Effect of Fiber Orientation on Viscoelastic Properties of Polymer Matrix Composites Subjected to Thermal Cycles

Transcription

1 Effect of Fiber Orientation on Viscoelastic Properties of Polymer Matrix Composites Subjected to Thermal Cycles Onur Coban, 1 Mustafa Ozgur Bora, 1 Tamer Sinmazcelik, 1,2 Volkan Gunay 2 1 Department of Mechanical Engineering, Kocaeli University, Kocaeli 41040, Turkey 2 TUBITAK-MAM, Material Ens., Gebze/Kocaeli, Turkey Fibers in polymer composites can be designed in various orientations for their usage in service life. Various fiber orientated polymer composites, which are used in aeroplane and aerospace applications, are frequently subjected to thermal cycles because of the changes in body temperatures at a range of 260 to 1508C during flights. It is an important subject to investigate the visco-elastic properties of the thermal cycled polymer composite materials which have various fiber orientations during service life. Continuous fiber reinforced composites with a various fiber orientations are subjected to 1,000 thermal cycles between the temperatures of 0 and 1008C. Dynamic mechanic thermal analysis (DMTA) experiments are carried out by TA Q800 type equipment. The changes in glass transition temperature (T g ), storage modulus (E 0 ), loss modulus (E 00 ) and loss factor (tan d) are inspected as a function of thermal cycles for different fiber orientations. It was observed that thermal and dynamic mechanical properties of the polymer composites were remarkably changed by thermal cycles. It was also determined that the composites with [458/2458] s fiber orientation presented the lowest dynamic mechanical properties. POLYM. COMPOS., 31: , ª 2009 Society of Plastics Engineers Correspondence to: Onur Coban; onur_coban@yahoo.com DOI /pc Published online in Wiley InterScience ( VC 2009 Society of Plastics Engineers INTRODUCTION Carbon fiber-reinforced plastics (CFRPs) are widely used in aeronautics and aerospace industry, on account of their high stiffness and strength and their low density. These materials present specific properties such as stiffness/weight and strength/weight ratios higher than those of metallic materials [1]. Polymer composites are designed and manufactured with different fiber orientations according to load directions which are probably subjected in service life. It is expected that polymer composites which have different fiber orientations present different mechanical properties. It is known that polymer composites are being widely used in aerospace industry as structural parts. These composite materials are subjected to different atmospheric conditions during flights. During the service life, because of sudden temperature changes (2608C/þ1008C) in airplane body called as thermal cycles, thermal and residual stresses have occurred in material. According to the literature survey, it was seen that the influence of fiber orientation and thermal cycling on mechanical properties of polymer composites were investigated individually. The effect of water storage and thermal cycling on the flexural properties of linear poly(butyl methacrylate) (PBMA) and crosslinked poly(methyl methacrylate) (PMMA)-sized unidirectional fiber reinforced composites containing different quantities of the surfacecleaned and silanized fibers were investigated [2]. It was reported that flexural properties of FRCs improved with increasing fiber content, whereas the flexural properties were not influenced significantly by water and thermal cycling. Analytic method was used to calculate humidity concentration through the thickness of plates of E-glass/epoxy and carbon/epoxy (T ) composites [3]. Obtained results reported that the lowest values of humidity concentration calculated through the thickness of the plate correspond to the angle of 908, for the angle of 08 the values of the concentration were distinctly larger. They concluded that the diffusion of the humidity with lower angles was much easier than with higher angles. Dynamic Mechanical Analysis (DMA) experiments as well as tensile tests at three different strain rates and three different temperatures below T g were performed on off-axis specimens of three different orientations [4]. The storage modulus and tan d versus temperature for all off-axis specimens were investigated. Four phenolic resins were inves- POLYMER COMPOSITES -2010

