In the development of launch vehicles, the weight reduction of cryogenic propellant tanks is a great concern because it occupies most of their weight. Carbon fiber reinforced composites are promising materials for this purpose. The systematic procedure, however, is inevitable from the development of composite material for cryogenic use to its application to actual structure, which is the main goal of this research.
For the purpose of developing carbon fiber reinforced composites suitable for cryogenic use, the mechanical properties of 6 types of matrix resin systems, which had been developed through formulating 2 component epoxy resins with different additives, were measured both at RT (room temperature) and -150 ℃ after thermo-mechanical cycles from RT to -150 ℃. As a result, it was found that the longitudinal tensile stiffness was little influenced as the resin composition changed and additives were added to their resin composition. The longitudinal tensile strength, however, was greatly influenced by the resin composition. Furthermore, the tensile strength of the composite to which newly developed resin had been applied increased considerably even after it was exposed to cryogenic condition for 108 hours, which was 30% higher than that of a baseline material.
In addition, the cryogenic bonding characteristics of adhesives, which are necessary to bond the composite and metal liner in a Type 3 cryotank, were investigated. For this purpose, three film types of adhesives were selected, and the joint strength was measured at RT and -150 ℃ through double-lap joint tests. From the experimental results, it was shown that the excellence of bulk adhesive strength did not always guarantee that of the joint strength, and bonding characteristics largely depended on service temperature and adherends.
Various design factors of a Type 3 cryotank were evaluated using ring specimens, which can simulate geometrical shape and continuity of reinforcing fibers of the actual tank structure as well as practical fabrication process. Composite ring specimens and composite/aluminum ring specimens with hoop winding were fabricated and their cryogenic behaviors such as thermal strain and thermal stress were investigated taking cryogenic operating conditions into consideration. From the experimental results, the behaviors of the Type 3 cryotank were understood. In addition, the effect of curing temperature on composite/aluminum ring specimens was examined. As a result, it was shown that higher curing temperature is desired for Type 3 cryotank structures. Through these tests, the behaviors of the Type 3 cryotank were predicted.
For the actual application of composites to the Type 3 cryotank, a prototype was fabricated, and a pressurized liquid nitrogen storage test was carried out. Before the test, the effects of curing temperature and autofrettage pressure on the prototype cryotank were investigated by thermo-elastic analysis. From the analysis results, it was shown that higher curing temperature is desirable and while the autofrettage pressure should be minimized. Therefore, appropriate tradeoff between the viewpoint of improving failure strength of the tank and that of elevating fatigue life of aluminum liner is needed. On the other hand, the delamination occurred between hoop and helical layers during pressurized liquid nitrogen storage test of the prototype of the Type 3 cryotank. Several attempts were made to investigate this phenomenon both analytically and experimentally. From microscopy of the composite/aluminum ring specimens, many voids were observed even before $LN_2$ immersion test, which was thought to be the main reason for the delamination in the cryotank, and some complementary measures to establish the optimal curing cycle were suggested.
우주 발사체 개발에 있어서 발사체 무게의 대부분을 차지하는 극저온 추진제 탱크의 무게 감량은 효율적인 운용을 위한 필수불가결한 요소이다. 이를 위해 탄소섬유로 보강된 복합재료(carbon fiber reinforced plastics)가 주목을 받고 있지만 실제적인 적용에 앞서 극저온 환경에서의 거동파악과 이에 적합한 재료선정이 요구된다. 또한 실제로 복합재를 Type 3 극저온 추진제 탱크에 적용할 경우에 설계 및 운용과정에서 발생하는 인자들에 대한 이해가 필요하다. 따라서 본 연구에서는 이에 대한 체계적인 연구를 수행하였다.
우선 극저온 환경에서 복합재의 거동을 파악하고 극저온 환경에 적합한 재료를 선정하기 위해, 수지조성과 첨가제를 조절하여6종류의 복합재 모델을 개발하였고 이들의 극저온에서의 기계적 물성 특성을 연구하였다. 이를 위해 실제 극저온 추진제 탱크가 경험하는 환경을 고려하여 상온부터 -150 ℃까지의 열하중 사이클을 부가하였으며, 그 결과 인장강성은 수지조성과 첨가제에 큰 영향을 받지 않았으나 인장강도는 동일한 보강섬유에도 불구하고 상당한 차이를 보이는 것을 확인하였다. 이에 대한 원인은 수지조성에 따른 섬유와 모재사이의 계면강도 차이에 있음을 모재지배물성의 실험결과로부터 확인할 수 있었다. 한편, Type 3 탱크의 복합재와 금속 라이너의 접합을 위한 접착제의 극저온 특성을 살펴보기 위해 양면 겹치기 조인트(double-lap joint) 시험을 수행하였으며 그 결과 접착제의 접착강도는 벌크(bulk) 접착제의 강도와 많은 차이를 보이며, 온도와 피접착물(adherends)에 상당히 의존적임을 확인하였다.
또한 Type 3 극저온 탱크의 다양한 설계 변수에 대한 이해를 위해 링 시편을 이용한 실험을 수행하였다. 이로부터 극저온 환경에서의 복합재와 라이너의 열응력 분포를 확인하였으며, 자긴압력(autofrettage pressure)과 복합재의 성형온도가 Type 3 극저온 탱크에 미치는 영향을 실험적으로 관찰하였다. 그 결과 자긴압력은 Type 3 극저온 탱크의 파손강도에 해로운 영향을 미치며 성형온도는 높은 것이 바람직하다는 것을 확인하였다.
마지막으로, 복합재를 Type 3 탱크에 적용하여 실제 운용 환경에서의 거동을 예측하고 실험하는 연구를 수행하였다. 이를 위해 Type 3극저온 탱크의 열탄성해석을 수행하여 자긴 압력과 성형온도에 따른 극저온 탱크의 거동을 예측하였으며 해석결과와 링 시편의 실험 결과가 잘 일치하는 것을 확인하였다. 또한, Type 3 프로토타입 탱크를 설계, 제작하고 가압된 액체질소를 저장하는 실험을 수행하였다. 실험 과정에서 복합재 층 내부에서 발생한 층간분리(delamination) 현상의 원인을 규명하기 위해 해석적, 실험적 방법으로 접근하였으며 그 결과 실제 탱크 제작 공정에 적합한 성형 방법의 정립이 필요함을 확인하였다.
