Tensile properties and fatigue characteristics of hybrid composites with non-woven carbon tissue

International Journal of Fatigue 24 (2002) 397–405 www.elsevier.com/locate/ijfatigue

Tensile properties and fatigue characteristics of hybrid composites with non-woven carbon tissue Seung-Hwan Lee a, Hiroshi Noguchi b

a,*

, Seong-Kyun Cheong

b

a Faculty of Engineering, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka-shi, Fukuoka 812-8581, Japan Department of Mechanical Engineering, Seoul National University of Technology, 172 Gongneung-dong, Nowon-gu, Seoul 139-743, South Korea

Abstract The mechanical characteristics of hybrid composites with non-woven carbon tissue (NWCT) are investigated under static tensile and tension–tension fatigue loadings. The hybrid composites are made by stacking NWCT and CFRP prepregs. Thirteen kinds of composites are studied; i.e. NWCT composites, CFRP longitudinal [0]8, transverse [90]12, off-axis [45]12 and angle-ply [±45]3S, hybrid longitudinal ([0/0]4, [/0/0/]4), transverse ([90/90]6, [/90/90/]6), off-axis ([45/45]6, [/45/45/]6), and angle-ply ([+45/⫺45]3S, [/+45/⫺45/]3S). The symbol ‘/’ means that the NWCT is located between the CFRP layers. To estimate the stiffness of hybrid composites, the rule of mixtures is used. The effects of NWCT on the tension–tension fatigue life, the residual strength and stiffness of hybrid specimens are evaluated. The fatigue damage and failure mechanisms of the hybrid composites are analyzed with an optical microscope.  2002 Elsevier Science Ltd. All rights reserved. Keywords: Hybrid composites; Non-woven carbon tissue; Rule of mixtures; Fatigue strength; Residual strength and stiffness

1. Introduction Composite materials are easily found in many kinds of structures. Carbon fiber reinforced composite materials have been used for the structure of automobile, ships, aircraft, satellite, sporting goods and so on [1]. The use of advanced hybrid materials in composite structures has become more popular nowadays [2–5]. Composite structures possess numerous beneficial characteristics, such as stability, lightweight and economy. The mechanical behavior of fiber reinforced, multilayered composite laminates differ considerably from that of homogeneous and isotropic materials. Various techniques [6–8] can be employed to increase the mechanical properties of fiber reinforced composite materials. The strengthening technique [9–11] using non-woven tissue (NWT) is one method. The NWT has been used for the exterior materials to protect the surface scratch in composite structures and increase corrosion resistance in chemical plant and electronic circuit plates

[12]. There are several kinds of materials like polyester, glass, carbon, aramid fiber NWT and so on. Fig. 1 shows the hybrid prepreg made by overlapping non-woven carbon tissue (NWCT) and unidirectional prepregs. Cheong and Lee et al. [9,10] evaluated tensile properties of CFRP composite with NWCT different from the present study. In this study, the tensile and fatigue characteristics of hybrid composites with NWCT and the effects of interleaving NWCT are investigated. Compared with the

* Corresponding author. Tel.: +81-92-6423375; fax: +81-926419744. E-mail address: [email protected] (H. Noguchi). 0142-1123/02/$ - see front matter  2002 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 1 1 2 3 ( 0 1 ) 0 0 0 9 5 - 0

Fig. 1.

Schematics of hybrid prepreg.

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CFRP composites, the hybrid composites exhibit peculiar characteristics under tensile and fatigue loadings.

3. Experiments 3.1. Materials processing

2. Estimation of hybrid composites stiffness A new estimation method is proposed in this paper to estimate the stiffness of hybrid composites with NWCT. A two-dimensional rule of mixture is used with the ordinary rule of mixture. The 2-D rule of mixture considering the Poisson’s ratio is based on Hooke’s law. The hybrid composites have two different Poisson’s ratios; the one is for the NWCT composites and the other for the CFRP composites. Fig. 2 shows the schematic figure for the explanation of the 2-D rule of mixture. The following equation is obtained from the equilibrium condition of the y-direction. sPy ⫻tP⫽sTy ⫻tT

(1)

Here, sPy and sTy are the stresses caused by the difference of Poisson’s ratios. tP and tT are the thickness of CFRP and NWCT composites, respectively. Eq. (2) is obtained from the compatibility between CFRP and NWCT, and Eq. (3) from the equilibrium of hybrid composites in the loading direction. eHx ⫽ePx ⫽eTx , eHy ⫽ePy ⫽eTy sHx ⫽

Px tPsPx tTsTx ⫹ ⫽ W(tP+tT) tP+tT tP+tT

(2) (3)

The Young’s modulus EHx and Poisson’s ratio vHxy of hybrid composites defined by Eq. (4) can be obtained from Eqs. (1)–(3) and the 2-D Hooke’s law with the Poisson’s ratios vPxy and vT. EHx ⫽

sHx H eHy , v ⫽⫺ eHx xy eHx

Fig. 2.