2 tigated: a resol/novolac blend, a phenolic/furan, novolac/ resol graft copolymer, a novolac, and a resol [5]. The results indicated that thermal ageing at 1808C for 1 day let to a more complete cure of all four phenolic resins as indicated by an increase in the temperature of the maximum of plots of both loss modulus (E 00 ) and tan d versus temperature. Thermal aging let to an increase in storage modulus (E 0 ) at higher temperatures and the magnitude of E 0 at a given temperature decreased with increasing exposure time. The magnitude of E 00 and tan d decreased with aging time for all resins, although E 00 and tan d were larger for the blend and copolymer composite than for the novolac and resol composites. DMA is one of the most reliable techniques for analyzing the viscoelastic properties of polymer matrix composite materials since it is relatively rapid and particularly suitable for quality control applications [6]. On the other hand, very limited number of studies can be found related with the influence of thermal cycling and fiber orientation on viscoelastic properties of polymer composites. In this study, it is aimed to investigate the variation of viscoelastic properties in different fiber oriented composites, which are subjected to thermal cycling. MATERIALS AND METHODS TABLE 1. Symbolic presentation of samples which have different fiber orientations. Laminate Code [(08/908) 3 /08] s A [(158/2758) 3 /158] s B [(308/2608) 3 /308] s C [(458/2458) 3 /458] s D [(608/2308) 3 /608] s E [(758/2158) 3 /758] s F [(908/08) 3 /908] s G Material Hot pressed continuous carbon fiber reinforced polyetherimide (PEI) composites which were used in experiments were supplied from TenCate Advanced Composites (Nijverdal/Hollanda). Commercial code of the composite laminate was CD5150. Polyacrylonitrile (PAN)-based carbon fibers in the composite laminate (T 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 as illustrated in Fig. 1. Composite laminate has 14 plaques. Each plaque s weight and thickness were 222 g/m 2 and 0.14 mm, respectively. To analyze the thermal cycling effects on viscoelastic properties of polymer composites, DMA samples cut from the composite laminate with different angles (08, 158, 308, 458, 608, 758, 908) respect to the (08) direction in dimensions of mm 3. DMA samples having seven different orientations are presented in Table 1. Also the samples are coded to follow the results easily. Thermal Cycling Exposure Samples were subjected to thermal cycling in a test rig composed of two separate tanks full of boiling water and an ice-water. The temperature of water was kept at 1008C (618C) for the boiling water tank and 08C (618C) for the ice-water tank. DMA samples were placed into sample holder and immersed into the boiling water tank for 5 s, and then the sample holder was immediately immersed into the ice-water tank for 5 s. This operation takes 10 s in total and corresponds to one thermal cycle. Dynamic Mechanical Analysis TA Instruments Q800 type tester was used for the experiments for dynamic mechanical analysis investigations. Original and 1,000 thermal cycled samples with different fiber orientations were tested under three point bending mode at 1 Hz frequency between the temperature ranges of C. Heating rate was 58C/min. During test, preload was chosen as 0.5 N. RESULTS The viscoelastic properties of fiber oriented and thermal cycled samples were investigated by DMA. DMA results of the composites were expressed by using storage modulus (E 0 ), loss modulus (E 00 ), and loss factor (tan d) and T g. FIG. 1. Fiber orientation of carbon/pei composite. Fiber Orientation Effects Fiber orientation effects on storage modulus of the composites are illustrated in Fig. 2. Storage modulus (E 0 ) 412 POLYMER COMPOSITES DOI /pc

3 FIG. 2. Effect of fiber orientation on storage modulus of original samples. [Color figure can be viewed in the online issue, which is available at of the composites having different fiber orientations remain at approximately constant value in glass region until the onset of the glass transition. Of the particular interest were the value of the storage modulus at onset of the glass transition and the shape of the glass transition region for all fiber orientation. The storage modulus values at onset of the glass transition were varied with the fiber orientations of the composites. As seen in Fig. 2; the storage modulus values were obtained at peak points of the samples. Not surprisingly the storage moduli of the composites were shown remarkable dependence on fiber orientations. The storage modulus of the samples are ordered as sample A [ B [ F [ G [ C[ E [ D. Not surprisingly the highest bending