• 한국안전학회지
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• 제11권2호
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• pp.16-24
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• 1996

In this study we simulate the fatigue test of a compact tension specimen and obtain the displacements, stresses and strains by using the finite element method. And we examine the path independency of $\Delta$J integral values and compare it with $\Delta$J integral values calculated from load-load line displacement curve. From the results of this study, we can find that $\Delta$J integral show the path Independency for saturated materials. We can also find that the path independency of $\Delta$J Is not satisfied when different material Is assumed near the crack tip, but the difference in $\Delta$J is small. And $\Delta$J integral values calculated from load-load line displacement is very analogous with those from integration path but always have lower values than those from integration paths. In the case of crack closing, we found that $\Delta$J integral values from load-load line displacement should be calculated with the load Increment values based on the crack opening point. The unsaturated material is also simulated and its $\Delta$J shows different values according to the path, but the difference is small.

• Journal of Welding and Joining
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• 제14권1호
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• pp.38-46
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• 1996

High temperature low cycle fatigue crack growth behavior is investigated over a range of two temperatures and various frequencies in SUS 304 stainless steel. It is found that low frequency and temperature can enhance time-dependent crack growth. With high temperature, low frequency and long crack length, ${\Delta}J_c/{\Delta}J_ f$, the ratio of creep J integral range to fatigue J integral range is increased and time-dependent crack growth is accelerated. Interaction between ${\Delta}J_f$ and ${\Delta}J_c$ is occured at high frequency and low temparature and ${\Delta}J_c$, creep J integral range is fracture mechanical parameter on transition from cycle-dependent to time dependent crack growth in creep temperature region.

• 한국정밀공학회지
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• 제13권12호
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• pp.70-77
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• 1996

Fatigue tests were carried out at high temperature on a Cr-Mo-V steel in order to assess the fatigue life of components used in power plants. The characteristics of high temperature fatigue were divided in terms of cycle-dependent fatigue and time-dependent fatigue, each crack propagation rate was examined with respect to fatigue J-integral range, .DELTA. J$_{f}$and creep J-integral range, .DELTA. J$_{c}$. The fatigue life was evaluated by analysis of J-integral value at the crack tip with a dimensional finite element method. The results obtained from the present study are summarized as follows : The propagation characteristics of high temperature fatigue cracks are determined by .DELTA. J$_{f}$for the PP(tensile plasticity-compressive plasticity deformation) and PC(tensile plasticity - compressive creep deformation) stress waveform types, and by .DELTA. J$_{c}$for the CP(tensile creep- compressive plasticity deformation) stress waveform type. The crack propagation law of high temperature fatigue is obtained by analysis of J-integral value at the crack tip using the finite element method and applied to examine crack propagation behavior. The fatigue life is evaluated using the results of analysis by the finite element method. The predicted life and the actual life are close, within a factor of 2.f 2.f 2.

• 한국전산구조공학회논문집
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• 제23권5호
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• pp.483-491
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• 2010

본 논문에서는 점탄성 재료의 피로균열 성장을 분석하기 위해 ${\Delta}J$-적분을 이용하였다. J-적분의 계산 시 기존의 수치해석 방법이 아닌 해석적인 적분 해를 도출하여 계산시간을 절감하고 정확도를 크게 높였다. 계산 시 응력확대계수는 특정 균열에 대해 참조하는 방법이 아닌 유한요소해석을 통해 구하는 방법을 사용하였다. 기존의 ${\Delta}K$를 이용한 피로균열 예측과는 달리 크리프 변형계수, 단 두 개의 피로성장 모델 변수만을 가지고 다양한 하중과 하중주기에서의 피로균열 성장을 성공적으로 분석할 수 있었다.

• 대한기계학회논문집
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• 제19권9호
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• pp.2133-2141
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• 1995

The thermo-mechanical fatigue tests were performed on the specimens extracted from 1.5Cr-0. 67Mo-0.33V alloy. The characteristics of thermo-mechanical fatigue crack propagation were examined and reviewed in view of fracture mechanics. The results obtained from the present study are summarized as follows : (1) The propagation characteristics of isothermal low-cycle fatigue crack are dominated by .DELTA.J$_{f}$ in case of PP waveform, and .DELTA.J$_{c}$ in case of CP waveform. (II)The propagation characteristics of thermo-mechanical fatigue crack are dominated by .DELTA.J$_{c}$ for in-phase case, and by .DELTA.J$_{c}$ for out-of-phase. The present results were in good agreement with the equation of propagation law for isothermal low-cycle fatigue crack in case of thermo-mechanical fatigue.tigue.e.

• 대한기계학회논문집
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• 제14권6호
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• pp.1568-1575
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• 1990

본 연구에서는 중앙균열을 가진 SB41강의 평판이 하중제어와 변형률제어 피로 상태에서 소규모 항복, 대규모 항복, 또는 전면 항복의 각 응력에서 피로균열 진전 속 도와 J적분 범위, .DELTA.J와의 관계를 조사하는 한편, 피로균열 진전 속도를 지배하는 국 소적 역학 인자인 균열선단 개구변위, COD와 균열 진전 속도와 관계 및 그 특성을 고 찰하였다.

