Updating the API 579-1/ASME FFS-1 Stress Intensity Factor for Straight Surface Crack in a Round Bar (KBSCS)

Abstract This paper documents newly developed stress intensity factor (K) solutions for a straight front surface crack in a round bar. In API 579-1/ASME FFS-1 2021 Part 9, K solutions for through-wall membrane and bending stress (KBSCS) are already available for this crack type. However, the membrane and bending stress K solutions can be inadequate for round bars with stress concentrating features (e.g. threads) and thermal stresses. In addition, the current KBSCS solution only provides results for the deepest point along the crack front. Therefore, K solutions using global bending and a through-wall 4th order polynomial stress distribution have been developed to provide K solutions for the entire crack front. These were developed by conducting finite element analysis on several cracked bar geometries. This work contributes to an ongoing effort to expand and refine K solutions for all cracked round bar configurations. A parametric study of various crack depths was conducted utilizing linear elastic finite element models. The results of this were used to calculate influence coefficients for an arbitrary stress distribution and a 4th order polynomial stress distribution. The influence coefficients for a membrane and global bending stress distribution were also independently calculated and compared to published values in API 579-1/ASME FFS-1 2021 and Bremberg 6. Lastly, this paper provides recommendations on when the existing API 579-1 Annex 9B three parameter surface crack weight function method should and should not be used for KBSCS.