Explanation for two-dimensional rule of mixture.

(4)

The materials used in this study are unidirectional CFRP-prepreg (SK-Chemicals, USN125 Series) and NWCT of 12 g/m2 (TFP, Optimat 203 Series). The Hybrid composites are made by stacking the NWCT and CFRP prepregs. Thirteen kinds of composites were studies; i.e. NWCT composites, CFRP longitudinal [0]8, transverse [90]12, off-axis [45]12 and angle-ply [±45]3S, hybrid longitudinal ([0/0]4, [/0/0/]4), transverse ([90/90]6, [/90/90/]6), off-axis ([45/45]6, [/45/45/]6), and angle-ply ([+45/⫺45]3S, [/+45/⫺45/]3S). The symbol ‘/’ means that the NWCT is located between the CFRP layers. The [0/0]4, [90/90]6, [45/45]6 and [+45/⫺45]3S hybrid composites are designated as the A-type, and the [/0/0/]4, [/90/90/]6, [/45/45/]6 and [/+45/⫺45/]3S as the B-type. The processing lay-ups of the hybrid composite laminates are outlined in Fig. 3. The CFRP and hybrid laminates were cured in an autoclave under the same curing cycle. The specimens were cut and polished according to the ASTM standard [13]. The cut edges of specimens were polished for microscopic observations. The specimens were end-tabbed for tensile testing, bonding Eglass tabs of 50 mm length and 1.6 mm thickness with a high strength epoxy adhesive. Moreover, an emery paper(#200) was interleaved between a specimen and tab to suppress a stress concentration from the end-tab in transverse [90]12, off-axis [45]12 and angle-ply [±45]3S of the CFRP and hybrid specimens except for longitudinal [0]8. The geometry of tensile and fatigue test specimens is given in Fig. 4. Fig. 5 shows cross-sections of the CFRP and hybrid specimens. The fiber volume fractions Vf [14] in the NWCT and CFRP layers are about 0.11 and 0.65, respectively, in a fully consolidated composite. 3.2. Experimental procedures The tensile test was performed using a screw driven test machine (Shimadzu, AG5000A) at constant crosshead rates of 2.0 and 3.0 mm/min. The results of tensile tests were determined using the data of five samples per type. An extensometer and strain gauge were used for the tensile properties. An extensometer with a gauge length of 50 mm and strain gauge (L-type rosettes) with a gauge length of 1 mm were attached to the center of specimen to measure the strain response. Chord modulus was used in this study, and the tensile Young’s modulus is defined as a gradient of stress versus strain. The fatigue tests were investigated with a servohydraulic test machine by Shimadzu servopulser (4825)control system. The results of fatigue tests were determined, using the data of 7–10 samples per type. The tension–tension fatigue tests were performed in a load-con-

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Fig. 3.

399

Processing lay-ups of CFRP and hybrid composite laminates.

temperature. The method of least squares was used to determine the coefficients for the S–N curve formula, which was supposed linear in S–log N. The fracture surfaces and the side surface of the specimens during the fatigue were observed with an optical microscope, and then the failure mechanism was discussed.

4. Results and discussion 4.1. Tensile properties

Fig. 4.

