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Assessment of the Failure Probability of the HTPB(Hydroxyl-terminated polybutadiene) based Composite under Hygrothermal Conditions

  • Jaehee Jeong (정재희, 대학원 기계공학과 기계공학전공)

초록/요약

In this study, using HTPB (Hydroxyl-terminated polybutadiene) composite, which is the most widely used mixed solid propellant binder material, the diffusion of oxygen and moisture into the propellant was visualized. Also, the probability of failure after 30 years was analyzed. It was confirmed that the diffusion rate was changed according to the content and size of AP (Ammonium Perchlorate) and aluminum powder bound to the HTPB matrix. Since the diffusion rate of oxygen is faster than moisture, it was found that after 5 years or more, oxygen diffused throughout the propellant to cause hardening. Meanwhile, the effect of moisture gradually increased, resulting in a softening effect. In this paper, we proposed a new modified Layton equation that can predict mechanical properties by introducing the results of diffusion analysis considering the effect of moisture. Two equations considering the softening effect have been proposed and compared: (1) softening coefficient as a constant (2) Function of moisture concentration distribution and time. Since the equation was proposed through MATLAB and ABAQUS CAE, it is necessary to verify it through experiments with the actual propellant material in the future. Through this, changes in mechanical properties were predicted in hygrothermal environment. The thermal stress and strain were derived by applying temperature cycle of Korea and applied to the stress-strength analysis to derive the probability of failure. When the softening coefficient is constant, the time at which the total survival rate is 50% over a 30-year period is about 14 years. Meanwhile, in the case of a function of time, the lifetime of the propellant is about 10.3 years.

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초록/요약

본 연구에서는 현재 가장 많이 쓰이는 혼합형 고체 추진제 바인더 소재인 HTPB(Hydroxyl-terminated polybutadiene) 계 소재를 이용하여 추진제 내부로의 산소 및 수분의 확산 현상을 가시화하고 장기간 후의 추진제의 파손 확률을 분석하였다. HTPB matrix에 결합 되어있는 AP(Ammonium Perchlorate) 및 Aluminum powder의 함량과 사이즈에 따라 확산속도가 변화하는 것을 확인하였다. 산소의 확산속도는 수분보다 빠르기 때문에, 5년 이상이 지나면 산소는 추진제 내부 전체적으로 확산이 되어 경화를 일으키고, 점차 수분의 영향이 커짐으로써 연화 효과가 나타나는 것을 알 수 있었다. 본 논문에선 수분의 영향을 고려하기 위하여 확산 해석 결과를 도입하여 복합환경 하에서의 기계적 물성을 예측을 할 수 있는 새로운 Layton 식을 제안하였다. 연화 계수를 상수인 경우와 수분의 확산 농도 분포를 이용하여 연화 효과를 고려한 2개의 연화식을 비교하고 이를 통해 온도와 습도의 복합환경 하에서의 기계적 물성의 변화를 예측하였다. 우리나라의 온도 변화 주기를 적용하여 열 응력 값을 도출하고, 이를 stress-strength 분석에 적용하여 장기간 후의 파손확률을 도출하였다. 연화식을 함수가 아닌 상수로 대입하면 전체 30년 기간 동안 생존율이 50%가 되는 시기가 약 14년인 반면, 함수로 제안했을 때 수명은 약 10.3년으로 더욱 보수적으로 수명을 평가할 수 있었다.

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목차

국문 초록 i
Abstract iii
Contents v
List of Tables vii
List of Figures ix
Chapter 1 Introduction 1
1.1 Introduction 1
1.2 Research backgrounds 4
Chapter 2 Experiment and Modeling setup 8
2.1 Materials 8
2.2 Diffusion modeling for Analysis 11
2.2.1 Setup for diffusion experiments 11
2.2.2 . Parametric study of diffusion into the HTPB based composite 14
2.3 Setup for tensile test 19
2.4 Evaluation of failure rate by applying aging properties 21
2.4.1 Modified Layton equation 21
2.4.2 Calculation of failure probability 26
Chapter 3 Results and Discussion 29
3.1 Result of the experiments using inert HTPB based composite 29
3.1.1 Result of diffusion coefficient of the inert HTPB based composite 29
3.1.2 Tensile test results 31
3.2 Diffusion modelling – small scale 33
3.2.1 Interfacial diffusion effect 33
3.2.2 Shape effect 36
3.2.3 Random distribution modelling 40
3.3 Failure Probability analysis 51
3.3.1 Thermal stress analysis considering temperature cycle of Korea 51
3.3.2 Constant softening coefficient case of modified Layton equation 54
3.3.3 Proposal of the Softening equation based on the diffusion analysis 56
3.3.4 Stress-strength analysis 61
Chapter 4 Conclusion 64
References 66
Acknowledgement 70

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