4 s specimens; this latter value being slightly over half that achieved (1.8%) at the compressive face of specimens tested under flexure [5]. Through comparison of the total area under each stress-strain curve, the S4 s composite was noted to possess a significantly higher work of fracture when compared to the other specimens, with the S2C2 s composite possessing the lowest value (approximately 55% that of the S4 s case). This result was in contrast to the work of fracture calculated during flexural loading [5] which produced a maximum value for the composite with equal amounts of C and SiC fibre. In addition, the compressive work of fracture exhibited by all specimens containing C fibre was similar, i.e., 55 65% of the S4 s specimens, - 3 -

4 and again suggested that the presence of C fibre controlled/limited the compressive properties of the composites. The largest specific work of fracture was achieved by the S4 S composite, despite its higher density, and followed by the C4 S specimens (approximately 79% of the S4 S case) and then the S3C S and SC3 S specimens (approximately 66 68% of the S4 S case). 4. Conclusions Unidirectional hybrid fibre-reinforced polymer composites containing SiC fibres and/or C fibres within an epoxy matrix were tested under compressive loading. The following conclusions were drawn from the present work: (i) No significant increase in compressive strength, elastic modulus, and work of fracture was noted for the case of composites containing a mixture of C and SiC fibres. (ii) Due to the presence of higher density SiC fibres, both the specific compressive strength and specific elastic modulus decreased with increasing SiC fibre content. (iii) Failure modes of the tested specimens were classified into two main groups, namely global crushing for the S4 s specimens and global shear for the other configurations, with the presence of fibre buckling and longitudinal splitting being noted in both cases. Acknowledgements The authors wish to express their sincere thanks to Dr. M. Shibuya of Ube Industries, Ltd., for provision of the Tyranno Si-Ti-C-O fibres and Mr. T. Ohki for specimen preparation. References 1. Weeton JW, Peters DM, and Thomas KL. Engineer s Guide to Composite Materials, ASM International, Metals Park, Ohio, USA (1990) pp Ishikawa T. Recent development of the SiC fibre (Nicalon) and its composites, including properties of SiC fibre (Hi-Nicalon) for ultra-high temperature. Compos. Sci. Technol. 1994;51: Schwart MM. Composite Materials Handbook (2nd edition), McGraw Hill, New York, USA (1992) Güemes JA, Menendez JM, Frövel M, Fernandez I, and Pintado JM. Experimental analysis of buckling in aircraft skin panels by fibre optic sensors. Smart. Mater. Struct. 2001;10(3): Davies IJ and Hamada H. Flexural properties of a hybrid polymer matrix composite containing carbon and silicon carbide fibres. Adv. Compos. Maters. 2000;10: Hancox NL. The compression strength of unidirectional carbon fibre reinforced plastic. J. Maters. Sci. 1975;10: Lee SH and Waas AM. Compressive strength and failure of fiber reinforced unidirectional composites. Int. J. Fracture 1999;100: Summerscales J and Short D. Carbon fibre and glass fibre hybrid reinforced plastics. Composites 1978;9: Sanparote PW and Lakkad SC. Mechanical properties of carbon/fibre reinforced hybrids. Fibre Sci. Technol. 1982;16: Hiroyuki O and Tsujihata H. Tyranno fibre. Goseijyushi Kogyo (Industrial Synthetic Resins) 1991;4: Yerramalli CS and Waas AM. Compression behavior of hybrid composites. AIAA American Institute of Aeronautics and Astronautics (2003). 12. Oya N and Johnson DJ. Longitudinal compressive behaviour and microstructure of PAN-based carbon fibres. Carbon 2001;39: Annual Book of ASTM, Vol 15.03, ASTM International, Conshohocken, USA (2002) Oya N and Hamada H. Effects of reinforcing fibre properties on various mechanical behaviour of unidirectional carbon/epoxy laminates. Sci. Eng. Comp. Mater. 1996;5: Haberle JG and Matthews FL. An improved technique for compression testing of unidirectional fibre-reinforced plastics: developments and results. Composites 1994;25(5):

5 16. Haberle JG and Matthews FL. The influence of test piece preparation on the compressive strength of unidirectional fiber-reinforced plastic. J. Testing and Evaluation. 1994;22(4): Yang F and Pitchumani R. Effects of interphase formation on the modulus and stress concentration factor of fiber-reinforced thermosetting-matrix composites. Compos. Sci. Technol. 2004;64: Cristensen RM. Mechanics of Composite Materials, John Wiley Interscience, New York, USA (1979) pp Figure 1: Schematic representation of the stacking configuration in the composite specimens under investigation. The subscript s refers to the stacking sequence being symmetrical

6 Figure 2: Effect of SiC fibre volume fraction (as a fraction of total fibre content) on the compressive properties of a hybrid fibre-reinforced polymer composite: (a) compressive strength, (b) compressive modulus, and (c) maximum compressive strain

Figure 3: Representative optical micrographs illustrating the different failure modes observed in the specimens under investigation: (a) global shear failure (C4s, SC3s,

Table 1: Property data for the fibre and matrix components utilised in the hybrid fibre-reinforced polymer composites under investigation (data supplied by manufacturer

50 180 3.3 3.46 iv C fibre ii 1.80 6.75 237 5.3 2.4 v Epoxy matrix iii 1.16 30 0.063 0.132 Tyranno Si-Ti-C-O fibre, Type S1, Ube Industries Ltd.

7 Figure 3: Representative optical micrographs illustrating the different failure modes observed in the specimens under investigation: (a) global shear failure (C4s, SC3s, S2C2s, and S3Cs), (b) global crushing (S4s), (c) kink band noted in the S2C2s specimen, and (d) kink band and longitudinal splitting noted in the S4s specimen. Table 1: Property data for the fibre and matrix components utilised in the hybrid fibre-reinforced polymer composites under investigation (data supplied by manufacturer unless otherwise stated) [5,12]. Materials Density (g.cm-3) Diameter (μm) Elastic Modulus (GPa) Tensile strength (GPa) Compressive strength (GPa) SiC fibre i iv C fibre ii v Epoxy matrix iii Tyranno Si-Ti-C-O fibre, Type S1, Ube Industries Ltd., Ube City, Japan T700S, Toray Industries, Inc., Tokyo, Japan iii Epikote 828, Yuka Shell Epoxy Co. Ltd., Tokyo, Japan iv Data taken from Davies and Hamada [5]. v Data taken from Oya and Johnson [12]. i ii -7-

SPECIMEN SIZE EFFECT ON THE IN-PLANE SHEAR PROPERTIES OF SILICON CARBIDE/SILICON CARBIDE COMPOSITES

5 SPECIMEN SIZE EFFECT ON THE IN-PLANE SHEAR PROPERTIES OF SILICON CARBIDE/SILICON CARBIDE COMPOSITES T. Nozawa 1, E. Lara-Curzio 2, Y. Katoh 1,, L.L. Snead 2 and A. Kohyama 1, 1 Institute of Advanced