- Email: [email protected]
glass fibre hybrid composite
Composites Science and Technology 69 (2009) 2218–2224
Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Deformation micromechanics of a model cellulose/glass fibre hybrid composite Kenny Kong, Marek Hejda, Robert J. Young, Stephen J. Eichhorn * Materials Science Centre, School of Materials and the Northwest Composites Centre, Grosvenor Street, University of Manchester, Manchester M1 7HS, UK
a r t i c l e
i n f o
Article history: Received 16 March 2009 Received in revised form 3 June 2009 Accepted 9 June 2009 Available online 14 June 2009 Keywords: A. Glass fibres A. Hybrid composites B. Interface B. Mechanical properties C. Stress transfer D. Raman spectroscopy
a b s t r a c t Interfacial stress transfer in a model hybrid composite has been investigated. An Sm3+ doped glass fibre and a high-modulus regenerated cellulose fibre were embedded in close proximity to each other in an epoxy resin matrix dumbbell-shaped model composite. This model composite was then deformed until the glass fibre fragmented. Shifts of the absolute positions of a Raman band from the cellulose fibre, located at 1095 cm1, and a luminescence band from a doped glass fibre, located at 648 nm, were recorded simultaneously. A calibration of these shifts, for both fibres deformed in air, was used to determine the point-to-point distribution of strain in the fibres around the breaks in the glass fibre. Each break that occurred in the glass fibre during fragmentation was shown to generate a local stress concentration in the cellulose fibre, which was quantified using Raman spectroscopy. Using theoretical model fits to the data it is shown that the interfacial shear stress between both fibres and the resin can be determined. A stress concentration factor (SCF) was also determined for the regenerated cellulose fibre, showing how the presence of debonding reduces this factor. This study offers a new approach for following the micromechanics of the interfaces within hybrid composite materials, in particular where plant fibres are used to replace glass fibres. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Replacing glass with plant fibres in glass fibre reinforced plastics (GFRP) has many benefits, including cost, weight-saving opportunities, and the reduction of the wear on processing machinery [1]. Unfortunately, all too often, the mechanical properties of natural fibres do not match up to those of glass fibres. One solution to this problem is to replace only a fraction of the glass fibres, thus making a hybrid material. One of the first studies on a hybrid glass fibre/plant fibre composite was by Clark and Ansell [2]. This paper showed that the toughness of jute and glass fibre hybrid laminates was maximised when the plant material was sandwiched between the glass. Later studies have shown how banana [3] and sisal [4] can be used in combination with glass fibres in composites. A review of the impact properties of hybrid glass–plant fibre composites has recently been published by Santulli [5]. The deformation micromechanics of natural fibre composites have been studied extensively using Raman spectroscopy. The principle of the technique relies on the measurement of the position of a characteristic band within a Raman spectrum as a function of the tensile deformation of the fibre. This shift in the peak position is indicative of the direct molecular deformation of the polymer backbone [6]. The first demonstration of Raman band * Corresponding author. Tel.: +44 0 161 306 5982; fax: +44 0 161 306 3586. E-mail address: [email protected] (S.J. Eichhorn). 0266-3538/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2009.06.006
shifts in cellulose fibres was in 1997 by Hamad and Eichhorn on regenerated cellulose filaments [7] and this was followed by work on hemp and flax fibres [8]. In recent times it has been shown that the interface between a regenerated cellulose fibre and a range of resin materials can be determined using model composite systems [9–11]. It has also been shown that Raman spectroscopy can be used to follow the deformation micromechanics of wood fibres [12] and chemically-modified plant fibres [13]. Significantly less has been published on the direct measurement of stress or strain in glass fibre composites. Previous attempts to do this have focussed on the use of strain-sensitive coatings applied to the surface of the glass fibres, such as a polyurethanediacetylene [14] and carbon nanotubes [15]. Another approach has been to use half-fringe photoelastic measurements to obtain the local stress field in the resin material surrounding the fibre [16]. It has been shown that doping glass with samarium fluoride (SmF3) enables the local stress state of the material to be followed using either photo- or electro-luminescence [17]. Local residual stresses in optical fibres doped with Er3+ have also been determined using the same technique [18]. A recent study on the use of SmF3 to enable the local stress state in a glass fibre and epoxy resin composite to be followed using luminescence spectroscopy has been reported [19]. It was shown that the fragmentation of a glass fibre embedded in epoxy resin followed classical composite micromechanics theory. Using both Raman spectroscopy and lumi-
K. Kong et al. / Composites Science and Technology 69 (2009) 2218–2224
nescence spectroscopy, it is possible to simultaneously determine the local stress for both glass [19] and polymeric fibres [20]. It ought, therefore, to be possible to place a polymeric fibre and a glass fibre together in order to determine the effect of one component of the reinforcement on the other and, at a fibre break, the interfacial shear stress between a matrix material and both fibres. In this study, a model hybrid composite has been constructed by placing a regenerated cellulose fibre and a glass fibre next to each other in an epoxy resin matrix. By fracturing the glass fibre in the composite, it is possible to investigate the effect of a break in the neighbouring region upon an intact regenerated cellulose fibre. We show that it is possible to simultaneously map point-topoint strains along two very different fibres embedded in a transparent epoxy resin matrix. This offers a new approach for the study of hybrid composite materials.
2219
2.2. Raman and luminescence spectroscopy
2. Experimental
A Renishaw system 1000 spectrometer coupled to a 633 nm HeNe laser was used to record Raman spectra from the regenerated cellulose fibres. The laser was focused on the sample to a spot size of 2 lm using a 50 lens and an Olympus microscope system. All spectra were recorded using an exposure time of 30 s and four accumulations, and then fitted using a mixed Gaussian/Lorentzian function, and an algorithm based on the work of Marquardt [23] to find the peak position. The same Renishaw Raman system and Olympus microscope were also used to record luminescence spectra from the glass fibres, this time excited using a 514 nm Ar+ laser. It is important to note that Raman spectroscopy was not used, as the luminescence effect is a different phenomenon where the absolute wavelength is measured. All luminescence spectra were recorded using an exposure time of 5 s and three accumulations.
2.1. Materials and sample preparation
2.3. Spectroscopy and deformation
Glass fibres were prepared using a composition (wt.%) of 64% SiO2, 6.5% Al2O3, 1.6% CaO, 10.7% Na2O, 0.4% MgO, 16.3% B2O3, and 0.5% SmF3. This composition was then heated in a furnace to 1450–1500 °C, and glass rods were produced. The glass investigated in this study has the same composition as for samples previously reported [19]. The glass rods were then stretched in a gas flame and were then drawn into fibres. The regenerated cellulose fibre investigated was a commercial high-modulus type produced by ACORDIS (called BocellTM) using a non-derivitised source of cellulose dissolved in an anisotropic phosphoric acid solution [21]. The mechanical properties for both fibres have been previously reported, and are summarised in Table 1 [9,19]. It is worth noting that there is a significant difference in the breaking strain for the regenerated cellulose fibres (5.5%) and the glass fibres (0.8%). This difference was expected to lead to a fracture of the glass fibre in the model composite before the regenerated cellulose fibre. Young’s modulus of the regenerated cellulose fibre is about half that of the glass fibre. The hybrid model composite was prepared by embedding both fibres in an epoxy resin dumbbell composite, using a similar method to fragmentation samples, which have been described previously [22]. A schematic diagram of a typical sample is shown in Fig. 1a. Both fibres were placed by hand, with the aid of a light microscope, parallel to each other with a small separation distance between them. An epoxy resin, cured at room temperature for 7 days, was used for the matrix material; namely AralditeÒ LY/HY 5052 (Ciba-Geigy 5052). This resin comes in two-parts, a butan1, 4, -diol diglycidyl ether resin (LY 5052) and an isophorone diamine hardener (HY 5052) which were mixed in the ratio 50:19. The exact spacing between both fibres cannot be controlled, as the separation distance varied slightly during the curing cycle. A small strain gauge was fitted to the surface of the cured dumbbell specimens, using cyanoacrylate adhesive, to record the strain of the specimen proximal to the fibre.
