A Study of Simulated Fatigue Load Spectra Generation Method for Fatigue Life Evaluation and Testing

항공기 주구조 설계과정에서 건전성 평가를 위하여 피로하중에 대한 내구성 또는 손상 허용성 해석이 요구된다. 이것은 피로손상누적이나 균열성장에 대한 해석적 분석이나 구조시험을 통하여 수행하게 되고 이때 사용수명 중 예상되는 신뢰성 있는 모사 피로하중스펙트럼 산출이 정확한 평가결과를 얻는데 중요한 관건이 된다. 항공기에 작용되는 불규칙 변동하중은 확정적 하중과 통계적 추론하중으로 구분할 수 있다. 비행 중 발생되는 돌풍이나 지상활주 등과 같은 하중은 후자에 속하게 되며 이러한 하중스펙트럼은 예상되는 실제하중 형태와 유사하게 모사하기가 매우 어려운 일이다. 특히 불규칙 변동하중 하에서 하중사이클 발생이력이 균열성장이나 피로손상누적에 큰 영향을 주는 것으로 알려져 있으며 따라서 이러한 추론적 하중은 방대한 측정자료로 부터 통계적 확률처리를 거쳐 실제와 유사한 하중스펙트럼 형태로 모사할 필요가 있다. 이러한 관점에서 본 연구는 마르코프기법을 활용하여 측정하중의 분석자료를 기초로 확률적 랜덤처리 과정을 거쳐 불규칙하중스펙트럼을 구성하는 알고리즘을 고찰하고 계산과정을 전산프로그램화 한다.Increasing emphasis on fatigue life and damage tolerance evaluation in aircraft structural design requires a more precise analytical definition and simulation of the fatigue load spectrum. The need for clear understanding and generation method of simulated flight load spectrum, has been emphasized by developments that clarify the role of fatigue load sequences and interaction in the fatigue failure process. The load spectrum applied to airframes can be classified into two kinds, such as deterministic and stochastic load. The latter case , for example gust or ground load, is not easy to be simulated as real load profile in the consideration of probabilitic occurrances of load level. A method is presented for deriving realistic fatigue load spectrum using the technique of Markov process on the basis of distribution of load level occurrance frequencies from the measured random load data.Increasing emphasis on fatigue life and damage tolerance evaluation in aircraft structural design requires a more precise analytical definition and simulation of the fatigue load spectrum. The need for clear understanding and generation method of simulated flight load spectrum, has been emphasized by developments that clarify the role of fatigue load sequences and interaction in the fatigue failure process. The load spectrum applied to airframes can be classified into two kinds, such as deterministic and stochastic load. The latter case , for example gust or ground load, is not easy to be simulated as real load profile in the consideration of probabilitic occurrances of load level. A method is presented for deriving realistic fatigue load spectrum using the technique of Markov process on the basis of distribution of load level occurrance frequencies from the measured random load data

Evaluation of High Cycle Fatigue Strength for Notched Steel Components under Combined Fatigue Loads

