Abstract

Hybrid composite laminates, composed of carbon and glass fiber in epoxy matrix, were made.  Specimens were grouped according to the relative amount of glass/carbon fibers and laminate geometry.  Tensile and compressive properties and failure criteria have been investigated using three methods: mechanical test, finite element analysis and scanning electron microscope. Emphasize was given upon the effects of relative proportion of the two fiber types and laminate geometry on tensile and compressive properties.

Keywords: Hybrid composites; carbon fiber; glass fiber; hybrid ratio; hybrid effect; finite element analysis; scanning electron microscope.

 1. Introduction

Hybrid composites comprise at least two to more than two reinforcement fibers in a common matrix. Hybrid composites can be classified as inter-ply or laminated hybrids, intra-ply or tow-by-tow hybrids, randomly mixed hybrids, and other types of mixtures. Some key parameters that affect the performance of the material from reinforcement prospective include: length of fiber, fiber orientation, fiber shape and fiber material 1, 2. Since there is a significant difference between the properties of carbon and glass fibers, and interfacial properties between the fibers and matrix the hybridization effects would vary significantly for the hybrid composites 3-5. Fibers provide toughness, impact resistance, and energy absorption to the composite 6.

Plenty of works have been carried out on mechanical properties evaluation of glass/carbon hybrid composites 7-12. Research on fiber hybrid composites started several decades ago. After the invention of carbon fibers in the 1960s 13, the high price was their main drawback. In an attempt to reduce the price, while still exploiting the exceptional properties of carbon fiber, hybridization became a highly active research area in the 1970s and 1980s. In general, the purpose of utilizing two fiber types in a single composite is to maintain the advantages of both fibers and improve some disadvantages. For instance, replacing carbon fibers in the middle of a laminate by cheaper glass fibers can significantly reduce the cost, while the flexural properties remain almost unaffected. If a hybrid composite is loaded in the fiber direction in tension, then the more brittle fibers will fail before the more ductile fibers. In a work the authors hybridized woven glass and carbon fiber and found improvements in failure strain of carbon fiber, ranging between 10% and 30% 10. In another work the authors found that the ultimate tensile strength of unidirectional glass/flax composites increased by 15% if the dispersion was improved 11. Experimental results show that during tensile loading, composites sustain greater loads as the angle between the fiber orientation and the load direction increases 14. In a work the authors investigated the tensile and compressive behaviors of hybrid glass/carbon fabric composites and reported that higher tensile strength and ultimate tensile strain could be obtained when glass fabric layers are placed at the exterior and the carbon fabric layers in the interior as sandwich configuration 15. From the data they presented, better performance can be noticed for compressive properties either 15.

In the present study, unidirectional non-crimp carbon, glass and glass/carbon hybrid fabrics with epoxy resin matrix have been used for fabricating hybrid composite laminates. Effects of different proportions of the two fibers and two different laminate geometries (intra-layer and inter-layer) on the tensile and compressive responses have been investigated experimentally and computationally using a commercial software package ABAQUS/standard. Scanning electron microscopy (SEM) was used to investigate the damage criterion and damage morphology of the specimens.

2. Materials and methods

2.1. Materials, manufacturing and mechanical tests

Unidirectional non-crimp plain carbon (T620S) and plain E-glass and glass/carbon hybrid preforms were used to reinforce epoxy (EPIKOTETM MGS^® RIMR135) cured with hardener. Vacuum assisted resin infusion (VARI), Fig. 1(b), was used to fabricate eight different composite laminates with five different glass/carbon ratios. Composite laminates were cured in a closed chamber oven at a constant pressure of 0.1 MPa and constant temperature of 70°C for 7 hours. Hybrid laminates were categorized into three groups according to different compositions. Each of these groups (I, II, and III) comprises two specimens; one intra-layer hybrid C:G-n:n and one inter-layer hybrid specimen Cn/Gn, where n = 1,2,3, Fig. 1(a). The overall fiber volume fractions of the composites are furnished in Table 1. The ISO 527:5 and ISO 14126 standards were used for the tensile and compression tests using a universal testing machine at room temperature. The average loading rate was 0.03 mm/sec for both tensile and compression tests. The thickness of the cured composite was between 1.5 mm and 2.5 mm. The specimen dimension of the test specimens as per ISO standards are given in Fig. 2(a-b).  At least five tests were carried out for each specimen. The SEM micrographs were taken from a Hitachi TM 3000 microscope.

