탄소섬유 강화 플라스틱의 기공률 증가에 따른 초음파 후면신호의 상관관계 분석 = Analysis of the correlation between ultrasonic back signal and increased porosity of Carbon Fiber Reinforced Plastic（CFRP）

국문 초록 (Abstract)

탄소섬유강화플라스틱（CFRP은 탁월한 강도 대 중량비, 내식성 및 설계 유연성 덕분에 항공우주 및 자동차 분야를 비롯한 고성능 산업에서 필수적인 구조 재료로 자리 잡았다.
예를 들어, 보잉 787 드림라이너는 기체 무게의 약 50%를 CFRP로 구성하고 있다.
그러나 CFRP의 기계적 건전성은 내부 결함, 특히 기공에 매우 민감하다. 제조 과정에서 발생하는 기공（보이드）은 하중 분포를 방해하고, 층간 전단 강도를 감소시키며, 파괴 인성을 저하시켜 전체 구조의 안전성을 위협할 수 있다.

본 연구에서는 지연 팁 접촉 변환기를 사용하는 비파괴 펄스 에코 초음파 검사 기법을 이용하여 두께가 다른 CFRP 적층판에서 기공 함량과 초음파 후면 에코（BWE） 신호의 변화 양상 간의 상관관계를 조사한다. 주요 목적은 기공 수준과 초음파 신호 특성의 측정 가능한 변화 사이의 정량적 및 정성적 관계를 규명하는 것이다.

실험은 보잉에서 제조한 CFRP 기공 참조 샘플을 사용하여 진행되었으며, 8, 16, 24겹의 세 가지 두께에서 0%, 약 2%, 3–5%, 7–8%의 네 가지 기공 수준으로 준비되었다.
결과적으로, 모든 두께의 적층판에서 초음파 후면벽 에코 측정값이 기공 함량에 따라 일관되게 증가함을 확인하였다.

이러한 경향은 기공이 수지의 일부를 대체함에 따라 효과적인 음향 속도가 감소하고, 초음파 파동의 산란 및 회절로 인한 경로의 연장이 원인으로 분석되었다.
이 연구는 지연 팁 변환기를 이용한 초음파 검사 방식이 CFRP의 기공 평가에 실용적이고 민감한 방법임을 보여준다.

후면 에코의 변위는 모든 두께의 적층판에서 기공과 명확한 양의 상관관계를 보였으며, 이는 기공 평가를 위한 비파괴 지표로 활용될 수 있다.
향후 연구는 기공의 형태 특성화 및 컴퓨터 모델링을 통합하여 초음파 신호 반응과 미세 구조 결함 매개변수 간의 관계를 더욱 정교하게 할 예정이다.

목차 (Table of Contents)

• I.서 론 ················································································· 1
• 1. 탄소섬유 강화 플라스틱 ····················································· 1
• 2. 복합재（탄소섬유 강화 플라스틱） 비행기 산업 현황 ················ 1
• 3. 탄소섬유 강화 플라스틱（CFRP）의 기계적 성능과 기공률 ········ 3
• 4. 초음파 검사 기술 ······························································ 3
• 5. 선행 연구 현황 ································································· 4
• 6. 연구 목표·········································································· 5
• II. 이론 ················································································ 6
• 1. 초음파 탐상 검사 기본 개념················································ 6
• 2. 초음파 탐상검사의 분류······················································ 7
• 1） 펄스밥사법（Pulse Echo Technique） ······························· 7
• 2） 투과법（Through Transmission Ultrasound） ·················· 8
• 3. 초음파의 종류와 성질························································· 8
• 1） 종파 （Longitudinal Wave） 정의······································· 8
• 2） 횡파 （Transverse Wave） 정의········································· 9
• 3） 표면파 （Surface Wave） 정의··········································· 9
• 4） 판파 （Plate Wave） 정의 ·················································· 9
• 4. 파형 정류 ········································································ 10
• 5. 초음파 신호 표시 방식······················································· 11
• 1） A-scan ········································································ 11
• 2） B-scan·········································································· 11
• 3） C-scan ········································································ 12
• 6. 음장의 특성 ···································································· 14
• 1） 근거리 음장 ··································································· 14
• 2） 원거리 음장 ································································· 14
• 7. 파의 진행 특성·································································· 15
• 1） 굴절의 법칙···································································· 15
• 2） 음향 임피던스································································· 17
• 3） 수직 입사시의 반사와 통과 ············································· 17
• 4） 결함에서의 구형 홈（기공） 반사 ········································ 18
• 5） 산란 및 감쇠 ································································· 19
• 6） 데시벨（dB）······································································ 20
• 8. 탐촉자 ············································································ 21
• 1） 직선（종방향 파형） 탐촉자 （Straight-beam Contact Type）···21
• 2） 각도 탐촉자 （Angle-beam Contact Type）·························22
• 3） 복합 탐촉자 （Dual-Element Units）·································· 23
• 4） 지연 팁 탐촉자 （Delay-Tip Units） ····································24
• 5） 침적식 탐촉자 （Immersion Type）····································· 25
• Ⅲ 실험 방법·········································································· 26
• 1. 시험 방식 채택 ································································· 26
• 1） 시험 채택 배경································································ 26
• 2） 시험 방식 ······································································ 26
• 3） 지연 팁 탐촉자································································ 27
• 4） 펄스 에코（Pulse Echo） 방식············································· 30
• 2. 시험 장치 구성 ································································· 31
• 1） 사용 장비 : 주기기·························································· 31
• 2） 탐촉자（프루브） ······························································· 32
• 3） 기타 ············································································· 33
• 3. 시험편 ············································································· 33
• 4. 저면 신호 확인법 ······························································ 36
• 5. 측정 방식 및 데이터 구성··················································· 37
• 1） 음극 반파 정류를 이용한 측정··········································· 37
• 2） 측정방식 ········································································ 38
• 3） 측정 영역 및 횟수 ·························································· 38
• 4） 추가 게인 상승 ······························································· 39
• Ⅳ 실험결과············································································ 41
• 1. 데이터 분석 ······································································ 41
• 1） 8 플라이········································································· 41
• 2） 16 플라이 ······································································ 44
• 3） 24 플라이 ······································································ 47
• 2. C-Scan 참고 측정······························································ 50
• Ⅴ 결론 및 고찰 ···································································· 53
• 1. 플라이 수에 따른 기준 측정값 및 절대 변화량 비교··············· 53
• 2. 기공률과 측정값 간의 상관관계: 플라이 수별 비교················· 54
• 3. 플라이 수별 비파괴 평가 지표로서의 종합 고찰····················· 55
• 참고문헌 ·············································································· 56
• ABSTRACT··········································································· 58