resistance achieved from [(08/908) 3 /08] s sample. Sample B follows the sample A. Because the fiber orientation angle in the first layer is 158 in sample B. This angle has a close orientation to the 08 fibers but gives lower bending resistance compared with 08 orientation. Briefly, when evaluating the storage modulus of the samples, it is possible to say that the orientation of the fibers and their location in composite laminate (ply location, the sequence of the plies from surface to neutral axis) strictly affect the results. The effect of fiber orientation and ply sequence in laminate can be easily observed from the storage modulus changes of the couples of sample B F, C E, and A G. The storage modulus of sample B is higher than that of sample F. The main reason of this difference is the stacking sequence of the 158 oriented fibers. In sample B, these plies located at the outer places compared with sample F and give higher storage modulus values. Not surprisingly, the number of the plies which the fibers are oriented perpendicular to the span direction gives the highest resistance against bending. The changes in storage modulus of samples C E and A G have similar reasons with sample B and F. The illustration of the obtained loss modulus for original samples that has different fiber orientations was presented in Fig. 3. The loss modulus of the samples tends to be low and stay constant at low temperatures. At higher temperatures approximately between 175 and 1958C loss modulus curves began to rise to a pronounced peak at the onset of the glass transition, which corresponds to the maximum heat dissipation per unit deformation [7]. The peaks are more pronounced for sample A, B, F, G and less pronounced for sample C, D, and E. These less pronounced peaks can be attributed to many laminaes that have fiber orientations of 308, 458, and 608. It was aforementioned that the maximum storage modulus value at peak point was seen for sample A and also maximum loss modulus value at peak point was seen for sample A. That is; the evidence of maximum ability to dissipate energy as heat was observed for sample A. The loss modulus values obtained at peak points of the samples are ordered as A [ B [ G [ F [ C [ E [ D, very similar to storage modulus values order. That means, when fibers oriented closer to orientation of 08 (perpendicular to the span direction) outer plies became to a material with a higher bending resistance. Also during the bending, FIG. 3. Effect of fiber orientation on loss modulus of original samples. [Color figure can be viewed in the online issue, which is available at FIG. 4. Effect of fiber orientation on loss factor of original samples. [Color figure can be viewed in the online issue, which is available at DOI /pc POLYMER COMPOSITES

4 TABLE 2. matrix. Characteristic properties of carbon fiber and polyetherimide Properties Carbon fiber PEI matrix E 1 (GPa) E 2 (GPa) 40 3 G 12 (GPa) G 23 (GPa) t t a 1 (10 26 /8C) a 2 (10 26 /8C) q (kg/m 3 ) more energy is lost as heat dissipation. The mechanism and the descriptions of storage modulus and fiber orientation relationship are strictly similar to loss modulus of the samples. The most important parameter in DMA is tan d. Tan d values are calculated by Eq. 1 and plotted by means of program (TA Universal Analysis). tan d ¼ E 00 =E 0 ð1þ In Fig. 4, the tan d curves of different fiber oriented samples are plotted against temperature. The most dominant feature of the tan d curve is its peak in the glass transition region which corresponds to high damping due to initiation of motion in long segments of the main polymer chain [7]. The peak point values of tan d curves of different fiber oriented samples were compared and ordered as sample G [ A [ B [ C [ D [ E [ F. The tan d values of all samples had maximum levels in the glass transition regions. This indicated that higher portion of mechanical energy was dissipated in the materials as heat in the transition regions. This order means that from sample G to F the higher mechanical energy was dissipated in the materials as heat in the transition regions. The temperature at which the peak occurs is commonly quoted as the glass transition temperature [8 12]. The effect of fiber orientation on T g values of samples could be seen in Fig. 4. The obtained T g values of different fiber oriented samples are ordered as sample A [ G [ B [ F [ E [ D [ C. This order also should be estimated from the storage modulus values order. There is an 108C difference between maximum and minimum T g values