능직 탄소/아라미드 복합재료의 환경변화에 따른 파괴인성 및 파손특성에 관한 연구

Abstract

Since the industrial revolution, renewable energy and eco-friendly materials have been required worldwide due to global warming and depletion of fossil fuels. Composite materials are materials that overcome the disadvantages of existing metal materials such as strength, stiffness, corrosion resistance, and heat resistance. A hybrid fiber composite is manufactured using two or more fibers such as carbon fiber, aramid fiber, and glass fiber. The aramid fiber has high toughness and heat resistance and high elasticity, and is used in various fields such as industrial protective materials, bulletproof helmets and vests. However carbon fiber composites are somewhat vulnerable to the impact of external objects even though it has excellent specific properties. On the other hand, the aramid fiber tends to decrease in elastic modulus and strength when applied to the epoxy matrix and if the aramid fiber is exposed to water or heat, its mechanical properties are often deteriorated. Thus, the carbon and aramid fiber are selected to hybrid for the complementary relationship. In this study, the tensile and flexural test were accomplished on the twill woven carbon/aramid fiber hybrid composites to investigate the effects of high temperature and water absorption. And a delamination fracture toughness was investigated under mode I loading by 2 kinds of MBT and MCC deduction. An acoustic emission (AE) evaluation was conducted for the carbon fiber/aramid fiber hybrid composites degraded and damaged by high-temperature seawater environments. The test specimen was molded using an autoclave. The flexural test specimen was fabricated as non treated (N), high temperature (H), impacted (I), water absorption (W). And mode Ⅰ test specimen was fabricated with 20 hybrid fabric plies. The initial crack was made by inserting the teflon tape in the center plane with a0/W=0.5 length. The test specimen was immersed in 70°C seawater for 224 days. In order to imitate the damage, a hole was machined with a diameter of 3 mm using a diamond drill. And the specimen with a hole was repaired by patch attachment processing. The flexural experiment was implemented by a three-point bending and an AE sensor with a resonance frequency of 150 kHz was attached to evaluate the damage and the effect of patch attachment. From the tensile test of the twill carbon fiber/aramid fiber hybrid composite, the elastic modulus was 23.4 GPa, the poisson's ratio was 0.032, and the tensile strength was 539 MPa. On the flexural test, the impact damage with absorbing moisture showed the largest a reduction rate of flexural strength among the IW, H300, I specimen. And from the results of the MBT and MCC evaluation methods for the energy release rate, the MCC evaluation showed a difference of about 4 to 12% in the two evaluation values according to on the crack growth length, and the fracture toughness value was 0.1849 kJ/m2 and 0.1916 kJ/m2 in MBT and MCC, respectively. With resin impregnation and patch fiber lamination in the hole damage area of the SWH specimen, the maximum load was recovered up to 99% of SW specimen. AE accumulative counts obtained at the maximum load were 69.2, 67.1, and 91.2 for SW, SHH, SWR. AE maximum amplitude was detected as almost same with low values. The results obtained in this study are expected to be useful for designing safety and health structures and mechanical facilities composed of twill weave carbon fiber/aramid fiber hybrid composites.