Session: DA-08-03 Practical Applications of FFS

Paper Number: 105485

105485 - A Critical Initial Flaw Size Analysis Approach for Cleavage Fracture in the Circumferential Welds of Layered Pressure Vessels 

The National Aeronautics and Space Administration (NASA) has been using carbon steel, layered pressure vessels (LPVs) since the 1950s as a critical part of their space flight center and research center operations. Many of these LPVs were designed and manufactured prior to ASME Boiler and Pressure Vessel (B&PV) code requirements, during a time in which the field of fracture mechanics was in its infancy. Several of the steels used in these non-code vessels are at a risk of cleavage fracture at temperatures within the operating temperature ranges of the NASA sites where the vessels are installed. Additionally, missing construction records and the use of proprietary materials makes fitness-for-service assessments and traditional evaluations of these non-code vessels a significant challenge. The replacement cost of these vessels is prohibitive, and thus NASA has engaged in a multi-year effort to determine the continued fitness-for-service of these aging, non-code LPVs through extensive material characterization, non-destructive evaluation (NDE) methods and technology development, and detailed computational analysis.

 

One activity within this multi-year effort has been to compute critical flaw sizes for key regions in the LPVs, such as the circumferential welds, to guide NDE inspections. Cleavage fracture toughness, , changes as a function of crack-front length, and standard procedures for computing the critical initial flaw size (CIFS) in the NASGRO® fatigue and fracture control software, which assumes fracture toughness is invariant to crack-front length, must be revised. This paper describes the CIFS analysis approach for cleavage fracture in circumferential welds developed using NASGRO and provides an example of the analysis for a demonstration vessel within NASA’s LPV fleet.

 

As used here, the CIFS grows to just reach a fracture critical size/shape at completion of a specified load schedule that drives fatigue crack growth. In this study, the initial flaw is a semi-elliptical, surface-breaking crack with a user-defined, initial aspect ratio, . The critical crack size (CCS) occurs when the maximum stress-intensity factor along the front of the growing crack, , reaches the user-defined material toughness, here taken in terms of the cleavage value . In general, increasing levels of the nominal stress field decrease the CIFS as smaller initial cracks become necessary to sustain the full load schedule before growing to become critical. The current version of NASGRO does not support two features needed in this study: (1) a fracture toughness value dependent on crack-front length, and (2) a varying  ratio during the CIFS computations to accommodate effects of the weld residual stresses that have a strong spatial variation on the crack plane. Consequently, an alternative iterative scheme was developed that leveraged the existing NASGRO capabilities for computing fatigue crack growth and the stress intensity factor after fatigue crack growth, , to find the CIFS consistent with the front-length dependent fracture toughness and loading schedule.

Presenting Author: Matthew Kirby Southwest Research Institute

Presenting Author Biography: Matt Kirby is a research engineer in the Mechanical Engineering Division at Southwest Research Institute (SwRI). His technical background and interests include applied mechanics, composite materials, and probabilistic modeling. Matt received his bachelor’s degree in mechanical engineering from Baylor University and his master’s degree in mechanical engineering from the University of Texas at San Antonio. His focus at SwRI is using advanced computational techniques and tools to perform deterministic and probabilistic analyses of structural and mechanical components, systems, and processes.

Authors:

Matthew Kirby Southwest Research Institute
David Riha Southwest Research Institute
Joseph Cardinal Southwest Research Institute
Joel Hobbs NASA Marshall Space Flight Center
Brian Stoltz NASA Marshall Space Flight Center
Benjamin Melia NASA Marshall Space Flight Center