Special Issue on Fracture Mechanics. Development of a Simplified J-Estimation Scheme Based on the Reference Stress Method.

In fracture mechanics, several J-estimation schemes are based on the reference stress approach. This approach has been developed initially in the frame of the R5 rule for creep and R6 rule for elasto-plastic fracture assessments. Later other methods, based on the reference stress concept, where derived like the Js method introduced in the French RSE-M code en 1997 and the Enhanced Reference Stress (ERS) method in Korea around 2001. However these developments are based on the J2 deformation plasticity theory and well established for a pure power hardening law. Even in this latter case, the reference stress depends on the hardening exponent. Js and ERS attempt to minimize this dependence and propose some corrections for recorded behavior laws which cannot be fitted by a power law. However their validation has been established mainly on cases where the material behavior is governed by a Ramberg-Osgood (R0) law. The question may be raised, as for the bilinear hardening law case, of the existence of a reference stress for non RO laws.

Download Full-text

Determination of the Elastic-Plastic Fracture Mechanics Z-Factor for Alloy 182 Weld Metal Flaws for Use in the ASME Section XI Appendix C Flaw Evaluation Procedures

Volume 1: Codes and Standards ◽

10.1115/pvp2007-26733 ◽

2007 ◽

Author(s):

G. Wilkowski ◽

H. Xu ◽

D.-J. Shim ◽

D. Rudland

Keyword(s):

Finite Element ◽

Fracture Mechanics ◽

Weld Metal ◽

Alloy Steel ◽

Limit Load ◽

Full Scale ◽

Low Alloy Steel ◽

Evaluation Procedures ◽

Estimation Scheme ◽

J Estimation

One of the ways that the ASME Section XI code incorporates elastic-plastic fracture mechanics (EPFM) in the Section XI Appendix C flaw evaluation procedures for circumferential cracks is through a parameter called Z-factor. This parameter allows the simpler limit-load (or net-section-collapse) solutions to be used with a multiplier from EPFM analyses. Traditionally the EPFM solution was determined by using the GE-EPRI J-estimation scheme to determine the maximum load by EPFM, and Z = limit load / EPFM solution. The Z-factor is a function of the material toughness as well as the pipe diameter. With the advent of primary water stress-corrosion cracks (PWSCC) in pressurized water reactor (PWR) dissimilar metal welds (DMW), there is a need to develop Z-factors for Alloy 82/182 nickel-based alloy welds that are susceptible to such cracks. Although there have been Z-factor solutions for cracks in stainless and ferritic pipe butt welds, the DMW are somewhat different in that there is a much lower yield strength material on one side of the weld (typically forged or wrought 304 stainless steel) and on the other side of the weld the low alloy steel has a much higher strength than even the weld metal. This paper shows how 3D finite element analyses were used for a particular pipe size to determine the sensitivity of the crack location in the Alloy 182 weldment (crack in the center of weld, or closer to the stainless or low alloy steel sides), and how an appropriate stress-strain curve was determined for use in the J-estimation schemes. A Z-factor as a function of the pipe diameter was then calculated using the LBB.ENG2 J estimation scheme using the appropriate stress-strain curves from the finite element analysis. The LBB.ENG2 analysis was used rather than the GE-EPRI estimation scheme since it has been found that the LBB.ENG2 analysis is more accurate when compared with full-scale pipe tests. From past work, the GE-EPRI method was found to be the most conservative of the J-estimation schemes in predicting the maximum loads for circumferential flaws when compared to full-scale circumferentially cracked-pipe tests. The proposed Z-factor relationship should be restricted to normal operating temperatures (above 200C) with low H2 concentrations, where the Alloy 182 weld metal exhibits high toughness.

Download Full-text