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polyethylene intraply fabric composites
Composites Science and Technology (1998) 1621±1628 # 1998 Elsevier Science Ltd. All rights reserved 58
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THE EFFECTS OF HYBRIDIZATION ON THE MECHANICAL PERFORMANCE OF ARAMID/POLYETHYLENE INTRAPLY FABRIC COMPOSITES Rohchoon Park & Jyongsik Jang*
Department of Chemical Technology, Seoul National University, San 56-1, Shinlimdong Kwanakgu, Seoul, Korea (Received 16 June 1997; revised 20 October 1997; accepted 20 November 1997)
been extensive interest in such materials, particularly those which are based on a combination of a cheap ®bre with a stier ®bre.4±6 In high-technology ®elds, the question of cost may be insigni®cant by comparison with the advantages of optimizing properties.7,8 Polyethylene-®bre-reinforced composites are attractive materials because of their high impact resistance and they are much used in military applications such as for helicopter and armor body. On the other hand, this material has the disadvantage that its tensile, ¯exural, and thermal properties are poor.9±11 To compensate for this defect, aramid/polyethylene hybrid composites have been designed. One of the most important purposes of using aramid/polyethylene hybrid composite is to combine the good mechanical property of aramid ®bre with the excellent impact resistance of polyethylene ®bre. Depending on the geometric pattern of ®bre arrangements, hybrid composites can be classi®ed as interply(interlaminated) and intraply(intermingled).12,13 In intraply hybrid composites, each fabric consists of two or more kinds of ®bre. The mechanical properties of intraply hybrid composites should be systematically studied for applications as structural components. Among the mechanical properties, the ¯exural properties, interlaminar shear strength, and impact properties are especially important because the information about them is applied to real systems and makes design with the new material possible. The volume ratio and stacking sequence of dierent components in the hybrid material act as determining factors in the variation of the mechanical properties. When the stier ®bres are placed in the outer parts of the laminate, the hybrid composite exhibits a higher ¯exural modulus.14 In addition, for wider structural application it is necessary to investigate the eects of stacking sequence and loading direction on the mechanical properties of these materials. In this study, the ¯exural properties, interlaminar shear strength, and impact properties of aramid/polyethylene intraply hybrid composites have been investigated as a function of lamination stacking sequence and aramid ®bre content. The changes in mechanical property with dierent loading direction are also examined.
Intraply hybrid composites have been fabricated by the open leaky-mold method in order to examine the eects of hybridization on the mechanical performance of aramid/ polyethylene hybrid composite. The ¯exural properties, interlaminar shear strength (ILSS), and impact properties of these composites have been studied as a function of aramid ®bre content and loading direction. Two dierent stacking sequences ([0/0]4 and [0/90]4) were selected for this study, and composites of four dierent aramid ®bre contents (100, 75, 50, and 0% by volume) have been studied. In addition, scanning electron microscopy (SEM) was used to examine the fracture surfaces of the intraply hybrid composites. The ¯exural strength and modulus increased with the volume fraction of aramid ®bre, and negative deviations from the rule of mixture were observed in all intraply laminates. When the load was applied in the aramid direction, the highest ¯exural strength was obtained. This is attributed to the fact that the ¯exural strength mainly depends on whichever ®bre is present in the longitudinal direction of the specimen. In contrast with the ¯exural property results, intraply hybrid composites exhibited lower ILSS values than polyethylene-®bre composites because of the dierence in the failure modes in the ¯exural and ILSS tests. In addition, intraply hybrid composites exhibited lower impact resistance than aramid ®bre composites because the level of maximum load had a major eect on the impact absorption energy. # 1998 Elsevier Science Ltd. All rights reserved Keywords: intraply hybrid composite, aramid ®bre Abstract
composite, ¯exural strength, strength, impact property
interlaminar
shear
1 INTRODUCTION
Hybrid composites consist of two or more types of ®bres in a common matrix.1±3 In recent years there has *To whom correspondence should be addressed. Fax: 82-2888-1604. 1621
R. Park, J. Jang
1622
2.3 Composite manufacturing
2 EXPERIMENTAL DETAILS 2.1 Materials
The fabric used for these intraply hybrid composites was the plain type of aramid ®bre and polyethylene (PE) ®bre. Two dierent intraply fabrics were used. The ®rst fabric was composed of aramid ®bre in the warp and polyethylene in the weft, so that the volume ratio of aramid ®bre to polyethylene ®bre was 1:1. The second fabric was composed of aramid ®bre in the warp, and aramid and polyethylene ®bres, by turns, in the weft, so that the volume ratio of two ®bres was 3:1. The aramid ®bre was Kevlar-29 from E. I. Du Pont de Nemours in the form of a 4800 denier, 480 ®lament yarn. The polyethylene ®bre was Spectra-900 from Allied Signal Co. in the form of a 3000 denier, 300 ®lament yarn. The structures of the two fabrics are shown in Fig. 1. The matrix resin was a styrene-based XSR-10 vinylester resin supplied by National Synthesis Co. (Korea). Benzoyl peroxide (BPO) was added to the matrix resin as initiator. The physical properties of Kevlar-29, Spectra900, and vinylester resin are given in Table 1. 2.2 Prepreg preparation
The hybrid composite prepeg was prepared by using vinylester resin with 2 wt% benzoyl peroxide. Each fabric was well impregnated with a solution of this mixture in acetone by means of a hand roller. The resinimpregnated fabrics were aged for 2 days in a hood at room temperature to allow thickening of the resin.
