DETERMINATION OF IMPROVED CRITERIA FOR CRACK PROPAGATION IN THE PRESENCE OF HIGH SURFACE STRESSES

Open Access
Author:
Smith, Katelyn J
Graduate Program:
Engineering Mechanics
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
April 19, 2011
Committee Members:
  • Albert Eliot Segall, Thesis Advisor
Keywords:
  • stress intensity factors
  • semielliptical cracks
  • weight function
  • surface stresses
  • finite element analysis
Abstract:
In the petrochemical industry, both the aging process and the economic pressure to optimize operation make accurately determining the structural integrity of equipment a principal concern. Standards are used to establish appropriate methods for design assessments to ensure safe and reliable long-term operation of equipment. The American Petroleum Industry developed a standardized fitness-for-service document titled API 579-1/ASME-1 Recommended Practice for Fitness-for-Service to provide technically sound assessment procedures. The current methods for determining critical crack dimensions in this standard compare a calculated stress intensity value from a through-thickness stress distribution with the material fracture toughness at surface and depth locations of the crack. In cases where complex stress distributions exist such as in residual stress fields, the influence of stress variations along the surface direction of the crack should to be considered. As such, the goal of this research was to use analytical techniques and finite element analysis methods to develop a weight function to calculate the stress intensity factors for Mode I loading applied to a planar semielliptical surface crack in the presence of stress fields that varying in the surface length direction. Finite element modeling was used to generate a battery of reference cases and analytical methods were then used to calculate a matrix of ‘M’ parameters necessary for the weight function application. Functions were then surface-fit over the range of solutions to accurately characterize the behavior of the constants over the domain. Comparative methods were used to validate both the weight function development and the functions characterizing the three ‘M’ parameters for each location. The evaluations performed on this work returned errors of less than 4.25% on all assessments to validate the ability of the developed weight function method to accurately predict stress intensity values from length-varying stress distributions for surface semielliptical cracks.