Jae Jun Han

Mismatch Effect of Creep Properties on Steady-State Stresses for Welded Straight Pipes: Quantification and Application

This paper describes steady-state stresses on welded straight pipes with the heat-affected zone (HAZ) using detailed two dimensional elastic creep finite element analyses. In our previous studies [9,10], it was found that the mis-match effect in creep on steady-state stresses within the weld metal for a various branch junction could be uniquely quantified by the mis-match factor, defined as a function of creep exponent and constants. The present study expands the findings to be applicable for welded straight pipes, not only the most widely used but also relatively simple compared to branch junctions. To see the effects of mis-match in creep properties and weld configuration, the parametric studies have been performed for various mismatched creep properties and three different groove angles, respectively. Internal pressure, tension and its combined cases are applied to investigate the effect of the loading mode. It is found that steady-state creep stresses can be quantified as mis-match factor and creep exponent. In conclusion, validation of the findings is presented and discussed through an application to the CMV pipe.Copyright

Mismatch Limit Loads of Circumferential Cracked Pipes With V-Groove Welds

The present work reports mis-match limit loads for V-groove welded pipe for a circumferential crack using finite element (FE) analyses. In our previous paper [14], closed-form solutions of mis-match limit loads were proposed for idealized butt weld configuration as a function of the strength mis-match ratio with only one geometry-related slenderness parameter. To integrate the effect of groove angles on mis-match limit loads, the geometry-related slenderness parameter has to be modified by relevant geometric parameters including groove angle, crack depth and root opening based on plastic deformation patterns in theory of plasticity. Circumferential through-wall cracks are located at the centre of the weld considering two different groove angles (45°, 90°). With regards to loading conditions, axial (longitudinal) tension is applied for all cases. For the parent and weld metal, elastic-perfectly plastic materials are used to simulate under-matching and over-matching conditions in plasticity. The overall results from the proposed solutions agree well with FE results.Copyright

Comparison of J Resistance Curves From Toughness Testing Specimens With Those From Various Cracked Pipe Tests

J contour integral still has great importance to predict fracture of both small specimen and full-scaled pipes. However, it is difficult to obtain experimental J resistance curve of full-scaled pipes due to the differences of defect shape and complexity of loads. Due to the recent development of the FE damage analysis to predict fracture of full-scaled pipes, it is also possible to predict J resistance of full-scaled pipes. To use this FE damage model for fracture estimation, it is necessary to verify the validity of this model by compared with toughness testing specimens.In this paper, J resistance curves of full-scaled pipes using FE damage analysis were compared with various toughness testing specimens from Pipe Fracture Encyclopedia performed by Battelle. And the J contour integral were calculated from FE analysis using the element-size-dependent damage model recently proposed by the authors. Compared results showed that J calculation using FE damage analysis could be used for J-estimation of full-scaled pipes by compared with fracture toughness testing specimens.Copyright

Effect of Creep Mismatch Factor on Stress Redistribution in Welded Branch Pipes

This paper describe steady state stress distribution into the weld metal of welded branch components using detailed three dimensional elastic creep finite element analyses. In order to show the effect of the loading mode, this research is carried out under various loading conditions such as internal pressure, in-plane bending to the branch pipe and out-of-plane bending to branch pipe. Also, to generalize the unique aspect, three geometries of branch components including welded large bore branch, medium bore branch, and trunnion are considered. It is a well-known fact that the creep strain rate of welds material is faster than that of parent material. Therefore, the creep exponent and constants for the parent and weld metal are systematically varied to analyze under-matching, even-matching and over-matching conditions in creep. It can be shown that mismatch effect can be quantified as mismatch factor with specific characteristics.Copyright

Evaluation of limit loads for circumferentially cracked pipes under combined loadings

Since the Fukushima nuclear accident, several researchers are extensively studying the effect of torsion on the piping systems In nuclear power plants. Piping installations in power plants with a circumferential crack can be operated under combined loading conditions such as bending and torsion. ASME Code provides flaw evaluations for fully plastic fractures using limit load criteria for the structural integrity of the cracked pipes. According to the recent version of Code, combined loadings are provided only for the membrane and bending. Even though actual operating conditions have torsion loading, the methodology for evaluating torsion load is not established. This paper provides the results of limit load analyses by using finite element models for circumferentially cracked pipes under pure bending, pure torsion, and combined bending and torsion with tension. Theoretical limit load solutions based on net-section fully plastic criteria are suggested and verified with the results of finite element analyses.