기계부품이나 구조물부재의 파손은 전체 시스템운용에 큰 영향을 미칠 수 있으며 대부분의 원인이 피로파괴에 기인한 것으로 알려지고 있다. 따라서 이러한 부재들의 설계과정에서 피로강도의 평가가 필수적으로 요구된다. 일반적으로 이러한 부재들은 기능상 여러가지 형태의 노치를 포함하게 되고 노치 부근의 응력집중으로 인하여 피로강도의 저하를 가져오게 된다. 그러므로 이러한 노치국부 형상에 따라 피로강도 특성이 달라지게 되고 따라서 설계과정에서 이러한 부재들에 대해 일일이 시험을 통하여 피로강도를 평가한다는 것은 많은 시간과 비용이 소요되는 일이다. 본 연구는 구조물용 철강 및 탄소강 재료의 여러 형태의 노치재에 대하여 참고문헌에서 제시된 고사이클 피로?쳬? 결과를 분석하여 얻은 상대응력변화율과 노치부의 피로강도 사이의 상관관계를 바탕으로 노치재의 장수명 피로강도를 추정하는 평가기법을 제시하였다. 또한 기계부품이나 구조물 부재에 작용하는 피로하중은 일반적으로 복합적인 하중형태가 많으며 이러한 복합하중 하에서의 피로강도를 등가응력 형태로 고찰하였다.In the design of machines and structural components, which are made of various steels, cyclic loading and resulting fatigue damage necessitate estimates of fatigue strength for a designed service life to ensure safety and durability. Moreover, it is inevitable to involve notch-like geometric shapes on the components, and also to be situated under combined loading conditions in real structures. Therefore it is often a difficult task to evaluate allowable strength to resist fatigue problems. And if component tests should be performed to verify fatigue design for every fatigue critical items, it will be absolutely tedious and ineffective during design work. In this reason, a simple and effective procedure is required to evaluate fatigue strength for given shapes and dimensions. Assessments of allowable fatigue strength in these problems, are based on the variation and distribution of stresses near the notch root at most critical location. A number of studies have shown that fatigue damage on local notch root does not directly depend on the maximum local stress, but rather on the relative stress gradient at near notch root. This paper shows how the stress gradient is related to the allowable fatigue strength for notched members under combined loading conditions, using various test data which have been published on several references.In the design of machines and structural components, which are made of various steels, cyclic loading and resulting fatigue damage necessitate estimates of fatigue strength for a designed service life to ensure safety and durability. Moreover, it is inevitable to involve notch-like geometric shapes on the components, and also to be situated under combined loading conditions in real structures. Therefore it is often a difficult task to evaluate allowable strength to resist fatigue problems. And if component tests should be performed to verify fatigue design for every fatigue critical items, it will be absolutely tedious and ineffective during design work. In this reason, a simple and effective procedure is required to evaluate fatigue strength for given shapes and dimensions. Assessments of allowable fatigue strength in these problems, are based on the variation and distribution of stresses near the notch root at most critical location. A number of studies have shown that fatigue damage on local notch root does not directly depend on the maximum local stress, but rather on the relative stress gradient at near notch root. This paper shows how the stress gradient is related to the allowable fatigue strength for notched members under combined loading conditions, using various test data which have been published on several references

Rivet Transfer Load Analysis in Center-Cracked Stiffened Panel

항공기 보강박판 구조의 손상허용성 평가를 위해서는 보강박판의 피로균열성장과 잔여강도해석이 선행되어야 하며 이러한 작업을 수행하기 위해서는 응력강도계수의 계산이 필요하며 이것은 리벳 전달하중을 계산함으로써 얻을 수 있다. 복잡한 형상의 구조에서는 유한요소법을 활용하여 응력강도 구조해석을 많이 수행하나 항공기구조의 초기설계 과정에서는 잦은 설계 변경이나 형상 및 치수의 비교분석 계산이 빈번하게 발생되므로 매번 유한요소해석을 수행 하기에는 번거롭고 계산시간이 많이 소요되게 된다. 따라서 본 연구에서는 중첩법을 활용하여 표피와 보강재 리벳연결부의 변위적합조건으로 부터 균열이 있는 리벳 보강박판의 리벳 전달하중을 기존의 탄성해를 통하여 구하는 과정을 고찰하였다. 특히 리벳의 유연도와 보강재 리벳구멍의 진단면적 고려 그리고 영향계수 계산시 간략화의 영향 등을 분석하였다. 이 방법의 적용범위는 비교적 형상이 간단한 보강판넬에 국한되므로 물론 유한요소해석에 비하여 제한적일 수 밖에 없으나 해석을 위한 입력 자료의 준비 과정이 쉽고 계산시간이 훨씬 짧으므로 균열성장에 따라 많은 계산을 반복적으로 수행해야하는 균열성장해석이나 잔류강도해석에 매우 효과적이라고 할 수 있다.It is indispensable to perform the residual strength analysis and crack propagation life evaluation for the damage tolerance design and assessment of cracked stiffened panels. Residual strength analysis of stiffened skin structure is predicted on the principles of linear elastic fracture mechanics. And also crack growth rate is a function of stress intensity factor range. Straight-forward finite element analysis to solve these problems, with detailed numerical modeling, would be prohibitively expensive, even in the linear elastic regime because whenever the configuration regarding to the structure, crack or load changes, the entire analysis must be repeated. This paper presents the formulation and computer programming for closed form solution using the structural superposition principal to obtain the rivet transfer force according to the precedent literatures. The effects of rivet flexibility, stiffener rivet hole net section, and approximated influence coefficient on the rivet force distribution, are also discussed comparing the calculation results.It is indispensable to perform the residual strength analysis and crack propagation life evaluation for the damage tolerance design and assessment of cracked stiffened panels. Residual strength analysis of stiffened skin structure is predicted on the principles of linear elastic fracture mechanics. And also crack growth rate is a function of stress intensity factor range. Straight-forward finite element analysis to solve these problems, with detailed numerical modeling, would be prohibitively expensive, even in the linear elastic regime because whenever the configuration regarding to the structure, crack or load changes, the entire analysis must be repeated. This paper presents the formulation and computer programming for closed form solution using the structural superposition principal to obtain the rivet transfer force according to the precedent literatures. The effects of rivet flexibility, stiffener rivet hole net section, and approximated influence coefficient on the rivet force distribution, are also discussed comparing the calculation results

