Materials and Design 36 (2012) 75–80

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Materials and Design

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Hybrid composite laminates reinforced with glass/carbon woven fabricsfor lightweight load bearing structures

Jin Zhang, Khunlavit Chaisombat, Shuai He, Chun H. Wang ⇑Sir Lawrence Wackett Aerospace Research Centre, School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Melbourne, VIC 3083, Australia

a r t i c l e i n f o

Article history:Received 6 September 2011Accepted 3 November 2011Available online 12 November 2011

Keywords:A. CompositesB. LaminatesH. Selection for material properties

0261-3069/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.matdes.2011.11.006

⇑ Corresponding author. Tel.: +61 3 99256115; fax:E-mail address: chun.wang@rmit.edu.au (C.H. Wan

a b s t r a c t

Light-weight structure utilising novel design and advanced materials is one of the keys to improving thefuel efficiency and reducing the environmental burden of automotive vehicles. To ensure the low cost ofapplying fibre-reinforced materials in automotive vehicles, it is proposed to selectively incorporate car-bon fibres to enhance glass fibre composites along main loading path. This paper investigates the influ-ences of stacking sequence of on the strength of hybrid composites comprising materials with differingstiffness and strength. Hybrid composite laminates were manufactured using varying ratio of glasswoven fabric and carbon woven fabric in an epoxy matrix. Static tests including tension, compressionand three-point-bending were carried out to composite coupons containing various ratios of carbonfibres to glass fibres. The results show that hybrid composite laminates with 50% carbon fibre reinforce-ment provide the best flexural properties when the carbon layers are at the exterior, while the alternatingcarbon/glass lay-up provides the highest compressive strength. The tensile strength is insensitive to thestacking sequence. Analytical solutions are also developed and are shown to provide good correlationwith the experimental data, which allow the optimisation of stacking sequence of hybrid compositesto achieve the maximum strength.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

The shortage of fossil fuel supply and the new imperative ofenvironmental sustainability to combat global warming have ex-erted tremendous pressure on transforming the current materialsdesign and manufacturing technologies for human transportation(air, land and sea). Lightweight is rapidly becoming a necessityfor structures, as a result of its less energy consumption from thevehicles which take into account of this critical issue. While plasticand composite materials are used in automobiles today, they con-stitute only approximately 7.5% of total vehicle mass and the appli-cations are generally not for the primary vehicle structure [1].Important reasons for the growing adoption of polymer matrixcomposites [2] are the reduced weight (30–40% lighter than steelparts of equal strength), part consolidation opportunities, designflexibility, reduced tooling cost, better damage and corrosion resis-tance, material anisotropy and improved internal damping, etc.

Although the great advantages of polymer matrix compositeshave been recognised by automotive industry, there exist a numberof critical challenges before their wide use in primary automotivestructures. Concerns about high material costs, slow productionrates and recyclability and the auto-industry’s lack of experience

ll rights reserved.

+61 3 9925 6108.g).

with composite materials, are the main problems automakers arefacing currently [1,3,4]. The most popular reinforcements for poly-mer matrix composites are carbon and glass fibres. It has been esti-mated that the use of glass fibre reinforced polymer matrix (GFRP)composites as structural components could yield a 20–35% reduc-tion in vehicle weight; more significantly, the use of carbon fibrereinforced polymer matrix (CFRP) composites could yield a 40–60% weight reduction [5]. While fibreglass composites have beenincreasingly used to replace steel in automotive industry [6,7],the adoption rate for carbon fibre composites which are much light-er, stronger and stiffer than glass fibre composites, remains low. Themain reason is the high cost of carbon fibres; that is also the reasonwhy CFRP composites are only popular in concept and luxury carsand aerospace vehicles. Different from the aerospace sector, whereadditional payload capacity and engine efficiency are the para-mount issues, the high-volume production of automotive industryresults in the most important consideration of being cost-effectivethat overwrites any technical concerns [8]. Cost reduction continu-ously to be the number one challenge facing automobile manufac-turers while fuel economy and technology transformation arebecoming more important [9].

