• 항공우주시스템공학회지
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• 제1권2호
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• pp.27-36
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• 2007

The Damage Tolerant Design is developed to help alleviate structural failure and cracking problems in aerospace structures. Recently, the Damage Tolerant Design is required and recommended for most of aircraft design. In this paper, the damage tolerant design is applied to tilt rotor UAV. First of all, the fatigue load spectrum for the tilt rotor UAV is developed and fatigue analysis is performed for the flaperon joint which has FCL (fatigue critical location). Tilt rotor UAV has two modes: helicopter mode when UAV is taking off and landing; fixed wing mode when the tilt rotor UAV is cruising. To make fatigue load spectrum, FELIX is used for helicopter mode. TWIST is used for fixed wing mode. Fatigue analysis of flaperon joint is performed using fatigue load spectrum. E-N curve approach is used for picking crack initiation point. The LEFM(Linear Elastic Fracture Method) is considered for analyzing crack growth or propagation. Finally, including the crack initiation and propagation, the fatigue life is evaluated. Therefore the Damage Tolerant Design can be done.

• 대한조선학회지
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• 제21권1호
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• pp.43-47
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• 1984

The purpose of this paper is to investigate theoretically the effect of residual stress due to welding in stress intensity factor of a plate containing the Model I Crack in different crack size and location, and on fatigue crack growth rate. The initiation of crack is found to be possible only in the region of tensile residual stress. The most dangerous crack has the values of d/b and a/b equal to about 0.6 and 1.0, respectively, where d/b is the ratio of distance from the crack to welding bead and the width of tensile residual stress region and a/b is the ratio of crack length and tensile residual stress region. The crack perpendicular to and on the line of welding bead and with a/b equal to about 0.6 has maximum stress intensity factor. The theoretical fatigue crack growth rate under residual stress and applied stress, which is obtained from Forman's Law by stress superposition, is relatively in good agreement with Glinka's[8] experimental value. The fatigue crack growth is shown to be retarded due to residual stress distribution.

• Journal of Advanced Marine Engineering and Technology
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• 제33권4호
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• pp.533-538
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• 2009

The magnitude of constraint effect $A_2$ values on the non-linear elastic plastic fracture toughness was experimentally estimated by using displacement at various measuring positions near crack tip. Constraint effect $A_2$ value was dependent on specimen configuration and on the measured displacement near crack front. The crack tip opening displacement in the vicinity of the crack tip front should be estimated within plastic region when appropriately constraint effect was calculated. It was found that the magnitude of constrain effect |$A_2$| is below 8.0 at the crack tip. But an appropriate location to measure the effective constraint effects $A_2$ at the critical value of J that crack initiation is characterizable by is r = 2mm and ${\theta}=90^{\circ}$ away from original crack tip, and the constraint effect |$A_2$| estimated is 5.3.

• 대한기계학회논문집A
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• 제29권11호
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• pp.1472-1479
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• 2005

Multiple surface crack distributed randomly along a weld toe influences strongly on the fatigue crack propagation life of welded joint. It is investigated by using statistical approaches based on series of systematic experiments. From the statistical results, initial crack numbers and its locations follow the normal distribution, and the probability of initial crack depths and lengths can be described well by tile Weibull distribution. These characteristics are used to calculate the fatigue crack propagation life, in which the mechanisms of mutual interaction and coalescence of the multiple cracks are considered as well as the Mk-factors obtained from a parametric study on the crack depths and lengths. The automatic calculation is achieved by the NESUSS, where the parameters such as the number, location and size of the cracks are all treated as random variables. The random variables are dealt through the Monte-Carlo simulation with sampling random numbers of 2,000. The simulation results show that the multiple cracks lead to much shorter crack propagation life compared with those in single crack situation. The sum of the simulation and tile fatigue crack initiation life derived by the notch strain approach agrees well with the experiments.

• 한국강구조학회 논문집
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• 제11권1호통권38호
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• pp.33-42
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• 1999

궤도용 차량의 안전성 확보를 위한 연구의 일환으로서 레일강의 균열 발생 조건과 모재와 용접부에 대한 정적파괴거동 및 단일모드 하중하의 피로균열진전거동을 검토하였다. 레일에서 횡방향 균열의 원점은 표면하층균열이며 이는 최대전단응력에 의해 발생하였다. 또한 표면하층균열의 크기가 증가함에 따라 균열의 진전은 전단모드에서 혼합모드로 천이될 가능성이 증가하였다. 용접부의 평면변형률 파괴인성은 조직의 조대화와 경도의 상승으로 인하여 모재에 비하여 약 10% 저하하였다. 용접부의 제 2단계 영역의 피로 균열진전속도는 낮은 ${\Delta}K$ 영역에서 모재에 비하여 저하하였으나 높은 ${\Delta}K$영역에서는 이의 차이가 소멸되었으며 이러한 경향은 R=0.1의 낮은 응력비에서 현저하였다. 이는 용접부의 미시조직이 모재에 비하여 성장하였기 때문이라고 판단된다.