Analysis of Fatigue Crack Crowth Behavior for Damage Tolerance Design of Fuselage Bulkhead Structure

압력차단벽 구조는 보강박판 구조형태로 이루어져 있으며 초기 설계과정에서 균열성장평가를 위한 응력강도해석에 유한요소해석법을 널리 사용하고 있으나 반복되는 계산작업이 매우 번거롭고 많은 시간이 소요되게 된다. 특히 압력차단벽은 내압을 효과적으로 감당하기 위하여 보통곡면 보강구조 형태로 설계되며 이러한 구조적 특성상 보강평판과는 달리 내압에 의해 표피구조에 이축응력이 발생하고 균열면을 따라 부풀어오름(bulging)현상이 발생되기 때문에 설계과정에서 손상허용해석이 까다롭게 된다. 이에 본 연구는 보강평판 해석에 적용한 바 있는 중첩법을 활용하여 압력차단벽 구조의 응력강도해석을 수행하고 유한요소해석 결과와 비교함으로써 중첩법의 활용 가능성을 검토하였다. 또한 내압차단벽의 손상허용 범위를 중앙보강재가 건재한 경우와 파손된 경우로 나누어 표피균열 끝단의 응력강도 해석 및 균열성장해석을 수행함으로써 손상허용성 확장에 따른 균열성장수명을 비교하였다.The fuselage pressure bulkhead is one of the important primary structures which should be designed satisfactorily damage tolerant. The calculation of fatigue crack growth in this study formed part of the preliminary analysis of damage tolerance design for the fuselage rear bulkhead of a medium civil transport. On the basis of superposition technique, stress intensity solution was derived for the cracked stiffened panel of the pressure bulkhead. The solution used is that of Poe for a built-up skin stiffener construction having a skin crack originating at a failed stiffener location. Both cases of intact and broken center stiffener were considered on the calculation of crack tip stress intensity factor. The only loading on the bulkhead is that of pressure, which is consisted of intermal cabin pressurization and external suction. The finite element analysis was also carried out for the stress intensity factor solution and its results were compared to that of closed form solution. The influence of center stiffener broken and biaxial stress state of skin, and also the effect of skin bulging due to internal pressure on crack growth behaviors, are also studied.The fuselage pressure bulkhead is one of the important primary structures which should be designed satisfactorily damage tolerant. The calculation of fatigue crack growth in this study formed part of the preliminary analysis of damage tolerance design for the fuselage rear bulkhead of a medium civil transport. On the basis of superposition technique, stress intensity solution was derived for the cracked stiffened panel of the pressure bulkhead. The solution used is that of Poe for a built-up skin stiffener construction having a skin crack originating at a failed stiffener location. Both cases of intact and broken center stiffener were considered on the calculation of crack tip stress intensity factor. The only loading on the bulkhead is that of pressure, which is consisted of intermal cabin pressurization and external suction. The finite element analysis was also carried out for the stress intensity factor solution and its results were compared to that of closed form solution. The influence of center stiffener broken and biaxial stress state of skin, and also the effect of skin bulging due to internal pressure on crack growth behaviors, are also studied

