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Effect of Fiber Orientation on Scratch Resistance in Unidirectional Carbon-Fiber-Reinforced Polymer Matrix Composites Mustafa Ozgur Bora, Onur Çoban, Tamer Sinmazcelik and Volkan Gunay Journal of Reinforced Plastics and Composites 2010 29: 1476 originally published online 2 June 2009 DOI: 10.1177/0731684409103953 The online version of this article can be found at: http://jrp.sagepub.com/content/29/10/1476

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Effect of Fiber Orientation on Scratch Resistance in Unidirectional Carbon-Fiber-Reinforced Polymer Matrix Composites _ _ IK MUSTAFA OZGUR BORA,* ONUR C¸OBAN AND TAMER SINMAZCEL Mechanical Engineering Department, Kocaeli University Vezirog˘lu Campus, 41040, Izmit/Turkey

VOLKAN GUNAY TUBITAK-MAM, Materials Institute, P.O. Box 21, 41470, Gebze/Turkey ABSTRACT: Polymer composites have been widely used in industrial applications because of their high-specific strength and modulus. During their maintenance and service life small scratches can be formed on the surface of the composite material. These small scratches can result in crack initiation causing material failure and also some esthetic defects. When we consider that polymer composites may have various fiber orientations, it is possible to develop a remarkable increase in their scratch resistance. For this reason it is necessary to investigate the effect of fiber orientation on scratch resistance for polymer composites. In this study, scratch resistance of continuous carbon-fiberreinforced polyetherimide composites were investigated as a function of fiber orientations by means of CSM microscratch tester machine. During the experiments the relation between the scratch resistance and operational parameters are determined as a function of scratch hardness, penetration depth, and coefficient of friction. KEY WORDS: fiber orientation, polymer composite, scratch resistance, scanning electron microscopy.

INTRODUCTION continue to replace non-reinforced polymers and metallic materials in engineering applications. The substitution of polymers with reinforced polymers is related to superior modulus–strength combination of polymer composites, while replacement of metals is encouraged because of density considerations [1]. Carbon-fiber-reinforced polymer composites have attracted significant attention because they are characterized by significantly higher modulus and strength compared to their unreinforced counterparts. This characteristic has led to their wide use, especially for structural and tribological applications, in the aeroplane and aerospace industry. Some small scratches may occur on the surface of polymeric materials due to many reasons such as lack of maintenance, dust, small particles, etc. A scratch on the surface

R

EINFORCED POLYMERIC MATERIALS

*Author to whom correspondence should be addressed. E-mail: [email protected] Figures 2, 4 and 5 appear in color online: http://jrp.sagepub.com

Journal of REINFORCED PLASTICS

AND

COMPOSITES, Vol. 29, No. 10/2010

0731-6844/10/10 1476–15 $10.00/0 DOI: 10.1177/0731684409103953 ß The Author(s), 2010. Reprints and permissions: http://www.sagepub.co.uk/journalsPermissions.nav