The geometry of tensile and fatigue tests specimen.

trolled mode with a sinusoidal load waveform [15]. The fatigue tests were conducted at a frequency of 10 Hz with a constant stress ratio of R(smin/smax)=0.1. Most S– N relationships were obtained in the whole range of fatigue loading to Nf=106 cycles. Some tests were continued until the specimen failed in order to establish S– N curves. The other tests were terminated at specified stages of fatigue life to allow measurements of residual tensile strength and stiffness as well as the investigations of damage mechanism. All tests were performed at room

Tensile stress–strain curves of CFRP composites are not linear [16]. The tensile Young’s modulus of CFRP composites is defined as the slope of the stress–strain curve in the initial linear region. Chords are made by selecting any two strain points on the tensile curve. Since the curve always gets steeper with increasing loads or strains, it is imperative that the readings are taken at certain fixed points. The chord moduli of micro strain between 1000 and 6000 or between 1000 and 3000 were used in the tensile tests. The tensile properties of NWCT composites are shown in Table 1. Fig. 6 and Table 2 show the stress–strain curve and tensile properties for the longitudinal CFRP and hybrid specimens. The Young’s modulus and strength of longitudinal A-type specimen are about 27% and 31% lower than those of the CFRP specimen, respectively, and in the B-type specimen about 32% and 31% lower. The comparisons between CFRP and hybrid specimens show that the Young’s modulus and strength of the NWCT specimen are lower than those of the CFRP specimen.

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Fig. 5.

Sections of CFRP and hybrid specimens. (a) CFRP specimen; (b) hybrid specimen.

Table 1 Tensile properties of NWCT composites NWCT (g/m2)

Poisson’s ratio n

Young’s modulus E (GPa)

Tensile strength s (MPa)

Fiber volume fraction Vf (%)

12

0.41

14

215

11

Fig. 7.

Fig. 6.

Mean longitudinal [0]8 tensile stress–strain curve.

Table 2 Longitudinal Young’s modulus and strength of hybrid laminates Longitudinal [0]8

CFRP specimen A-type Ordinary hybrid and 2-D, R.O.M Experiment B-type Ordinary hybrid and 2-D, R.O.M Experiment

Poisson’s ratio (n12)

0.30 0.33

Young’s Tensile modulus E1 strength s1 (GPa) (MPa) 146 106

Mean transverse [90]12 tensile stress–strain curve.

The ordinary and 2-D rule of mixtures were used in order to estimate the longitudinal stiffness and strength of hybrid specimens, where it is supposed that the fracture strain of hybrid specimens are the same as that of the CFRP specimen. The estimated values are compared with the experimental results in Table 2. The estimated values of longitudinal hybrid specimen stiffness are almost equal to experimental results. Fig. 7 and Table 3 show the stress–strain curve and tensile properties for transverse CFRP and hybrid specimens. The Young’s modulus and strength of the transverse A-type specimen are about 19% and 37% higher Table 3 Transverse Young’s modulus and strength of hybrid laminates Transverse [90]12

Young’s modulus E2 (GPa)

Tensile strength s2 (MPa)

CFRP specimen A-type hybrid Ordinary R.O.M 2-D, R.O.M Experiment B-type hybrid Ordinary R.O.M 2-D, R.O.M Experiment

9.4 10.9 11.6 11.2 11.0 11.9 11.4

70 – – 96 – – 116

2600 1800

0.33 0.34

107 99

1800 1800

0.34

99

1800

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Fig. 10. Fig. 8.

401

Mean angle-ply [±45]3S tensile stress–strain curve.

Mean off-axis [45]12 tensile stress–strain curve.

than those of the CFRP specimen, respectively, and in the B-type specimen about 21% and 66% higher, respectively. The transverse Young’s modulus and strength of A and B-type hybrid specimens are improved by the interleaving NWCT. In particular, the transverse strength of B-type hybrid specimen is markedly increased. It seems that the transverse hybrid specimens have higher Young’s modulus and strength than that of the CFRP by the reinforcing effect of NWCT. The ordinary and the 2-D rule of mixtures were used to estimate stiffness of the transverse hybrid specimens. The estimated results were compared with those experiments and the 2-D rule of mixtures, and the validity is confirmed. Fig. 8 shows the stress–strain curve for off-axis CFRP and hybrid specimens. The Young’s modulus and strength of the off-axis A-type specimen are about 5% and 44% higher than those of the CFRP specimen, respectively, and in the B-type specimen about 6% and 60% higher, respectively. As expected from the result of the transverse specimen, the strength of hybrid specimens is markedly increased. Fig. 9 shows the shear stress–strain curve for off-axis