The composites were made by the open leaky-mold method. The eight-ply composites were then cured in a hot press for 20 min at 43 C and 50 min at 90 C at a pressure of 7 MPa (1000 psi). The thickness of the composites was approximately 3.5 mm. They were laminated with two dierent stacking sequences, [0/0]4 and [0/90]4. In the [0/0]4 lay-up, the evaluation of mechanical properties was done in two loading directions, the aramid and the PE directions (aramid direction means that the aramid ®bre lies parallel to the longitudinal direction of specimen). In the [0/90]4 lay-up, the aramid and PE ®bres were cross-plied and the loading direction for measurement of mechanical properties was denoted as the hybrid direction. Schematic diagrams of each loading direction in the (1/1) intraply hybrid composite are shown in Fig. 2. 2.4 Flexural properties
The ¯exural strength and modulus of the hybrid composites were measured in three-point bending tests according to the ASTM standard method D 790. The composites specimens, of dimensions 80220 mm, were tested with a support span of 54 mm at a crosshead speed of 1.3 mm minÿ1. 2.5 Interlaminar shear strength (ILSS)
The interlaminar shear strengths of the intraply hybrid composites were determined according to the ASTM D
The structure of two fabrics with dierent volume ratios of aramid ®ber to polyethylene ®ber: (a) 1:1; (b) 3:1.
Fig. 1.
Table 1. Physical properties of Kevlar-29, Spectra-900, and vinylester resin
Physical properties Density (g cmÿ3) Tensile modulus (GPa) Tensile strength (MPa) Maximum strain (%)
Kevlar-29 Spectra-900 Vinylester 1.44 62.00 2760.00 4.00
0.97 117.30 2500.00 3.50
1.15 3.71 63.30 6.30
A schematic diagram of the loading directions in (1/1) intraply hybrid composites: (a) PE direction; (b) aramid direction; (c) hybrid direction. Fig. 2.
Aramid/polyethylene intraply hybrid composite
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dierent kinds of ®bres determines the ¯exural properties of the hybrid composites. Figure 3 shows the ¯exural strength of intraply hybrid composite as a function of the volume fraction of aramid ®bre. The dotted line represents the values predicted by the rule of mixtures. The ¯exural strength increases with increasing aramid volume fraction. In addition, there is a negative deviation from the rule of mixtures in all intraply laminates. Because the intraply hybrid fabric contains two dierent ®bres in each layer, much residual stress is induced within the composite because of the dierence in thermal expansion coe15 cient of the two ®bres. Choy reported that the thermal expansion coecient of PE ®bre is larger than that of the aramid ®bre by a factor of 6 in the ®bre axial direction and by a factor of 4 in the ®bre radial direction. The increase in aramid ®bre content reduces aramid/PE contacts and residual thermal stresses, which contributes to the improvement in ¯exural strength. The residual stress, which is a maximum near the interface between the dierent ®bres, prevents load transfer between ®bres and matrix. Therefore, when the load is applied to a hybrid composite, the interphase region between the two dierent ®bres becomes weak and the composite fails initially in this region. This results in the negative hybrid eect. The ¯exural strengths and moduli of the (1/1) intraply hybrid composite in the dierent loading directions shown in Fig. 4. The ¯exural loading introduces tensile and compressive stresses and strains across the thickness of the composites. The applied load is initially distributed in the longitudinal direction of the specimen on the compressive side, and is transferred to the tension side from the compressive side through the thickness. Therefore, the ¯exural strength of a intraply hybrid mainly depends on which ®bre is present in the length direction of specimen.16,17 In the case of a laminate loaded in the aramid direction, the highest ¯exural strength is obtained because most of the load is carried by the aramid ®bre, whereas when the loading is in the PE direction, the lowest value is obtained. For the composite loaded in the hybrid direction, the ¯exural strength is nearly the mean value of those of the two laminates. This is attributed to the fact that the ¯exural strength decreases with large numbers of o-axis plies
2344 speci®cation. Specimens were of dimensions 20210 mm with a support span of 14 mm, and the crosshead speed was 2 mm minÿ1. 2.6 Impact property
Unnotched Izod impact test specimens were prepared according to the ASTM D256-84 standard. The capacity of this machine was 80 kgf cmÿ1 with a hammer striking velocity of 3.3 msÿ1. The dimensions of specimen were 621 cm.
et al.
2.7 Scanning electron microscopy (SEM)
Scanning electron microscopy (SEM) was used to study the fracture surfaces of the hybrid composites. The instrument used in this study was a JEOL JSM-35 and all specimens were coated with a thin layer of gold to eliminate charging eects.
3 RESULTS AND DISCUSSION 3.1 Flexural properties
Table 2 summarizes the results for the ¯exural strength and modulus of the intraply hybrid composites as a function of aramid ®bre content and loading direction. From the previous discussion of interply hybrids, it is clear that a aramid ®bre laminate exhibits higher ¯exural strength and modulus than a PE ®bre laminate because of the excellent ¯exural properties of aramid ®bres.7 The intraply (1/1) hybrid composite shows a lower ¯exural strength and modulus than the PE ®bre laminate, but the (3/1) intraply hybrid gave values between those of the aramid laminate and the PE laminate. This is due to the increase in the volume fraction of the aramid ®bre. In addition, the ¯exural behavior of the hybrid composites changes markedly when the stacking sequence and loading direction are altered in the hybrids. The stacking sequence has an eect on the relative locations of the dierent kinds of ®bre across the thickness of laminate. For example, in a [0/0]4 laminate of the (1/1) intraply hybrid composite, aramid/ aramid and PE/PE ®bre contacts occur across the whole thickness of the composite. However, in the [0/90]4 laminate aramid-PE contacts across the thickness of sample. The dierence in position and disposition of the
Table 2. The ¯exural strength and modulus of intraply hybrid composites as functions of aramid ®ber content and loading direction
Type
Notation
Lamination stacking sequence
Loading direction
Flexural strength (MPa)
Flexural modulus (GPa)
Aramid only
A
[0/0]4
Aramid
Intraply (1/1)
B1 B2 B3
[0/0]4 [0/0]4 [0/90]4
Aramid PE Hybrid
Intraply (3/1)
C1 C2 C3
[0/0]4 [0/0]4 [0/90]4
Aramid PE Hybrid
PE only
D
[0/0]4
PE
140.0 75.0 33.7 48.4 102.5 82.1 89.2 73.5
11.5 5.3 4.4 3.4 7.1 7.0 6.5 5.9
R. Park, J. Jang
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The ¯exural strength of intraply hybrid composites as a function of the volume fraction of aramid ®ber.