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of a polymeric material can act as a stress-raiser undermining the longevity of the material during tensile, impact, or fatigue loading. Thus, improvement in scratch resistance of polymeric materials is important for maximizing their applications and the service life of the components [2,3]. Angle-ply laminates have been used extensively in industrial applications of polymeric composites because of their superior properties in multidirectional loadings. Despite their extensive use, the damage occurrence in this type of laminate is not fully understood, particularly under scratch loadings. There are very limited studies about the effect of fiber orientation on the scratch resistance in literature. On the other hand, in literature, it is seen that extensive study has been carried out to characterize the scratch resistance of polymers and reinforced polymers [4–21]. Experimental study of scratch deformation, conducted using a modified pendulum sclerometer, on a poly(methyl methacrylate) (PMMA) surface was reported [4]. It was shown that the depth of a scratch was a crucial parameter determining the damage mode, and that the variation of imposed depth extended the range of the observed generic deformation behavior associated with a particular scratch velocity and nominal indenter geometry. It would appear that imperfections in the tip geometry (the tip defect) might be a contributing factor to the variation in response as a function of depth. The determining role of scratch indenter radius on surface deformation of high density polyethylene and calcium-carbonate-reinforced composite was examined [5]. The results showed that the scratch behavior of neat polyethylene and polyethylene–calcium carbonate composite system was sensitive to the indenter radius. However, the scratch resistance was not linearly related to the radius of indenter, but depended on the geometry of the indenter. Briscoe et al. examined the influence of applied load, sliding velocity, angle of indenter, and lubrication [6]. Scratch deformation processes of diverse types including fully elastic, elastic–plastic, ironing, wedging, crazing–tearing, grooving, edge-cracking, and chipping were observed depending on the normal load, cone angle, and scratch velocity. The scratch-induced surface damage of neat and calcium-carbonate-reinforced high density polyethylene was described in terms of characteristics of scratch morphology and scratch deformation parameters [7]. The results showed that a comparative assessment of scratch damage in terms of maximum depth of the scratch, average scratch roughness, and scratch hardness suggested increase in resistance of calcium-carbonate-reinforced polyethylene composite to scratch deformation. Despite the large number of works in this field open questions remain and there is no simple way available to correlate variables, such as scratch velocity, applied load, and angle of indenter within fiber orientation in a composite plate, that are important for the performance of the composite. Only limited studies have so far been devoted to the influence of fiber orientation on scratch deformation of composite laminates. The friction and wear characteristics for AS4/3501-6 composites were tested via single scratch tests by a Vickers diamond indenter [8]. Variables for the tests were the orientation of the surface-ply fibers with respect to the scratch direction and the normal load applied to the indenter. Traces of the wear furrows showed that the case of scratching parallel to the fiber orientation, the width of the furrow appeared to be fairly constant, while in the off-axis cases, surface-fiber pullout and bridging caused the width of the furrow to vary noticeably along its length. Lhymn and Park [9] found that the friction force was greatest for the normal direction, that is, for a surface in which the fibers are aligned perpendicular to the scratch direction, and least for the parallel direction, where the fibers align with the scratch direction, in silicon carbide/aluminum oxide and carbon/carbon composites.


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From the foregoing discussion it is clear that a more detailed study is needed to understand the influence of fiber orientation on surface scratch deformation of laminated composites, and to determine the key governing parameters such as scratch hardness, scratch velocity, penetration depth, and coefficient of friction. This article summarizes that the results of surface scratch tests carried out at different fiber orientations ranging from 08 to 908 on continuous carbon-fiber-reinforced PEI laminates. The results obtained from the different fiber-oriented polymer composites were correlated with a function of scratch hardness, penetration depth, and coefficient of friction. Moreover, scanning electron microscopy and optical microscopy were used to examine the scratch deformation morphologies of different fiber-oriented carbon/PEI composites. MATERIALS AND METHODS Material Continuous carbon-fiber-reinforced PEI composites were supplied from Ten Cate Advanced Composites (Nijverdal/Hollanda). Composite laminates were manufactured by hot press process. Polyacrylonitrile (PAN) based carbon fibers in the composite laminate (T300 12 K 309 NT type) were manufactured by Amoco. Fiber volume fraction was 60%. The fiber orientation of composite laminate was [(08/908)3/08]s. Composite laminate has 14 plaques. Each plaque’s weight and thickness were 222 g/m2 and 0.14 mm, respectively. Commercial code of the laminate was CD5150. Fiber orientation of the composite laminate is shown in Figure 1 which can be provided in following link (http:// mf.kocaeli.edu.tr/makina/ozgurbora/Figure 1.htm). In order to analyze the influence of fiber orientation on scratch resistance of polymer composite material, samples were cut from the composite laminate in different angles (0, 15, 30, 45, 60, 75, and 908) with respect to the top plaque’s fiber direction (08). Dimensions of the scratch samples were 30 mm 5 mm 2 mm. Polymer composites with seven different fiber orientations are coded as represented in Table 1.

90 °Fiber orientation angle

0 °Fiber orientation angle

Figure 1. Fiber orientation of carbon/PEI composite.