To further reduce vehicle weight without excessive cost in-crease, one technique is to incorporate carbon fibre reinforcementinto glass fibre composites and innovatively design by selectivelyreinforcing along the main load path [10–12]. Hybrid composites

0.0 0.5 1.0 1.5 2.00

100

200

300

400

500a

5

4

3

2

1

1: [C]8 2: [C2G2]s 3: [CG3]s 4: [C/G/C/G]s 5: [G]8

Tens

ile S

tress

(MPa

)

Tensile Strain (%)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.50

100

200

300

400

500

5

432

1

b 1: [C]8 2: [C2G2]s 3: [CG3]s 4: [C/G/C/G]s 5: [G]8

Com

pres

sive

Stre

ss (M

Pa)

Compressive Strain (%)

76 J. Zhang et al. / Materials and Design 36 (2012) 75–80

are composed of more than one type of reinforcement and they canbe classified into interply or laminated hybrid, intraply or tow-by-tow hybrid, intimately mixed hybrid, and other types of mix-tures. Some of the influential factors that affect the compositemechanical performance from the reinforcement include thelength of fibre, fibre orientation, fibre shape and fibre material[13,14]. Since the mechanical properties of glass and carbon fibres,and the interfacial properties between reinforcement and matrixdiffer greatly, the hybridisation effects would vary too for the hy-brid composites [15–17]. A comprehensive review on the mechan-ical properties of hybrid fibre reinforced plastics especially onglass/carbon hybrid composites showed that the longitudinal ten-sile moduli are generally in good agreement with the rule of mix-ture, which is due to the strain compatibility through the thicknessof a hybrid material [18]. The review also showed that the shape ofthe stress–strain curves for hybrid composites varies with the dif-ferent type of fibres and resins used, the overall fibre fraction andthe orientation of the reinforcement in the material. Positive hy-brid effect was observed in flexure tests of specimens with highglass versus carbon fibre ratio [19]; however, the compressionand fracture energy of glass/carbon epoxy matrix compositesexhibited negative hybrid effect [18]. In a recent study, the tensileand compressive loading cases on hybrid glass/carbon fabric com-posites have been investigated and the main observation of thiswork is that by placing glass fabric layers in the exterior and car-bon fabric layers in the interior, higher tensile strength and ulti-mate tensile strain were obtained [20].

In the present study, E-glass and carbon fibre woven fabricswith epoxy resin matrix have been used for fabricating hybrid lam-inates. Different glass/carbon ratios and stacking sequences wereinvestigated against the tensile, compressive and flexural re-sponses of hybrid composite laminates.

Fig. 1. Tensile and compressive stress–strain curves for different compositelaminates.

2. Experimental procedure

Colan™ E-glass plain weave fabric and Sigmatex™ carbon 2/2twill weave fabric (T300, 3 K Tow, 199 GSM) were used to reinforceWest system epoxy 105 cured with slow hardener 206. Wet lay-upwas applied to fabricate laminates with five different lay-upschemes: [C]8, [C2G2]s, [CG3]s, [CGCG]s and [G]8, where C and G de-note carbon fibre and glass fibre respectively. The composite lam-inates were cured at ambient temperature for 24 h before cut intospecimens for mechanical tests. The overall fibre weight fraction ofthe composites was approximately 45%. The ASTM D3039-76 [21]and ASTM D 790 [22] standards were used for tension and three-point-bending tests; the compression tests were performed usinga NASA short block compression fixture. The thickness of the curedcomposite laminates was between 2.5 mm and 3.5 mm. The spec-imen dimensions were 250 mm � 25 mm for tensile and were100 mm � 25 mm for flexural tests. The compression specimenswere grinded to ensure perfectly horizontally parallel top and bot-tom edges, with the dimension of 52 mm � 25 mm. The opticalmicrographs were taken from a Leica EZ4D and a DP 71 opticalmicroscope.

3. Results and discussion

3.1. Hybrid composites under tensile and compressive loading

With the variation of composition, the density for five types ofcomposites varied from 1.508 g/cm3 for the plain glass fibre com-posite [G]8 to 1.460 g/cm3, 1.327 g/cm3, 1.316 g/cm3 and 1.237 g/cm3

for the [CG3]s, [C2G2]s, [CGCG]s and [C]8 composites, respectively.The addition of carbon fibre reinforcement reduced the density ofthe composites. The tensile and compressive stress–strain curves

