Understanding Equivalent Margins Analysis for Required Upper Shelf Energy

Equivalent Margins Analysis (EMA) involves the calculation of an alternative minimum reactor pressure vessel (RPV) upper shelf energy (USE) when the projected value falls below current limits codified in Title 10, Code of Federal Regulations, Part 50 (10 CFR 50), Appendix G. One set of calculation methodologies for performing the analysis are provided in the Nuclear Regulatory Commission’s (NRC’s) Regulatory Guide (RG) 1.161 and American Society of Mechanical Engineers Boiler & Pressure Vessel Code (ASME Code), Section XI, nonmandatory Appendix K. Careful application of fracture mechanics principles is necessary in order to properly carry out the evaluation. This is particularly the case for demonstrating compliance with the ductile crack growth stability criterion. This paper discusses robust implementation of EMA calculations and identifies recommended changes to RG 1.161 and ASME Code, Section XI, nonmandatory Appendix K.

T-Stresses Across Static Crack Kinking

This paper is concerned with the T-stress change before and after crack kinking in two-dimensional elastic solids. By using asymptotic analysis and the Westergaard stress function method, approximate analytical formulas for calculating the T-stress as well as stress intensify factors of an infinitesimal kink are given. Contributions from the T-stress before crack kinking, to the T-stress and the stress intensity factors of the kinked crack, are clearly described. It is noted that since the sign of the T-stress of a kinked open crack might be different from that of a main crack, simply using the sign of the T-stress before crack kinking is not sufficient to determine crack growth stability as observed in recent experiments.

The effect of crack growth stability induced by residual compressive stresses on strength variability

Rising T- (or R-) curve behavior is increasingly being used in order to improve the mechanical reliability of ceramic materials. In this study, the possibility of inducing such behavior using residual compressive stresses is analyzed. The T-curves obtained for certain residual stress profiles induce crack stability when the stress minima (compressive stress maxima) lie away from the surface of the sample. The consequences of this stabilization on the strength characteristics are a significant reduction in the strength variability and strength insensitivity to the initial flaw size. In addition to these desirable features, considerable strengthening is also obtained. Hence, suitably engineered compressive stress profiles are shown to be a novel and alternative means of enhancing mechanical reliability.