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Effect of Fiber Orientation on Scratch Resistance Table 1. Symbolic representation of composite samples. Orientation of laminates

Codes

[(08/908)3/08]s [(158/758)3/158]s [(308/608)3/308]s [(458/458)3/458]s [(608/308)3/608]s [(758/158)3/758]s [(908/08)3/908]s

A B C D E F G

Also, the three-dimensional illustration of the seven different fiber-oriented composites and the scratch directions are given in Figure 2. Scratch Tests In this study the scratch resistance of different fiber-oriented composite plates was examined using CSM microscratch tester. Rockwell S-218 type indenter having a spherical diamond and a contact radius of 200 mM was used. The samples were fixed on a leveling platform attached to a displacement stage and normal load was applied by placing dead-weights on the indenter holder. Mechanically induced surface damage in the form of a scratch was introduced on the surface of the samples using loads at 30 N. The scratch velocity was 60 mm/min and the length of scratch was 10 mm. During the experiments, the relation between scratch direction and fiber orientation was inspected as a function of scratch hardness, penetration depth, and coefficient of friction. Scratch Deformation The surfaces of laminated polymer composites which were scratched by Rockwell S-218 type indenter were inspected using scanning electron microscope (SEM) and optical microscope (OM). The macrostructural evaluation associated with the scratch process using OM (Leica S6D Stereo Microscope-80) and the microstructural evolution associated with the scratch process was studied using field emission SEM (Jeol JSM-6335F FEG-600,000). The characteristics of the deformation process and micromechanisms involved on the scratched surface area were examined with regard to various fiber orientations. RESULTS AND DISCUSSION Scratch tests provide a qualitative measure for the tribological properties at the surface of the materials. The test proceeds by recording the depth profile and the coefficient of friction while the indenter tip is scratching over the surface. The penetration depth is determined by the contact length between the leading edge of the indenter tip and the material indented [10]. It is influenced by the hardness, the modulus, and the wear resistance of the material being scratched. Figure 3 represents the identification of the scratch widths and depths of a symbolic scratch test. SW1 is the inner width of the scratch groove according to Wong et al. [11]. SW2 is the outer width of the scratch groove, that is, the distance between the points where the slopes of the hills meet the unscratched plane. SD1 is the depth of the scratch groove


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M.O. BORA (a)

(b)

Fiber orientation direction

Scratch direction

ET AL.

Fiber orientation direction Scratch direction θ

θ=0°

θ=15°

((0°/−90°)3/0°)s

((15°/−75°)3/15°)s


(c)


(d)

θ

Scratch direction

θ Scratch direction

θ=30° θ=45°

((30°/−60°)3/30°)s

((45°/−45°)3/45°)s

Scratch direction

(e)

Scratch direction

(f) θ

θ


Fiber orientation direction θ=75°

θ=60°

((75°/−15°)3/75°)s

((60°/−30°)3/60°)s Scratch direction

(g) θ Fiber orientation direction θ=90°

((90°/0°)3/90°)s

Figure 2. Three-dimensional illustration of different fiber-oriented samples and scratch direction.



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Figure 3. Definitions of scratch widths and scratch depths [11].

