Kunio Hasegawa

Fatigue Lives of Multiple Flaws in Accordance With Combination Rule

When multiple flaws are detected in structural components, remaining lives of the components are estimated by fatigue flaw growth calculations using combination rules in fitness-for-service codes. ASME, BS7910 and FITNET Codes provide different combination rules. Fatigue flaw growth for adjacent surface flaws in a pipe subjected to cyclic tensile stress were obtained by numerical calculations using these different combination rules. In addition, fatigue lives taking into account interaction effect between the two flaws were conducted by extended finite element method (X-FEM). As the calculation results, it is found that the fatigue lives calculated by the X-FEM are close to those by the ASME Code. Finally, it is worth noticing that the combination rule provided by the ASME Code is appropriate for fatigue flaw growth calculations.Copyright

Experimental Study on Failure Estimation Method for Circumferentially Cracked Pipes Subjected to Multi-Axial Loads

When a crack is detected in a piping line during in-service inspections, failure estimation method provided in ASME Boiler and Pressure Vessel Code Section XI or JSME Rules on Fitness-for-Service for Nuclear Power Plants can be applied to evaluate the structural integrity of the cracked pipe. The failure estimation method in the current codes accounts for the bending moment and axial force due to pressure. The torsion moment is not considered.Recently, analytical investigations have been carried out by several authors on the limit load of cracked pipes considering multi-axial loads including torsion and two failure estimation methods for multi-axial loads including torsion moment with different ranges of values have been proposed. In this study, to investigate the failure behavior of cracked pipes subjected to multi-axial loads including the torsion moment and to provide experimental support for the failure estimation methods, failure experiments were performed on 20 mm diameter stainless steel pipes with a circumferential surface crack or a through-wall crack under combined axial force and bending and torsion moments. Based on the experimental results, the proposed failure estimation methods were confirmed to be applicable to cracked pipes subjected to multi-axial loads.Copyright

Development of Stress Intensity Factors for Deep Surface Cracks in Pipes and Plates

Stress intensity factors (SIFs) for pipes with semi-elliptical cracks containing large aspect ratios were calculated by finite element analysis. The cracks were circumferential and axial surface cracks inside the pipes. The parameters of the SIFs are crack aspect ratio, crack depth and the ratio of pipe radius to wall thickness. In comparing SIFs for plates and pipes, it can be clarified that SIFs for both plates and thin pipes with t/Ri ≤1/10 are almost the same, and the SIFs for plates are used as a substitute for pipes with t/Ri ≤1/10, where t is the pipe wall thickness and Ri is the inner radius of the pipe. This means that it is not necessary to provide SIF solutions for pipes with t/Ri ≤1/10, and it is suggested that number of tables for influence coefficients G values for pipes can significantly reduce.Copyright

Failure Experiments on Pipes With Local Wall Thinning Subjected to Multi-Axial Loads

Piping lines in nuclear power plants may experience multi-axial loads including tensile force, bending and torsion moments during operation. There is a lack of guidance for failure evaluation under the multi-axial loads including torsion moment. ASME B&PV Code Section XI Working Group is currently developing fully plastic failure evaluation procedures for pressurized piping items containing local wall thinning subjected to multi-axial loads. A failure estimation method for locally wall thinned pipes subjected to multi-axial loads including torsion moment has been proposed through numerical analyses. In this study, in order to investigate the failure behavior of the pipes with local wall thinning subjected to multi-axial loads including the torsion, failure experiments were performed on 20 mm diameter carbon steel pipes with a local wall thinning. Based on the experimental results, the proposed failure estimation method is confirmed to be applicable to pipes with local wall thinning.© 2015 ASME

Technical Basis for Extension of Section XI Appendix C Pipe Flaw Evaluation Procedures to Pipe Diameters Less Than NPS 4

A lower limit restriction on nominal pipe size (NPS) was originally set as NPS 4 (DN 100) in ASME Section XI, Appendix C pipe flaw evaluation procedures (1983 Edition). Pipe less than NPS 4 that contained flaws could not be evaluated to the code and had to be repaired or a relief request submitted. The primary basis for this analysis restriction was that stress analysis information for small diameter lines required for flaw evaluation would be limited for a detailed flaw evaluation. The plant owner would opt to repair such flawed pipe rather than perform an evaluation. However, current evaluation procedures are generally applicability to smaller pipe sizes. Because ASME Appendix C procedures are referenced in code cases that cover small diameter pipe, and because small diameter pipe can be difficult to repair due to cost and radiation exposure, a proposed revision to ASME Section XI has extended Appendix C procedures to pipe sizes down to NPS 1 (DN 25). This improvement is now in the 2013 Edition of Section XI and gives the plant owner the option for evaluation for such situations.© 2015 ASME