that obtained from different fiber oriented samples. In fact, the tan d peak values of samples A and B were very close to 1. This means that almost half of the mechanical energy supplied by the oscillating force would be lost as a heat in the bulk of the samples at the temperatures where the tan d peaks appears. The observed decrease in storage modulus as well as a shift in the tan d peaks to lower temperatures for the samples indicates a decreased bending resistance of the samples. Thermal Cycling Effects Interlaminar and fiber/matrix interface weakness can occur from built-in stresses during thermal cycling exposure of composite material. Such residual stresses essentially arise due to mismatch in coefficient of thermal expansion of fiber and matrix. And also these thermal stresses are most pronounced in cross-ply laminates. The thermal expansion coefficients of composite components (fiber-matrix) are thus an important factor particularly if the composite are subject to thermal gradients [13]. Characteristic properties of carbon fiber and polyetherimide matrix materials were represented in Table 2 [14]. For each laminae, the thermal residual stress in longitudinal fiber direction can be calculated with stated formula which is shown below. r 11 ¼ ðe 1f 3 a 1f E 1m 3 a 1m Þ 3 DT ðmpaþ ð2þ where r 11 is thermal residual stress in longitudinal fiber direction, E 1f is elastic modulus of longitudinal fiber direction, E 1m is elastic modulus of matrix, a 1f is coefficient of thermal expansion of fiber in longitudinal fiber direction, a 1m is coefficient of thermal expansion of PEI matrix in longitudinal fiber direction. For each laminae if the thermal residual stress in longitudinal fiber direction was calculated by utilizing Eq. 2, situated value is found as MPa. The resultant residual thermal stress is MPa which is similar for each laminae. The schematic illustration of residual stresses for first two laminae, resultant residual thermal FIG. 5. laminae Symbolic vectorial projections of thermal stresses of each TABLE 3. Angles between resultant thermal residual stress and horizontal direction. Laminate a Angle (8) A 45 B 30 C 15 D 0 E 15 F 30 G POLYMER COMPOSITES DOI /pc

5 FIG. 6. Variation of storage modulus values obtained at peak points versus orientation angle of original and 1,000 thermal cycled samples. [Color figure can be viewed in the online issue, which is available at stress value and a angle is given in Fig. 5 for sample A. When we called 08 as a horizontal direction between the two span in DMA tester, it is believed that viscoelastic properties of the samples having residual stresses may dependent on the a angle between the resultant force direction and 08 (or horizontal) direction (Table 3). The comparison of storage modulus of thermal cycled and original samples which have different fiber orientations were illustrated in Fig. 6. As seen in Fig. 6; variation of storage modulus values due to the thermal cycling was calculated around 10% levels. Beside this; the storage modulus values obtained at the onset of glass transition points were compared according to fiber orientation angle of upper ply of the samples; it is seen that the resultant thermal residual stress ( MPa) was negligible compared with longitudinal tensile stress (1,890 MPa) [15]. FIG. 8. Variation of tan d values at peak points versus orientation angle of original and 1,000 thermal cycled samples. [Color figure can be viewed in the online issue, which is available at wiley.com.] Therefore; it could be said that thermal cycling exposure has no remarkable effect on the storage modulus of composite samples. Maximum variation was observed for sample A, this result can be attributed the maximum a angle values of 458 between the resultant thermal residual stress and horizontal direction. Also; the minimum variation was observed for sample D this result can also be attributed to the minimum angle value of 08 between the resultant thermal residual stress and horizontal direction. Briefly, the difference in the storage modulus of the original and 1,000 times thermal cycled sample gives similar results with variation of a angle. Loss modulus values of different fiber oriented original and thermal cycled samples presented in Fig. 7. Variation of loss modulus values due to thermal cycling was calculated around 11% levels. The results of thermal cycling FIG. 7. Variation of loss modulus values at peak points versus orientation angle of original and 1,000 thermal cycled samples. [Color figure can be viewed in the online issue, which is available at wiley.com.] FIG. 9. Variation of T g values versus orientation angle of original and 1,000 thermal cycled samples. [Color figure can be viewed in the online issue, which is available at DOI /pc POLYMER COMPOSITES

