Session: CS-06-01 The Martin Prager Memorial Session on API 579/ASME Code Fitness-for-Service Activities

Paper Number: 122536

122536 - A New Stress-Intensity Factor Solution for an Internal Surface Crack in Spheres 

This paper describes a new stress-intensity factor (SIF) solution for an internal surface crack in a sphere that expands capabilities for this common pressure vessel geometry. The SIF solution employs the weight function (WF) methodology that enables rapid calculations of SIF values. The WF methodology determines SIF values from the nonlinear stress variations extracted in the uncracked geometry, e.g., from service stresses and/or residual stresses. The current approach supports two degrees of freedom that denote the two crack tips located at the deepest location and the surface of the sphere. The geometric formulation of this solution enforces an elliptical crack front, maintains normality of the crack front with the free surface, avoids non-physical crack shapes with protruding “ears”, and supports two degrees of freedom for fatigue crack growth from an internal crack tip and a surface crack tip. The new SIF solution enables all spherical geometries with the exterior diameter greater than or equal to four times the thickness. This WF SIF solution has been combined with stress variations common for spherical pressures vessels: uniform internal pressure on the interior surface, uniform tension on the crack plane, and uniform bending on the crack plane. These stress variations facilitate solution usability. This work builds on earlier efforts for an external surface crack (PVP2021-61397) and provides a complete solution capability for spheres.

This paper provides a complete overview of this solution. We present for the first time the geometric formulation of the crack front that enables the new functionality and set the geometric limits of the solution, e.g., the maximum size and shape of the crack front. The paper discusses the bivariant WF formulation used to define the SIF solution and details the finite element analyses employed to calibrate terms in the WF formulation. A summary of verification efforts demonstrates the credibility of this solution against independent results from finite element analyses. We also compare results of this new solution against independent SIFs computed by finite element analyses, legacy SIF solutions, API 579, and FITNET. These comparisons indicate that the new WF solution compares favorably with results from finite element analyses. Finally, we summarize the capabilities of this solution in NASGRO®.

Presenting Author: James Sobotka Southwest Research Institute

Presenting Author Biography: James Sobotka, Ph.D., PMP is a Lead Engineer at Southwest Research Institute® (SwRI®). He graduated from the University of Illinois at Urbana-Champaign with a Ph.D. from the Structural Mechanics and Engineering program under Prof. Robert H. Dodds. He then spent two years on a post-doc at Illinois working on hypersonic aerospace structures. He worked for 16 months at Bechtel Marine Propulsion Corporation in the Naval Nuclear Propulsion Program. He has worked at Southwest Research Institute since January 2014, currently in the Computational Materials Integrity Section. His work includes the development of new stress-intensity factor solutions, constitutive modeling, certification of higher-criticality parts built by additive manufacturing. and manufacturing process modeling. He is a co-manager of NASGRO®, the number one fracture mechanics and fatigue crack growth software in the world. He is a lead designer of DARWIN®, a software tool that assesses the risk of fracture from inherent anomalies in metallic components.

Authors:

James Sobotka Southwest Research Institute
Yi-Der Lee Southwest Research Institute
Joseph Cardinal Southwest Research Institute
R. Craig Mcclung Southwest Research Institute