calculated from the unscratched plane. SD2 is the height of the peak to the trough of the scratch groove. For spherical indenters, the scratch grooves generally show a symmetric cross-sectional profile [11]. It is possible to predict the resistance to scratch deformation. The greater the value of scratch depth, the greater is the suspectibility to enhanced plastic deformation. Hot pressed plaques were supplied from Ten Cate. The plaques have smooth surfaces. From the SEM studies, we observe that composite laminate has a very thin polymer layer (skin thickness) between the outer surface of the laminate and the fibers located in the first ply. Not surprisingly, the indenter contacts this thin polymeric layer first. On the other hand, in order to eliminate the surface-roughness effect and the effect of variation in skin thickness, the indenter performs a pre-scratch under the load of 0.03 N. Typical parameters, including the penetration depth of the indenter during the scratch testing for different fiber-oriented polymer composite plates are illustrated in Figure 4 in the following link (http://mf.kocaeli.edu.tr/makina/ozgurbora/Figure 4.htm). There is a close relationship between penetration depth and the contact angle (between the indenter’s scratch direction and top ply’s fiber orientation of polymer composite). In Figure 4, as a result of increase in the contact angle, the penetration depth values are increased. It is evident that sample A exhibited maximum scratch resistance followed by other samples. Sample G exhibited the minimum scratch resistance (having maximum penetration depth). On the basis of scratch depth, resistance to scratch deformation of composite samples are in a sequence as follows: A 4 B 4 C 4 D 4 E 4 F4 G. Each plaque thickness of the composite samples was 0.14 mm as it is emphasized in section ‘Material’. Figure 4 shows that the indenter scratched only the first and the second laminae of the samples. When we consider the first two laminaes, though, the fiber orientation of sample G [(908/08)3/908]s is similar to fiber orientation of sample A [(08/908)3/08]s. Similar to Sample A and G, B–F, and C–E sample couples have similar (symmetric) fiber orientations, but the penetration depth values are found to be different due to the angle between indenter’s scratch direction and the first laminae’s fiber direction. It can be concluded that the angle between the first laminae’s fiber direction and the indenter’s scratch direction is the main parameter, which strictly influences the scratch deformation characteristics of the composite sample. In literature the scratch deformation of the polymeric materials can be described in terms of the stick–slip motion between the indenter and the surface of the sample. During the stick stage, there is no relative motion between the indenter tip and the surface of the material, but the indenter continues to apply stress on the sample surface resulting in


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(a)

ET AL.

(b) Sample A θ=0°

Sample B θ=15°

(d)

(c)

Sample D θ=45°

Sample C θ=30°

(e)

(f) Sample E θ=60°

Sample F θ=75°

(g) Sample G θ=90°

Figure 4. The influence of fiber orientation on penetration depth.

deformation of the material underneath the indenter. The tangential or horizontal stress acting during the stick stage is less than the critical stress, but increases with time. Once the stress applied by the indenter on the sample surface exceeds the required critical stress, the slip stage initiates, resulting in relative motion between the tip of the indenter and the sample surface. The slip stage terminates once the applied stress falls below the critical



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stress, resulting in subsequent onset of the stick stage when the indenter and material surface come into contact again. The material plastically deforms and accumulates in front of the indenter during the slip stage [3,5,12,13]. The coefficient of friction is another parameter of scratch resistance, which is determined from the ratio between the lateral to the normal force. Therefore, the coefficient of friction indicates the resistance of the material to the indenter penetration in the tangential direction [10,14,15]. The variation of coefficient of friction due to the contact angles is illustrated in Figure 5 in the following link (http://mf.kocaeli.edu.tr/makina/ozgurbora/Figure 5.htm). It can be concluded that the coefficient of friction is increased by the increase in angle between the scratch direction and fiber orientation. It is also observed that sample A exhibited minimum oscillations in friction coefficient during the scratch testing. On the other hand sample G exhibited the maximum oscillations. Increase in the coefficient of friction indicates the amount of surface deformation of the material. In the case of lower contact angles, Figure 5(a)–(c), there are small changes in coefficient of friction values during the scratch testing. The coefficient of friction line has small fluctations during the test. There are very small oscillations between the peak and valley points; the record shows approximately a linear characteristic. The lower-level regions of the frictional forces reflected testing conditions at which scratching occurred only in the polymer matrix between the fibers. If the indenter scratched only over the unfilled fiber, the scratch-damaged region appeared small, and plastic ploughing with micro-cracking was dominant. The main reason for small changes in coefficient of friction values can be related to stick–slip behavior of the polymers as discussed above [3,5,12,13,16]. At small angles there is not much fiber fracture and the indenter can be‘more in contact with the polymer matrix. This is the main reason for small fluctations in coefficient of friction during the tests. Figure 5(d) represents the results for a contact angle of 458 (sample D), which has a boundary characteristic between the lower and the higher contact angles. At higher contact angles (Figure 5(e)–(g)), coefficient of friction values increased drastically, i.e. very high peaks were observed at some locations. Due to the increase in penetration depth of the diamond indenter, the scratch depth was detected to be more than one fiber layer, hence fiber de-bonding took place in larger region, thus causing a higher frictional force and a great amount of deformation [16]. Optical microscopy was used to investigate the effect of the angle between the indenter’s scratch direction and fiber orientation of the polymer composite. The variation of scratch damages due to different fiber orientations is shown in Figure 6 in the following link (http://mf.kocaeli.edu.tr/makina/ozgurbora/Figure 6.htm). The most characteristic compressive damage on fiber-reinforced composites is kink bands which are formed by fiber bundles [16]. In Figure 6(a), kink bands were observed at the center of the scratched region due to the fiber direction of sample A and scratch direction of indenter (contact angle, ¼ 08). As seen in Figure 6, increase in the contact angle makes the scratch trace of the samples wider. There is a close correlation between scratch width and scratch hardness. The lower scratch width on the surface of the sample is indicative of reduced susceptibility to scratch hardness and scratch deformation. The lowest scratch width was observed at Sample A (Figure 6(a)). In Figure 6(a), the scratch direction and the fiber orientation is parallel. Similar to abrasive wear behavior of the continuous fiber composites [16], there is not much fiber deformation during the scratch. The indenter forced the fibers to bend. Fractured fibers leave their positions with some amount of matrix. On the other hand, the indenter can slip on the fibers. Although the fibers cracked, the load-bearing