for all five types of composite laminates are shown in Fig. 1 andthe calculated mechanical property data are included in Table 1.As expected, the [C]8 composite had the highest tensile and com-pressive strength and the [G]8 composite had the lowest tensileand compressive strength, where the average tensile and compres-sive strength of plain glass fibre composites account for almost 50%of the plain carbon fibre composites for both tension and compres-sion loading cases. The compressive strength of [C]8 was 38% lowerthan its tensile strength and the compressive strength of [G]8 was42% lower than its tensile strength. At the same glass/carbon fibreratio, [C2G2]s and [C/G/C/G]s showed similar tensile strength;however, higher compressive strength and strain were found forthe [C/G/C/G]s than the [C2G2]s. With the addition of 25% of carbonfibres in the exterior layers, [CG3]s exhibited only slight improve-ment in both tensile and compressive strength than plain glasscomposite [G]8. The compressive strain resulted from the alternat-ing lay-up scheme ([CGCG]s) showed the highest value above allother composites. The enhanced compression strength for thealternating stacking sequence may be caused by the bridging effectof the carbon fibre layer between the failed glass fibre layers.Kretsis has concluded that in hybrid composites, the weakestlow-elongation fibres break first to form cracks that are ‘bridged’by the surrounding high-elongation composite, thus allowing thestronger low-elongation fibres to reach their ultimate strength[18]. A similar explanation has also been given by Manders whopointed out that the hybrid effect arises from a failure to realisethe full potential strength of the fibres in all-carbon fibre compos-ites, rather than from an enhancement of their strength in thehybrids [23].

Table 1Mechanical properties of composite laminates under tensile and compressive loading.

Lay-upscheme

Tensile strength(MPa)

Ultimate tensilestrain (%)

Young’s modulus fortension (GPa)

Compressionstrength (MPa)

Ultimate compressivestrain (%)

Young’s modulus forcompression (GPa)

[C]8 420 (±57) 1.07 (±0.13) 38.39 (±2.43) 260 (±12) 3.06 (±0.21) 10.97 (±0.18)[C2G2]s 260 (±7) 1.18 (±0.06) 22.04 (±1.50) 171 (±38) 2.63 (±0.30) 7.40 (±0.83)[CG3]s 206 (±9) 1.41 (±0.14) 18.09 (±0.54) 189 (±33) 3.01 (±0.20) 7.72 (±1.59)[C/G/C/G]s 263 (±11) 1.40 (±0.38) 22.27 (±0.84) 217 (±42) 3.13 (±0.21) 8.22 (±1.75)[G]8 200 (±3) 1.87 (±0.29) 11.01 (±2.11) 117 (±8) 2.76 (±0.20) 5.14 (±0.17)

J. Zhang et al. / Materials and Design 36 (2012) 75–80 77

The linear rule of mixture (ROM) was used for calculating thetensile and compressive strength. Denoting the carbon fibre ratioof all fibre reinforcement is a, the Young’s modulus can be ex-pressed using the simple rule of mixtures as:

E ¼ aEC þ ð1� aÞEG ð1Þ

where E denotes the modulus of hybrid composite, EC denotes themodulus of carbon fibre composite and EG denotes the modulus ofglass fibre composite. So the stress (r) of the hybrid compositecan be expressed as

r ¼ ðECtC þ EGtGÞec=ðtC þ tGÞ ð2Þ

Assuming a equals to the percentage of the thickness of carbonlayers (tC) in the whole laminate (tC + tG) and the composite failswhen the elongation of laminates reaches ec. However, in real case,the glass fibres can still carry load until the ultimate strain of glassfibre composite is reached. Therefore the maximum stress of hy-brid composites is

300

400

500

5

4

3

2

1

1: [C]8 2: [C2G2]s 3: [CG3]s 4: [C/G/C/G]s 5: [G]8

l Stre

ss (M

Pa)

rmax ¼ max ecðECtC þ EGtGÞ=ðtC þ tGÞ; EGtGeG=ðtC þ tGÞ½ � ð3Þ

As it can be seen from Fig. 2, the tensile strength results agreedwell with analytical results; however, the compressive strengthexhibited a negative hybridisation effect, where the hybridisationeffect is shown by the deviation from the ROM behaviour. Steva-novic et al. found similar results when they investigated the tensileproperties of unidirectional carbon fibric /non wovon glass mat/polyester resin hybrid composite laminates. The tensile moduli oftheir tested composites agree with values calculated by the ROM[19]. The ROM could not demonstrate the difference in the com-pressive strength of [C2G2]s and [CGCG]s composites, due to thesame carbon/glass fibre ratio.