Development of a Short-term Failure Assessment of High Density Polyethylene Pipe Welds - Application of the Limit Load Analysis -

In the US, the number of cases of subterranean water contamination from tritium leaking through a damaged buried nuclear power plant pipe continues to increase, and the degradation of the buried metal piping is emerging as a major issue. A pipe blocked from corrosion and/or degradation can lead to loss of cooling capacity in safety-related piping resulting in critical issues related to the safety and integrity of nuclear power plant operation. The ASME Boiler and Pressure Vessel Codes Committee (BPVC) has recently approved Code Case N-755 that describes the requirements for the use of polyethylene (PE) pipe for the construction of Section III, Division 1 Class 3 buried piping systems for service water applications in nuclear power plants. This paper contains tensile and slow crack growth (SCG) test results for high-density polyethylene (HDPE) pipe welds under the environmental conditions of a nuclear power plant. Based on these tests, the fracture surface of the PENT specimen was analyzed, and the fracture mechanisms of each fracture area were determined. Finally, by using 3D finite element analysis, limit loads of HDPE related to premature failure were verified. † Corresponding Author, kimy0308@korea.ac.kr C 2015 The Korean Society of Mechanical Engineers 류호완 · 한재준 · 김윤재 · 김종성 · 김정현 · 장창희 406

Prediction of creep crack initiation and growth for P91 at 600°C using MOD-NSW model

In this paper, predictions of creep crack initiation times and growth rates using the NSW-MOD model are compared with experimental data for the parent and weld metals of P91 at 600°C. For the weld metal, creep crack growth rates are found to be bounded by upper and lower bounds of the predictions, but creep crack initiation times are close to the upper bound. For the parent material, creep crack growth rates are bounded by upper and lower bounds of the predictions using the minimum creep rates, but creep crack initiation times are bounded by those using the average creep rates.

Ductile Fracture Simulation of CRIEPI STPT 410 Pipe With Circumferential Crack

This paper provides simulation of ductile crack growth in full-scale cracked pipe tests using an element-size dependent damage model. This method is based on the stress-modified fracture strain damage model. The stress-modified fracture strain model is determined to be incremental damage in terms of stress triaxiality and fracture strain for dimple fracture from tensile test result with FE analyses technique. To validate the proposed method, this research analyses STPT 410 cracked pipes test at 300°C taken from CRIEPI (Central Research Institute of Electric Power Industry). In order to calibrate the stress-modified fractures strain model, tensile tests and fracture toughness tests were compared with simulated results using element-size dependent damage model. Tensile specimen and fracture toughness specimen were extracted from STPT 410 steel pipe. The calibrated damage model predicts ductile crack growth in 5 type circumferential cracked pipes bending test. And these results were compared with the experimental results. The results show that the proposed method can simulate ductile crack growth in full-scale cracked pipe tests.© 2014 ASME

Effect of weld geometry on mis-match limit load analyses for circumferential surface cracked pipes in the centre of welds

The weld metal compared to the base metal, is known to be more vulnerable to cracking. Thus, the strength mismatch limit load analysis is necessary for defect assessment. In our previous paper, closed-form solutions of mis-match limit loads were proposed for idealized butt weld configuration as a function of the strength mis-match ratio with geometry-related slenderness parameter. The actual weld geometry is not idealized band configuration but shape of V-grooved welds. The surface crack is more common and significant than through wall crack. The present work provides mis-match limit loads for circumferential surface cracked pipes in the centre of V-groove welds using finite element (FE) analysis. Various crack depth (a/t=0.35∼0.8) and groove angles (20°∼90°) are considered for systematic investigation. With regards to loading conditions, axial tension is applied for all cases.Copyright

Limit Loads for Circumferentially Cracked Pipes Under Combined Bending and Torsion With Tension

In power plants, piping installations with a circumferential crack can be operated under combined bending and torsion. ASME Boiler and Pressure Vessel code Section XI non-mandatory Appendix C provides the flaw evaluations for fully-plastic fracture using limit load criteria for the structural integrity of the cracked pipes. According to the recent version of Code, combined loading is provided only for membrane and bending. Even though actual operating conditions have torsion loading, the methodology for evaluating torsion load is not established, and assumed torsion is relatively small compared to membrane and bending and can be ignored in the evaluation.This paper provides the results of limit load analyses by using finite element models for circumferentially cracked pipes under pure bending, pure torsion and combined bending and torsion with tension. Theoretical limit load solutions based on net-section fully-plastic criteria are compared with the results of finite element analyses. The validation of theoretical limit loads for combined bending and torsion with tension is discussed.Copyright