In this study, the effect of stacking sequence on the flexural and fracture properties of carbon/basalt/epoxy hybrid composites was investigated. Two types of carbon/basalt/epoxy hybrid composites with a sandwich form were fabricated: basalt skin-carbon core (BSCC) composites and carbon skin-basalt core (CSBC) composites. Fracture tests were conducted and the fracture surfaces of the carbon/basalt/epoxy hybrid composites were then examined using scanning electron microscopy (SEM). The results showed that the flexural strength and flexural modulus of the CSBC specimen respectively were ~32% and ~245% greater than those of the BSCC specimen. However, the interlaminar fracture toughness of the CSBC specimen was ~10% smaller than that of the BSCC specimen. SEM results on the fracture surface showed that matrix cracking is a dominant fracture mechanism for the CSBC specimen while interfacial debonding between fibers and epoxy resin is a dominant fracture process for the BSCC specimen.
It is known that hybrid composites that consist of two or more fibers in a single matrix have unusual properties compared to conventional single fiber reinforced composites. Accordingly, many studies have been performed to investigate the mechanical properties of hybrid composites [1-4]. For instance, Dong
In recent years, basalt fiber has received increasing attention as a substitute for glass fiber on the basis of its ecologically benign and superior mechanical properties. Basalt fiber, which is made from basalt rock, has superior properties relative to glass fiber such as high tensile strength (~4800 MPa), thermal performance (−259℃ to 960℃), and chemical resistance and it also considerably more economical carbon fiber. For this reason, basalt fiber is widely used in various industries and many studies have been carried out to understand the mechanical behavior of basalt fiber reinforced composites [9-12]. However, few studies have been made to investigate the mechanical properties of basalt/carbon hybrid composites [13,14]. In particular, no research results on the fracture behavior of basalt/carbon hybrid composites have been reported to date.
In this study, the effect of stacking sequence on the flexural and fracture properties of carbon/basalt/epoxy hybrid composites was investigated. For this purpose, three-point flexural tests and mode I interlaminar fracture tests were conducted using basalt skin-carbon core (BSCC) composites and carbon skin-basalt core (CSBC) composites. Following fracture tests, the fracture surfaces of both composites were examined using a scanning electron microscope (SEM) to investigate the fracture mechanisms depending on the stacking sequence.
Woven type carbon fibers (CF332NON, Hcarbon, Korea) and basalt fibers (EcoB4-F260, Secotech, Korea) were used as reinforcing materials. The epoxy used was diglycidyl ether of bisphenol A (YD-115, Kukdo Chemical, Korea), and the curing agent was polyamidoamine (G-A0533, Kukdo Chemical). Eight plies of basalt and carbon fabrics with two different stacking sequences were stacked by the hand lay-up method. The stacking sequences used were [carbon/carbon/basalt/basalt]s and [basalt/ basalt/carbon/carbon]s. In this study, the composites with [carbon/ carbon/basalt/basalt]s and [basalt/basalt/carbon/carbon]s stacking sequences are denoted as CSBC and BSCC, respectively. The stacked CSBC and BSCC specimens were impregnated with a matrix made of epoxy resin mixed with curing agent (2:1 v/v) and then cured in a hot press at 15 MPa and 80℃ for 2 h.
Flexural tests were performed according to the ASTM D-790 standard in a three-point bending mode at a cross-head rate of 0.5 mm/min using 130 × 12.7 × 1.6 mm3 CSBC and BSCC specimens. Mode I interlaminar fracture toughness tests were performed using double-cantilever beam (DCB) CSBC and BSCC specimens according to the ASTM D5528-01 standard. For DCB fracture specimens (width 25 mm and length 200 mm), an initial crack was made by inserting a 20 mm Kapton film (thickness: 13 μm) between the fourth and fifth plies. In order to measure crack extension, one side of the DCB specimens was coated with a correction fluid and marked with thin vertical lines. The cross-head speed was set at 2 mm/min. The crack length, displacement, and fracture load values were measured to calculate fracture toughness. Four flexural and fracture tests, respectively, were performed to ensure reliability of the test results. Fig. 1 shows schematic diagrams of the CSBC and BSCC DCB specimens and the test equipment.
Fig. 2 shows the flexural load-displacement curves of the CSBC and BSCC hybrid composites. It can be seen in the figure that both composites exhibited almost linear elastic behavior before the reaching the maximum applied load. It also can be seen in the figure that the CSBC composites, which have carbon fabrics at the skin, displayed stiffer behavior than the BSCC composites, which have basalt fabrics at the skin. The figure also shows that the flexural strength and the flexural modulus of the carbon/basalt/epoxy hybrid composites are influenced by the stacking sequence of the basalt and carbon fabrics. Fig. 3 compares the flexural strength and the flexural modulus of the CSBC composites with those of the BSCC composites. The flexural strength and the flexural modulus were determined using the following equations:
In the above equations, “
The critical energy release rate, which is the fracture toughness, indicates the crack resistant force at the moment a new surface is created at a crack tip. Mode I fracture toughness,
In the equation, “
A SEM analysis was conducted on the fracture surface of CSBC and BSCC composites to investigate the fracture mechanism of carbon/basalt/epoxy hybrid composites due to stacking sequence. Fig. 6a shows the fracture surface of the BSCC composites, and Fig. 6b shows the fracture surface of the CSBC composites. For the BSCC composites, as shown in Fig. 6a, the carbon fibers were relatively clean, showing that carbon fibers were removed from the epoxy matrix. Compared to the BSCC composites, some basalt fibers were fractured, whereas they sufficiently adhered to the epoxy matrix in the CSBC composites (Fig. 6b). It also can be seen that the epoxy matrix was fractured in a hackle pattern. This indicates that matrix cracking and fiber breakage are dominant fracture mechanisms for CSBC composites while debonding between fibers and the matrix plays an important role in the BSCC composites.
In this study, the effect of stacking sequence on the flexural and fracture properties of carbon/basalt/epoxy hybrid composites was investigated through flexural and mode I interlaminar fracture tests. The conclusions obtained from this study were as follows. The flexural strength and flexural modulus of the CSBC composites were ~32% and ~245% greater than those of the BSCC composites, respectively. On the other hand, the fracture toughness of the CSBC composites was ~10% smaller than that of the BSCC composites. Fiber breakage and matrix cracking were the dominant fracture mechanisms for the CSBC composites and debonding between fibers and the matrix was the dominant fracture mechanism for the BSCC composites.