Tensile behavior of hybrid epoxy composite laminate containing carbon and basalt fibers

2.1 Materials

In the present study, we used plain woven carbon fiber (C120-3K; fabric weight=200±10 g/m²; fabric thickness=0.25±0.02 mm) purchased from Hyun Dai Fiber Co. Ltd. (Korea), and plain woven basalt fiber (EcoB4 F210; fabric weight=210±10 g/m²; fabric thickness=0.19±0.20 mm) provided by Secotech (Korea). The resin matrix used was a modified bisphenol A epoxy resin (HTC-667C; specific gravity=1.16±0.02; viscosity=1.2±0.5 kg/m.s) with a modified aliphatic amine hardener and was supplied by Jet Korea Co. (Korea).

2.2 Composite fabrication

The panels of laminates were manufactured by a vacuum-assisted resin transfer molding (VARTM) process. VARTM is an adaptation of the resin transfer molding (RTM) process that exploits vacuum pressure of <101.32500 kPa to draw off resin to the impregnate preforms. VARTM presents many benefits in composite fabrication such as low cost, low void contents, and stable product thickness [22, 26, 27]. The schematic of the present VARTM process is shown in Figure 1. In this work, a bronze plate with dimensions of 300 mm×300 mm was prepared and oiled with a liquid wax (for safe release) on the top of plate. Sealant tape was then placed around the plate. The carbon and basalt fibers were both cut with a dimension of 250 mm×250 mm and arranged on the mold according to the laminate design. Next, epoxy resin with a hardener mixture ratio of 5:1 after degassing in vacuum desiccators (at -70 cmHg for 40 min), was directly injected into the impregnated preform at a pressure of -80 kPa using a vacuum pump (Airtech Ulvac G-100D, ULVAC Kiko Incorporated, Japan). The panel was then dried inside an oven at 65^oC for at least 2 h. In this work, we laminated 10 layers of fibers in every panel, constituting about 62 wt% of the hybrid composite. The thickness of panels manufactured through VARTM was approximately 2 mm. The details of the combination of the fibers are shown in Table 1.

Figure 1

Schematic layout of VARTM: (1) bronze plate, (2) laminate fiber, (3) release films, (4) breather net, (5) vacuum tube, (6) plastic bag, and (7) sealant tape.

2.3 Tensile test and characterization

In the present study, tensile tests were performed to determine the stress-strain behavior of each composite (carbon-basalt fibers/epoxy) in accordance with ASTM D 638 [28]. Five dog-bone-type specimens were cut for each composite panel using water-jet machining (see Figure 2). The tensile tests were performed in a universal testing machine (Unitect-M, R&B Research and Business, Korea) at a constant crosshead speed of 2.0 mm/min at room temperature. The strain was measured through an extensometer with a gage length of 50 mm (Extensometer model 3542-0200-50-ST, Epsilon Tech. Corp, WY, USA). The failed surface characterization of each specimen was investigated and analyzed using SEM. Microscopic analyses were performed aiming to recognize the failure mode of each hybrid composite.

Figure 2

Schematic of dog-bone-type specimen for tensile test.

3.1 Tensile properties

Figure 3 illustrates the typical stress-strain curves obtained from the tensile test for each laminate, and Table 2 gives the summary of the mechanical properties. Here, the basalt fabric layers were placed between carbon fabric layers (i.e., B1–B5). It can be directly noticed that the behaviors of each laminate all showed a linear trend. The slope of the stress-strain curves demonstrated a proportional decrease with the increase in the number of basalt fibers in the composite laminates. However, the tensile strain showed an increasing trend with the increase in the number of basalt fabric layers. This signifies that at the highest number of basalt layers, i.e., B5, the composite laminate showed the highest tensile strain. In contrast, the tensile strength of the composites with the basalt fiber has values slightly lower than that of CFRP but much higher than BFRP (see Figure 3). In other words, the enhancement depending on the content of basalt fiber has a significant impact on the ultimate strength, elastic modulus, and strain of the composite laminate. As a result of basalt reinforcement in CFRP, the interply hybrid composites could take more strain before incurring failure [1, 29].

Figure 3

Stress-strain curves of CFRP, BFRP, and hybrid composites with different numbers of basalt fiber layers.