0 20 40 60 80 1000

100

200

300

400

500

[CGCG]s

[C2G2]sMax

imum

Stre

ss (M

Pa)

Carbon Fibre Ratio (%)

σT-experimental

σT-analytical

σC-experimental

σC-analytical

Fig. 2. Tensile and compressive strength of composite laminates affected by thecarbon fibre ratios in the reinforcement (both experimental and analytical results).rT is the tensile strength and rC is the compressive strength.

3.2. Hybrid composites under flexural loading

The flexural stress–strain curves for all five types of compositelaminates are shown in Fig. 3 and the calculated mechanical prop-erty data are included in Table 2. As expected, the [C]8 compositehad the highest flexural strength and lowest flexural strain, whichis the opposite case to the [G]8 composite. Placing two carbon lay-ers at the exterior effectively increased the flexural strength, asshown in the stress–strain curve for the [C2G2]s composite. Theflexural strength of the [C2G2]s was 406 MPa, accounting for 89%of the plain carbon fibre laminate [C]8. By placing the high stiffnesscarbon fibre away from the neutral axis and the low stiffness glassfibre at the neutral axis, the flexural modulus is enhanced signifi-cantly, which is also proved by other researchers work [24].

Based on the plane section assumption, the strain distributionin the laminate can be expressed in terms of the curvature j, withthe origin of the coordinate being at the mid plane of the compos-ite laminate,

eðzÞ ¼ �jz ð4Þ

where z denotes the coordinate along the thickness direction(Fig. 4). Different fibre reinforced composite plies can exhibit signif-icantly different failure strains. For instance, unidirectional glass/polyester laminae can reach an axial failure strain of approximately

0.0 0.5 1.0 1.5 2.0 2.5 3.00

100

200

Flex

ura

Flexural Strain (%)

Fig. 3. Flexural stress–strain curves for different composite laminates.

Table 2Mechanical properties of composite laminates under flexural loading.

Lay-upscheme

Flexural strength(MPa)

Ultimate flexuralstrain (%)

Flexural modulus(GPa)

[C]8 455 (±35) 1.69 (±0.04) 29.03 (±2.09)[C2G2]s 406 (±17) 1.68 (±0.04) 27.31 (±1.19)[CG3]s 339 (±15) 1.78 (±0.05) 21.98 (±0.91)[C/G/C/G]s 348 (±9) 1.81 (±0.08) 22.47 (±0.70)[G]8 218 (±9) 3.00 (±0.10) 11.12 (±0.46)

Fig. 4. The rectangular beam under bending.

0 20 40 60 80 1000

100

200

300

400

500

600

[CGCG]s

[C2G2]s

Max

imum

Stre

ss (M

Pa)

Carbon Fibre Ratio (%)

σF-experimentalσF-analytical

Fig. 5. Flexural strength of composite laminates affected by the carbon fibre ratiosin the reinforcement (both experimental and analytical results).

78 J. Zhang et al. / Materials and Design 36 (2012) 75–80

2%, the strain-to-failure for type I carbon/epoxy unidirectional lam-inae is approximately 0.5% [25], and the natural fibre/epoxy com-posite strain can reach 5.2% when jute is used as reinforcement[26]. So for a hybrid composite laminate under pure bending load,the maximum bending moment it can carry corresponds to the loadwhen a certain ply in the laminate reaches its failure strain. Due tothe potentially very large difference in failure strains of differenttypes of composite materials, critical location may not always bethe surface ply. The maximum curvature at failure of the laminate is

a b

d

Fig. 6. Optical micrographs of failed composite laminates under tensile loading. (a) [C]8

failed tensile specimens and the rest show the facture surfaces.

jmax ¼minecðziÞ

zi;etðziÞ

zi

� �; i ¼ 1;2;3 . . . N ð5Þ

where ec(zi) denotes the strain of the ith ply under compression andet(zi) denotes the stress of the ith ply under tension. It is clear thatmaximum deformation (curvature) occurs when plies with thehighest failure strain are placed close to the surface, far removedfrom the neutral axis.

