• 대한기계학회:학술대회논문집
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• 대한기계학회 2008년도 추계학술대회A
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• pp.725-726
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• 2008

This paper proposes elastic stress intensity factors and crack opening displacements (CODs) for a slanted axial through-wall cracked cylinder under an internal pressure based on detailed 3-dimensional (3-D) elastic finite element (FE) analyses. Based on the elastic FE results, the stress intensity factors along the crack front and CODs through the thickness at the center of the crack were provided. These values were also tabulated for three selected points, i.e., the inner and outer surfaces and at the mid-thickness. The present results can be used to evaluate the crack growth rate and leak rate of a slanted axial through-wall crack due to stress corrosion cracking and fatigue. Moreover, the present results can be used to perform a detailed Leak-Before-Break analysis considering more realistic crack shape development.

• 한국유체기계학회 논문집
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• 제11권4호
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• pp.14-21
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• 2008

The prediction for gaseous leak flows through a narrow crack is important for a leak-before-break (LBB) analysis. Therefore, the methodology to obtain the flow characteristics of gaseous leak flow in a narrow crack for the wide range by using the product of friction factor and Reynolds number correlations (fRe) for a micro-channel is developed and presented. The correlation applied here was proposed by the previous study. The fourth-order Runge-Kutta method was employed to integrate the nonlinear ordinary differential equation for the pressure and the regular-Falsi method was also employed to find the inlet Mach number. A narrow crack whose opening displacement ranges from 10 to $100{\mu}m$ with a crack length in the range from 2 to 200mm was chosen for sample prediction. The present results are compared with both numerical simulation results and available experimental measurements. The results are in excellent agreement with them. The leak flow rate can be approximately predicted by using proposed methodology.

• 대한기계학회논문집A
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• 제24권11호
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• pp.2796-2803
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• 2000

For traditional Leak-Before-Break(LBB) analyses, symmetric conditions were assumed for a pipe-nozzle interface to simplify the analysis in calculating J-integral. However. this assumption could result in an overly conservative design criteria for a pipe-nozzle interface, Since the pipe-nozzle interface is asymmetric due to the difference of stiffness between pipe and nozzle, it is required to develop a new methodology considering the nozzle effect. The objective of this paper is to evaluate the effect of nozzle no the development of LBB design criteria for nuclear pipings. For this purpose, extensive finite element analysis were performed to evaluate the effect of nozzle on Crack Opening Area(COA), Detectable Leakage Crack(DLC) length and J-integral values. In conclusion, it was proven that the application of LBB concept could be extended for more nuclear piping system by considering the nozzle.

• 대한기계학회논문집A
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• 제28권10호
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• pp.1612-1620
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• 2004

This study presents methods to estimate elastic-plastic crack opening displacement (COD) fur circumferential through-wall cracked pipes for the Leak-Before-Break (LBB) analysis of pressurized piping. Proposed methods are based not only on the GE/EPRI approach but also on the reference stress approach. For each approach, two different estimation schemes are given, one for the case when full stress-strain data are available and the other fur the case when only yield and ultimate tensile strengths are available. For the GE/EPRI approach a robust way of determining the Ramberg-Osgood (R-O) parameters is proposed, not only fur the case when detailed information on full stress-strain data is available but also for the case when only yield and ultimate tensile strengths are available. The COD estimates according to the GE/EPRI approach, using the R-O parameters determined from the proposed R-O fitting procedures, generally compare well with the published pipe test data. For the reference stress approach, the COD estimates according to the method based on both full stress-strain data and limited tensile properties are in good agreement with pipe test data. In conclusion, experimental validation given in the present study provides sufficient confidence in the use of the proposed method to practical LBB analyses even though when information on material's tensile properties is limited.

• 한국해양공학회지
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• 제13권3호통권33호
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• pp.36-48
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• 1999

Fatigue life and crack retardation behavior after penetration were experimentally examined using surface pre-cracked specimens of aluminium alloy 5083. The Wheeler model retardation parameter was used successfully to predict crack growth behavior after penetration. By using a crack propagation rule, the change in crack shape after penetration can be evaluated quantitatively. Advanced, waveform-based acoustic emission (AE) techniques have been successfully used to evaluate signal characteristics obtained form fatigue crack propagation and penetratin behavior in 6061 aluminum plate with surface crack under fatigue stress. Surface defects in the structural members are apt to be origins of fatigue crack growth, which may cause serious failure of the whole structure. The nondestructive analysis on the crack growth and penetration from these defects may, therefore, be one of the most important subjects on the reliability of the leak before break (LBB) design. The goal of the present study is to determine if different sources of the AE could be identified by characteristics of the waveforms produced from the crack growth and penetration. AE signals detected in four stages were found to have different signal per stage. With analysis of waveform and power spectrum in 6061 aluminum alloys with a surface crack, it is found to be capabilities on real-time monitoring for the crack propagation and penetration behavior of various damages and defects in structural members.