Analysis of Flapping and Lagging Natural Frequency on Rotating Hingeless Rotor Blade

운용속도로 회전하고 있는 로터깃의 구조적 안정성은 로터깃의 고유진동 특성과 밀접한 관계가 있다. 특히 공력하중에 의한 가진주파수와 깃의 고유진동수가 일치할 경우 깃에 공진현상이 발생하게 되어 갓구조나 허브 또는 연결구조 등에 심각한 파손을 유발할 수가 있다. 따라서 운용속도로 회전하는 로터깃에 대한 고유진동해석이 필수적이며 이 경우 회전원심력 효과를 고려하여 계산을 수행하여야 한다. 여기서는 플래핑운동과 래깅운동의 특성을 기초로 전달행렬(transfer matrix)을 이용하여 비교적 간단한 방법으로 회전하는 무한지깃의 플래핑과 래깅운동에 대한 깃의 고유진동수 해석을 수행하였다.The structural stability of rotor blades which is rotating at operational speed, are closely related to their natural vibration characteristics. Especially, if the frequencies of aerodynamic exciting force become equal to natural frequencies of blade, the resonance of blade can occur and cause blades, hubs or their joint structures to fail. So it is necessary to have the analysis of blades checked and the effect of centrifugal force should be considered to perform the calculation. This paper presents the characteristics of flapping and lagging of rotating hingeless rotor blade and relatively simple method for analysis of natural frequencies using transfer matrix.The structural stability of rotor blades which is rotating at operational speed, are closely related to their natural vibration characteristics. Especially, if the frequencies of aerodynamic exciting force become equal to natural frequencies of blade, the resonance of blade can occur and cause blades, hubs or their joint structures to fail. So it is necessary to have the analysis of blades checked and the effect of centrifugal force should be considered to perform the calculation. This paper presents the characteristics of flapping and lagging of rotating hingeless rotor blade and relatively simple method for analysis of natural frequencies using transfer matrix

Damage Tolerance Test and Evaluation on Aircraft Butt Lap Joint Structure Under Flight-by-Flight Load Spectrum

본 연구는 항공기 주익이나 동체 표피구조의 Butt Lap Joint 연결부 볼트구멍 주변에 내재된 짧은 모서리 균열을 가진 시험편 구조에 대하여 설계 예상수명 동안의 비행하중스펙트럼하에서 피로균열성장 시험을 수행하고 모서리 균열에 대한 기존의 3차원 응력강도계수 계산식을 이용하여 균열성장해석을 수행한 결과와 실험결과를 비교함으로써 Butt Lap Joint 구조의 손상허용성 평가를 수행하였다.The prediction of fatigue crack growth on the basis of the linear fracture mechanics is in accordance with the cases of through-thickness-cracks or long cracks, however the crack growth pattern of corner crack which is located at joint holes is different. The behaviours of such a short corner crack play an important role on the damage tolerance life of aircraft structures. In this paper fatigue crack growth tests are carried out on the butt lap joint specimens with small corner crack on the bolt hole under expected flight-by-flight load spectrum. And the damage tolerance evaluation is also performed for the design condition of 2 life times.The prediction of fatigue crack growth on the basis of the linear fracture mechanics is in accordance with the cases of through-thickness-cracks or long cracks, however the crack growth pattern of corner crack which is located at joint holes is different. The behaviours of such a short corner crack play an important role on the damage tolerance life of aircraft structures. In this paper fatigue crack growth tests are carried out on the butt lap joint specimens with small corner crack on the bolt hole under expected flight-by-flight load spectrum. And the damage tolerance evaluation is also performed for the design condition of 2 life times

