Significance of Ms and Validation of Reference Stress Solutions for Crack Like Flaws: Part 2

Abstract

This is Part 2 of two papers discussing the significance of two key factors of crack like flaw assessment in the Fitness for Service assessment. While FEM analysis technology has been advancing amazingly in recent years, and FEM based fitness-for-service assessment of a damaged components, such as crack like flaws and local metal loss assessment, has become mainstream in assessments, it is still important to understand the reference stress solution based on a limit load analysis and the role of each factor in the failure mode to control the damaged component safely until the end of its life. In API 579-1/ASME FFS-1[1], Part 9, Assessment of Crack like Flaws, those reference stress solutions were developed based on the limit load analysis using Folias factor Mt and surface correction factor Ms. Folias factor Mt and surface correction factor Ms, are factors that account for the bulging effect around flaws. Those factors enable prediction of a maximum allowable pressure of a damaged cylindrical shell from a simple flat plate model that contain same size of a damaged area. As for Folias factor, Mt, it is well known to express the relationship between the reference stress of a through-wall crack flat plate and a through-wall crack cylinder. The application of Mt is clearly defined in ASME/API 579 FFS-1 part 9C, as well as papers by Folias et al. The the significance of the surface correction factor for surface flaw, Ms, has not been commonly understood well enough in general. Unfortunately, API 579-1/ASME FFS-1 also does not clearly mention its significance and how Ms is to be applied in the stress analysis. At a glance, Ms looks like a similar factor to Mt, and it is tempting to simply apply Ms to primary membrane stress term like Mt, but that is not correct. Eventually, an incorrect application of Ms would lead to an incorrect discussion of a flaw characterization. Often, there is a question about ASME/API 579 FFS-1 Part 9C reference stress solutions, especially for ASME/API 579 FFS-1 eq. 9C.76, from the misunderstanding meaning of the Ms factor. Addressing this issue is important to maintain the integrity of the Fitness-For-Service technology. In this Part 2 of two papers, validation of equations obtained in Part 1 are discussed and proven based on FEM analysis.