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¯ fiber direction. Figure 5. The influence of fiber orientation on coefficient of friction at 0


ET AL.

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capacity of the fiber is higher in parallel direction compared to other orientations. From Figure 6(b)–(g), the contact angle rises with respect to the fiber directions. Especially in Figure 6(g), similar to scratch deformation of methanol-plasticized poly(methylmetacrylate) [17], it is clear that the indenter produces a great deformation on material surface. As a result, perpendicular contact (between the indenter and fibers) results in bending failure of the fibers. There are remarkable tearings and fiber–matrix de-bondings perpendicular to the scratch direction observed. At oblique angles (angle between scratch direction and fibers), easily broken fibers due to bending loads result in wider scratch deformation.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

Figure 6. OM images showing surface morphologies after scratch deformation.


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To characterize scratch resistance, scratch hardness can also be used to determine the material’s resistance to scratch deformation. Scratch hardness, which is also a measure of the scratch resistance, was calculated using [3,5,7,18–20]: H¼

Fn A

ð1Þ

where H is the scratch hardness in N/mm2, Fn is the scratch load in N, and A is the projected load-supporting area, in mm2, which is w2/4, and w is the residual scratch width. Thus, the scratch hardness is given by: H¼x

4:Fn w2

ð2Þ

where x is a parameter that assumes a value of 1 for purely elastic contact and 2 when the contact is plastic. Other types of material behavior involving visco-elastic and visco-plastic contacts will have a value for the parameter x between 1 and 2 [20]. In the present study, x is assumed to be 1. It may be noted that Equation (2) involving scratch width is sensitive to the nature of the material. There is a direct relationship between the scratch width and scratch hardness. Materials with higher scratch hardness are expected to exhibit higher scratch resistance to scratch damage. Table 2 presents the calculated scratch hardness of each different fiber-oriented polymer composite sample. Average scratch width of each sample is measured as shown in Figure 6. Figure 7 represents the correlation between scratch hardness and scratch width with respect to contact angles. The scratch hardness and scratch width values were taken from Table 2 and Figure 6, respectively. As seen in Figure 7, the scratch hardness values of the composites have values between 130 and 170 N/mm2. Also, the scratch hardness value has a great dependence on the contact angle. There is a remarkable difference between the values for angles between 08–158 and 758–908. Therefore, it can be concluded that the contact angle between the indenter and the fibers is the most important parameter which affects the deformation characteristics. As seen in Figure 8 which is provided in the following link (http://mf.kocaeli.edu.tr/ makina/ozgurbora/Figure 8.htm), the coefficient of friction values increase due to increase in the contact angle between the indenter’s scratch direction and top ply’s fiber orientation. Similar to scratch hardness, the coefficient of friction value has a great dependence on the fiber orientation.

