국부수침수조를 이용한 일체형 복합재 항공기 구조물의 곡률 부위 초음파검사 기법 개발 = Development of Ultrasonic Inspection Technique for Curved Sections of Integrated Composite Aircraft Structures Using Local Immersion Tanks

국문 초록 (Abstract)

항공 산업은 유럽연합(EU)의 탄소배출권 거래 제도(Emission Trading System, ETS) 강화와 탄소중립 실현을 위한 글로벌 이슈에 따라, 경량화와 연료 효율성 향상을 핵심 과제로 삼고 있다. 이에 따라 복합재료, 특히 탄소섬유 강화 복합재(Carbon Fiber Reinforced Plastics, CFRP)는 우수한 비강도와 비강성을 기반으로 항공기의 연료 효율성과 운항 성능을 개선하는 데 중요한 역할을 하고 있다. 그러나 복합재료는 제조 과정에서 발생할 수 있는 내부 결함과 고도의 이방성으로 인해 설계 허용 물성값 정의와 비파괴 검사(Non-Destructive Inspection, NDI) 기술의 적용에서 도전 과제를 안고 있다.
본 연구는 항공기 복합재료 제작 공정을 개발하여 적용 중인 OOA(Out of Autoclave) 공정을 기반으로 제작된 고굴곡 대형 복합재 구조물 스파 부위의 내부 건전성 평가 기법을 개발하였다. 기존 A-scan 방식은 검사원의 숙련도에 따라 결과의 신뢰성이 달라지는 한계를 가지며, 대형 구조물의 정밀한 검사에는 부적합하다. 이를 극복하기 위해 본 연구는 초음파 빔의 입사 각도와 결함 탐지 정확도 간의 관계를 분석하고, ±4° 범위에서 100% 결함 검출이 가능함을 입증하였다. 이를 바탕으로 설계된 국부 수침 수조(Local immersion tank)는 가변형 웨지 구조를 도입하여 초음파 빔이 결함에 항상 수직으로 입사되도록 검사 정확도를 향상시켰다.
실험 결과, 국부 수침 수조와 C-scan 방식을 활용한 검사는 기준 결함 대비 99% 이상의 높은 정확도로 결함 크기와 위치를 정밀하게 측정할 수 있었다. 또한, 기존 대형 수침 수조와 추가 장비 없이 고굴곡 대형 구조물에 대해 효율적 검사 가능성을 입증하였다. 국부 수침 수조는 소형화된 설계를 통해 장비의 휴대성과 경제성을 극대화하였으며, 3D 프린팅 기술을 활용하여 제작 비용과 시간을 절감하였다. 특히, 복잡한 곡률을 가진 항공기 구조물의 협소한 부위에서도 적용 가능성을 확보하였고, 검사 준비와 유지보수를 간소화하여 산업적 실용성을 높였다.
본 연구는 고굴곡 대형 복합재 구조물의 초음파 비파괴 평가를 위한 새로운 기술적 대안을 제시하였다. 국부 수침 수조 기반 C-scan 검사는 항공기 제조 및 유지보수 공정에서 높은 신뢰성과 효율성을 제공할 수 있는 혁신적인 솔루션으로, 항공 산업의 지속가능성과 경쟁력을 강화할 것으로 기대된다.

목차 (Table of Contents)

• 1. 서 론 ··························································································································· 1
• 1.1 연구배경 ··············································································································· 1
• 1.2 연구동향 ··············································································································· 9
• 1.3 연구목적과 논문구성 ······················································································· 11
• 2. 이론적 배경 ············································································································· 13
• 2.1 복합재료 결함 ··································································································· 13
• 2.2 초음파 빔 신호 분석 및 해석 ········································································ 16
• 3. 실험장비 및 결함 시편 설계 ··············································································· 23
• 3.1 초음파 빔의 최적 각도 설계 ·········································································· 23
• 3.2 국부수침수조 ····································································································· 34
• 3.3 초음파검사 결함 시편 설계 ············································································ 46
• 4. 결과 및 고찰 ············································································································ 55
• 4.1 일체형 스파 결함 시편 제작 ·········································································· 55
• 4.2 소형단품 시편 초음파검사 ·············································································· 64
• 4.3 일체형 스파 초음파검사 ················································································· 75
• 4.3.1 상부 270° 곡률부 ······················································································ 76
• 4.3.2 하부 90° 곡률부 ························································································ 88
• 4.4 상용 일체형 스파 제품 초음파검사 ······························································ 97
• 4.4.1 상부 270° 곡률부 ······················································································ 99
• 4.4.2 하부 90° 곡률부 ······················································································ 102
• 5. 결 론 ······················································································································ 107
• 참고문헌 ······················································································································ 109
• Abstract ····················································································································· 119