Fig. 3.
Fig. 4.
®bre interface across the thickness resists the applied load, and a higher ¯exural modulus is obtained. The ¯exural strengths and moduli of the (3/1) intraply hybrid composite in dierent loading directions are shown in Fig. 5. The trends of the variations of the ¯exural properties are found to be similar to those of Fig. 4. In the intraply hybrid composite, the stress changes continuously across the thickness of specimen, and the extent of employed stress changes with the nature of the ®bre/®bre contacts.14,20,21 In the hybrid direction, the stress is concentrated at the interface between the aramid and PE ®bres, and the composite cannot eectively resist the applied load. Therefore, an absence of an aramid/PE ®bre interface across the thickness, such as in the aramid and PE directions, results in higher ¯exural modulus. Figure 6 shows load/displacement curves for various composites in bending. The aramid laminate shows the highest maximum load, rapid load rise, and steep initial slope, and these results are attributed to the low elongation and brittle nature of the aramid ®bre. However, the PE ®bre laminate exhibits the slow load rise, high yield displacement, and lower maximum load. These features suggest that the composite bears the applied load up to higher displacements, and failure occurs in a ductile manner because of the high elongation of the PE ®bre. The (3/1) intraply hybrid composite shows an initial slope and maximum load between those of the aramid and PE laminates. The initial curve of the laminate resembles that of the aramid-®bre composite, and even after yielding the laminate supports the load continuously because of the presence of the PE ®bre. However, the (1/1) intraply hybrid composite exhibits the lowest maximum load and initial slope, and follows the behavior of the PE ®bre composite. Figure 7 represents the load/displacement curves of the (1/1) intraply hybrid composites in dierent loading directions. When the load is applied in the aramid
The ¯exural strength and modulus of the (1/1) intraply hybrid composite in dierent loading directions.
because most of the loads are carried by the 0 plies.18,19 In case of the ¯exural modulus, a slightly dierent result is obtained. The ¯exural modulus is a measure of the resistance to deformation of the composite in bending, and relates to the stiness on the compressive side. The ¯exural modulus is a critical function of the laminate stacking sequence and ¯exural failures may occur initially at dierent axial locations.18,19 When the load is in the hybrid direction, the failure initiates in the interface between the aramid and PE ®bres across the thickness of laminate. Therefore, the laminate cannot carry the load continuously and shows the lowest ¯exural modulus. However, in a laminate loaded in the aramid direction or the PE direction, the stier aramid/aramid
Fig. 5.
The ¯exural strength and modulus of the (3/1) intraply hybrid composites in dierent loading directions.
Aramid/polyethylene intraply hybrid composite
Load/displacement curves of various composites in bending: (A) aramid-®ber composite, (B) PE-®ber composite, (C) (1/1) intraply hybrid composite, (D) (3/1) intraply hybrid composite. Fig. 6.
direction, the rapid rise of load is very similar to the results for the aramid-®bre laminate, as shown in Fig. 6. In addition, in a laminate loaded in the PE direction, the load increases gradually as shown in the PE/®bre composite. This indicates that in bending most of the load is carried by ®bres in the longitudinal direction of the specimen. When the load is applied in the hybrid direction, the composite gives the lowest initial slope and an intermediate maximum load. Figure 8 shows the fracture surfaces of intraply hybrid composites obtained by SEM. The microscopy of ¯exural failures in hybrid composites requires attention because of the various possible failure modes.
7. Load/displacement curve of (1/1) intraply hybrid composite in dierent loading directions: (A) aramid-®ber direction, (B) PE-®ber direction, (C) hybrid direction. Fig.
1625
Figure 8(a)±(d) are the fracture surfaces of the intraply (1/1) hybrid composite, and (e)±(h) are those of the (3/1) intraply hybrid composite. The thick ®bre is the polyethylene ((b),(d),(f), and (h)), and the thin ®bre is the aramid ((a),(c),(e), and (g)). Figure 8(a), (b), (e), and (f) are laminated in the [0/90]4 sequence, and contact between aramid and PE ®bres exists across the thickness. As discussed above, the aramid/PE ®bre interface induces residual thermal stresses, and acts as a weak region on ¯exural loading. This results in adhesive failure of the intraply hybrid composite. Figure 8(a) and (b) shows clean ®bre surfaces, indicating that adhesive failure of the composite initiates in this region. Compared with Fig. 8(a), however, Fig. 8(e) shows much ®bre ®brillation and matrix cracking on the ®bre surface, and this is attributed to the fact that aramid/aramid ®bre interfaces exist across the thickness of composite and cohesive failure is induced. Although the two intraply hybrids have the same stacking sequence, the (3/1) intraply hybrid exhibits the higher ¯exural strength as a consequence of the introduction of aramid-aramid interfaces across the thickness. Figure 8(c), (d), (g), and (h) is laminated in the [0/0]4 sequence, and it is clear that aramid/aramid and PE/PE interfaces are introduced into the composite. The fracture surface shows matrix adhering to the ®bre surfaces and the curvature of the ®bres (Fig. 8(c)), ®brillation of the PE ®bres (Fig. 8(d) and (h)), and many ®bre ®brils and matrix cracked by cohesive failure (Fig. 8(g)). These aspects of the fracture behavior result in higher ¯exural strength and modulus. 3.2 Interlaminar shear strength (ILSS)
Table 3 summarizes the results for the ILSS of intraply hybrid composites as a function of aramid ®bre content and loading direction. The above experimental results may also be understood by following the ILSS tests of the hybrids. The trend of the variation of ILSS is found to be similar to that of the ¯exural properties in Table 2. As discussed earlier in relation to the ¯exural response, the aramid laminate exhibits a higher ILSS than the PE laminate because of the better resistance to shear of the aramid ®bre, compared with that of the PE ®bre. In contrast with the results for ¯exural properties, however, the intraply hybrid composites show lower ILSS values than the PE/®bre laminate. This is due to the dierence in the failure modes in the ¯exural and ILSS tests. In the ¯exural mode, only the normal stress is applied to the laminate, and shear de¯ection is minimized. This means that the ¯exural strength depends primarily on the ®bre properties rather than on the ®bre/matrix interfacial strength. The intraply hybrid composite therefore exhibits more or less higher ¯exural strength than the PE laminate, in proportion to the aramid ®bre content. However, in a short-beam shear test, the maximum normal stress is quite low, while the shear stress is maximized. The beam will fail in the interlaminar shear mode by cracking along a horizontal plane between the laminate. This indicates that ILSS
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Fig. 8.
