EVALUATION OF THE CRITERIA FOR CRACK PROPAGATION IN THE PRESENCE OF HIGH SURFACE STRESSES

Open Access
Author:
Smith, Katelyn J
Area of Honors:
Engineering Science
Degree:
Bachelor of Science
Document Type:
Thesis
Thesis Supervisors:
  • Albert Eliot Segall, Thesis Supervisor
  • Christine Masters, Honors Advisor
Keywords:
  • stress intensity factor
  • critical crack criteria
  • residual stress
  • finite element analysis
Abstract:
The petrochemical industry is largely dependent on the continuous operation of equipment to turn the maximum profit. Thus, there exists an intricate balance between the financial gains of the business and the structural integrity of the equipment. Numerous standards are therefore used to establish an appropriate degree of conservatism in the design process to ensure safe and reliable long-term operation. The standard under evaluation in this thesis research is the API 579-1/ASME FFS-1. The present methods for determining critical crack dimensions in this standard compare a calculated stress intensity value from the surface and depth locations with the material fracture toughness. In cases where high, residual stresses exist on the surface, this treatment may result in an overly conservative estimation of the likelihood of crack propagation due to the rapidly diminishing stresses as the crack face moves away from any high surface stresses. Since over-conservatism can be problematic, the goal of this project is to evaluate the critical crack assessment in the presence of a stress field that diminishes rapidly in the through-thickness direction. Through the use of finite element analysis and other analytical tools, the depth and surface locations of a semi-elliptical surface crack were examined in terms of stress intensity solutions. A comparison between the standard and the finite element analysis confirmed that the critical crack-like flaw assessment provides an additional degree of conservatism at the depth location of the flaw; this inherent conservatism is equivalent to a safety factor of approximately 1.375 or higher for the cases reviewed. The surface location was unable to be properly assessed due to inconsistencies in the mesh applied at this location. Thus, further modeling work is necessary to accurately evaluate the level of conservatism at the surface location of the semi-elliptical crack.