In order to determine the fibre strain in a hybrid composite sample, both single filaments were first deformed in tension (see Fig. 1b for a schematic of the sample). Regenerated cellulose fibres were deformed in tension using strain increments of 0.2%. At each deformation step a Raman spectrum was taken from the cellulose fibres, and the position of a Raman band located at 1095 cm1 was monitored. Similarly, the glass fibre was deformed in tension using strain increments of 0.01%, and the position of a luminescence peak located at 648 nm was also monitored at each strain step. The dumbbell composite specimens (see Fig. 1a) were deformed in tension using a MINIMATTM Miniature Materials straining rig (Polymer Laboratories Ltd., UK), which was placed onto the microscope stage of the Raman spectrometer. The surface strain of the composite was determined using a strain gauge attached to the matrix. The Raman and luminescence spectra were recorded along the fibres within the composite in the vicinity of a break in the glass fibre, using the same exposure times and accumulations as mentioned in Section 2.2.
Table 1 Mechanical properties of regenerated cellulose and glass fibres, where d is the fibre diameter; E is the Young’s modulus, rf is the fibre breaking stress and ef is the breaking strain of the fibres [9,19]. d (lm)
E (GPa)
rf (GPa)
ef (%)
55.4 ± 1.0
2.6 ± 0.3
5.5 ± 1.4
99.0 ± 3.0
0.9 ± 0.1
0.8 ± 0.1
Regenerated cellulose fibre 11.83 ± 0.03 Glass fibre 55
3. Results and discussion 3.1. Raman and luminescence band shifts of the cellulose and glass fibres A typical shift in the peak position for the 1095 cm1 Raman band of a regenerated cellulose fibre is shown in Fig. 2a indicating direct molecular deformation of the cellulose backbone structure. A shift in the position of this peak has been observed in many studies of the micromechanics of different types of regenerated cellulose fibres [24–26]. The shift in the central position of the 1095 cm1 band is shown in Fig. 2b. The shift rate, determined from a linear fit to these data, was found to be 1.10 cm1 %1. This value of the shift rate is similar to one reported by Eichhorn et al. [24], and can be used to determine the point-to-point variation of the strain of a cellulose fibre embedded in an epoxy resin. A typical shift in the peak position of the 648 nm luminescent band is shown in Fig. 3a. It is clear that the central position of the peak shifts towards a lower wavelength, an effect which has been previously reported [19]. The peak position of this band as a function of strain is shown in Fig. 3b. The shift rate, as determined by a linear fit to these data, is found to be 0.07 nm %1. This value is the same as that reported previously using a similar methodology [19] and can be used as a calibration in order to determine the point-to-point strain in a glass fibre when it is embedded in the matrix material.
2220
K. Kong et al. / Composites Science and Technology 69 (2009) 2218–2224
(a)
Regenerated cellulose fibre
9 mm
20 mm
40 mm
30 mm
30 mm
Glass fibre
(b) Sellotape Regenerated cellulose fibre or glass fibre
Gauge length
Epoxy Resin
Deformation axis Fig. 1. Schematic diagrams of (a) a model hybrid composite dumbbell specimen (not to scale but dimensions are labelled) and (b) a single fibre strain calibration sample.
3.2. Deformation micromechanics of the cellulose/glass fibre hybrid composites A typical Raman spectrum from the regenerated cellulose fibre embedded in epoxy resin, highlighting the 1095 cm1 band, is shown in Fig. 4a. The Raman scattering from the resin indicates that there are two bands located close to the 1095 cm1 band (at 1080 and 1130 cm1), which may have a small influence on the fitting procedure. The influence of these bands was reduced by placing the regenerated cellulose fibre close to the surface of the epoxy resin, thus minimising the scattering from the resin. A typical luminescence spectrum from a SmF3 doped glass fibre embedded in epoxy resin, focussing on the 648 nm peak, is shown in Fig. 4b. This spectrum also shows the resin scattering in the same region. The resin scattering gives rise to only a low intensity background, and so should have a negligible effect on the fitting of the luminescence peak from the glass. During deformation, the glass fibre within the composite sample was analysed carefully in order to visually detect any fracture. Typical data from one specimen will be presented. For this specimen, the first visible fracture of the glass fibre was observed at a matrix strain of 1.1%, and an image obtained from the optical microscope of a crack in this glass fibre is shown in Fig. 5. It can be seen that a crack starting in the glass fibre passes through the resin and into the regenerated cellulose fibre. It is thought that, if the regenerated cellulose fibre remains intact, this crack will cause a localised tensile stress concentration in the fibre. The separation distance between the cellulose and glass fibres was found to be 38 lm, which is a little less than the diameter of the glass fibre. This region of the hybrid composite in the vicinity of the crack was then mapped, using both Raman and luminescence spectros-
copy to determine a point-to-point variation in fibre strains for both the regenerated cellulose fibre and the glass fibre. These data are reported in Fig. 6a. It is noted that there is more scatter in the data for the regenerated cellulose fibre than for the glass fibre which may be due to interference from the epoxy resin. The data for the regenerated cellulose fibre clearly indicate a stress concentration, which occurs at the same position as the break in the glass fibre. The strain in the glass fibre appears to go into compression at the break point, which may be due to recoil of the fibre ends after fracture. The maximum strain in the centre of the stress concentration of the cellulose fibre is about 2.2%, which is twice that of the matrix strain, but below the failure strain of the fibre (see Table 1). The strain profiles for both fibres appear to mirror each other. The strain profile for the glass fibre follows classical composite micromechanics theory, where a model fit is seen to follow these data, according to the equation [27]
ef ¼ em ½1 a coshðnfx l=2gÞ
ð1Þ
where ef is the fibre strain, em is the matrix strain, x is the distance along the fibre or fragment and n, l and a are fitting parameters. A similar form of this equation has been used before to analyse the local fibre strain distribution around fibre ends in model glass fibre composites using luminescence spectroscopy [19]. The form of Eq. (1) is however not suitable for the analysis the stress concentration in the regenerated cellulose fibre. A modification of this equation is required in order to fit the form of the data in Fig. 6a. This equation now becomes
ef ¼ em ½1 a coshðnfx l=2gÞ þ eo where eo is the baseline of the profile.
ð2Þ
2221
K. Kong et al. / Composites Science and Technology 69 (2009) 2218–2224
(a)
(a)18000
Intensity (Arbitrary Units)
16000
Intensity (Arbitrary Units)
400000
Strain = 0.0 % Strain = 3.6 %
14000 12000 10000 8000
350000 300000 250000 200000 150000 100000
Strain = 0.0% Strain = 0.8%
50000
6000 1060
1100
1080
1120
635
1140
640
645
650
655
Luminescence Wavelength (nm)
-1
Raman wavenumber (cm )
(b) 647.54 -1
-1
Raman band shift (cm )
0
Gradient = 1.10 cm % 2 R = 0.99
-1
Luminescence Wavelength (nm)
(b)
-1 -2 -3 -4 -5
Gradient = 0.07 nm % 2 R = 0.94
647.52
647.50
647.48
647.46
647.44
0
1
2
3
4
-1
0.0
5
0.2
0.4
0.6
0.8
Strain (%)
Fibre strain (%) Fig. 2. (a) A typical shift in the position of the 1095 cm1 Raman band of a regenerated cellulose fibre under tensile deformation and (b) the calibration of the Raman band shift as a function of tensile strain.
