적층 UHMWPE/CFRP 하이브리드 복합재의 층간 파괴인성에 관한 연구

Abstract

Many studies on the light weighting of mechanical devices and structures have been carried out in order to save energy. As results, the studies on many composite materials have been conducted and applied to various industries. Composite materials can be manufactured so as to have the designed strength and stiffness by tailoring various materials depending on the type of reinforced materials and matrix, molding method. Reinforcement fibers, in particular, have an impact on the performance of composite, from single fiber to hybrids. The carbon fiber has a thermal expansion coefficient close to zero, so that a structure can be manufactured in which the size of the carbon fiber does not substantially change with the change in circumstantial temperature. However, carbon fiber reinforced plastic (CFRP) has the restrictive toughness due to the low fracture strain of high strength and rigid carbon fibers. In order to solve this problem, various hybrid composite materials using a combination of polymer fibers having a high failure strain rate on carbon fibers have been needed. UHMWPE(Ultra High Molecular Weight Polyethylene) fiber has low density and excellent cutting resistance and chemical resistance. However, UHMWPE fibers are relatively weak in compressive strength, vulnerable to resins, and have low thermal and creep resistance. Therefore, hybridization of carbon fiber and UHMWPE fiber can show very complementary performance improvement. Therefore, it is necessary to evaluate the characteristics of hybrid composite materials through fracture mode experiments. In this study, to evaluate the mechanical characteristics of UHMWPE and CFRP hybrid composite, the basic mechanical properties were evaluated by the tensile and fracture test. And the interlaminar fracture toughness evaluation and the failure behavior were investigated in mode I, mode II and mixed mode I/II. The following conclusions were obtained :

(1) The modulus of elasticity of the specimens obtained as a result of the tensile test was calculated to be 68 GPa for CFRP, 5.8 GPa for UHMWPE, and 28 GPa for UHMWPE/CFRP hybrid composite materials. Poisson's ratio was obtained at 0.1 for CFRP, 1.21 for UHMWPE, and 0.1 for UHMWPE/CFRP hybrid composite materials. Also, the fractured specimens showed that the direction of fracture occurred almost perpendicularly from the load direction.

(2) In the mode I experiment with DCB test specimen, we checked the crack growth characteristics of the interface between the CFRP layer and the UHMWPE layer, and found that the crack between the flat fabrics penetrates into each material due to the characteristics of the interfacial crack in the dissimilar material and again grows through the matrix surface. As internal cracks occurred, parts of CFRP weft fiber were torn off and cracks advanced to UHMWPE, fiber strands were pulled out due to delayed separation, resulting in fiber bridging. The bridge between fiber and fiber was found to have increased and decreased loads generated during the fracture test, and to cause perturbation caused by interference of laminated fiber, and the range of KIc was found to be between MPa·m1/2. KIc at the time of initial cracking was obtained at 2.52 MPa·m1/2, 2.39 MPa·m1/2 and 2.26 MPa·m1/2 at a0/L = 0.3, 0.4, 0.5, respectively. The longer the initial crack length, the smaller the value of KIc at the beginning of crack growth. The energy release rate G was obtained at 84.29 kJ/m2, 69.95 kJ/m2 and 65.89 kJ/m2 for initial crack length a0/L = 0.3, 0.4 and 0.5, respectively. The shorter the initial crack length, the higher the energy release rate. This shows a similar trend with the results of the stress intensity factor, K, which showed that as the initial crack length increases, the load required for crack growth is required to be smaller, and the value of the stress intensity factor K and the energy release rate GⅠ is reduced.

(3) The fracture characteristics specificity of the interface between the UHMWPE laminate and the CFRP laminate was observed in mode II using ENF specimens. In mode II, multiple fractures occur above and below the interface due to in-plane shear by material rather than interfacial crack propagation. The GIIc at the time of initial cracking was shown at a0/L = 0.3, 0.4 and 0.5 to 2.17 kJ/m2, 1.71 kJ/m2, and 1.37 kJ/m2. It can be seen that the longer the initial crack length, the smaller the critical interlaminar energy release rate GIIc value becomes at the beginning of crack growth. The interlaminar fracture toughness KIIc was obtained from a0/L = 0.3 to 23.83 MPa·m1/2, a0/L = 0.4 to 21.13 MPa·m1/2 and a0/L = 0.5 to 18.76 MPa·m1/2. The longer the initial crack length, the smaller the stress intensity factor KII at the beginning of crack growth. The smaller the initial crack length, the slower the timing of the initial crack growth was confirmed, and this shows that the load to generate separation between layers depends on the initial crack length.

(4) Through the mixed mode I/II fracture experiment with MMB test specimen, GIc represents the fracture interlaminar critical energy release rate and was obtained at 107.3 J/m2, 70.43 J/m2 and 19.39 J/m2 from a0/L = 0.3, 0.4 and 0.5, respectively. As the length of the crack increases, the energy release rate tends to decrease dramatically from a0/L = 0.3 and 0.4 and 0.5 to 14.32 J/m2, 6.32 J/m2 and 5.12 J/m2. It can be seen that as the initial crack length increases, the energy release rate initially shows a maximum value and then decreases. GIIc represents values of interlaminar critical energy rate, obtained from a0/L = 0.3 and 0.4 and 0.5 to 0.157 J/m2, 0.089 J/m2 and 0.0035 J/m2, respectively, showing small values compared with GI. As the initial crack length increases, the energy release rate, like the GI component, initially represents the maximum value and continues to decrease as the crack length increases. To evaluate the relationship of energy release rate GI, GII, the slope of GI/GII was checked to confirm that the slope gradually decreases as the initial crack length increases. The slope of GI/GII was obtained 559, 540 and 513 as the crack lengths increased from a0/L = 0.3, 0.4 and 0.5, and found to be significantly affected by mode I among the energy release rate values. As the initial crack length increases, it is confirmed that the effect of mode II is increasing a little further.

(5) The result values of NL, VIS, and 5% offset were compared by checking the correlation of the fracture according to the crack measurement method. The deviation of each measurement method was greatest when a0/L = 0.3 and the deviation of each measurement method was found to be the smallest when a0/L = 0.4.

(6) Fracture behaviors show the different appearances according to the loading direction for the UHMWPE/CFRP laminated hybrid composite. In case of the mode II fracture, multiple fracture occurred up and down the interfacial surface due to in-plane shear by material rather than interfacial crack growth, resulting in failure occasionally at the intersections between the weft fiber. In case of mixed mode I/II fracture, interlaminar delamination occurred rapidly due to mode I load distribution together with the interfacial crack. It can be seen that the fracture toughness was different accordint to the loading direction.