R. Park, J. Jang
The fracture surface of the (1/1) intraply hybrid composites ((a)±(d)) and (3/1) intraply hybrid composites ((e)±(h)): [0/90]4 (a),(b),(e),(f); [0/0]4 (c),(d),(g), and (h).
Aramid/polyethylene intraply hybrid composite
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Table 3. The interlaminar shear strength (ILSS) of intraply hybrid composites as a function of aramid ®ber content and loading direction
Type
Notation Lamination Loading ILSS (MPa) stacking direction sequence
Aramid only
A
[0/0]4
Aramid
Intraply (1/1)
B1 B2 B3
[0/0]4 [0/0]4 [0/90]4
Aramid PE Hybrid
Intraply (3/1)
C1 C2 C3
[0/0]4 [0/0]4 [0/90]4
Aramid PE Hybrid
PE only
D
[0/04]4
PE
12.35 6.14 2.22 3.87 9.01 5.89 6.08 8.23
depends primarily on the matrix properties and the ®bre/matrix interfacial strength rather than on the ®bre properties. As a result, the intraply hybrid composite has a lower ILSS than the PE composite because of the low interfacial strength caused by the introduction of aramid/PE ®bre interfaces. Figure 9 shows the ILSS values for intraply hybrid composites in dierent loading directions. The trend of the variation in the ILSS is found to be similar to that for the ¯exural properties as shown in Figs 4 and 5. When the load is applied in the aramid direction, the highest ILSS is obtained because most of the load is carried by the aramid ®bre, whereas when the loading is in the PE direction, the lowest value is obtained. For the composite loaded in the hybrid direction, the ILSS has approximately the mean value of those of the two laminates. This is attributed to the fact that the laminate contains both aramid and PE ®bre in the specimen longitudinal direction. In addition, the (3/1) intraply hybrid composite exhibits higher ILSS values than the intraply (1/1) hybrid composite.
Fig. 9.
ILSS of intraply hybrid composites in dierent loading directions.
3.3 Impact properties
The eects of aramid ®bre content and loading direction on the impact energy absorption in intraply hybrid composites are shown in Fig. 10. Total impact energy absorption is the sum of the energy absorbed up to the maximum load (initiation energy) and the absorbed energy after the maximum load (propagation energy).22 The impact resistance of composite is therefore governed by the maximum load on impact loading and the displacement by delamination. The PE composite shows the highest impact energy, and this suggests that the high degree of delamination and plastic deformation is the major source of impact-energy absorption. PE ®bre is a ductile ®bre with a high elongation, and it absorbs much impact energy by ®bre pull-out, delamination, and plastic deformation.10 The aramid-®bre composite exhibits relatively low impact energy compared with the PE/®bre composite. This is attributed to the fact that the increase in maximum load compensates for the decrease in delamination in the aramid composite. The
Variation of the impact energy adsorption of intraply hybrid composites with aramid-®ber content and loading direction. Fig. 10.
composite shows a low extent of delamination but large maximum load because of the high tensile strength and modulus of the aramid ®bre. In both composites, no ®bre breakage and through-thickness penetration were observed. A great amount of energy was consumed in deforming the laminate and creating delaminations between the plies. Intraply hybrid composites exhibit lower impact resistance than aramid/®bre laminates. This is attributed to the fact that although the degree of delamination is increased by virtue of the low interfacial strength of the intraply hybrid composite, the maximum load, by contrast, exhibits a very low value. In these materials, the level of maximum load has a major eect
R. Park, J. Jang
1628
on the impact resistance. Therefore, the (3/1) intraply hybrid composite shows better impact resistance than the (1/1) intraply hybrid as a consequence of the increase in maximum load. On the other hand, on changing the loading direction, when the load is applied in the aramid direction a higher impact resistance occurs because of the increase in the maximum load.
4 CONCLUSIONS
The mechanical properties of aramid/polyethylene intraply hybrid composites have been investigated as functions of aramid ®bre content and loading direction. The ¯exural strength increased with the aramid volume fraction. This was attributed to the fact that the increase in aramid ®bre content reduced the aramid/PE ®bre contacts and residual thermal stresses. The ¯exural behavior of the hybrid composites also changed signi®cantly with stacking sequence and loading direction. Flexural strength mainly depended on which ®bre was present in the longitudinal direction of the specimen, whereas the ¯exural modulus was aected by the presence of the stier aramid/aramid ®bre interfaces across the thickness. When the load was applied in the hybrid direction, the lowest ¯exural modulus was obtained because the load was concentrated on the interfaces between aramid and PE ®bres across the thickness. The trend of variation of ILSS was very similar to that of the ¯exural properties. In contrast with the results for ¯exural properties, however, intraply hybrid composites exhibited lower ILSS values than the PE-®bre composite. This was due to the dierence in the failure modes in the two tests. The ¯exural strength depended mainly on the ®bre properties, whereas the ILSS depended primarily on the matrix properties and ®bre/matrix interfacial strength rather than on the ®bre properties. Intraply hybrid composites exhibited lower impact resistances than the aramid-®bre laminate because, although the degree of delamination increased as a result of the low interfacial strength of the intraply hybrid composite, the maximum load decreased.
Fiber Composite Hybrid Materials
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This work was supported by Korea Science and Engineering Foundation (No. 95-0300-02-04-3).
