• 오토저널
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• 제8권1호
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• pp.40-51
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• 1986

A study has been made of the corrosion fatigue of high strength low alloy steel in 3.5% NaCl solution under tension stress for solution temperature being 25.deg. C, 55.deg. C and 85 .deg. C. The main results obtained are as follows; 1) The corrosion fatigue crack growth rate curve could be divided into the First Region, the Second Region and the Third Region. 2) The corrosion fatigue crack growth rates in the First Region and the Second Region were Arrhenius temperature-dependent in this test range. The apparent activation energies for the corrosion fatigue cack growth rate were found to be 2000cal/mol in the First Region and 3700 cal/mol in the Second Region. 3) Hematite (Fe$_{2}$O$_{3}$) as the hexahedral crystal and magnetite (Fe$_{3}$O$_{4}$) as the octahedral crystal were observed in the corrosion products on the corrosion fatigue fracture surface at 85.deg. C and the anode fusion seem to be generated in the crack tip region at high temperature. 4) The complex environment effect ratio which was defined by the ratio of fatigue crack growth rate in corrosion environment to that in air might be considered not only a criterion estimating the effect of environment quantitatively but also an important parameter in the selection of the design stress for the fail safe design. The complex environment effect was not greater than ten in this test.

• 한국해양공학회지
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• 제22권2호
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• pp.50-57
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• 2008

The effect of weld improvement on the corroded fatigue life of welded structures was investigated. Toe grinding, TIG dressing and weld profiling were used as the geometric improvement methods. Fatigue tests under the corroded condition in artificial seawater were carried out to investigate the corrosion fatigue behavior of API 2W Gr.50T steel plate produced by POSCO. The test results in weld improved conditions were compared with those in as-welded condition. The test results were also compared with the design curves in UK DEn Class F. Corroded fatigue life of weld improved specimens was longer than that of as-welded specimen. Especially, the corroded fatigue life exceeded the mean SN curve in air of UK DEn Class F.

• 대한토목학회논문집
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• 제6권3호
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• pp.53-62
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• 1986

소재시험편(素材試驗片)과 맞대기 용접시험편(鎔接試驗片)을 대상(對象)으로 피로시험(疲勞試驗)을 행(行)하여 작용최대응력(作用最大應力)-피로수명(疲勞壽命)(S-N)선도(線圖), 소성변형율(塑性變形率)-하중반복회수(荷重反復回數)(${\varepsilon}_p$-N)선도(線圖), 초기균열발생시(初期龜裂發生時)의 소성변형율(塑性變形率)-피로수명(疲勞壽命)(${\varepsilon}_p-N_c$)선도(線圖) 및 균열성장율(龜裂成長率)-응력확대계수변동범위(應力擴大係數變動範圍)(da/dN-${\Delta}K$)선도(線圖)를 그려서 강구조물(鋼構造物) 용접부(鎔接部)에서의 여러가지 피로거동(疲勞擧動)을 검토(檢討)하였다. 본(本) 연구(硏究)에서 얻어진 결과(結果)를 요약(要約)하면 다음과 같다. 하중반복회수(荷重反復回數) $2{\times}10^6$ cycle에서의 피로강도(疲勞强度)는 용접부(鎔接部)가 소재부(素材部)의 약(約) 60%였으며 강복응력(降伏應力)에 대한 피로강도(疲勞强度)의 비(比)는 소재부(素材部)와 용접부(鎔接部)가 각각(各各) 0.72, 0.65로 나타났다. 용접시험편(鎔接試驗片)의 S-N 선도(線圖)는 피로강도(疲勞强度)의 감소(減少)가 완만(緩慢)한 구간(區間)과 피로강도(疲勞强度)의 감소(減少)가 큰 구간(區間)으로 분류(分類)되어 작용최대응력(作用最大應力)이 작을 수록 피로강도(疲勞强度)의 감소(減少)가 커지는 현상(現象)을 보였다. 두 구간(區間)으로 구분(區分)되지 않은 S-N 선도(線圖)에 의한 피로강도(疲勞强度)에 비(比)해 $2{\times}10^6$ cycle에서의 피로강도(疲勞强度)가 약(約) 83%에 해당(該當)하였다. 이는 낮은 응력(應力)에서 용접부(鎔接部) 내부결함(內部缺陷)의 영향(影響)이 크게 작용(作用)하는 때문이 아닌가 생각된다. 용접시험편(鎔接試驗片)의 경우(境遇) 소재시험편(素材試驗片)에 비(比)해, 또 작용최대응력(作用最大應力)이 클수록 초기균열발생(初期龜裂發生)이 빠르고 균열발생직전(龜裂發生直前)의 소성변형율(塑性變形率)의 증가(增加)가 급격(急激)한 것을 알 수 있었다. 소재시험편(素材試驗片)에서 ${\varepsilon}_p-N_c$ 관계식(關係式)의 상수(常數) ${\alpha}$값이 0.42 로 Manson, Coffin의 연구결과(硏究結果)와 거의 일치(一致)하였고 용접시험편(鎔接試驗片)에서는 이 값이 0.28 로 이들의 연구결과(硏究結果)와 큰 차이(差異)를 나타내었는데, 이는 피로(疲勞)에 의한 용접부(鎔接部) 소성변형(塑性變形)의 급격(急激)한 변화(變化)와 내부결함(內部缺陷)의 영향(影響)이 작용(作用)했기 때문이 아닌가 생각된다. 이와 같은 결과(結果)로 볼 때 기존(旣存)의 강구조물(鋼構造物) 및 향후(向後) 신설(新設)될 강구조물(鋼構造物) 용접부(鎔接部)의 품질(品質) 및 피로(疲勞)에 대한 안전성(安全性)에 적절(適切)한 대책(對策)이 요망(要望)되며, 이 분야(分野)에 관한 더 많은 연구(硏究)가 행(行)해져야 되리라고 사료(思料)된다.