Figure 4 shows photographic images of the failed samples after the tensile test. The laminate with only one layer of basalt fiber in CFRP (Figure 4A) showed a brittle mode of failure. However, the damage modes of composite laminates B2, B3, B4, and B5 (Figure 4B–D) demonstrated many dispersed failure fibers. B4, with four plies of basalt fiber, incurred flat damage on carbon layers after the tensile test and disperse fiber failure on the basalt layer (Figure 4D), which means that the interply hybrid composites can take more strain before failure under tensile loading. Similar results were also reported by other research groups [30, 31] when they investigated the behavior of FRP and hybrid FRP.

Figure 4

Optical images of failed hybrid composite under tensile loading: (A) B1, (B) B2, (C) B3, (D) B4, and (E) B5.

From Table 2, the results of the mechanical properties depending on the basalt content show a linearly decreasing tensile strength and Young’s modulus values of the hybrid composites with the increase in the number of basalt layers. This signifies that the incorporation of basalt fiber significantly affected the tensile behavior of hybrid composites. However, an increase of basalt fibers also raises the strain behavior of hybrid composite.

Here, we can make the approximate relations for the tensile strength and the Young’s modulus of the interply hybrid composites as a function of the number of basalt fiber layers, x. It has been derived as follows:

3.2 Hybridization effect

To investigate whether the arrangement of basalt fibers in the hybrid composite laminate has some effect on its tensile properties, we tested several types of specimens that have different arrangements, i.e., BC, CB, and B4 (see Table 3). In all cases, the specimens were made by using four and six plies of basalt and carbon fibers, respectively. In the present study, placing the basalt fibers in the outermost layer and the carbon layer in the innermost layers showed an increase in tensile properties of the composite compared to B4. Figure 5 shows the stress-strain curves of hybrid composite for B4, BC, and CB. Here, the hybrid composite with basalt layers placed in the exterior of BC has higher tensile strength and strain compared with the carbon layers placed in the exterior of B4. The tensile strength values of hybrid composite were about 571 and 536 MPa for hybrid composites BC and B4, respectively. At the same time, the Young’s modulus of BC was about 49.5 GPa, which was higher than that of B4 (~47 GPa). Furthermore, the hybrid composite with dispersed stacks of basalt layers between the carbon layers, i.e., CB, had a tensile strength and Young’s modulus of about 556 MPa and 47.5 GPa, respectively, which were lower compared with that of BC, but higher than that of B4 (see Table 3). The tensile strains of hybrid composites of B4, BC, and CB have almost similar values (Table 3). This result indicates that the variation of the basalt-reinforcing position has a major effect on the tensile strength and Young’s modulus of this type of hybrid composite. Tensile strength and Young’s modulus of BC are higher than that of B4 by 6.5% and 5.3%, respectively.

Figure 5

Stress-strain curves of the hybrid composites with different stacking varieties: B4, BC, and CB under tensile loading.

3.3 Fracture characterization

Figures 6 and 7 show the SEM images of the fractured surfaces of the present samples after the tensile test. Figure 6A shows cross-sectional damage of the BFRP, which shows rumpled failure features, whereas in Figure 6B, we can observe that the CFRP failed via a tensile mode failing by flat features. Figure 7 shows the SEM images of cross sections of the failed tensile specimens. B4 (Figure 7A) shows few carbon fiber-matrix debonding and basalt fiber shows damage in dispersed failures. In the hybrid composite BC (Figure 7B), there are some carbon fiber pullouts, debonding, and fiber-matrix delamination. Some longitudinal delaminations also occurred in basalt layers. In this case, the interfaces between carbon fiber and basalt fiber were delaminated caused by the effect of residual stress between the layers during tensile loading. At present, the carbon layers failed in a flat manner and basalt layers failed in a rumpled manner. This is due to the fact that carbon fiber is brittle and basalt fiber is relatively ductile. In the hybrid composite, with an alternatively sequenced manner (Figure 7C), some rumpled failures occurring on the basalt layer, causing delamination between the carbon layer and basalt layers, were observed, as similarly attempted by Wei et al. [19]. This made its tensile strength lower than that of other hybrid composite arrangements, even though the strain on this arrangement was a little higher (see Table 3). This phenomenon is attributed to the brittle failure of carbon fibers, where the specimen broke through all layers with an abundant rupture [32].

Figure 6

Low- and high-magnification SEM images for the cross section of the fracture surface of the hybrid composites: (A) BFRP and (B) CFRP.

Figure 7

Low- and high-magnification SEM images for the cross section of the fracture surface of the hybrid composites: (A-A’) B4, (B-B’) BC, and (C-C’) CB.