In recent years, the Office of Safety and Mission Assurance (OSMA) Pressure Systems program accomplished a number of efforts to facilitate broader use of NASGRO for the analysis and sustainment of pressure systems. By expanding the use of NASA’s perpetual, royalty-free NASGRO license, centers across the agency will realize a significant cost savings. 

“The agency has a long history with this software, but by expanding its capabilities, we open it to a wider audience and reduce the amount of software needed agencywide,” said Cliff Arnold, Pressure Systems program executive. 

Background

For many years, NASA engineers performing fracture analyses of ground-based pressure vessels and piping systems typically used specialized software developed by various third-party vendors. In aggregate, NASA centers have spent well into six figures per year for such software licenses.

At the same time, NASA had a sophisticated fracture and fatigue analysis package in the form of NASGRO —a suite of programs initially developed in the 1980s by Lockheed Martin Services under contract to NASA Johnson Space Center to address space flight fracture control needs and currently maintained by Southwest Research Institute¹. As mentioned, this license is both perpetual and royalty-free. Originally developed² to address the needs of the space flight community, this software didn’t include the features required for assessment of ground-based pressure systems using techniques contained in the applicable codes and standards.

Since 2014, the Pressure Systems program has been incrementally funding software enhancements to enable the efficient use of NASGRO to perform Fitness-for-Service (FFS) analyses consistent with industry-accepted practices for ground-based pressure systems as outlined by the American Society of Mechanical Engineers and American Petroleum Institute standards. These standards employ the Failure Assessment Diagram (FAD) as a key means of presenting failure criteria in an easy to apply  manner. While the FAD previously existed in NASGRO, it had not been exploited for easy application to ground-based pressure systems.

Efforts to Broaden NASGRO Use

A plotting utility to display the FAD for easy application, rather than providing the analytical results only in tabular form, was a key initial implementation task. The Pressure Systems program added FAD capability for two key pressure vessel geometries (axial and circumferential surface cracks in cylinders). Subsequently, it added FAD capability for three through crack models in a sphere and cylinder, as well as the capability to separately account for secondary cyclic stresses in a fatigue crack growth analysis. This fulfils an important need since it is required by both the FITNET (used in Europe) and American Society of Mechanical Engineers (ASME)/American Petroleum Institute (API) FAD approaches.

In 2017, the program implemented the API-579/ASME FFS-1 FAD approach in NASGRO (ASME Level 3, Methods A & B). The program also implemented a tool to fit stress-strain data to the Ramberg-Osgood equation to facilitate use of ASME Level 3 Method B. The program performed a series of benchmark stress intensity factor comparisons between detailed finite element analyses, NASGRO and API-579 models to investigate differences between these approaches and to identify potential inconsistencies in results. More recently, in 2019, the program made revisions within NASGRO’s FAD methodologies in order to be consistent with the 2016 edition of API 579. 

The Pressure Systems program implemented failure assessment diagram capabilities for the NASGRO bivariant weight function models for a corner crack, surface crack and embedded crack, and also derived new limit load solutions for these models. Since these models can handle a two-dimensional (2D) bivariant stress distribution, they now provide a unique capability to handle 2D weld residual stress distributions in a FAD analysis. This is a key capability in the FFS evaluations of NASA’s fleet of Layered Pressure Vessels (LPVs). The program added residual stress input capability to two commonly used pressure vessel models: an axial surface crack in a cylinder and a circumferential surface crack in a cylinder. In addition, it developed a new capability to input residual stresses via polynomial coefficients, which is available for all NASGRO weight function models; it enables easy use of weld residual stress distributions specified in the API-579 and other codes.

In 2018, the program started work on NASFAD, the first new analysis module added to the NASGRO suite of programs since 2000. The NASFAD module enables the NASGRO user to employ the FAD approach for assessment of crack-like flaws. The features of the module share many of the capabilities contained in other NASGRO modules, including geometry selection and definition, material selection, stress intensity factor and limit load calculation, and FAD plotting and output. The primary objective of the NASFAD module is to provide the capability to compute and plot assessment points for known (detected or assumed) crack sizes and graphically compare them to the FAD Failure Assessment Line. There is no crack growth capability in NASFAD, that is available in the NASFLA module using the FAD failure criteria. The NASFAD module provides the following capabilities as options: (a) plotting assessment point(s) vs FAD failure assessment line, and (b) computing critical crack size (for a given load and material). The NASFAD module was first available in NASGRO v9.2 and development is continuing in 2020.

The program developed a new bivariant Weight Function solution for an external surface crack in a sphere in 2019. The geometry is similar to the existing NASGRO sphere model, but the new solution provides additional functionality due to the weight function formulation that will enable the use of residual stresses and FAD application. This model will be available in NASGRO v10.0. Currently, work is underway to develop a corresponding solution for an internal surface crack in a sphere. 

In 2020, the program will incorporate material properties and test data obtained on the NASA LPV Risk Mitigation Project (RMP) into the databases of the NASGRO software. These data were obtained in recent years through an extensive material characterization testing program conducted at the NASA Marshall Space Flight Center. The goal of this effort is to ensure long-term availability of the material property data developed in the LPV RMP and to provide easy access to these data sets for use in future analyses. This objective will be accomplished by adding the LPV material property data to the databases of the NASGRO fracture and fatigue crack growth analysis software. This will ensure consistent use of the same data by many users and securely maintain the content of the data within the NASGRO material property databases.

NASA has used NASGRO agencywide as the primary fatigue and fracture analysis software for space flight hardware for over 25 years; the developments described have now leveraged existing NASA software capabilities to make them available royalty-free to the NASA Pressure Systems community with enhancements specifically tailored to ground-based pressure systems. 

“Enhancement of the NASGRO software is a continuous process, nevertheless, the milestones achieved over the past six years have put in place technology necessary to substantiate the integrity of NASA pressure systems using an established NASA software framework that will be supported and maintained long-term,” said Arnold. 

Questions about this article can be directed to Cliff Arnold. Questions about the NASGRO software can be directed to Joe Cardinal, Southwest Research Institute.