• 대한기계학회논문집
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• 제10권2호
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• pp.188-197
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• 1986

본 연구에서는 응력이 변동되는 경우 내부크랙 전파특성을 중심으로 내부크랙 전파거동을 표면크랙 전파거동과 비교 검토하였다.

• Structural Engineering and Mechanics
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• 제69권4호
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• pp.361-369
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• 2019

The determination of Paris' law parameters based on crack growth experiments is an important procedure of fatigue life assessment. However, it is a challenging task because it involves various sources of uncertainty. This paper proposes a novel probabilistic method, termed the S-N Paris law (SNPL) method, to quantify the uncertainties underlying the Paris' law parameters, by finding the best estimates of their statistical parameters from the S-N curve data using a Bayesian approach. Through a series of steps, the SNPL method determines the statistical parameters (e.g., mean and standard deviation) of the Paris' law parameters that will maximize the likelihood of observing the given S-N data. Because the SNPL method is based on a Bayesian approach, the prior statistical parameters can be updated when additional S-N test data are available. Thus, information on the Paris' law parameters can be obtained with greater reliability. The proposed method is tested by applying it to S-N curves of 40H steel and 20G steel, and the corresponding analysis results are in good agreement with the experimental observations.

• 한국세라믹학회지
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• 제33권10호
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• pp.1095-1100
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• 1996

• Journal of Advanced Research in Ocean Engineering
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• 제1권4호
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• pp.239-251
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• 2015

In this study, a spectral fatigue analysis was performed for the topside structure of an offshore floating vessel. The topside structure was idealized using beam elements in the SACS program. The fatigue analysis was carried out considering the wave and wind loads separately. For the wave-induced fatigue damage calculation, motion RAOs calculated from a direct wave load analysis and regular waves with different periods and unit wave heights were utilized. Then, the member end force transfer functions were generated covering all the loading conditions. Stress response transfer functions at each joint were produced using the specified SCFs and member end force transfer functions. fatigue damages were calculated using the obtained stress ranges, S-N curve, wave spectrum, heading probability of each loading condition, and their corresponding occurrences in the wave scatter diagrams. For the wind induced fatigue damage calculation, a dynamic wind spectral fatigue analysis was performed. First, a dynamic natural frequency analysis was performed to generate the structural dynamic characteristics, including the eigenvalues (natural frequencies), eigenvectors (mode shapes), and mass matrix. To adequately represent the dynamic characteristic of the structure, the number of modes was appropriately determined in the lateral direction. Second, a wind spectral fatigue analysis was performed using the mode shapes and mass data obtained from the previous results. In this analysis, the Weibull distribution of the wind speed occurrence, occurrence probability in each direction, damping coefficient, S-N curves, and SCF of each joint were defined and used. In particular, the wind fatigue damages were calculated under the assumption that the stress ranges followed a Rayleigh distribution. The total fatigue damages were calculated from the combination with wind and wave fatigue damages according to the DNV rule.

• 한국융합학회논문지
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• 제5권1호
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• pp.1-8
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• 2014

본 논문의 목적은 진동 피로해석 기법을 적용하여 다수의 용접점을 포함하고 있는 점용접 구조물의 강성변화를 고려하여 피로수명을 평가하는 것이다. 먼저 모재부 인장특성 및 점용접부의 인장 및 피로특성을 획득하였다. 유한요소법을 적용하여 S-N 선도를 획득하여 피로해석에 적용하였다. 주파수 응답해석을 수행하여 구조물의 전달함수를 획득하였고, 주파수영역에서 0.11의 PSD를 선정하여 수행하였다. 강성변화 및 고유진동수의 변화를 알아보기 위하여 점용접부의 총 6개 지점 중 최소수명이 발생하는 지점을 기준으로 1개씩 제거하여 반복 해석을 수행하였다. 따라서 강성이 낮아질수록 고유주파수도 낮아지는 것을 확인하였다. 이러한 조건하에서 진동피로해석을 수행하여 피로손상의 누적에 따른 전달합수의 변화를 고려한 진동피로해석을 수행하였다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2004년도 추계학술대회
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• pp.342-347
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• 2004

To evaluate the fatigue lives of welded joints taking into residual stress relaxation, two approaches are applied. One is based on the conventional local strain analyses. The other is based on a model developed by the authors. In the first approach, the Ramberg-Osgood relation, Lawrence model and S.W.T. parameter are used. In the second approach, The S-N curve for a welded joint is deduced from that of the parent material. Residual stress relaxation obtained by finite element analysis is considered. Finally, we evaluate the fatigue lives for four weld details using the two approaches.

• Journal of Mechanical Science and Technology
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• 제16권10호
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• pp.1246-1252
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• 2002

Ultrafine grained (UFG) low carbon (0.15 wt.% C) steel produced by equal channel angula. pressing (ECAP) was tested for investigating the effect of load ratio on the fatigue crack growth rate. Fatigue crack growth resistance and threshold of UFG steel were lower than that of asreceived coarse grained steel. It was attributed to the less tortuous crack path. The UFG steel exhibited slightly higher crack growth rates and a lower △Kth with an increase of R ratio. The R ratio effect on crack growth rates and △Kth was basically indistinguishable at lower load ratio (R >0.3), compared to other alloys, which indicates that contribution of the crack closure vanishes. The crack growth rate curve for UFG steel exhibited a longer linear extension to the lower growth rate regime than that for the coarse grained as-received steel.