• 대한기계학회논문집A
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• 제28권5호
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• pp.524-531
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• 2004

To investigate relevance of the definition of the reference stress to estimate J and $C^{*}$ for surface crack problems, this paper compares FE J and $C^{*}$ results for surface cracked pipes with those estimated according to the reference stress approach using various definitions of the reference stress. Pipes with part circumferential inner surface crack and finite internal axial crack are considered, subject to internal pressure and global bending. The crack depth and aspect ratio are systematically varied. The reference stress is defined in four different ways using (i) the local limit load, (ii) the global limit load, (iii) the global limit load determined from the FE limit analysis, and (iv) the optimized reference load. It is found that the reference stress based on the local limit load gives overall excessively conservative estimates of J and $^{*}$. Use of the global limit load clearly reduces the conservatism, compared to that of the local limit load, although it can provide sometimes non-conservative estimates of J and $^{*}$. The use of the FE global limit load gives overall non-conservative estimates of J and $^{*}$. The reference stress based on the optimised reference load gives overall accurate estimates of J and $^{*}$, compared to other definitions of the reference stress. Based on the present finding, general guidance on the choice of the reference stress for surface crack problems is given.

• 대한기계학회논문집A
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• 제34권5호
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• pp.629-635
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• 2010

SGBEM-FEM 교호법이 Nikishkov, Park 및 Atluri 에 의하여 제안되었었다. 제안된 방법을 사용하면 임의 형태의 평면 혹은 비평면 삼차원 균열에 대하여 복합 모드의 응력강도계수를 구할 수 있다. 그러나 현장에서의 적용을 위해서는 이 방법의 정확성 및 신뢰성에 대한 검토가 더욱 필요하다. 따라서 본 논문에서는 응력강도계수에 영향을 주는 주요한 몇 가지 인자를 검토하였다. 그리고 원통의 내부 및 외부에 존재하는 원주방향 표면균열에 대한 응력강도계수를 구하여 기존의 해와 비교하였다. 그 결과 SGBEM-FEM 교호법은 이들 균열에 대하여 정확한 해를 주고 있음을 확인하였다.

• 대한기계학회논문집A
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• 제26권2호
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• pp.374-379
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• 2002

Corrosion fatigue tests were carried out to clarify the influence of acid fog as environmental factor on the fatigue strength of SM55C using rotary bending fatigue tester. The fatigue strength of acid fog specimen extremely decreased about 80% compared to that of distilled water specimen. In acid fog environment, a number of cracks commenced at corrosion pit and coalesced with the adjacent cracks during they propagate, and they formed a single non-propagating circumferential crack under the endurance stress of N=5$\times$10$\^$7/ cycles. Also, the depth of the crack is smaller than that of normal fatigue crack, so the crack has a veil small aspect ratio. The reason of this peculiar crack growth characteristics is that the crack opening-closure behaviors are hindered by corrosion products on the surface crack faces, and hence it is thinkable that the strong corrosion action like anodic dissolution for crack growth in depth direction is weaker compared with surface, resulting from faint pumping action of crack during loading-shedding processes.

• 한국가스학회지
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• 제4권4호
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• pp.25-33
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• 2000

최근 들어 가혹한 온도 및 압력조건에서 운전이 요구되는 고온배관들의 잔여수명예측은 상당히 중요한 관심사로 부각되고 있다. 이에 고온손상을 정량적으로 평가하여 설비를 안전하게 사용하기 위한 유지기술 및 수명평가 기술을 확보하는 것은 매우 중요하다. 한편 이러한 고온배관내 대부분의 균열은 반타원 표면균열의 형태로 형상화되어 파괴역학의 실제응용에서 발견할 수 있는 가장 복잡한 형상과 하중특성을 갖게 된다. 이를 배경으로 본 연구에서는 먼저 축방향 및 원주방향 반타원 표면균열이 존재하는 직관의 완전 3차원 유한요소망 자동생성 프로그램을 작성하였다. 이렇게 생성된 유한요소 입력자료를 이용하여 3차원 유한요소 해석을 수행하면 반타원 표면균열선단을 따른 고온 크리프 파괴 지배변수 $C^{\ast}$-적분값을 구할 수 있다. 또한 $C^{\ast}$-적분값에 준해 균열진전량을 계산하는 후처리 균열진전 모듈을 추가하여 사용자 편의 수명평가 프로그램을 개발하였다. 최종적으로 개발된 통합 수명 평가 프로그램을 이용하여 여러 가지 해석변수들의 반타원 표면균열을 갖는 고온 직관 수명에 대한 영향을 살펴보았다.