The point-to-point variation in the interfacial shear stress (ISS)
s is derived using the force balance equation [22] def ¼ 2s=Ef r dx
ð3Þ
where Ef is the fibre modulus, r is fibre radius and ef is the strain determined using Eq. (1). The resultant fits of Eqs. (1) and (2) to the data are reported in Fig. 6a. There is clear agreement between the model fits and the data, and so by using Eq. (3), further data reported in Fig. 6b are produced. These data indicate a continuous non-linear curve for the interfacial shear stress, indicative of a well-bonded interface. The maximum interfacial shear stress smax has values of 70 MPa and 25 MPa for the glass and cellulose fibres respectively. The value of smax for the glass fibre is quite high, exceeding the shear yield stress of the resin (ry = 40–50 MPa) [28]. The value of smax for the cellulose fibres suggests that there is a strong interface with the resin, and it is higher than a previously reported value of 17 MPa, for a flat-film of epoxy and this form of regenerated cellulose fibre [9]. These values for both fibres indicate that there is a strong and fully bonded interface in the cracking region. A similar study conducted by Hejda et al. reported a value of smax equal to 40 MPa for a glass fibre–epoxy composite [19], which is almost half of the value in this study. A value of 70 MPa has been reported for smax for a glass–epoxy composite system using a simulation method [29]. It was thought that this large value was due to
Fig. 3. (a) A typical shift in the position of the 648 nm luminescence band of a glass fibre under tensile deformation and (b) the calibration of the luminescence band shift as a function of tensile strain.
the presence of ‘‘penny-shaped cracks” [29], which could also occur in this fragmented composite sample. In order to describe the overloading of the regenerated cellulose fibre by the broken glass fibre, a stress concentration factor (SCF), K, is defined as the ratio between the maximum stress in the regenerated cellulose fibre (rmax) and the stress in the undisturbed region of the fibre (rundisturbed) using the equation
K¼
rmax rundisturbed
ð4Þ
A value of 2.3 is found for the SCF at a matrix strain of 1.1 % (cf. Fig. 6a), which is relatively high compared to a previous study on a multi-carbon-fibre/epoxy-resin composite [30] and a model hybrid composite containing a single carbon fibre surrounded by a groups of aramid fibres [31]. It is thought that when the glass fibre breaks, the relieved stress is then carried by the single regenerated cellulose fibre, leading to a large value of the SCF. Fig. 7a reports the local fibre strain within the same region for both fibres at a matrix strain of 1.5%. The regenerated cellulose fibre strain reaches a maximum of 3.0% at the same position as the break in the glass fibre. This value is double the matrix strain, indicating that the stress concentration is significant in this region. This value is however, lower than the breaking strain for this fibre (see Table 1), which suggests that the cellulose fibre has not fractured. Moreover, the linear nature of the strain distributions in this
2222
K. Kong et al. / Composites Science and Technology 69 (2009) 2218–2224
(a)
(a)
Regenerated cellulose fibre/epoxy resin Epoxy resin
27000
Intensity (Arbitrary units)
26000 25000
Fibre strain (%)
24000 23000 22000 21000 20000 19000 18000 17000 1060
1080
1100
1120
1140
2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8 -0.3
Matrix strain = 1.1% Regenerated cellulose fibre Glass fibre Model fit
-0.2
Raman wavenumber (cm-1)
(b)
700000
ISS (MPa)
Intensity (Arbitrary units)
(b) 70
800000
600000 500000 400000 300000 635
640
645
650
0.0
0.1
0.2
0.3
0.4
0.5
Distance along fibre (mm)
Glass fibre/epoxy resin Epoxy resin
900000
-0.1
655
Luminescence Wavelength (nm)
Matrix strain 1.1% Regenerated cellulose fibre Glass fibre
60 50 40 30 20 10 0
-10 -20 -30 -40 -50 -60 -0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
Distance along fibre (mm) Fig. 4. (a) Typical Raman spectrum for a regenerated cellulose fibre embedded in epoxy resin compared to pure epoxy resin in the same region and (b) a typical luminescence spectrum for the glass fibre embedded in epoxy resin compared to pure epoxy resin in the same region.
Fig. 6. (a) Fibre strain as a function of distance along the fragmented region of a glass fibre proximal to a regenerated cellulose fibre embedded in a model epoxy resin dumbbell composite specimen at 1.1% matrix strain. The solid lines are fits of Eqs. (1) and (2). (b) The corresponding interfacial shear stress (ISS) derived from the slope of a fit of Eqs. (1) and (2) to the data reported in (a), and the subsequent use of Eq. (3).
ef ¼
Fig. 5. An optical microscope image of the fracture point of a glass fibre. The fracture is seen to be affecting a neighbouring regenerated cellulose fibre.
region observed for the regenerated cellulose fibre, and one fragment of the glass fibre, is indicative of fibre–matrix debonding [30]. The glass fibre is thought to be debonded along a small section (0.1 mm) of the left-hand-side part of the fragment (cf. Fig. 7a). When a fibre debonds from a matrix material, the interfacial shear stress is assumed to be a constant along the interface, and can be determined using the equation
2s x þ eo Ef r
ð5Þ
where e0 is the fibre strain at the end of the debonded region. Eq. (5) is simply the form of a straight line with a slope (2s/Efr). A relatively easy determination of the interfacial shear stress can therefore be carried out graphically. The continuous ISS curves for both fibres are shown in Fig. 7b. It can be seen that the debonding regions have constant values of ISS. The calculated values of smax are about 32 MPa and 22 MPa for the glass fibre and the regenerated cellulose fibre respectively. A debonding of the interface occurs along a small section of the glass fibre which causes the ISS to drop to 32 MPa, leading to the discontinuity shown in Fig. 7b. The value of smax for the glass fibre is much lower at this elevated strain level than for the one obtained at 1.1% strain. This indicates that there is a breakdown in the interface caused by debonding. The value of smax for the cellulose fibre is however only slightly lower than that found at 1.1% strain, which suggests that a failure of the fibre–matrix interface may have occurred. It is worth noting that the value of the SCF in the cellulose fibre, obtained at 1.5% matrix strain (K = 1.9) is lower than the value obtained at 1.1% matrix strain. This is expected
K. Kong et al. / Composites Science and Technology 69 (2009) 2218–2224
(a)
stress of the matrix surrounding the fibre. Furthermore, the values of a SCF obtained from two different level of matrix strain have been reported. It was found that the SCF is higher when the fibre–matrix interface remains intact, and reduces when debonding occurs. This work has demonstrated that plant-based fibres are capable of bearing load following breaks in glass fibres in a model hybrid composite, and indicates that the technique could be useful for understanding more generally the interaction between glass and plant fibres in other hybrid systems.
Matrix strain = 1.5% Regenerated cellulose fibre Glass fibre Model Fit
3.0 2.5 2.0
Fibre strain (%)
2223
1.5
Acknowledgements 1.0
The authors would like to thank the EPSRC (EP/C002164) for funding this research. We would also like to thank Dr. Miroslav Rada of the Department of Glass and Ceramics, Institute of Chemical Technology, Prague (ICT) for assistance in preparing the doped glass compositions.