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ACKNOWLEDGEMENT
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P I I : S 0 2 6 6 - 3 5 3 8 ( 9 7 ) 0 0 2 2 8 - 5
Printed in Great Britain 0266-3538/98 $Ðsee front matter
THE EFFECTS OF HYBRIDIZATION ON THE MECHANICAL PERFORMANCE OF ARAMID/POLYETHYLENE INTRAPLY FABRIC COMPOSITES Rohchoon Park & Jyongsik Jang*
Department of Chemical Technology, Seoul National University, San 56-1, Shinlimdong Kwanakgu, Seoul, Korea (Received 16 June 1997; revised 20 October 1997; accepted 20 November 1997)
been extensive interest in such materials, particularly those which are based on a combination of a cheap ®bre with a stier ®bre.4±6 In high-technology ®elds, the question of cost may be insigni®cant by comparison with the advantages of optimizing properties.7,8 Polyethylene-®bre-reinforced composites are attractive materials because of their high impact resistance and they are much used in military applications such as for helicopter and armor body. On the other hand, this material has the disadvantage that its tensile, ¯exural, and thermal properties are poor.9±11 To compensate for this defect, aramid/polyethylene hybrid composites have been designed. One of the most important purposes of using aramid/polyethylene hybrid composite is to combine the good mechanical property of aramid ®bre with the excellent impact resistance of polyethylene ®bre. Depending on the geometric pattern of ®bre arrangements, hybrid composites can be classi®ed as interply(interlaminated) and intraply(intermingled).12,13 In intraply hybrid composites, each fabric consists of two or more kinds of ®bre. The mechanical properties of intraply hybrid composites should be systematically studied for applications as structural components. Among the mechanical properties, the ¯exural properties, interlaminar shear strength, and impact properties are especially important because the information about them is applied to real systems and makes design with the new material possible. The volume ratio and stacking sequence of dierent components in the hybrid material act as determining factors in the variation of the mechanical properties. When the stier ®bres are placed in the outer parts of the laminate, the hybrid composite exhibits a higher ¯exural modulus.14 In addition, for wider structural application it is necessary to investigate the eects of stacking sequence and loading direction on the mechanical properties of these materials. In this study, the ¯exural properties, interlaminar shear strength, and impact properties of aramid/polyethylene intraply hybrid composites have been investigated as a function of lamination stacking sequence and aramid ®bre content. The changes in mechanical property with dierent loading direction are also examined.
Intraply hybrid composites have been fabricated by the open leaky-mold method in order to examine the eects of hybridization on the mechanical performance of aramid/ polyethylene hybrid composite. The ¯exural properties, interlaminar shear strength (ILSS), and impact properties of these composites have been studied as a function of aramid ®bre content and loading direction. Two dierent stacking sequences ([0/0]4 and [0/90]4) were selected for this study, and composites of four dierent aramid ®bre contents (100, 75, 50, and 0% by volume) have been studied. In addition, scanning electron microscopy (SEM) was used to examine the fracture surfaces of the intraply hybrid composites. The ¯exural strength and modulus increased with the volume fraction of aramid ®bre, and negative deviations from the rule of mixture were observed in all intraply laminates. When the load was applied in the aramid direction, the highest ¯exural strength was obtained. This is attributed to the fact that the ¯exural strength mainly depends on whichever ®bre is present in the longitudinal direction of the specimen. In contrast with the ¯exural property results, intraply hybrid composites exhibited lower ILSS values than polyethylene-®bre composites because of the dierence in the failure modes in the ¯exural and ILSS tests. In addition, intraply hybrid composites exhibited lower impact resistance than aramid ®bre composites because the level of maximum load had a major eect on the impact absorption energy. # 1998 Elsevier Science Ltd. All rights reserved Keywords: intraply hybrid composite, aramid ®bre Abstract
composite, ¯exural strength, strength, impact property
interlaminar
shear
1 INTRODUCTION
Hybrid composites consist of two or more types of ®bres in a common matrix.1±3 In recent years there has *To whom correspondence should be addressed. Fax: 82-2888-1604. 1621
R. Park, J. Jang
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2.3 Composite manufacturing
2 EXPERIMENTAL DETAILS 2.1 Materials
The fabric used for these intraply hybrid composites was the plain type of aramid ®bre and polyethylene (PE) ®bre. Two dierent intraply fabrics were used. The ®rst fabric was composed of aramid ®bre in the warp and polyethylene in the weft, so that the volume ratio of aramid ®bre to polyethylene ®bre was 1:1. The second fabric was composed of aramid ®bre in the warp, and aramid and polyethylene ®bres, by turns, in the weft, so that the volume ratio of two ®bres was 3:1. The aramid ®bre was Kevlar-29 from E. I. Du Pont de Nemours in the form of a 4800 denier, 480 ®lament yarn. The polyethylene ®bre was Spectra-900 from Allied Signal Co. in the form of a 3000 denier, 300 ®lament yarn. The structures of the two fabrics are shown in Fig. 1. The matrix resin was a styrene-based XSR-10 vinylester resin supplied by National Synthesis Co. (Korea). Benzoyl peroxide (BPO) was added to the matrix resin as initiator. The physical properties of Kevlar-29, Spectra900, and vinylester resin are given in Table 1. 2.2 Prepreg preparation
The hybrid composite prepeg was prepared by using vinylester resin with 2 wt% benzoyl peroxide. Each fabric was well impregnated with a solution of this mixture in acetone by means of a hand roller. The resinimpregnated fabrics were aged for 2 days in a hood at room temperature to allow thickening of the resin.