6 effect on loss modulus of fiber oriented samples were determined as similar with storage modulus values. As seen in Fig. 7 maximum variation was observed for sample A this result can be attributed the maximum a angle value of 458 between the resultant thermal residual stress and horizontal direction. Also; the minimum variation was observed for sample D this result can also be attributed to the minimum a angle value of 08. Also, as concluded in Fig. 6, similar trend is obtained in Fig. 7. It is observed that higher variations in loss modulus values are obtained in orientations which are having the higher a angles. Variation of tan d values of different fiber oriented samples due to the thermal cycling was presented in Fig. 8. As seen in Fig. 8, the average variation value of tan d was obtained approximately as 10% levels. Also variation of T g values of different fiber oriented samples due to thermal cycling was presented in Fig. 9. Approximately, T g values show 28C decrease at thermal cycled samples for same orientations. Variations in T g values between the original and thermal cycled samples also show a similar sequence with variation of a angle. CONCLUSIONS Influence of thermal cycling and fiber orientation on viscoelastic properties of continuous carbon fiber reinforced polyetherimide (PEI) composites were studied by DMA. The changes in glass transition temperature (T g ), storage modulus (E 0 ), loss modulus (E 00 ) and loss factor (tan d) are inspected as a function of thermal cycles for different fiber orientations. The storage modulus of the samples showed a great dependence to the fiber orientations. DMA results showed that storage modulus ordered as sample A [ B [ F [ G [ C [ E [ D. The fibers oriented in 08 direction, which are located at the outer lamina of the composite (nearby the surface) gives the highest bending resistance. The mechanism and the descriptions of storage modulus and fiber orientation relationship were strictly similar to loss modulus of the samples. The loss modulus values obtained at peak points of the samples were ordered as sample A [ B [ G [ F [ C [ E [ D, which is very similar to order of storage modulus values. That means when the fibers oriented closer to orientation of 08 outer plies show a higher bending resistance, also during the more energy is loss as heat dissipation. The peak point values of tan d curves of different fiber oriented samples were compared and ordered as sample G [ A [ B [ C [ D [ E [ F. This order means that from sample G to F, the higher mechanical energy was dissipated in the materials as heat in the transition regions. The obtained T g values of different fiber oriented samples were ordered as sample A [ G [ B [ F [ E [ D [ C. This order also should be estimated from the storage modulus values order. There is an 108 difference between maximum and minimum T g values that obtained from different fiber oriented samples. Briefly, it is observed that the fiber orientations strictly affect the viscoelastic properties of the composites. As calculated, the residual stresses after thermal cycling have very small values compared to strength of the composites. So the effect of residual stresses on viscoelastic properties of the composites may negligible. On the other hand, the ageing of the polymer matrix during thermal cycles may be very limited. Because the material exposed to lower temperatures during the thermal cycles compared with its T g temperature in short period (for only a few hours). As a result, there is no remarkable difference observed in viscoelastic properties at each fiber orientations after thermal cycling. So, this material is very suitable for applications which have the thermal cycles between the lower temperatures than T g value. REFERENCES 1. T. Gómez-del Río, R. 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Scheirs, Compositional and Failure Analysis of Polymers, Wiley, London (2000) ISBN: R. Akkerman and R.S. de Vries, Thermomechanical Properties of Woven Fabric Composites, in The Proceedings of the International Conference on Fibre Reinforced Composites FRC 98, Woodhead, Newcastle, United Kingdom, 422 (1998). 15. Tencate product data sheet. Available atwww.tencate.com (accessed on May 12, 2008). 416 POLYMER COMPOSITES DOI /pc