Table 1. Average tensile properties of composite laminates

Composite laminate	Overall fiber volume fraction (Vf)	Strain (%)	Stress (MPa)	Modulus (GPa)	Specific strength (KNm/kg)	Specific stiffness (MNm/kg)	Density (g/cc)	Hybrid ratio (r)
GFRP	0.539	2.51 (±0.11)	835 (±11.65)	33.26	571.938	22.78	1.460	0.00
Group I
C:G-1:4	0.412	2.28 (±0.26)	987.4 (±8.79)	43.30	745.207	33.67	1.325	0.20
C1/G4	0.413	2.21 (±0.27)	942.1 (±9.12)	42.62	711.018	32.16
Group II
C:G-1:2	0.422	2.11 (±0.18)	1021 (±7.4)	48.38	786.594	37.27	1.298	0.33
C1/G2	0.426	2.04 (±0.14)	1003.3 (±5.2)	49.18	772.958	37.88
Group III
C:G-1:1	0.437	1.96 (±0.19)	1282 (±8.01)	65.41	1007.148	51.38	1.273	0.50
C1/G1	0.431	1.88 (±0.14)	1214.3 (±7.1)	64.59	953.888	50.73
CFRP	0.563	1.70 (±0.18)	1785 (±7.98)	105.01	1443.27	84.89	1.237	1.00

2.2. Finite element analysis

The tensile and compressive behaviour were further studied using a commercial software package

ABAQUS/Standard (Version 6.11). The parts used for the analysis were 3D deformable solid of extrusion type. The input material properties to the FE models calculation were the density, mechanical properties and the Hashin damage criteria. The lamina properties including the longitudinal modulus E₁₁, the transverse moduli E₂₂ and E₃₃, and the shear moduli G₁₂, G₁₃ and G₂₃ were derived by Hashin’s model 16. Each part was assigned the respective material properties and assembled together. Meshing of the model was done with the eight-node reduced integration element (C3D8R), Fig. 3. The element size (0.5mm) was kept constant for all the specimens under both tensile and compression tests. Hashin formulation does not consider the failure by delamination. Therefore, in order to see the delamination artificially, a surface to surface cohesive zone modeling was created at the interfaces and the quadratic traction damage initiation criterion for cohesive surfaces was included in the Hashin damage models. Appropriate boundary conditions were applied and the specimens were loaded (at a rate of 0.03 mm/sec) to mimic the experimental conditions.

Table 2. Average compressive properties of composite laminates

Composite laminate	Strain (%)	Stress (MPa)	Modulus (GPa)	Specific strength (KNm/kg)	Specific stiffness (MNm/kg)
GFRP	1.51 (±0.09)	489.8 (±9.2)	32.43	335.4	22.21
Group I
C:G-1:4	1.36 (±0.11)	572.5 (±5.1)	42.09	432.1	31.76
C1/G4	1.27 (±0.21)	547.1 (±6.3)	43.07	412.9	32.50
Group II
C:G-1:2	1.16 (±0.22)	683.0 (±7.3)	58.87	526.2	45.35
C1/G2	1.10 (±0.14)	633.3 (±5.5)	57.57	487.9	44.35
Group III
C:G-1:1	1.02 (±0.21)	797.2 (±5.0)	78.18	626.2	61.41
C1/G1	0.97 (±0.15)	747.0 (±7.9)	77.01	586.8	60.49
CFRP	0.70 (±0.18)	1081 (±9.1)	154.36	873.5	124.78

 
3. Results and discussion

The tensile stress-strain curves for all eight types of composite laminates are shown in Fig. 4(a) and Fig. 4(c) and the calculated mechanical property data are furnished in Table 1.  The purpose of the static tensile test was to determine the effect of relative proportion of glass/carbon and laminate geometry on tensile stress, modulus, strain-to-failure, specific strength and specific stiffness in longitudinal direction. The laminates exhibit a slight nonlinear elastic behaviour until breakage and the slop of the stress-strain curve increases as the relative proportion of CF content decreases. All laminates failed catastrophically except C:G-1:1.