0.5 0.0 -0.5 -1.0 -1.0
-0.5
0.0
0.5
1.0
Distance along fibre (mm)
ISS (MPa)
(b)
Matrix strain 1.5% Regenerated cellulose fibre Glass fibre
30 25 20 15 10 5 0 -5 -10 -15 -20 -25 -30 -1.0
-0.5
0.0
0.5
1.0
Distance along fibre (mm) Fig. 7. (a) Fibre strain as a function of distance along the fragmented region of a glass fibre proximal to a regenerated cellulose fibre embedded in a model epoxy dumbbell composite specimen at 1.5% matrix strain. The solid lines are fits of Eqs. (1) and (2). (b) The corresponding interfacial shear stress (ISS) derived from the slope of a fit of Eqs. (1) and (2) to the data reported in (a) and the subsequent use of Eqs. (3) and (5).
since debonding of the fractured fibre is thought to reduce the SCF [32]. 4. Conclusions The simultaneous use of Raman and luminescence spectroscopy has been shown to be useful for following the local micromechanics of a model cellulose/glass-fibre reinforced model composite. Shifts in the peak positions of a Raman band located at 1095 cm1 and a luminescence band at 648 nm for fibres deformed in tension have been shown to give accurate calibrations of the point-to-point strains within filaments embedded in a transparent epoxy resin. The local micromechanics of a fractured glass fibre, in proximity to a regenerated cellulose fibre, has been determined. It has been shown that the fracture in the glass fibre influences the micromechanics of the cellulose fibre by generating a stress concentration, and an interfacial shear stress in excess of the yield
References [1] Eichhorn SJ, Baillie CA, Zafeiropoulos N, Mwaikambo LY, Ansell MP, Dufresne A, et al. Review: current international research into cellulosic fibres and composites. J Mater Sci 2001;36(9):2107–31. [2] Clark RA, Ansell MP. Jute and glass-fiber hybrid laminates. J Mater Sci 1986;21(1):269–76. [3] Pothan LA, Potschke P, Habler R, Thomas S. The static and dynamic mechanical properties of banana and glass fiber woven fabric-reinforced polyester composite. J Compos Mater 2005;39(11):1007–25. [4] Kalaprasad G, Francis B, Thomas S, Kumar CR, Pavithran C, Groeninckx G, et al. Effect of fibre length and chemical modifications on the tensile properties of intimately mixed short sisal/glass hybrid fibre reinforced low density polyethylene composites. Polym Int 2004;53(11):1624–38. [5] Santulli C. Impact properties of glass/plant fibre hybrid laminates. J Mater Sci 2007;42(11):3699–707. [6] Young RJ, Eichhorn SJ. Deformation mechanisms in polymer fibres and nanocomposites. Polymer 2007;48(1):2–18. [7] Hamad WY, Eichhorn S. Deformation micromechanics of regenerated cellulose fibers using Raman spectroscopy. ASME J Eng Mater Technol 1997;119(3):309–13. [8] Eichhorn SJ, Hughes M, Snell R, Mott L. Strain induced shifts in the Raman spectra of natural cellulose fibers. J Mater Sci Lett 2000;19(8):721–3. [9] Mottershead B, Eichhorn SJ. Deformation micromechanics of model regenerated cellulose fibre–epoxy/polyester composites. Compos Sci Technol 2007;67(10):2150–9. [10] Tze WTY, Gardner DJ, Tripp CP, O’Neill SC. Cellulose fiber/polymer adhesion: effects of fiber/matrix interfacial chemistry on the micromechanics of the interphase. J Adhes Sci Technol 2006;20(15):1649–68. [11] Tze WTY, O’Neill SC, Tripp CP, Gardner DJ, Shaler SM. Evaluation of load transfer in the cellulosic-fiber/polymer interphase using a micro-Raman tensile test. Wood Fiber Sci 2007;39(1):184–95. [12] Gierlinger N, Schwanninger M, Reinecke A, Burgert I. Molecular changes during tensile deformation of single wood fibers followed by Raman microscopy. Biomacromolecules 2006;7(7):2077–81. [13] Peetla P, Schenzel KC, Diepenbrock W. Determination of mechanical strength properties of hemp fibers using near-infrared Fourier transform Raman microspectroscopy. Appl Spectrosc 2006;60(6):682–91. [14] Young RJ, Thongpin C, Stanford JL, Lovell PA. Fragmentation analysis of glass fibres in model composites through the use of Raman spectroscopy. Composites: Part A 2001;32(2):253–69. [15] Zhao Q, Wood JR, Wagner HD. Stress fields around defects and fibers in a polymer using carbon nanotubes as sensors. Appl Phys Lett 2001;78(12):1748–50. [16] Zhao FM, Hayes SA, Patteson EA, Jones FR. Phase-stepping photoelasticity for the measurement of interfacial shear stress in single fibre composites. Composites: Part A 2006;37(2):216–21. [17] Pezzotti G. Probing nanoscopic stresses in glass using luminescent atoms. Micros Anal 2003;17(May):13–5. [18] Leto A, Pezzotti G. Probing nanoscale stress fields in Er3+-doped optical fibres using their native luminescence. J Phys Condens Matter 2004;16(28): 4907–20. [19] Hejda M, Kong K, Young RJ, Eichhorn SJ. Deformation micromechanics of model glass fibre composites. Compos Sci Technol 2008;68(3–4):848–53. [20] Young RJ. Monitoring deformation processes in high-performance fibres using Raman spectroscopy. J Text Inst 1995;86(2):360–81. [21] Boerstoel H, Maatman H, Westerink JB, Koenders BM. Liquid crystalline solutions of cellulose in phosphoric acid. Polymer 2001;42(17):7371–9. [22] Young RJ. Evaluation of composite interfaces using Raman spectroscopy. Interfacial effects in particulate, fibrous and layered composite materials. Key Eng Mater 1996;116–117:173–92.
2224
K. Kong et al. / Composites Science and Technology 69 (2009) 2218–2224
[23] Marquardt DW. An algorithm for least-squares estimation of nonlinear parameters. J Soc Ind Appl Math 1963;11(2):431–41. [24] Eichhorn SJ, Young RJ, Davies RJ, Riekel C. Characterisation of the microstructure and deformation of high modulus cellulose fibres. Polymer 2003;44(19):5901–8. [25] Eichhorn SJ, Young RJ, Yeh WY. Deformation processes in regenerated cellulose fibers. Text Res J 2001;71(2):121–9. [26] Kong K, Eichhorn SJ. Crystalline and amorphous deformation of processcontrolled cellulose-II fibres. Polymer 2005;46(17):6380–90. [27] Cox HL. The elasticity and strength of paper and other fibrous materials. Br J Appl Phys 1952;3:72–9. [28] Andrews MC. Stress transfer in aramid/epoxy model composites. PhD thesis, UMIST, Manchester Materials Science Centre; 1994.
[29] Feillard P, Desarmot G, Favre JP. A critical-assessment of the fragmentation test for glass epoxy systems. Compos Sci Technol 1993;49(2):109–19. [30] van den Heuvel PWJ, Peijs T, Young RJ. Failure phenomena in two-dimensional multi-fibre microcomposites. 2. A Raman spectroscopic study of the influence of inter-fibre spacing on stress concentrations. Compos Sci Technol 1997;57(8):899–911. [31] van den Heuvel PWJ, Goutianos S, Young RJ, Peijs T. Failure phenomena in fibre-reinforced composites. 6. A finite element study of stress concentrations in unidirectional carbon fibre reinforced epoxy composites. Compos Sci Technol 2004;64(5):645–56. [32] Ochiai S, Schulte K, Peters PWM. Strain concentration factors for fibers and matrix in unidirectional composites. Compos Sci Technol 1991;41(3): 237–56.
Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Deformation micromechanics of a model cellulose/glass fibre hybrid composite Kenny Kong, Marek Hejda, Robert J. Young, Stephen J. Eichhorn * Materials Science Centre, School of Materials and the Northwest Composites Centre, Grosvenor Street, University of Manchester, Manchester M1 7HS, UK
a r t i c l e
i n f o
Article history: Received 16 March 2009 Received in revised form 3 June 2009 Accepted 9 June 2009 Available online 14 June 2009 Keywords: A. Glass fibres A. Hybrid composites B. Interface B. Mechanical properties C. Stress transfer D. Raman spectroscopy
a b s t r a c t Interfacial stress transfer in a model hybrid composite has been investigated. An Sm3+ doped glass fibre and a high-modulus regenerated cellulose fibre were embedded in close proximity to each other in an epoxy resin matrix dumbbell-shaped model composite. This model composite was then deformed until the glass fibre fragmented. Shifts of the absolute positions of a Raman band from the cellulose fibre, located at 1095 cm1, and a luminescence band from a doped glass fibre, located at 648 nm, were recorded simultaneously. A calibration of these shifts, for both fibres deformed in air, was used to determine the point-to-point distribution of strain in the fibres around the breaks in the glass fibre. Each break that occurred in the glass fibre during fragmentation was shown to generate a local stress concentration in the cellulose fibre, which was quantified using Raman spectroscopy. Using theoretical model fits to the data it is shown that the interfacial shear stress between both fibres and the resin can be determined. A stress concentration factor (SCF) was also determined for the regenerated cellulose fibre, showing how the presence of debonding reduces this factor. This study offers a new approach for following the micromechanics of the interfaces within hybrid composite materials, in particular where plant fibres are used to replace glass fibres. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Replacing glass with plant fibres in glass fibre reinforced plastics (GFRP) has many benefits, including cost, weight-saving opportunities, and the reduction of the wear on processing machinery [1]. Unfortunately, all too often, the mechanical properties of natural fibres do not match up to those of glass fibres. One solution to this problem is to replace only a fraction of the glass fibres, thus making a hybrid material. One of the first studies on a hybrid glass fibre/plant fibre composite was by Clark and Ansell [2]. This paper showed that the toughness of jute and glass fibre hybrid laminates was maximised when the plant material was sandwiched between the glass. Later studies have shown how banana [3] and sisal [4] can be used in combination with glass fibres in composites. A review of the impact properties of hybrid glass–plant fibre composites has recently been published by Santulli [5]. The deformation micromechanics of natural fibre composites have been studied extensively using Raman spectroscopy. The principle of the technique relies on the measurement of the position of a characteristic band within a Raman spectrum as a function of the tensile deformation of the fibre. This shift in the peak position is indicative of the direct molecular deformation of the polymer backbone [6]. The first demonstration of Raman band * Corresponding author. Tel.: +44 0 161 306 5982; fax: +44 0 161 306 3586. E-mail address: [email protected] (S.J. Eichhorn). 0266-3538/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2009.06.006
shifts in cellulose fibres was in 1997 by Hamad and Eichhorn on regenerated cellulose filaments [7] and this was followed by work on hemp and flax fibres [8]. In recent times it has been shown that the interface between a regenerated cellulose fibre and a range of resin materials can be determined using model composite systems [9–11]. It has also been shown that Raman spectroscopy can be used to follow the deformation micromechanics of wood fibres [12] and chemically-modified plant fibres [13]. Significantly less has been published on the direct measurement of stress or strain in glass fibre composites. Previous attempts to do this have focussed on the use of strain-sensitive coatings applied to the surface of the glass fibres, such as a polyurethanediacetylene [14] and carbon nanotubes [15]. Another approach has been to use half-fringe photoelastic measurements to obtain the local stress field in the resin material surrounding the fibre [16]. It has been shown that doping glass with samarium fluoride (SmF3) enables the local stress state of the material to be followed using either photo- or electro-luminescence [17]. Local residual stresses in optical fibres doped with Er3+ have also been determined using the same technique [18]. A recent study on the use of SmF3 to enable the local stress state in a glass fibre and epoxy resin composite to be followed using luminescence spectroscopy has been reported [19]. It was shown that the fragmentation of a glass fibre embedded in epoxy resin followed classical composite micromechanics theory. Using both Raman spectroscopy and lumi-
K. Kong et al. / Composites Science and Technology 69 (2009) 2218–2224
nescence spectroscopy, it is possible to simultaneously determine the local stress for both glass [19] and polymeric fibres [20]. It ought, therefore, to be possible to place a polymeric fibre and a glass fibre together in order to determine the effect of one component of the reinforcement on the other and, at a fibre break, the interfacial shear stress between a matrix material and both fibres. In this study, a model hybrid composite has been constructed by placing a regenerated cellulose fibre and a glass fibre next to each other in an epoxy resin matrix. By fracturing the glass fibre in the composite, it is possible to investigate the effect of a break in the neighbouring region upon an intact regenerated cellulose fibre. We show that it is possible to simultaneously map point-topoint strains along two very different fibres embedded in a transparent epoxy resin matrix. This offers a new approach for the study of hybrid composite materials.
2219
2.2. Raman and luminescence spectroscopy
2. Experimental
A Renishaw system 1000 spectrometer coupled to a 633 nm HeNe laser was used to record Raman spectra from the regenerated cellulose fibres. The laser was focused on the sample to a spot size of 2 lm using a 50 lens and an Olympus microscope system. All spectra were recorded using an exposure time of 30 s and four accumulations, and then fitted using a mixed Gaussian/Lorentzian function, and an algorithm based on the work of Marquardt [23] to find the peak position. The same Renishaw Raman system and Olympus microscope were also used to record luminescence spectra from the glass fibres, this time excited using a 514 nm Ar+ laser. It is important to note that Raman spectroscopy was not used, as the luminescence effect is a different phenomenon where the absolute wavelength is measured. All luminescence spectra were recorded using an exposure time of 5 s and three accumulations.
2.1. Materials and sample preparation
2.3. Spectroscopy and deformation
Glass fibres were prepared using a composition (wt.%) of 64% SiO2, 6.5% Al2O3, 1.6% CaO, 10.7% Na2O, 0.4% MgO, 16.3% B2O3, and 0.5% SmF3. This composition was then heated in a furnace to 1450–1500 °C, and glass rods were produced. The glass investigated in this study has the same composition as for samples previously reported [19]. The glass rods were then stretched in a gas flame and were then drawn into fibres. The regenerated cellulose fibre investigated was a commercial high-modulus type produced by ACORDIS (called BocellTM) using a non-derivitised source of cellulose dissolved in an anisotropic phosphoric acid solution [21]. The mechanical properties for both fibres have been previously reported, and are summarised in Table 1 [9,19]. It is worth noting that there is a significant difference in the breaking strain for the regenerated cellulose fibres (5.5%) and the glass fibres (0.8%). This difference was expected to lead to a fracture of the glass fibre in the model composite before the regenerated cellulose fibre. Young’s modulus of the regenerated cellulose fibre is about half that of the glass fibre. The hybrid model composite was prepared by embedding both fibres in an epoxy resin dumbbell composite, using a similar method to fragmentation samples, which have been described previously [22]. A schematic diagram of a typical sample is shown in Fig. 1a. Both fibres were placed by hand, with the aid of a light microscope, parallel to each other with a small separation distance between them. An epoxy resin, cured at room temperature for 7 days, was used for the matrix material; namely AralditeÒ LY/HY 5052 (Ciba-Geigy 5052). This resin comes in two-parts, a butan1, 4, -diol diglycidyl ether resin (LY 5052) and an isophorone diamine hardener (HY 5052) which were mixed in the ratio 50:19. The exact spacing between both fibres cannot be controlled, as the separation distance varied slightly during the curing cycle. A small strain gauge was fitted to the surface of the cured dumbbell specimens, using cyanoacrylate adhesive, to record the strain of the specimen proximal to the fibre.