The composites were made by the open leaky-mold method. The eight-ply composites were then cured in a hot press for 20 min at 43 C and 50 min at 90 C at a pressure of 7 MPa (1000 psi). The thickness of the composites was approximately 3.5 mm. They were laminated with two dierent stacking sequences, [0/0]4 and [0/90]4. In the [0/0]4 lay-up, the evaluation of mechanical properties was done in two loading directions, the aramid and the PE directions (aramid direction means that the aramid ®bre lies parallel to the longitudinal direction of specimen). In the [0/90]4 lay-up, the aramid and PE ®bres were cross-plied and the loading direction for measurement of mechanical properties was denoted as the hybrid direction. Schematic diagrams of each loading direction in the (1/1) intraply hybrid composite are shown in Fig. 2. 2.4 Flexural properties
The ¯exural strength and modulus of the hybrid composites were measured in three-point bending tests according to the ASTM standard method D 790. The composites specimens, of dimensions 80220 mm, were tested with a support span of 54 mm at a crosshead speed of 1.3 mm minÿ1. 2.5 Interlaminar shear strength (ILSS)
The interlaminar shear strengths of the intraply hybrid composites were determined according to the ASTM D
The structure of two fabrics with dierent volume ratios of aramid ®ber to polyethylene ®ber: (a) 1:1; (b) 3:1.
Fig. 1.
Table 1. Physical properties of Kevlar-29, Spectra-900, and vinylester resin
Physical properties Density (g cmÿ3) Tensile modulus (GPa) Tensile strength (MPa) Maximum strain (%)
Kevlar-29 Spectra-900 Vinylester 1.44 62.00 2760.00 4.00
0.97 117.30 2500.00 3.50
1.15 3.71 63.30 6.30
A schematic diagram of the loading directions in (1/1) intraply hybrid composites: (a) PE direction; (b) aramid direction; (c) hybrid direction. Fig. 2.
Aramid/polyethylene intraply hybrid composite
1623
dierent kinds of ®bres determines the ¯exural properties of the hybrid composites. Figure 3 shows the ¯exural strength of intraply hybrid composite as a function of the volume fraction of aramid ®bre. The dotted line represents the values predicted by the rule of mixtures. The ¯exural strength increases with increasing aramid volume fraction. In addition, there is a negative deviation from the rule of mixtures in all intraply laminates. Because the intraply hybrid fabric contains two dierent ®bres in each layer, much residual stress is induced within the composite because of the dierence in thermal expansion coe15 cient of the two ®bres. Choy reported that the thermal expansion coecient of PE ®bre is larger than that of the aramid ®bre by a factor of 6 in the ®bre axial direction and by a factor of 4 in the ®bre radial direction. The increase in aramid ®bre content reduces aramid/PE contacts and residual thermal stresses, which contributes to the improvement in ¯exural strength. The residual stress, which is a maximum near the interface between the dierent ®bres, prevents load transfer between ®bres and matrix. Therefore, when the load is applied to a hybrid composite, the interphase region between the two dierent ®bres becomes weak and the composite fails initially in this region. This results in the negative hybrid eect. The ¯exural strengths and moduli of the (1/1) intraply hybrid composite in the dierent loading directions shown in Fig. 4. The ¯exural loading introduces tensile and compressive stresses and strains across the thickness of the composites. The applied load is initially distributed in the longitudinal direction of the specimen on the compressive side, and is transferred to the tension side from the compressive side through the thickness. Therefore, the ¯exural strength of a intraply hybrid mainly depends on which ®bre is present in the length direction of specimen.16,17 In the case of a laminate loaded in the aramid direction, the highest ¯exural strength is obtained because most of the load is carried by the aramid ®bre, whereas when the loading is in the PE direction, the lowest value is obtained. For the composite loaded in the hybrid direction, the ¯exural strength is nearly the mean value of those of the two laminates. This is attributed to the fact that the ¯exural strength decreases with large numbers of o-axis plies
2344 speci®cation. Specimens were of dimensions 20210 mm with a support span of 14 mm, and the crosshead speed was 2 mm minÿ1. 2.6 Impact property
Unnotched Izod impact test specimens were prepared according to the ASTM D256-84 standard. The capacity of this machine was 80 kgf cmÿ1 with a hammer striking velocity of 3.3 msÿ1. The dimensions of specimen were 621 cm.
et al.
2.7 Scanning electron microscopy (SEM)
Scanning electron microscopy (SEM) was used to study the fracture surfaces of the hybrid composites. The instrument used in this study was a JEOL JSM-35 and all specimens were coated with a thin layer of gold to eliminate charging eects.
3 RESULTS AND DISCUSSION 3.1 Flexural properties
Table 2 summarizes the results for the ¯exural strength and modulus of the intraply hybrid composites as a function of aramid ®bre content and loading direction. From the previous discussion of interply hybrids, it is clear that a aramid ®bre laminate exhibits higher ¯exural strength and modulus than a PE ®bre laminate because of the excellent ¯exural properties of aramid ®bres.7 The intraply (1/1) hybrid composite shows a lower ¯exural strength and modulus than the PE ®bre laminate, but the (3/1) intraply hybrid gave values between those of the aramid laminate and the PE laminate. This is due to the increase in the volume fraction of the aramid ®bre. In addition, the ¯exural behavior of the hybrid composites changes markedly when the stacking sequence and loading direction are altered in the hybrids. The stacking sequence has an eect on the relative locations of the dierent kinds of ®bre across the thickness of laminate. For example, in a [0/0]4 laminate of the (1/1) intraply hybrid composite, aramid/ aramid and PE/PE ®bre contacts occur across the whole thickness of the composite. However, in the [0/90]4 laminate aramid-PE contacts across the thickness of sample. The dierence in position and disposition of the
Table 2. The ¯exural strength and modulus of intraply hybrid composites as functions of aramid ®ber content and loading direction
Type
Notation
Lamination stacking sequence
Loading direction
Flexural strength (MPa)
Flexural modulus (GPa)
Aramid only
A
[0/0]4
Aramid
Intraply (1/1)
B1 B2 B3
[0/0]4 [0/0]4 [0/90]4
Aramid PE Hybrid
Intraply (3/1)
C1 C2 C3
[0/0]4 [0/0]4 [0/90]4
Aramid PE Hybrid
PE only
D
[0/0]4
PE
140.0 75.0 33.7 48.4 102.5 82.1 89.2 73.5
11.5 5.3 4.4 3.4 7.1 7.0 6.5 5.9
R. Park, J. Jang
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The ¯exural strength of intraply hybrid composites as a function of the volume fraction of aramid ®ber.
Fig. 3.
Fig. 4.