• 비파괴검사학회지
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• 제27권1호
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• pp.38-47
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• 2007

항공기 구조를 모사하여 일련의 리벳구멍을 갖는 AA2024-T3 박판 구조를 대상으로 음향방출(AE)을 이용하여 피로균열을 탐지하고, 표면초음파(SAW)를 이용하여 균열길이를 측정하였다. 누적 AE 발생수 곡선은 단균열(short crack)의 발생과 성장에 따라 일정한 간격을 두고 급격히 증가하는 계단식 형태로 나타났으나 피로균열의 성장은 지수함수적인 증가를 보였다. SAW를 이용한 균열길이 측정은 균열길이가 다른 13개의 균열에 대하여 실시하였고, 측정된 데이터를 이동식 현미경으로 측정한 결과와 비교하였다. 그 결과 1 mm 이하의 단균열의 크기를 표면초음파 방법으로 측정하는 것은 거의 불가능하였으나, 1 mm 이상의 균열에서는 비교적 높은 신뢰도로 균열길이 측정이 가능함을 확인함으로써 실용적인 측면에서 이 방법이 유효하게 사용될 수 있는 영역이 존재함을 알 수 있었다.

• Structural Engineering and Mechanics
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• 제73권5호
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• pp.585-598
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• 2020

We develop design model within which nucleation and propagation of crack in a fibrous composite is described. It is assumed that under loading, crack initiation and fracture of material happens in the composite. The problem of equilibrium of a composite with embryonic crack is reduced to the solution of the system of nonlinear singular integral equations with the Cauchy type kernel. Normal and tangential forces in the crack nucleation zone are determined from the solution of this system of equations. The crack appearance conditions in the composite are formed with regard to criterion of ultimate stretching of the material's bonds. We study the case when near the fiber, the binder has several arbitrary arranged rectilinear prefracture zones and a crack with interfacial bonds. The proposed computational model allows one to obtain the size and location of the zones of damages (prefracture zones) depending on geometric and mechanical characteristics of the fibrous composite and applied external load. Based on the suggested design model that takes into account the existence of damages (the zones of weakened interparticle bonds of the material) and cracks with end zones in the composite, we worked out a method for calculating the parameters of the composite, at which crack nucleation and crack growth occurs.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2004년도 추계학술대회
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• pp.31-36
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• 2004

Defective components of interest include not only homogeneous components, but also components with weldments where tensile properties vary across the weldment. Noting that the region near the weldment is the most vulnerable place for crack initiation and subsequent growth, defect assessment methods for homogeneous structure. Moreover, weldment width and crack location also affects the deformation and fracture behavior of the welded joints. These weld characteristics can evaluate using plastic limit load. So in this paper, evaluate plastic limit load both full circumference part-throughwall cracked pipes and circumference through-wall cracked pipes considering weld characteristics. And using evaluate results, proposed J-integral and crack opening displacement(COD) estimate method based on reference stress method.

• 한국철도학회논문집
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• 제10권5호
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• pp.546-553
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• 2007

본 연구는 압입축에 프레팅이 발생할 경우 프레팅 마모에 의한 접촉형상의 변화가 접촉응력의 분포, 균열발생 위치에 미치는 영향을 분석하고자 하였다. 압입축의 프레팅 피로실험시 측정한 접촉면의 프로파일을 이용하여 유한요소 해석을 수행하고 피로 사이클별 마모형상 변화에 따른 접촉면의 응력 변화를 분석하였다. 접촉면의 응력 해석결과를 이용하여 프레팅 피로손상 파라미터와 다축 피로이론를 적용하여 마모에 따른 균열발생위치의 변화를 해석하고 실험과 비교, 분석하였다. 프레팅 마모에 의해 접촉 끝단의 응력집중은 초기에 급격하게 감소하며, 마모가 진행될수록 응력집중의 위치는 접촉끝단에서 안쪽으로 이동한다. 따라서 프레팅 마모에 의한 접촉응력의 변화가 균열발생 위치의 변차와 다중균열발생의 주요원인임을 명확히 하였다.

• Journal of Mechanical Science and Technology
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• 제20권12호
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• pp.2159-2167
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• 2006

In order to find out a physical quantity which controls the fatigue life of a structure and to predict the fatigue life of tires, a finite element simulation methodology to use the cracking energy density (CED) and the virtual crack closure technique (VCCT) was proposed and applied to three different tires of a similar size. CED was calculated to predict the location of a crack initiation, and VCCT was used to obtain the strain energy release rate (SERR) at the tip of an initiated crack. Finite element simulations showed that SERR oscillated in the circumferential direction with its minimum occurring just before the contact zone and its maximum occurring just after the center of the contact zone, and SERR was affected significantly by the frictional force acting on the crack surface. In addition, a durability test was conducted to measure the fatigue life of the three tires. The comparison of SERR values with the test data revealed that the fatigue life increased as the amplitude of SERR decreased or as the R-ratio of SERR increased.