In order to determine the fibre strain in a hybrid composite sample, both single filaments were first deformed in tension (see Fig. 1b for a schematic of the sample). Regenerated cellulose fibres were deformed in tension using strain increments of 0.2%. At each deformation step a Raman spectrum was taken from the cellulose fibres, and the position of a Raman band located at 1095 cm1 was monitored. Similarly, the glass fibre was deformed in tension using strain increments of 0.01%, and the position of a luminescence peak located at 648 nm was also monitored at each strain step. The dumbbell composite specimens (see Fig. 1a) were deformed in tension using a MINIMATTM Miniature Materials straining rig (Polymer Laboratories Ltd., UK), which was placed onto the microscope stage of the Raman spectrometer. The surface strain of the composite was determined using a strain gauge attached to the matrix. The Raman and luminescence spectra were recorded along the fibres within the composite in the vicinity of a break in the glass fibre, using the same exposure times and accumulations as mentioned in Section 2.2.
Table 1 Mechanical properties of regenerated cellulose and glass fibres, where d is the fibre diameter; E is the Young’s modulus, rf is the fibre breaking stress and ef is the breaking strain of the fibres [9,19]. d (lm)
E (GPa)
rf (GPa)
ef (%)
55.4 ± 1.0
2.6 ± 0.3
5.5 ± 1.4
99.0 ± 3.0
0.9 ± 0.1
0.8 ± 0.1
Regenerated cellulose fibre 11.83 ± 0.03 Glass fibre 55
3. Results and discussion 3.1. Raman and luminescence band shifts of the cellulose and glass fibres A typical shift in the peak position for the 1095 cm1 Raman band of a regenerated cellulose fibre is shown in Fig. 2a indicating direct molecular deformation of the cellulose backbone structure. A shift in the position of this peak has been observed in many studies of the micromechanics of different types of regenerated cellulose fibres [24–26]. The shift in the central position of the 1095 cm1 band is shown in Fig. 2b. The shift rate, determined from a linear fit to these data, was found to be 1.10 cm1 %1. This value of the shift rate is similar to one reported by Eichhorn et al. [24], and can be used to determine the point-to-point variation of the strain of a cellulose fibre embedded in an epoxy resin. A typical shift in the peak position of the 648 nm luminescent band is shown in Fig. 3a. It is clear that the central position of the peak shifts towards a lower wavelength, an effect which has been previously reported [19]. The peak position of this band as a function of strain is shown in Fig. 3b. The shift rate, as determined by a linear fit to these data, is found to be 0.07 nm %1. This value is the same as that reported previously using a similar methodology [19] and can be used as a calibration in order to determine the point-to-point strain in a glass fibre when it is embedded in the matrix material.
2220
K. Kong et al. / Composites Science and Technology 69 (2009) 2218–2224
(a)
Regenerated cellulose fibre
9 mm
20 mm
40 mm
30 mm
30 mm
Glass fibre
(b) Sellotape Regenerated cellulose fibre or glass fibre
Gauge length
Epoxy Resin
Deformation axis Fig. 1. Schematic diagrams of (a) a model hybrid composite dumbbell specimen (not to scale but dimensions are labelled) and (b) a single fibre strain calibration sample.
3.2. Deformation micromechanics of the cellulose/glass fibre hybrid composites A typical Raman spectrum from the regenerated cellulose fibre embedded in epoxy resin, highlighting the 1095 cm1 band, is shown in Fig. 4a. The Raman scattering from the resin indicates that there are two bands located close to the 1095 cm1 band (at 1080 and 1130 cm1), which may have a small influence on the fitting procedure. The influence of these bands was reduced by placing the regenerated cellulose fibre close to the surface of the epoxy resin, thus minimising the scattering from the resin. A typical luminescence spectrum from a SmF3 doped glass fibre embedded in epoxy resin, focussing on the 648 nm peak, is shown in Fig. 4b. This spectrum also shows the resin scattering in the same region. The resin scattering gives rise to only a low intensity background, and so should have a negligible effect on the fitting of the luminescence peak from the glass. During deformation, the glass fibre within the composite sample was analysed carefully in order to visually detect any fracture. Typical data from one specimen will be presented. For this specimen, the first visible fracture of the glass fibre was observed at a matrix strain of 1.1%, and an image obtained from the optical microscope of a crack in this glass fibre is shown in Fig. 5. It can be seen that a crack starting in the glass fibre passes through the resin and into the regenerated cellulose fibre. It is thought that, if the regenerated cellulose fibre remains intact, this crack will cause a localised tensile stress concentration in the fibre. The separation distance between the cellulose and glass fibres was found to be 38 lm, which is a little less than the diameter of the glass fibre. This region of the hybrid composite in the vicinity of the crack was then mapped, using both Raman and luminescence spectros-
copy to determine a point-to-point variation in fibre strains for both the regenerated cellulose fibre and the glass fibre. These data are reported in Fig. 6a. It is noted that there is more scatter in the data for the regenerated cellulose fibre than for the glass fibre which may be due to interference from the epoxy resin. The data for the regenerated cellulose fibre clearly indicate a stress concentration, which occurs at the same position as the break in the glass fibre. The strain in the glass fibre appears to go into compression at the break point, which may be due to recoil of the fibre ends after fracture. The maximum strain in the centre of the stress concentration of the cellulose fibre is about 2.2%, which is twice that of the matrix strain, but below the failure strain of the fibre (see Table 1). The strain profiles for both fibres appear to mirror each other. The strain profile for the glass fibre follows classical composite micromechanics theory, where a model fit is seen to follow these data, according to the equation [27]
ef ¼ em ½1 a coshðnfx l=2gÞ
ð1Þ
where ef is the fibre strain, em is the matrix strain, x is the distance along the fibre or fragment and n, l and a are fitting parameters. A similar form of this equation has been used before to analyse the local fibre strain distribution around fibre ends in model glass fibre composites using luminescence spectroscopy [19]. The form of Eq. (1) is however not suitable for the analysis the stress concentration in the regenerated cellulose fibre. A modification of this equation is required in order to fit the form of the data in Fig. 6a. This equation now becomes
ef ¼ em ½1 a coshðnfx l=2gÞ þ eo where eo is the baseline of the profile.
ð2Þ
2221
K. Kong et al. / Composites Science and Technology 69 (2009) 2218–2224
(a)
(a)18000
Intensity (Arbitrary Units)
16000
Intensity (Arbitrary Units)
400000
Strain = 0.0 % Strain = 3.6 %
14000 12000 10000 8000
350000 300000 250000 200000 150000 100000
Strain = 0.0% Strain = 0.8%
50000
6000 1060
1100
1080
1120
635
1140
640
645
650
655
Luminescence Wavelength (nm)
-1
Raman wavenumber (cm )
(b) 647.54 -1
-1
Raman band shift (cm )
0
Gradient = 1.10 cm % 2 R = 0.99
-1
Luminescence Wavelength (nm)
(b)
-1 -2 -3 -4 -5
Gradient = 0.07 nm % 2 R = 0.94
647.52
647.50
647.48
647.46
647.44
0
1
2
3
4
-1
0.0
5
0.2
0.4
0.6
0.8
Strain (%)
Fibre strain (%) Fig. 2. (a) A typical shift in the position of the 1095 cm1 Raman band of a regenerated cellulose fibre under tensile deformation and (b) the calibration of the Raman band shift as a function of tensile strain.