Fatigue Crack Growth Behavior in Riveted Stiffened Sheet with Crack Closure Effects under Flight-by-Flight Load Spectrum

항공기 구조는 기능적 특성상 주구조의 대부분이 보강박판으로 이루어져 있으며 이러한 리벳연결 보강박판구조의 손상허용성(damage tolerance) 평가는 설계해석에서 필수적인 과제라고 할 수 있다. 손상허용성에 대한 구조적 건전성을 평가하기 위하여 비행하중스펙트럼 하에서 정확한 피로균열 성장수명 분석이 우선적으로 요구된다. 본 논문에서는 비행 대비행 하중스펙트럼 하에서 중앙균열을 가진 리벳연결 보강박판구조의 균열성장 거동을 예측하기 위하여 기존의 응력강도 해석에 소성유도 균열닫힘 모델인 Strip-yield 모델을 활용하여 보다 개선된 균열성장수명 해석을 시도하였다. 또한 응력강도 해석과 균열성장 해석과정을 연계시켜 전산프로그램화 하여 불규칙 변동하중 하에서 보강박판의 균열성장 수명해석을 효과적으로 수행하도록 하였다. 또한 보강박판 시험편구조에 대하여 균열성장수명 계산을 수행하고 계산결과와 시험결과를 비교 분석함으로써 개선된 결과를 확인하였다.It is required precise fatigue crack growth prediction to evaluate structural integrity for damage tolerance under flight load spectrum. In this paper, for predicting crack growth behaviour of riveted stiffened sheet with center crack under flight-by-flight load spectrum, it was performed to evaluate crack growth life with the effect of crack closure on the basis of strip-yield model. It was also carried out the crack growth life calculation and compared with experimental test results for the stiffened sheet specimen structure that consists of 2024-T3 aluminum alloy skin and 2024-T3 aluminum alloy stiffener.It is required precise fatigue crack growth prediction to evaluate structural integrity for damage tolerance under flight load spectrum. In this paper, for predicting crack growth behaviour of riveted stiffened sheet with center crack under flight-by-flight load spectrum, it was performed to evaluate crack growth life with the effect of crack closure on the basis of strip-yield model. It was also carried out the crack growth life calculation and compared with experimental test results for the stiffened sheet specimen structure that consists of 2024-T3 aluminum alloy skin and 2024-T3 aluminum alloy stiffener

Fatigue Durability Evaluation for an Exhaust Muffler Structure of Automobile

자동차 배기 머플러 구조는 주행 중 진동으로 인한 극심한 피로하중을 받게 되고 구조적으로 취약한 필렛 용접부에서 자주 파손이 발생한다. 따라서 설계개발 과정에서 피로수명에 대한 평가가 요구된다. 본 연구는 머플러 구조의 피로내구성을 평가하기 위하여 주행 시험 데이터로부터 국부변형률 스펙트럼을 산출하고 스트레인-피로수명 시험 데이터를 기초로 잔여강도 저하모델을 사용하여 피로내구성을 평가하였다.It is one of the important design assessment items to evaluate the fatigue life for the exhaust system of automobile. For this purpose, it is necessary to describe reliable load spectrum expected during the real operation of automobile. A study on the development of operational load spectrum for the exhaust system of automobile and the analysis of welded part were carried out to obtain the local strain spectrum on the spot where the fatigue damages are mostly anticipated. Using these results, fatigue life evaluation was performed on the basis of the strength degradation model with strain-life test data.It is one of the important design assessment items to evaluate the fatigue life for the exhaust system of automobile. For this purpose, it is necessary to describe reliable load spectrum expected during the real operation of automobile. A study on the development of operational load spectrum for the exhaust system of automobile and the analysis of welded part were carried out to obtain the local strain spectrum on the spot where the fatigue damages are mostly anticipated. Using these results, fatigue life evaluation was performed on the basis of the strength degradation model with strain-life test data

수명과 대사의 항상성 유지를 위한 p62의 필수적인 역할

학위논문(박사) - 한국과학기술원 : 생명과학과, 2012.2, [ iv, 109 p. ]한국과학기술원 : 생명과학과