In this intra-layer hybrid specimen, as also could be realized from Fig. 1(a), the low elongation CF is surrounded by the high elongation GF. The possible damage evolution in CF parts is constrained and hence it could be the reason why this intra-layer hybrid specimen did not fail catastrophically.

The compressive stress-strain plots, Fig. 5(a) and Fig. 5(c), also shows a slight nonlinearity and the final failure occurs catastrophically for all composite laminates except intra-layer specimen C:G-1:1. The main specific property of compressive stress-strain plots is that, the curves exhibit positive curvature which might be due to poor interface bonding between matrix and fiber.  Imperfect fiber-matrix bonding is a manufacturing deficiency that is created during curing process. This defect influences the role of the matrix that support the fibers when the composite laminates are under compression 17.

From the curves it seems that by increasing compression, this bonding becomes greater and the role of matrix against micro-buckling of fibers is more highlighted. As in figure, significant nonlinear deformation was often observed before the maximum stress, and this behaviour is associated to the plastic deformation of the polymeric matrix. Tensile and compressive behaviour were further studied by FEA. Fig. 4(b), Fig. 4(d), Fig. 5(b) and Fig. 5(d) show the experimental and FEA results, in general they are in good agreement. These figures also plot the tensile and compressive stress against hybrid ratio. As expected, CFRP had the highest tensile and compressive stress and lowest failure strain. The opposite is true for GFRP.  The results from the hybrid specimens exist in between those of CRPP and GFRP.  It is also showing that as the hybrid ratio increases, the tensile and compressive stresses increase.

Tensile strength of intra-layer hybrid specimen C:G-1:1 is 1.25 times and 1.3 times as that of C:G-1:2 and C:G-1:4, respectively, Table 1. Also, compressive strength of intra-layer hybrid specimen C:G-1:1 is 1.16 times and 1.39 times as that of C:G-1:2 and C:G-1:4, respectively, Table 2. A more or less similar result is seen for the inter-layer hybrid specimens. Similar effect of carbon fiber content ratio on tensile and compressive properties can be seen in a work, where the authors studied the tensile, compressive and flexural behaviour of glass/carbon fabric hybrid composites 10. Moduli, specific strengths and specific stiffness are also affected in a more or less same way. However, in case of tensile and compressive strain-to-failure, the opposite is true. Meaning that as the carbon content increases, the failure strain decreases, Table 1 and Table 2. The strain-to-failure is minimum when the proportion of carbon fiber content is substantial.

Percentage loss in strength of all hybrid specimens is noticed while compared to plain CFRP both under tension and compression. On the other hand, a significant percentage enhancement in tensile and compressive failure strain is noticed while compared to composite entirely reinforced with carbon fibers. The loss in strength and gain in failure strain are governed by the relative proportion of CF content since there is strength and strain compatibility of CF throughout the composites. Percentage loss in strength when compared to CFRP has also been reported in some other literatures 15. In a same work, the authors hybridized woven glass and carbon fiber and found improvements in failure strain, ranging between 10% and 30% 10.

In terms of laminate geometry, higher tensile and compressive properties are observed for all intra-layer hybrid specimens while compared to their counterpart inter-layer hybrid samples.  Tensile strength of intra-layer hybrid specimen C:G-1:1 is 1.05 times as that of inter-layer hybrid specimen C1/G1, Table 1. Compressive strength of intra-layer hybrid specimen C:G-1:1 is 1.06 times as that of inter-layer hybrid specimen C1/G1, Table 2. Other specimens also show a more or less same result. The reason for this higher tensile and compressive strengths could be due to intra-layer hybrids having a smaller interface/delaminated area, which should in principle result in better mechanical properties. The reason for the failure strain enhancement could be due to a more gradual failure as it can be seen in Fig. 4(a) and Fig. 5(a), the last parts of the stress-strain diagrams are not linear which to some extent have some sort of plateau near the end.