The point-to-point variation in the interfacial shear stress (ISS)
s is derived using the force balance equation [22] def ¼ 2s=Ef r dx
ð3Þ
where Ef is the fibre modulus, r is fibre radius and ef is the strain determined using Eq. (1). The resultant fits of Eqs. (1) and (2) to the data are reported in Fig. 6a. There is clear agreement between the model fits and the data, and so by using Eq. (3), further data reported in Fig. 6b are produced. These data indicate a continuous non-linear curve for the interfacial shear stress, indicative of a well-bonded interface. The maximum interfacial shear stress smax has values of 70 MPa and 25 MPa for the glass and cellulose fibres respectively. The value of smax for the glass fibre is quite high, exceeding the shear yield stress of the resin (ry = 40–50 MPa) [28]. The value of smax for the cellulose fibres suggests that there is a strong interface with the resin, and it is higher than a previously reported value of 17 MPa, for a flat-film of epoxy and this form of regenerated cellulose fibre [9]. These values for both fibres indicate that there is a strong and fully bonded interface in the cracking region. A similar study conducted by Hejda et al. reported a value of smax equal to 40 MPa for a glass fibre–epoxy composite [19], which is almost half of the value in this study. A value of 70 MPa has been reported for smax for a glass–epoxy composite system using a simulation method [29]. It was thought that this large value was due to
Fig. 3. (a) A typical shift in the position of the 648 nm luminescence band of a glass fibre under tensile deformation and (b) the calibration of the luminescence band shift as a function of tensile strain.
the presence of ‘‘penny-shaped cracks” [29], which could also occur in this fragmented composite sample. In order to describe the overloading of the regenerated cellulose fibre by the broken glass fibre, a stress concentration factor (SCF), K, is defined as the ratio between the maximum stress in the regenerated cellulose fibre (rmax) and the stress in the undisturbed region of the fibre (rundisturbed) using the equation
K¼
rmax rundisturbed
ð4Þ
A value of 2.3 is found for the SCF at a matrix strain of 1.1 % (cf. Fig. 6a), which is relatively high compared to a previous study on a multi-carbon-fibre/epoxy-resin composite [30] and a model hybrid composite containing a single carbon fibre surrounded by a groups of aramid fibres [31]. It is thought that when the glass fibre breaks, the relieved stress is then carried by the single regenerated cellulose fibre, leading to a large value of the SCF. Fig. 7a reports the local fibre strain within the same region for both fibres at a matrix strain of 1.5%. The regenerated cellulose fibre strain reaches a maximum of 3.0% at the same position as the break in the glass fibre. This value is double the matrix strain, indicating that the stress concentration is significant in this region. This value is however, lower than the breaking strain for this fibre (see Table 1), which suggests that the cellulose fibre has not fractured. Moreover, the linear nature of the strain distributions in this
2222
K. Kong et al. / Composites Science and Technology 69 (2009) 2218–2224
(a)
(a)
Regenerated cellulose fibre/epoxy resin Epoxy resin
27000
Intensity (Arbitrary units)
26000 25000
Fibre strain (%)
24000 23000 22000 21000 20000 19000 18000 17000 1060
1080
1100
1120
1140
2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8 -0.3
Matrix strain = 1.1% Regenerated cellulose fibre Glass fibre Model fit
-0.2
Raman wavenumber (cm-1)
(b)
700000
ISS (MPa)
Intensity (Arbitrary units)
(b) 70
800000
600000 500000 400000 300000 635
640
645
650
0.0
0.1
0.2
0.3
0.4
0.5
Distance along fibre (mm)
Glass fibre/epoxy resin Epoxy resin
900000
-0.1
655
Luminescence Wavelength (nm)
Matrix strain 1.1% Regenerated cellulose fibre Glass fibre
60 50 40 30 20 10 0
-10 -20 -30 -40 -50 -60 -0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
Distance along fibre (mm) Fig. 4. (a) Typical Raman spectrum for a regenerated cellulose fibre embedded in epoxy resin compared to pure epoxy resin in the same region and (b) a typical luminescence spectrum for the glass fibre embedded in epoxy resin compared to pure epoxy resin in the same region.
Fig. 6. (a) Fibre strain as a function of distance along the fragmented region of a glass fibre proximal to a regenerated cellulose fibre embedded in a model epoxy resin dumbbell composite specimen at 1.1% matrix strain. The solid lines are fits of Eqs. (1) and (2). (b) The corresponding interfacial shear stress (ISS) derived from the slope of a fit of Eqs. (1) and (2) to the data reported in (a), and the subsequent use of Eq. (3).
ef ¼
Fig. 5. An optical microscope image of the fracture point of a glass fibre. The fracture is seen to be affecting a neighbouring regenerated cellulose fibre.
region observed for the regenerated cellulose fibre, and one fragment of the glass fibre, is indicative of fibre–matrix debonding [30]. The glass fibre is thought to be debonded along a small section (0.1 mm) of the left-hand-side part of the fragment (cf. Fig. 7a). When a fibre debonds from a matrix material, the interfacial shear stress is assumed to be a constant along the interface, and can be determined using the equation
2s x þ eo Ef r
ð5Þ
where e0 is the fibre strain at the end of the debonded region. Eq. (5) is simply the form of a straight line with a slope (2s/Efr). A relatively easy determination of the interfacial shear stress can therefore be carried out graphically. The continuous ISS curves for both fibres are shown in Fig. 7b. It can be seen that the debonding regions have constant values of ISS. The calculated values of smax are about 32 MPa and 22 MPa for the glass fibre and the regenerated cellulose fibre respectively. A debonding of the interface occurs along a small section of the glass fibre which causes the ISS to drop to 32 MPa, leading to the discontinuity shown in Fig. 7b. The value of smax for the glass fibre is much lower at this elevated strain level than for the one obtained at 1.1% strain. This indicates that there is a breakdown in the interface caused by debonding. The value of smax for the cellulose fibre is however only slightly lower than that found at 1.1% strain, which suggests that a failure of the fibre–matrix interface may have occurred. It is worth noting that the value of the SCF in the cellulose fibre, obtained at 1.5% matrix strain (K = 1.9) is lower than the value obtained at 1.1% matrix strain. This is expected
K. Kong et al. / Composites Science and Technology 69 (2009) 2218–2224
(a)
stress of the matrix surrounding the fibre. Furthermore, the values of a SCF obtained from two different level of matrix strain have been reported. It was found that the SCF is higher when the fibre–matrix interface remains intact, and reduces when debonding occurs. This work has demonstrated that plant-based fibres are capable of bearing load following breaks in glass fibres in a model hybrid composite, and indicates that the technique could be useful for understanding more generally the interaction between glass and plant fibres in other hybrid systems.
Matrix strain = 1.5% Regenerated cellulose fibre Glass fibre Model Fit
3.0 2.5 2.0
Fibre strain (%)
2223
1.5
Acknowledgements 1.0
The authors would like to thank the EPSRC (EP/C002164) for funding this research. We would also like to thank Dr. Miroslav Rada of the Department of Glass and Ceramics, Institute of Chemical Technology, Prague (ICT) for assistance in preparing the doped glass compositions.