®bre interface across the thickness resists the applied load, and a higher ¯exural modulus is obtained. The ¯exural strengths and moduli of the (3/1) intraply hybrid composite in dierent loading directions are shown in Fig. 5. The trends of the variations of the ¯exural properties are found to be similar to those of Fig. 4. In the intraply hybrid composite, the stress changes continuously across the thickness of specimen, and the extent of employed stress changes with the nature of the ®bre/®bre contacts.14,20,21 In the hybrid direction, the stress is concentrated at the interface between the aramid and PE ®bres, and the composite cannot eectively resist the applied load. Therefore, an absence of an aramid/PE ®bre interface across the thickness, such as in the aramid and PE directions, results in higher ¯exural modulus. Figure 6 shows load/displacement curves for various composites in bending. The aramid laminate shows the highest maximum load, rapid load rise, and steep initial slope, and these results are attributed to the low elongation and brittle nature of the aramid ®bre. However, the PE ®bre laminate exhibits the slow load rise, high yield displacement, and lower maximum load. These features suggest that the composite bears the applied load up to higher displacements, and failure occurs in a ductile manner because of the high elongation of the PE ®bre. The (3/1) intraply hybrid composite shows an initial slope and maximum load between those of the aramid and PE laminates. The initial curve of the laminate resembles that of the aramid-®bre composite, and even after yielding the laminate supports the load continuously because of the presence of the PE ®bre. However, the (1/1) intraply hybrid composite exhibits the lowest maximum load and initial slope, and follows the behavior of the PE ®bre composite. Figure 7 represents the load/displacement curves of the (1/1) intraply hybrid composites in dierent loading directions. When the load is applied in the aramid
The ¯exural strength and modulus of the (1/1) intraply hybrid composite in dierent loading directions.
because most of the loads are carried by the 0 plies.18,19 In case of the ¯exural modulus, a slightly dierent result is obtained. The ¯exural modulus is a measure of the resistance to deformation of the composite in bending, and relates to the stiness on the compressive side. The ¯exural modulus is a critical function of the laminate stacking sequence and ¯exural failures may occur initially at dierent axial locations.18,19 When the load is in the hybrid direction, the failure initiates in the interface between the aramid and PE ®bres across the thickness of laminate. Therefore, the laminate cannot carry the load continuously and shows the lowest ¯exural modulus. However, in a laminate loaded in the aramid direction or the PE direction, the stier aramid/aramid
Fig. 5.
The ¯exural strength and modulus of the (3/1) intraply hybrid composites in dierent loading directions.
Aramid/polyethylene intraply hybrid composite
Load/displacement curves of various composites in bending: (A) aramid-®ber composite, (B) PE-®ber composite, (C) (1/1) intraply hybrid composite, (D) (3/1) intraply hybrid composite. Fig. 6.
direction, the rapid rise of load is very similar to the results for the aramid-®bre laminate, as shown in Fig. 6. In addition, in a laminate loaded in the PE direction, the load increases gradually as shown in the PE/®bre composite. This indicates that in bending most of the load is carried by ®bres in the longitudinal direction of the specimen. When the load is applied in the hybrid direction, the composite gives the lowest initial slope and an intermediate maximum load. Figure 8 shows the fracture surfaces of intraply hybrid composites obtained by SEM. The microscopy of ¯exural failures in hybrid composites requires attention because of the various possible failure modes.
7. Load/displacement curve of (1/1) intraply hybrid composite in dierent loading directions: (A) aramid-®ber direction, (B) PE-®ber direction, (C) hybrid direction. Fig.
1625
Figure 8(a)±(d) are the fracture surfaces of the intraply (1/1) hybrid composite, and (e)±(h) are those of the (3/1) intraply hybrid composite. The thick ®bre is the polyethylene ((b),(d),(f), and (h)), and the thin ®bre is the aramid ((a),(c),(e), and (g)). Figure 8(a), (b), (e), and (f) are laminated in the [0/90]4 sequence, and contact between aramid and PE ®bres exists across the thickness. As discussed above, the aramid/PE ®bre interface induces residual thermal stresses, and acts as a weak region on ¯exural loading. This results in adhesive failure of the intraply hybrid composite. Figure 8(a) and (b) shows clean ®bre surfaces, indicating that adhesive failure of the composite initiates in this region. Compared with Fig. 8(a), however, Fig. 8(e) shows much ®bre ®brillation and matrix cracking on the ®bre surface, and this is attributed to the fact that aramid/aramid ®bre interfaces exist across the thickness of composite and cohesive failure is induced. Although the two intraply hybrids have the same stacking sequence, the (3/1) intraply hybrid exhibits the higher ¯exural strength as a consequence of the introduction of aramid-aramid interfaces across the thickness. Figure 8(c), (d), (g), and (h) is laminated in the [0/0]4 sequence, and it is clear that aramid/aramid and PE/PE interfaces are introduced into the composite. The fracture surface shows matrix adhering to the ®bre surfaces and the curvature of the ®bres (Fig. 8(c)), ®brillation of the PE ®bres (Fig. 8(d) and (h)), and many ®bre ®brils and matrix cracked by cohesive failure (Fig. 8(g)). These aspects of the fracture behavior result in higher ¯exural strength and modulus. 3.2 Interlaminar shear strength (ILSS)
Table 3 summarizes the results for the ILSS of intraply hybrid composites as a function of aramid ®bre content and loading direction. The above experimental results may also be understood by following the ILSS tests of the hybrids. The trend of the variation of ILSS is found to be similar to that of the ¯exural properties in Table 2. As discussed earlier in relation to the ¯exural response, the aramid laminate exhibits a higher ILSS than the PE laminate because of the better resistance to shear of the aramid ®bre, compared with that of the PE ®bre. In contrast with the results for ¯exural properties, however, the intraply hybrid composites show lower ILSS values than the PE/®bre laminate. This is due to the dierence in the failure modes in the ¯exural and ILSS tests. In the ¯exural mode, only the normal stress is applied to the laminate, and shear de¯ection is minimized. This means that the ¯exural strength depends primarily on the ®bre properties rather than on the ®bre/matrix interfacial strength. The intraply hybrid composite therefore exhibits more or less higher ¯exural strength than the PE laminate, in proportion to the aramid ®bre content. However, in a short-beam shear test, the maximum normal stress is quite low, while the shear stress is maximized. The beam will fail in the interlaminar shear mode by cracking along a horizontal plane between the laminate. This indicates that ILSS
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Fig. 8.