The linear Rule-of-Mixture (RoM) was used for calculating the tensile and compressive strength. Denoting the proportion of carbon fiber content of all fiber reinforcement is r, the Young’s modulus can be expressed using the simple RoM as:

(1)

where E_H denotes the modulus of hybrid composite, E_C denotes the modulus of carbon fiber composite and  E_G denotes the modulus of glass fiber composite. So the tensile/compressive stress can be expressed as:

          
(2)
where ɛ_C  and ɛ_G are the tensile/compressive strains of CFRP and GFRP respectively.

As it can be seen from Fig. 6(a-b), both the tensile and compressive stresses exhibited a negative hybridization effect, where the hybridization effect is shown by the deviation from the RoM behaviour. The trend of the results obtained from intra-layer and inter-layer are quite similar; however, more negative hybrid effect can be noticed for the case of inter-layer hybrid specimens. Hybrid effects were also found in some other works where the authors investigated the tensile properties of unidirectional carbon fabric/non-woven glass mat/polyester resin hybrid composite laminate 18 and others 5, 19. Negative hybrid effect was noticed for tensile failure strain though the hybrid effect for tensile modulus was not very significant. In case of hybrid effects for compressive properties a similar type of result was seen.

4. Damage morphology

Fractured specimens were analyzed using SEM, specimens were sputtered with gold before observation. The SEM analysis of fractured surfaces after the mechanical tensile and compression tests are shown in Fig. 7(a -d). The matrix cracking, formation of fracture line, fiber pull out and fiber-resin compatibility were studied. The observations revealed the laceration on carbon fiber due to the applied loads during tests. The formation of voids due to fiber pull out was noticed in fibers because of poor resin compatibility of synthetic fibers. The matrix cracking, Fig. 7(c), and fracture lines were formed on the surfaces that exhibited poor interfacial bond. Though in some places excellent adhesion between fiber and matrix is seen, Fig. 7(a) which represents the failure of composite specimens occurred due to fiber breakage not because of poor adhesion between fiber and matrix.

Both under tension and compression, carbon fiber deformed first, Fig. 7(b, c), and as it happened there was stress drop which can be noticed in stress-strain curves and as the load continued to increase the stress increased until final failure. Meaning that the rest of the stress was carried by the remaining glass fibers.  Which also means that the catastrophic failure of composites could be avoided through hybridization.

The unidirectional arrangement of the fibers is clearly visible in Fig. 7(a , c,  d).  This kind of fiber arrangement yields maximum mechanical properties in the principal loading/longitudinal direction. FEA stress contour plots resemble with mechanical test specimens, i.e., the maximum stress is somewhere near the center of the specimen. Figure 8 shows the FEA stress contour plots of CFRP, GFRP and hybrid specimen C1/G1. Delamination is noticed in plain fiber reinforced laminates. However, in case of hybrid laminates no significant delamination was observed in hybrid laminates.

5. Conclusion

Mechanical experiments and computational investigations were conducted on GF/CF hybrid laminates made with different proportions of the two fiber types and two different laminate geometry. Tensile and compressive properties of hybrid laminates largely depend on the relative proportion of CF content and the intra-layer geometry further optimizes them. Failure strain enhancement for hybrid laminates was noticed under both tension and compression while compared to CFRP. SEM observation revealed that under tension and compression, CF fail first due to having a very low strain-to-failure and as it happens the stress drops as can be seen in stress-strain plots. The material continues extending meaning that it is the glass fibers that contribute to the rest of the stress. In some cases last part of the stress-strain diagram is not linear which is to some extent has some sort of plateau near the end- meaning that the hybrid laminates fail gradually and the catastrophic failure behaviour can be avoided.

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