0.5 0.0 -0.5 -1.0 -1.0
-0.5
0.0
0.5
1.0
Distance along fibre (mm)
ISS (MPa)
(b)
Matrix strain 1.5% Regenerated cellulose fibre Glass fibre
30 25 20 15 10 5 0 -5 -10 -15 -20 -25 -30 -1.0
-0.5
0.0
0.5
1.0
Distance along fibre (mm) Fig. 7. (a) Fibre strain as a function of distance along the fragmented region of a glass fibre proximal to a regenerated cellulose fibre embedded in a model epoxy dumbbell composite specimen at 1.5% matrix strain. The solid lines are fits of Eqs. (1) and (2). (b) The corresponding interfacial shear stress (ISS) derived from the slope of a fit of Eqs. (1) and (2) to the data reported in (a) and the subsequent use of Eqs. (3) and (5).
since debonding of the fractured fibre is thought to reduce the SCF [32]. 4. Conclusions The simultaneous use of Raman and luminescence spectroscopy has been shown to be useful for following the local micromechanics of a model cellulose/glass-fibre reinforced model composite. Shifts in the peak positions of a Raman band located at 1095 cm1 and a luminescence band at 648 nm for fibres deformed in tension have been shown to give accurate calibrations of the point-to-point strains within filaments embedded in a transparent epoxy resin. The local micromechanics of a fractured glass fibre, in proximity to a regenerated cellulose fibre, has been determined. It has been shown that the fracture in the glass fibre influences the micromechanics of the cellulose fibre by generating a stress concentration, and an interfacial shear stress in excess of the yield
References [1] Eichhorn SJ, Baillie CA, Zafeiropoulos N, Mwaikambo LY, Ansell MP, Dufresne A, et al. Review: current international research into cellulosic fibres and composites. J Mater Sci 2001;36(9):2107–31. [2] Clark RA, Ansell MP. Jute and glass-fiber hybrid laminates. J Mater Sci 1986;21(1):269–76. [3] Pothan LA, Potschke P, Habler R, Thomas S. The static and dynamic mechanical properties of banana and glass fiber woven fabric-reinforced polyester composite. J Compos Mater 2005;39(11):1007–25. [4] Kalaprasad G, Francis B, Thomas S, Kumar CR, Pavithran C, Groeninckx G, et al. Effect of fibre length and chemical modifications on the tensile properties of intimately mixed short sisal/glass hybrid fibre reinforced low density polyethylene composites. Polym Int 2004;53(11):1624–38. [5] Santulli C. Impact properties of glass/plant fibre hybrid laminates. J Mater Sci 2007;42(11):3699–707. [6] Young RJ, Eichhorn SJ. Deformation mechanisms in polymer fibres and nanocomposites. Polymer 2007;48(1):2–18. [7] Hamad WY, Eichhorn S. Deformation micromechanics of regenerated cellulose fibers using Raman spectroscopy. ASME J Eng Mater Technol 1997;119(3):309–13. [8] Eichhorn SJ, Hughes M, Snell R, Mott L. Strain induced shifts in the Raman spectra of natural cellulose fibers. J Mater Sci Lett 2000;19(8):721–3. [9] Mottershead B, Eichhorn SJ. Deformation micromechanics of model regenerated cellulose fibre–epoxy/polyester composites. Compos Sci Technol 2007;67(10):2150–9. [10] Tze WTY, Gardner DJ, Tripp CP, O’Neill SC. Cellulose fiber/polymer adhesion: effects of fiber/matrix interfacial chemistry on the micromechanics of the interphase. J Adhes Sci Technol 2006;20(15):1649–68. [11] Tze WTY, O’Neill SC, Tripp CP, Gardner DJ, Shaler SM. Evaluation of load transfer in the cellulosic-fiber/polymer interphase using a micro-Raman tensile test. Wood Fiber Sci 2007;39(1):184–95. [12] Gierlinger N, Schwanninger M, Reinecke A, Burgert I. Molecular changes during tensile deformation of single wood fibers followed by Raman microscopy. Biomacromolecules 2006;7(7):2077–81. [13] Peetla P, Schenzel KC, Diepenbrock W. Determination of mechanical strength properties of hemp fibers using near-infrared Fourier transform Raman microspectroscopy. Appl Spectrosc 2006;60(6):682–91. [14] Young RJ, Thongpin C, Stanford JL, Lovell PA. Fragmentation analysis of glass fibres in model composites through the use of Raman spectroscopy. Composites: Part A 2001;32(2):253–69. [15] Zhao Q, Wood JR, Wagner HD. Stress fields around defects and fibers in a polymer using carbon nanotubes as sensors. Appl Phys Lett 2001;78(12):1748–50. [16] Zhao FM, Hayes SA, Patteson EA, Jones FR. Phase-stepping photoelasticity for the measurement of interfacial shear stress in single fibre composites. Composites: Part A 2006;37(2):216–21. [17] Pezzotti G. Probing nanoscopic stresses in glass using luminescent atoms. Micros Anal 2003;17(May):13–5. [18] Leto A, Pezzotti G. Probing nanoscale stress fields in Er3+-doped optical fibres using their native luminescence. J Phys Condens Matter 2004;16(28): 4907–20. [19] Hejda M, Kong K, Young RJ, Eichhorn SJ. Deformation micromechanics of model glass fibre composites. Compos Sci Technol 2008;68(3–4):848–53. [20] Young RJ. Monitoring deformation processes in high-performance fibres using Raman spectroscopy. J Text Inst 1995;86(2):360–81. [21] Boerstoel H, Maatman H, Westerink JB, Koenders BM. Liquid crystalline solutions of cellulose in phosphoric acid. Polymer 2001;42(17):7371–9. [22] Young RJ. Evaluation of composite interfaces using Raman spectroscopy. Interfacial effects in particulate, fibrous and layered composite materials. Key Eng Mater 1996;116–117:173–92.
2224
K. Kong et al. / Composites Science and Technology 69 (2009) 2218–2224
[23] Marquardt DW. An algorithm for least-squares estimation of nonlinear parameters. J Soc Ind Appl Math 1963;11(2):431–41. [24] Eichhorn SJ, Young RJ, Davies RJ, Riekel C. Characterisation of the microstructure and deformation of high modulus cellulose fibres. Polymer 2003;44(19):5901–8. [25] Eichhorn SJ, Young RJ, Yeh WY. Deformation processes in regenerated cellulose fibers. Text Res J 2001;71(2):121–9. [26] Kong K, Eichhorn SJ. Crystalline and amorphous deformation of processcontrolled cellulose-II fibres. Polymer 2005;46(17):6380–90. [27] Cox HL. The elasticity and strength of paper and other fibrous materials. Br J Appl Phys 1952;3:72–9. [28] Andrews MC. Stress transfer in aramid/epoxy model composites. PhD thesis, UMIST, Manchester Materials Science Centre; 1994.
[29] Feillard P, Desarmot G, Favre JP. A critical-assessment of the fragmentation test for glass epoxy systems. Compos Sci Technol 1993;49(2):109–19. [30] van den Heuvel PWJ, Peijs T, Young RJ. Failure phenomena in two-dimensional multi-fibre microcomposites. 2. A Raman spectroscopic study of the influence of inter-fibre spacing on stress concentrations. Compos Sci Technol 1997;57(8):899–911. [31] van den Heuvel PWJ, Goutianos S, Young RJ, Peijs T. Failure phenomena in fibre-reinforced composites. 6. A finite element study of stress concentrations in unidirectional carbon fibre reinforced epoxy composites. Compos Sci Technol 2004;64(5):645–56. [32] Ochiai S, Schulte K, Peters PWM. Strain concentration factors for fibers and matrix in unidirectional composites. Compos Sci Technol 1991;41(3): 237–56.