The bending moment of a beam with rectangular section is gi-ven by,

M ¼ �Z t=2

�t=2rxðzÞzBdz ¼ j

Z t=2

�t=2zEðzÞBdz ð6Þ

where t is the thickness of the beam, B is the width of the structure,rx(z) is the stress either in compression or tension and E(z) denotesthe Young’s modulus of the material at coordinate z. It can be seenfrom Eq. (6) that the maximum bending moment of a hybrid lami-nate depends on through-thickness distribution of the strain-to-failure and lamina stiffness. Considering a symmetric laminate witheight plies, the moment is

M ¼ B2

XN

i¼1

ðt2i � t2

i�1Þ½EðiÞT þ EðiÞC �j ð7Þ

where ti denotes the locations of the ith ply of a laminate containinga total of 2N plies in a symmetric lay-up. Because for a rectangularsection, the maximum flexural stress can be written as

rmax ¼ M=S ¼ M=ðBt2=6Þ ¼ 6M=Bt2 ð8Þ

where S is the elastic section modulus. The maximum flexural stresscan be determined as

rmax ¼ ð3=t2ÞXN

i¼1

t2i � t2

i�1

� �½EðiÞT þ EðiÞC �j ð9Þ

If 50% of the reinforcement is carbon fibre, the maximum flex-ural stress can be obtained when two carbon fibre composite layersare located at the exterior of the laminate. Both the experimentaland analytical flexural strength data are shown in Fig. 5. The ana-lytical solutions were able to simulate the experimental trendinfluenced by hybrid composition and stacking sequence; how-ever, their calculated results were higher than the experimentaldata due to the shear stresses presented in the specimens which

c

e

, (b) [C2G2]s, (c) [CG3]s, (d) [CGCG]s and (e) [G]8. (b) and (c) show the edge views of

a

b c

d e

Fig. 7. Optical micrographs of failed composite laminates under compressive loading. (a) [C]8, (b) [C2G2]s, (c) [CG3]s, (d) [CGCG]s and (e) [G]8.

a b c

d e

Fig. 8. Optical micrographs of failed composite laminates under flexural loading. (a) [C]8, (b) [C2G2]s, (c) [CG3]s, (d) [CGCG]s and (e) [G]8.

J. Zhang et al. / Materials and Design 36 (2012) 75–80 79

caused additional displacements and led to reduced modulus. Forthe low span-to-depth ratios and high modulus materials, theseshear stresses are more significant [18,19].

3.3. Failure modes of hybrid composite laminates

The failed composite specimens were investigated under opticalmicroscopes. Fig. 5a, d and e presents the tensile fracture surface ofthe [C]8, [CGCG]s and [G]8 composites. The fibre pull-outs of theplain glass fibre composite were more significant than the plaincarbon fibre composite [C]8 and the alternating carbon/glass fibrelaid-up composites [CGCG]s. In comparison, the composites withone or two carbon fibre layers at the exterior did not break intotwo halves, with glass fibres bridging the ruptured carbon surfacelayers (Fig. 6b and c shows the edge view of the tensile specimens).The optical micrographs in Fig. 7 display the compression failure ofdifferent types of composites. It can be seen that the carbon fibrelayers failed with a transverse catastrophic mode, wherever the

glass fibre layer failed with the typical kinking–slitting mode. Byapplying the alternating lay-up scheme, the compression failurewas effectively suppressed. Fig. 8 presents the cross-sections ofthe failed flexural specimens along the central line of span. Theplain carbon fibre composite [C]8 exhibited brittle failure, wherethe specimen broke through all layers with abundant carbon fibrerupture. The other four types of composite laminates showed morematrix fracture other than reinforcement failure, in comparisonwith the compression failure modes. With the hybridisation ofthe reinforcement, the brittle and catastrophic failure mode ofthe all carbon fibre composite is avoided. It can be concluded thatimprovement in the balance of stiffness and toughness in compos-ite laminates can be realised through hybridisation [24,27].

4. Conclusion

Five types of composite laminates, i.e. [C]8, [C2G2]s, [CG3]s,[CGCG]s and [G]8 composites, were investigated under static load-

80 J. Zhang et al. / Materials and Design 36 (2012) 75–80

ing under tension, compression and three-point-bending. To effec-tively improve the tensile, compressive and flexural strength of theplain glass fibre composite, glass/carbon (50:50) fibre reinforce-ment was used either by placing the carbon layers at the exterioror by placing different fibre types alternatively. With the same hy-brid composition, the stacking sequence did not show noticeableinfluence on the tensile properties but affected the flexural andcompressive properties significantly. The current composite sys-tem exhibited more matrix failure under flexural loading and morereinforcement failure under compressive loading.

Acknowledgement

This work was supported by the ‘‘Lightweight Modular VehiclePlatform’’ project of Australian Cooperative Research Centre forAdvanced Automotive Technology (Auto CRC).

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