• Journal of Advanced Marine Engineering and Technology
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• 제24권6호
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• pp.24-33
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• 2000

This paper investigates the relationship between J-integral and crack tip opening displacement, ${\delta}_t$ using Gordens results of numerical analysis. Estimation were carried out for several strength levels such as ultimate, flow, yield, ultimate-flow, flow-yield stress to determine the influence of strain hardening and the ratio of crack length to width on the $J-{\delta}_t$ relationship. It was found that for SE(B) specimens, the $J-{\delta}_t$ relationship can be applied to relate J to ${\delta}_t$ as follows $J=m_j{\times}{\sigma}_i{\times}{\delta}_t$ where $m_j=1.27773+0.8307({\alpha}/W)$, ${\sigma}_i:{\sigma}_U$, ${\sigma}_{U-F}={\frac{1}{2}} ({\sigma}_U+{\sigma}_F$), ${\sigma}_F$, ${\sigma}_F}$ $Y=({\sigma}_F+{\sigma}_Y)$, ${\sigma}_Y$

• Structural Engineering and Mechanics
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• 제17권1호
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• pp.51-68
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• 2004

Since the linear elastic fracture analysis has been proved to be insufficient in predicting the failure of strain hardening materials, a number of fracture concepts have been studied which remain applicable in the presence of plasticity near a crack tip. This work thereby presents a new finite element model to predict the elastic-plastic crack-tip field and fatigue life of center-cracked panels(CCP) with ductile fracture under large-scale yielding conditions. Also, this study has been carried out to investigate the path-dependence of J-integral within the plastic zone for elastic-perfectly plastic, bilinear elastic-plastic, and nonlinear elastic-plastic materials. Based on the incremental theory of plasticity, the p-version finite element is employed to account for the accurate values of J-integral, the most dominant fracture parameter, and the shape of plastic zone near a crack tip by using the J-integral method. To predict the fatigue life, the conventional Paris law has been modified by substituting the range of J-value denoted by ${\Delta}J$ for ${\Delta}K$. The experimental fatigue test is conducted with five CCP specimens to validate the accuracy of the proposed model. It is noted that the relationship between the crack length a and ${\Delta}K$ in LEFM analysis shows a strong linearity, on the other hand, the nonlinear relationship between a and ${\Delta}J$ is detected in EPFM analysis. Therefore, this trend will be depended especially in the case of large scale yielding. The numerical results by the proposed model are compared with the theoretical solutions in literatures, experimental results, and the numerical solutions by the conventional h-version of the finite element method.

• 한국해양공학회지
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• 제2권2호
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• pp.61-70
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• 1988

In order to consider the concept of the fitness for purpose'in fatigue design of offshore structure, fracture mechanics is applied to evaluate initial or weld defects. Generally, linear elastic fracture mechanics has been applied to tstimate initial fatigue crack propagation rate as well as long fatigue crack propagation rate. But, initial fatigue crack propagation rate in elasto-plastic notch field may not be characterized by application of stress intensity factor range .DELTA. K, because plastic effect due to stress concentration of notch may contribute to initial crack propagation. Therefore, to introduce the plastic effect into fatigue crack driving force, in this studty, the evaluating method of J-integral range .DELTA. J, was developed by willson was modified for application to notch field. In calculation of .DELTA. J obtained from the modified J-integral, stress gradient and crack closure behavior in the notch field were considered. The initial crack propagation rates in the notch fields of mild steels and high tensile strength steels were correlated to .DELTA. J. As the result, it was cleared that the present .DELTA. J is applicable to charachterize the fatigue crack propagation rates in both the elastic and elasto-plastic notch fields.

• Journal of Advanced Marine Engineering and Technology
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• 제26권1호
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• pp.59-67
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• 2002

The comparison of $J_{Rice}$-resistance considering a few strength ratio in Rice J-integral formula and $J_{\delta}$-resistance curves converted from experimental CTOD using appropriate strength chosen according to strain hardening level, n=10.6 (A533B steel) and n=8.1 (BS4360 steel) is carried out. The optimal dimensionless strength ratio like the factor of revision, (see full text)reflecting strain hardening level in Rice\`s experimental formula is found out and the reliability of appropriate reference strength chosen according to strain hardening level in different materials is investigated through doing that CTOD is transformed from $J_{\delta}$-integral using relationship between J-integral and CTOD. The results are as follows; 1) The optimal factor of revision is when m equals to 3 in (see full text) for Rice's and the above optimal factor of revision multiplies by coefficient, η in Rice's experimental formula instead of n=2, 2) and the pertinent reference strength for high strain hardening material like BS4360 steel is ultimate strength, $\sigma_{u}$ and for material like A533B steel is ultimate-flow strength, $\sigma_{u-f}$. The incompatible of the behavior of both experimental J-resistance curves using Rice's formula and CTOD-resistance curves for A533B and BS4360 steel by Gordon, et al., could be corrected using the optimal factor of revision in Rice\`s and the pertinent reference strength in J=$m_{j}$${\times}$$\sigma_{i}$${\times}$CTOD.