Fitness-For-Service Assessments of Crack-Like Flaws Using NASGRO®

This report provides guidance in the use of the NASGRO® fracture mechanics and fatigue crack growth analysis software to perform fitness-for-service (FFS) assessments of crack-like flaws in typical pressure system components in accordance with procedures outlined in Part 9 of the 2021 edition of API 579-1/ASME FFS-1. While multiple sections of this report provide necessary background on the use of the various analytical modules of NASGRO, this report is not intended as a substitute for the NASGRO reference manual, nor is it intended as a substitute for API 579. Familiarity with both is required to understand and effectively apply the guidance provided in this document.

NASGRO has been enhanced to perform fitness-for-service analyses of safety-critical ground-based steel pressure vessels and other non-flight structures using approaches contained in the API and ASME standards. The guidelines for performing FFS assessment procedures for crack-like flaws outlined in this report are based on the well-known Failure Assessment Diagram (FAD). The objective of this report was to provide guidance in the use of NASGRO and the FAD to perform FFS assessments and fatigue crack growth analyses. Key features and recent enhancements to NASGRO that enable analyses to be performed aligned with API 579 approaches are reviewed. In some cases, there may be slight differences in formulations between NASGRO and API 579 and these are documented.

Four representative example analyses were presented in Section 13.0 and were intended to illustrate the use of NASGRO for different FFS analyses of “real-world” pressure vessel applications. While these examples considered typical pressure vessel geometries and materials, they should only be treated as “notional” demonstrations of the NASGRO software capabilities and features.

A key aspect of the examples presented herein is the use of different choices for material properties such as the fatigue crack growth relationship and the fracture toughness. In general, it was shown that the API 579 material property choices produce results that are conservative relative to the NASGRO database materials. It is recommended that analysts be aware of these differences when setting up an analysis.

Comparisons between results obtained with NASGRO and the INSPECT software were made for two of the examples. These benchmark comparisons produced reasonably good agreement between the SIFs and final crack sizes; however key differences were noted between the value of Lr used in the FAD. These differences are most likely due to the difference between a local and global failure computation used by the two programs. It is recommended that future NASGRO enhancements include the output of the reference stress and Lr values for both the local and global failure conditions, with an option for the analyst to choose which one to use for the FAD.

Lastly, additional benchmark analyses are recommended between NASGRO and other FFS software programs to identify and understand differences such as those noted above.