Table 2. The variation of scratch hardness of different fiber-oriented polymer composite samples. Sample codes A B C D E F G

Contact angle

Average scratch width (mM)

Scratch hardness (N/mm2)

08 158 308 458 608 758 908

123.0769 232.258 253.846 269.231 276.923 284.615 576.923

310.294 164.460 147.567 141.875 137.934 134.206 66.208


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Effect of Fiber Orientation on Scratch Resistance 0° fiber orientation

360

15° fiber orientation

320 Scratch hardness (N/mm2)

30° fiber orientation 280

45° fiber orientation

240

60° fiber orientation 75° fiber orientation

200

90° fiber orientation 160 120 80 40 0.2

0.1

0.0

0.3

0.5

0.4

0.6

Scratch width (mm)

Figure 7. The correlation between scratch hardness and scratch width of different fiber-oriented samples.

Coefficient of friction (mm)

0.6

θ = 0°

0.5

θ =15° θ =30° θ = 45°

0.4

θ = 60° θ = 75° θ = 90°

0.3

0.2 −10

0

10

20

30

40

50

60

70

80

90 100

Contact angle, θ (°) Figure 8. The correlation between coefficient of friction and contact angles.

Scanning electron microscopic observations were performed in order to identify the effects of fiber orientations on scratch-deformation mechanisms and trace morphology. Figure 9 presents the scratch morphologies and the different damage modes for different fiber-oriented polymer composite samples. It can be observed that increase in contact angle from 08 to 908 results in remarkably severe surface deformation on the samples (Figure 9). In the case of small contact angles, the matrix between the fibers was extruded along the scratch and pushed away from the composite with fractured fibers as large-sized fragments. Figure 9(a)–(f ) indicates that the dominating wear mechanism was in the plane of bending the fibers, which caused fiber fracture and removal of whole fiber packages


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Figure 9. SEM observations for different fiber-oriented polymer composites.

from the contact surface. In Figure 9(a), it can be seen that the scratch-damaged region appears smaller, with matrix cracks and depression of fibers [16]. On the other hand, in Figure 9(f) (leading to maximum wear), the fibers are subjected to shear, bending, and torsion loading by the scratch indenter, which could tear away the fibers, resulting in the highest wear [21]. CONCLUSION Scratch resistance of continuous carbon–fiber-reinforced PEI composites was determined as a function of fiber orientations by means of CSM microscratch tester. The relation between scratch direction and fiber orientation is inspected as a function of scratch hardness, penetration depth, and coefficient of friction. A close relationship between penetration depth and the contact angle (between the indenter’s scratch direction and the top ply’s fiber orientation of polymer composite) is observed. In the case of higher contact angles, higher penetration depths are achieved. On the basis of scratch depth, one can conclude that resistance to scratch deformation of composite samples follows the sequence which has a close relationship between the fiber orientations. The penetration-depth values are found to be different due to the angle between indenter’s scratch direction and first laminae’s fiber direction. It can be concluded that the angle between first laminae’s fiber direction and indenter’s scratch direction is the main parameter which determines the scratch deformation of the composite sample. As a result of increase in the contact angle, lateral force (frictional force) is increased. That means the coefficient of friction is increased due to increase in contact angle.



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The scratched surfaces of laminated polymer composites which were deformed by Rockwell S-218 type indenter were inspected using SEM and OM. OM observations showed that the increase in the contact angle makes the scratch width of the samples larger. The lower scratch width on the surface of the sample is indicative of reduced susceptibility to scratch hardness and scratch deformation. SEM observations show that the surface-deformation mechanisms of different fiber-oriented samples were influenced by the variation of contact angle. It was found that the dominating wear mechanism was in the plane of the bending of the fibers, which caused fiber fracture and removal of whole fiber packages from the contact surface.

ACKNOWLEDGMENT The work outlined in this paper which was implemented in Advanced Materials Lab of Kocaeli University was supported financially by DPT 2008 K 120800. The authors would like to express their gratitude for this support.

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