• 한국정밀공학회지
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• 제18권6호
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• pp.67-73
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• 2001

Three dimensional, elastic-plastic finite element(FE) analyses for circumferential through-wall cracked pipes under combined tension and bending are performed using actual tensile data of stainless steel, for two purposes. The first one is to validate the recently-proposed enhanced reference stress (ERS) method to estimate the J-integral and COD for circumferential through-wall cracked pipes under combined tension and bending. The second one is to compare those results with the GE/EPRI estimations. The FE results of the J-integral and the COD, resulting from six cases of proportional and non-proportional combined tension and bending, compare very well with those estimated from the proposed method. Excellent agreements of the proposed method with the detailed FE results provide sufficient confidence in the use of the proposed method to the Leak-Before-Break(LBB) analysis of through-wall cracked pipes under combined tension and bending.

• 대한기계학회논문집A
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• 제21권11호
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• pp.1786-1795
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• 1997

In order to perform leak-before-break design of nuclear piping systems and integrity evaluation of reactor vessels, full stress-strain curves and fracture resistance (J-R) curves are required. However it is time-consuming and expensive to obtain J-R curves experimentally. The objective of this paper is to modify two J-R curve prediction methods previously proposed by the authors and to propose an additional J-R curve prediction method for nuclear piping materials. In the first method which is based on the elastic-plastic finite element analysis, a blunting region handling procedure is added to the existing method. In the second method which is based on the empirical equation, a revised general equation is proposed to apply to both carbon steel and stainless steel. Finally, in the third method, both full stress-strain curve and finite element analysis results are used for J-R curve prediction. A good agreement between the predicted results based on the proposed methods and the experimental ones is obtained.

• 대한기계학회논문집A
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• 제25권4호
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• pp.741-747
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• 2001

Three-dimensional, elastic-plastic finite element analyses for circumferential through-wall cracked pipes are performed using actual tensile data of stainless steels, for two purposes. The first one is to validate the recently-proposed enhanced reference stress (ERS) method to estimate the J-integral and COD for circumferential through-wall cracked pipes. The second one is to compare those results with the GE/EPRI estimations. It is found that the J-integral and COD estimations according to the GE/EPRI method can be very sensitive to how the stress-strain data are fitted using the Ramberg-Osgood relation. Moreover, no tendency can be found regarding the most appropriate fitting range for the Ramberg-Osgood fit. On the contrary, the J-integral and COD estimations based on the ERS method give more accurate results than the GE/EPRI estimation. The present results provide confidence in applying the proposed method to the Leak-Before-Break(LBB) analysis.

• 대한기계학회논문집A
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• 제28권4호
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• pp.352-361
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• 2004

One important element of the Leak-Before-Break analysis of nuclear piping is how to determine relevant fracture toughness (or the J-resistance curve) for nonlinear fracture mechanics analysis. The practice to use fracture toughness from a standard C(T) specimen is known to often give conservative estimates of toughness. To improve the accuracy, this paper proposes a new method to determine fracture toughness using a nonstandard testing specimen, curved wide-plate in tension. To show validity of the proposed curved wide-plate test, the J-resistance curve from the full-scale pipe test is compared with that from the curved wide-plate test and that from the C(T) specimen. It is shown that the J-resistance curve form the curved wide-plate tension test is similar to, but that from the C(T) specimen is lower than, the J-resistance curve from the full-scale pipe test. Further validation is performed by investigating crack-tip constraint conditions via detailed 3-D FE analyses, which shows that the crack-tip constraint condition in the curved wide-plate tension specimen is indeed similar to that in the full-scale pipe under bending.

• International Journal of Ocean Engineering and Technology Speciallssue:Selected Papers
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• 제4권1호
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• pp.44-50
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• 2001

Existing surface defects in structural members often act as sites of fatigue crack initiation, and if undetected, these cracks may grow through the thickness of the member, leading to catastrophic failure of the structure. Thus, in-service monitoring of fatigue cracks through reliable and effective nondestructive techniques is an important ingredient in the leak-before-break (LBB) design and safe operation of defects critical structures. An advanced, waveform-based, acoustic emission (AE) technique has been used in this paper to study the characteristics of the signals emanating from the initiation, growth and through-the -thickness penetration of surface fatigue crack in a 6061 aluminum plate. The goal of this experimental study is to determine whether the evolution of the fatigue crocks could be identified from the properties of the waveforms produced during the tests. The AE waveform signals detected at different stages of crack growth was found to have different temporal and spectral characteristics. The data analysis technique presented here can be applied to real-time monitoring of the initiation and propagation of fatigue cracks in structural components.