CFRP and hybrid specimens. The chord shear modulus and 0.2%-offset strength [17] of the off-axis hybrid specimens were compared with the CFRP specimen. The results shows that the shear moduli of A and B-type hybrid specimens are nearly the same as that of the CFRP, and the 0.2%-offset shear strengths are about 17% and 23% higher, respectively. Moreover, the maximum shear strengths of A and B-type hybrid specimens are about 44% and 60% higher than that of the CFRP, respectively. The off-axis hybrid specimens seem to be effective to improve the shear properties. Fig. 10 and Table 4 show the stress–strain curves and tensile properties for angle-ply CFRP and hybrid specimens. The Young’s moduli of A and B-type angle-ply hybrid specimens are lower than that of the CFRP specimen, and the tensile strengths are both about 20% lower, respectively. In order to estimate the stiffness of angleply hybrid specimens, the ordinary and 2-D rule of mixtures were used. The estimated values were compared with the experimental results in Table 4, and the estimated stiffness values of angle-ply hybrid specimens are almost equal to experimental results. Fig. 11 shows the shear stress–strain curve for angleply CFRP and hybrid specimens. The chord shear modulus and 0.2%-offset shear strength [18] of the angle-ply hybrid specimens were compared with those of CFRP specimen. The shear moduli of A and B-types hybrid Table 4 Tensile Young’s modulus and strength of hybrid angle-ply laminates

Fig. 9. Shear stress–strain relation and 0.2%-offset strength from offaxis [45]12 tests.

Angle-ply [±45]3S

Young’s modulus Ex (GPa)

Tensile strength sx (MPa)

CFRP specimen A-type hybrid Ordinary R.O.M 2-D, R.O.M Experiment B-type hybrid Ordinary R.O.M 2-D, R.O.M Experiment

17.5 16.4 17.0 17.0 16.3 16.9 16.7

230 – – 185 – – 184

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Fig. 11. Shear stress–strain relation and 0.2%-offset strength from angle-ply [±45]3S tests.

Fig. 13.

S–N curves of [45]12 off-axis composites.

Fig. 12. S–N curves of [90]12 transverse composites.

specimens are nearly the same as that of the CFRP specimen, however the 0.2%-offset strengths are about 6% and 8% higher by the NWCT, respectively.

Fig. 14.

S–N curves of [±45]3S angle-ply composites.

4.2. Fatigue characteristics The fatigue strength at Nf was defined as the intersect point stress of the S–N formula and Nf values. Fig. 12Table 5 show the S–N curves and the fatigue strengths Table 5 Fatigue strength and tensile strengtha Specimen

Fatigue strength at Nf=106, S (MPa) Transverse Off-axis [45]12 Angle-ply [±45]3S [90]12

NWCT CFRP A-type hybrid B-type hybrid

125(215) 45(70) 60(96) 70(116)

a

Number in (); tensile strength.

125(215) 60(97) 80(140) 90(155)

125(215) 155(230) 110(185) 110(184) Fig. 15. Matrix crack densities in the CFRP [±45]3S and hybrid specimens under stress smax=110 MPa.

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Fig. 16. Side section of angle-ply CFRP and hybrid specimens with matrix cracks [(a), (c) and (e)], delamination [(b), (d) and (f)] and NWCT matrix crack [(d) and (f)] under maximum stress smax=110 MPa.

at Nf=106 of the NWCT composites and three kinds of the [90]12 transverse specimens, respectively. It is found from Fig. 12 that the fatigue strengths at Nf=106 of the A and B-types hybrid specimens are about 33% and 55% higher than that of the CFRP specimen. The fatigue strengths of transverse hybrid specimens are improved by the NWCT. Especially, the results show that the B-type hybrid specimen has the best fatigue performance in the range investigated. From Fig. 12, the fatigue ratio smax/sx(90) at Nf=106 is nearly equal to 0.6. The distri-

butions of the fatigue ratios in the transverse hybrid specimens are nearly as same as that of the NWCT composites. Fig. 13 and Table 5 show the S–N curve and the fatigue strength at Nf=106 of three kinds of the [45]12 off-axis specimens. The fatigue strength at Nf=106 of the A and B-type hybrid specimens are about 33% and 50% higher than that of the CFRP specimen, respectively. As expected from the result of the transverse specimen, the fatigue strength of the B-type specimen increases remarkably. Also the distributions of the fatigue ratios