R. Park, J. Jang
The fracture surface of the (1/1) intraply hybrid composites ((a)±(d)) and (3/1) intraply hybrid composites ((e)±(h)): [0/90]4 (a),(b),(e),(f); [0/0]4 (c),(d),(g), and (h).
Aramid/polyethylene intraply hybrid composite
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Table 3. The interlaminar shear strength (ILSS) of intraply hybrid composites as a function of aramid ®ber content and loading direction
Type
Notation Lamination Loading ILSS (MPa) stacking direction sequence
Aramid only
A
[0/0]4
Aramid
Intraply (1/1)
B1 B2 B3
[0/0]4 [0/0]4 [0/90]4
Aramid PE Hybrid
Intraply (3/1)
C1 C2 C3
[0/0]4 [0/0]4 [0/90]4
Aramid PE Hybrid
PE only
D
[0/04]4
PE
12.35 6.14 2.22 3.87 9.01 5.89 6.08 8.23
depends primarily on the matrix properties and the ®bre/matrix interfacial strength rather than on the ®bre properties. As a result, the intraply hybrid composite has a lower ILSS than the PE composite because of the low interfacial strength caused by the introduction of aramid/PE ®bre interfaces. Figure 9 shows the ILSS values for intraply hybrid composites in dierent loading directions. The trend of the variation in the ILSS is found to be similar to that for the ¯exural properties as shown in Figs 4 and 5. When the load is applied in the aramid direction, the highest ILSS is obtained because most of the load is carried by the aramid ®bre, whereas when the loading is in the PE direction, the lowest value is obtained. For the composite loaded in the hybrid direction, the ILSS has approximately the mean value of those of the two laminates. This is attributed to the fact that the laminate contains both aramid and PE ®bre in the specimen longitudinal direction. In addition, the (3/1) intraply hybrid composite exhibits higher ILSS values than the intraply (1/1) hybrid composite.
Fig. 9.
ILSS of intraply hybrid composites in dierent loading directions.
3.3 Impact properties
The eects of aramid ®bre content and loading direction on the impact energy absorption in intraply hybrid composites are shown in Fig. 10. Total impact energy absorption is the sum of the energy absorbed up to the maximum load (initiation energy) and the absorbed energy after the maximum load (propagation energy).22 The impact resistance of composite is therefore governed by the maximum load on impact loading and the displacement by delamination. The PE composite shows the highest impact energy, and this suggests that the high degree of delamination and plastic deformation is the major source of impact-energy absorption. PE ®bre is a ductile ®bre with a high elongation, and it absorbs much impact energy by ®bre pull-out, delamination, and plastic deformation.10 The aramid-®bre composite exhibits relatively low impact energy compared with the PE/®bre composite. This is attributed to the fact that the increase in maximum load compensates for the decrease in delamination in the aramid composite. The
Variation of the impact energy adsorption of intraply hybrid composites with aramid-®ber content and loading direction. Fig. 10.
composite shows a low extent of delamination but large maximum load because of the high tensile strength and modulus of the aramid ®bre. In both composites, no ®bre breakage and through-thickness penetration were observed. A great amount of energy was consumed in deforming the laminate and creating delaminations between the plies. Intraply hybrid composites exhibit lower impact resistance than aramid/®bre laminates. This is attributed to the fact that although the degree of delamination is increased by virtue of the low interfacial strength of the intraply hybrid composite, the maximum load, by contrast, exhibits a very low value. In these materials, the level of maximum load has a major eect
R. Park, J. Jang
1628
on the impact resistance. Therefore, the (3/1) intraply hybrid composite shows better impact resistance than the (1/1) intraply hybrid as a consequence of the increase in maximum load. On the other hand, on changing the loading direction, when the load is applied in the aramid direction a higher impact resistance occurs because of the increase in the maximum load.
4 CONCLUSIONS
The mechanical properties of aramid/polyethylene intraply hybrid composites have been investigated as functions of aramid ®bre content and loading direction. The ¯exural strength increased with the aramid volume fraction. This was attributed to the fact that the increase in aramid ®bre content reduced the aramid/PE ®bre contacts and residual thermal stresses. The ¯exural behavior of the hybrid composites also changed signi®cantly with stacking sequence and loading direction. Flexural strength mainly depended on which ®bre was present in the longitudinal direction of the specimen, whereas the ¯exural modulus was aected by the presence of the stier aramid/aramid ®bre interfaces across the thickness. When the load was applied in the hybrid direction, the lowest ¯exural modulus was obtained because the load was concentrated on the interfaces between aramid and PE ®bres across the thickness. The trend of variation of ILSS was very similar to that of the ¯exural properties. In contrast with the results for ¯exural properties, however, intraply hybrid composites exhibited lower ILSS values than the PE-®bre composite. This was due to the dierence in the failure modes in the two tests. The ¯exural strength depended mainly on the ®bre properties, whereas the ILSS depended primarily on the matrix properties and ®bre/matrix interfacial strength rather than on the ®bre properties. Intraply hybrid composites exhibited lower impact resistances than the aramid-®bre laminate because, although the degree of delamination increased as a result of the low interfacial strength of the intraply hybrid composite, the maximum load decreased.
Fiber Composite Hybrid Materials
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Polymer Testing
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This work was supported by Korea Science and Engineering Foundation (No. 95-0300-02-04-3).
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J. Appl. Mech.
J. Mater. Sci.
Polymer Composites
J. Appl. Polym. Sci.
Proceedings of 2nd Japan International SAMPE Symposium nol.
ACKNOWLEDGEMENT
J. Mater. Sci.
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