CNF TEST를 利用한 一方向 CFRP複合材의 모드Ⅱ 層間破壞引性에 關한 硏究

Abstract

이 논문은 현 학계에 잘 알려 지지 않은 CNF실험에 대한 연구를 소개와 더불어 CFRP 적층 시험편의 4-CNF(4-Point center noched flexure test)실험을 통한 층간파괴 인성치를 구하는 것을 목적으로 한다. 본 연구에서는 평직 직물형태로 짜여 적층시킨 평직 CFRP복합재의 적층분리에 대한 신뢰성과 안정성을 확보하기 위해 CNF시험편을 이용 4점 굽힘 실험하고자 한다 또한 초기 균열의 크기가 파괴 인성치에 미치는 영향을 평가 하고자 a0/L 비 값을 0.2, 0.3, 0.4로 구분하여 실시할 계획이다. 본 문에서는 초기 균열을 포함한 CFRP 적층 시험편에 대한 하중의 영향을 균열 확장의 관찰을 통해 분석하고, 4-CNF테스트의 시험편 제작 및 실험방법에 대해 설명하고자 한다.

Composite materials can become a third material with highly desirable properties by the combination of two or more disparate materials. Unidirectional Carbon Fiber Reinforced Plastics (CFRP) are advanced materials which combine the characteristics of the light weight, high stiffness, strength and chemical stability. They also have good corrosion resistance and thermal conductivity. For this reason, the use unidirectional CFRP have increased in jet fighters, aerospace structures. However, unidirectional CFRP composite have a lot of problems, especially delamination, compared with common materials such as steels and aluminums, and so forth. Therefore, the Interlaminar fracture toughness for a laminate composite is very important. In this study, The interlaminar fracture toughness was measured by using CNF test. The CNF specimen using unidirectional carbon prepreg was fabricated by a hot-press that is controlled with the gage pressure and temperature. And three kinds of a/L ratio was applied to these specimens. Here, we discuss the relations of the crack growth and the interlaminar fracture toughness under the four point bending CNF test. The experiment material used a commercial material of CU 175NS in unidirectional prepreg(Hankuk Carbon Co.). The measure tester used universal dynamic tester (50kN) of Hyunyang Co. The crack length was measured by traveling microscope. For plain woven CFRP composite, three-point bending test with the ENF specimen was conducted to evaluate the modeⅡ interlaminar fracture toughness, and the conclusions are as following. 1) In the fracture experiment of the CNF specimen, there were three a/L ratios: 0.2, 0.3, and 0.4. A relatively gradual growth was shown in load-displacement curve, despite the difference of a/L ratios, proving that the a/L ratio is not a significant factor in the CNF specimen. Later, the load was put to a certain point followed by a sudden reduction, which caused the crack extension length to grow rapidly. 2) In the experiment of the three a/L ratios, it was found that the higher the a/L ratio, the lower the loading point for the interlaminar fracture to occur. It was shown for the crack extension length that the higher the a/L ratio, the shorter the length of the crack growth. 3) GⅡC for the interlaminar fracture toughness was 5.33kJ/㎡ for a0/L=0.2, 2.9kJ/㎡ for a0/L=0.3 and 0.58kJ/㎡ for a0/L=0.4. The GⅡ was 118.49kJ/㎡ for a0/L=0.2, 47.69kJ/㎡ for a0/L=0.3 and 9.10kJ/㎡ for a0/L=0.4.