• 한국정밀공학회:학술대회논문집
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• 한국정밀공학회 1995년도 추계학술대회 논문집
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• pp.664-667
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• 1995

The purpose of this paper is to consider the disk failure phenomenon based on the second kind Fredholm integral equation and numerical inversion of Laplace transform when the head hit disk asperities at HDI under antiplane impact loading. The model for analysis is a two layeered half-space with a circumferential surface edge crack. The optimum design parameters to reduce the disk failure due to impact are presented

• 한국산학기술학회논문지
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• 제21권5호
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• pp.648-655
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• 2020

현대의 자동소총의 공이는 공이치기에 의해 타격을 받아 장전된 탄약의 뇌관을 기폭 시키는 역할을 한다. 이 과정에서 공이는 충격하중을 받게 되며 소총의 수명주기 동안 반복적인 힘을 받게 된다. 소총의 내구도 시험에서 전체의 96.26% 진행 중 공이가 조기에 파손되는 현상이 발생하였다. 이에 따라 원인분석과 재현시험을 통해 파손현상 사례연구를 실시하였다. 파손이 발생한 공이의 파단면을 현미경 및 SEM 분석결과 반복충격에 의해 표면 원주방향 전체에서 균열이 시작해 심부로 피로균열이 발생했다. 반복충격에 의해 균열이 성장하다 마지막에 피로파괴가 발생하였으며, 노치에 의한 것으로 추정되었다. 검증을 위해 원주방향 0.03mm의 노치를 생성한 공이로 재현시험결과 동일한 형태의 파단면을 가지면서 전체 수명의 64.25%에서 파손되었다. 파손사례연구를 위한 노치형태별 재현시험결과 한쪽 측면 노치 0.3mm, 0.5mm의 공이는 각각 65.53%, 50.76%에서, 6개 지점의 노치 0.03mm는 85.65%에서 파손되었다. 마지막으로 표면 거칠기가 거칠고 툴 마크가 육안으로 확인이 가능한 공이는 내구수명을 만족하며 381㎛의 내부균열이 진행되었다. 본 연구를 통해 노치형태별 파손에 대해 고찰하였으며, 반복충격을 받는 부품의 신뢰성 확보를 위해 노치와 표면 거칠기 품질관리가 중요한 것을 알 수 있다.

• 대한기계학회논문집A
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• 제30권8호
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• pp.993-1000
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• 2006

Operating experience of steam generators has shown that cracks of various morphology frequently occur in the steam generator tubes. These cracked tubes can stay in service if it is proved that the tubes have sufficient safety margin to preclude the risk of burst and leak. Therefore, integrity assessment using exact limit load solutions is very important for safe operation of the steam generators. This paper provides global and local limit load solutions for surface cracks in the steam generator tubes. Such solutions are developed based on three-dimensional (3-D) finite element analyses assuming elastic-perfectly plastic material behavior. For the crack location, both axial and circumferential surface cracks, and for each case, both external and internal cracks are considered. The resulting global and local limit load solutions are given in polynomial forms, and thus can be simply used in practical integrity assessment of the steam generator tubes.

• 대한기계학회논문집A
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• 제28권10호
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• pp.1483-1491
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• 2004

To assess the integrity of the pipeline is the most important problem to be solved first of all for prevention of any fracture accident of the pipeline. As a result of exerting such efforts, a number of plastic collapse assessment equations have been suggested, however, the scope of using or applying such assessment equations has not been exactly defined. In this study, the case that a surface crack existed in the circumferential direction in the external side of the natural gas pipeline and a bending load was applied to the pipeline was analytically identified as the most critical condition, and a plastic collapse assessment equation fur it was suggested. The flow stress of the API X65 linepipe was defined through the experiment conducted on SENT specimens. Also, a local assessing criterion of a 3-dimensional crack behavior considering not only the crack depth but also the crack length was suggested. Finally, a plastic collapse assessment equation for the API X65 linepipe was developed by performing the 3-dimensional finite element analysis.

• 한국생산제조학회지
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• 제8권1호
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• pp.21-27
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• 1999

In this study, stress intensity factors KI, KII, KIII are existing at the same time to a hollow cylindrical bar of three dimension inclination crack. In order to investigate by experimentally the effect of the inclination angle $\psi$ of crack, artificial inclination cracks in the circumferential direction are put in the surface of a hollow cylindrical bar made by the epoxy-resin. Experimentally, stress analysis methods of stress intensity factors were proposed. But, suitable method are the caustic method and the photoelastic stress freezing method. The mixed mode of KI, and KII, were determined by the photoelastic method of the classical approach method and the FORTRAN language program of the used smallest square method.

• Nuclear Engineering and Technology
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• 제48권6호
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• pp.1412-1422
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• 2016

Austenitic stainless steels (ASSs) are widely used for nuclear pipes as they exhibit a good combination of mechanical properties and corrosion resistance. However, high tensile residual stresses may occur in ASS welds because postweld heat treatment is not generally conducted in order to avoid sensitization, which causes a stress corrosion crack. In this study, round robin analyses on stress intensity factors (SIFs) were carried out to examine the appropriateness of structural integrity assessment methods for ASS pipe welds with two types of circumferential cracks. Typical stress profiles were generated from finite element analyses by considering residual stresses and normal operating conditions. Then, SIFs of cracked ASS pipes were determined by analytical equations represented in fitness-for-service assessment codes as well as reference finite element analyses. The discrepancies of estimated SIFs among round robin participants were confirmed due to different assessment procedures and relevant considerations, as well as the mistakes of participants. The effects of uncertainty factors on SIFs were deducted from sensitivity analyses and, based on the similarity and conservatism compared with detailed finite element analysis results, the R6 code, taking into account the applied internal pressure and combination of stress components, was recommended as the optimum procedure for SIF estimation.