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(smax/sx (off-axis 45)) in the off-axis hybrid specimens are nearly the same as that of the NWCT composites. Fig. 14 and Table 5 show the S–N curve and the fatigue strength at Nf=106 of three kinds of the [±45]3S angle-ply specimens. The fatigue strength at Nf=106 of the A and B-types hybrid specimens are both about 29% lower than that of the CFRP specimen. The fatigue ratios (smax/sx (angle-ply45)) in the angle-ply hybrid specimens are also nearly that of the NWCT composites. As in the case of the transverse and off-axis specimens, the S–N relations of angle-ply hybrid composites can be estimated by the fatigue characteristics of the NWCT composites and the tensile strength of angle-ply hybrid composites. Fig. 15 shows the matrix crack densities of the angleply CFRP and hybrid specimens under the maximum stress smax=110 MPa; the density is defined as the average number of matrix cracks in each CFRP-prepreg layer per unit length in the longitudinal direction. Fig. 16 shows the side surface of angle-ply CFRP and hybrid specimens with matrix crack, delamination and NWCT matrix crack under maximum stress smax=110 MPa. As shown in Fig. 16(a), the matrix crack appears in CFRP specimens during the N=103 cycles. On the other hand, the matrix cracks are not observed in the A and B-type hybrid specimens during the N=8×104 cycles. Fig. 16(c) and (e) show that the matrix crack of the hybrid specimen is observed at 105 cycles, and gradually increased from the N=2×105 cycles. The matrix cracks of both the angle-ply CFRP and hybrid specimens are nucleated in the central 45° plies of the angle-ply specimen. As shown in Fig. 16(d) and (e), the delaminations of CFRP and A-type hybrid specimens are observed to appear from the specimen surface at about N=3×104 and N=4×105 cycles, respectively. However, the B-type hybrid specimen is not observed at the N=8×105 cycles. Such a phenomenon is different from the characteristics of the NWCT, and matrix cracks appear early in the angle-ply specimen because of the shear characteristics of CFRP specimen. The damage initiation is delayed by the interleaving NWCT in the angle-ply hybrid specimens. The residual strength and stiffness of angle-ply specimens after N=106 cycles under smax=110 MPa are shown in Fig. 17. The cycle evolution results in the accumulation of multi-damage, rather than in the propagation of a single crack, which in turn progressively reduces the residual strength as well as the Young’s modulus. The result shows that the residual tensile strength and stiffness of CFRP specimen are about 17% and 23% lower than those before the fatigue, respectively. On the other hand, the residual tensile strength and stiffness of the Atype hybrid specimens are about 1% and 3% lower than those before the fatigue, respectively. Also, the B-type hybrid specimen is about 17% and 10% lower than those before the fatigue, respectively. Although the residual

Fig. 17. Comparison of residual strength and stiffness after 106 cycles under smax=110 MPa with those before fatigue. (a) Residual strength; (b) residual stiffness.

tensile strengths of A and B-type specimens are about 4% and 19% lower than those of the CFRP specimen, the residual tensile stiffnesses are about 22% and 11% higher than that of the CFRP specimen, respectively. The Mode II interlaminar fracture toughness of the hybrid specimens is about 3 times as high as the CFRP. From the results and characteristics of Fig. 10, it seems that the NWCT in the hybrid specimens delayed the matrix crack and delamination.

5. Conclusions The tensile properties and tension–tension fatigue characteristics of the hybrid composites made by NWCT have been studied experimentally. The results of hybrid composites have been compared with those of the CFRP

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composites. The conclusive remarks can be summarized as follows: 1. The two-dimensional rule of mixtures provides more precise estimation of the tensile stiffness for hybrid composites. The tensile properties of hybrid composites, the longitudinal Young’s moduli and strengths of hybrid specimens are lower than those of the CFRP specimen. However, the transverse and offaxis hybrid specimens are higher, respectively. The 0.2%-offset strengths of off-axis and angle-ply hybrid specimens are also improved by the NWCT. 2. The tension–tension fatigue strength of transverse and off-axis hybrid specimens is improved. However, the strength of angle-ply hybrid specimens is lower than that of CFRP specimens. Despite the decreased tensile and fatigue strength, the residual stiffness and strength behavior of the angle-ply hybrid specimens are better than the CFRP specimen. The matrix crack of angle-ply specimens appears from the central 45° plies, delaminations appear from the surface, and the damage initiation in the hybrid angle-ply specimen is delayed by the NWCT.

Acknowledgements The authors are grateful for the advice and assistance of Dr. Seung-Gyu Lim of SK-Chemicals Co. Ltd in Korea. References [1] Swanson SR. Introduction to design and analysis with advanced composite materials. Englewood Cliffs, NJ: Prentice Hall, 1997.

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