Linear elastic fracture mechanics based flaw evaluation procedures in ASME Section XI require calculation of the stress intensity factor. The method to calculate the stress intensity factor that is provided in the 2010 Edition of Appendix A of ASME Section XI is to fit the stress distribution ahead of the crack tip to a polynomial equation, and then use standardized influence coefficients. In PVP2011-57911, an alternate method for calculation of the stress intensity factor for a surface flaw that makes explicit use of the Universal Weight Function Method and does not require a polynomial fit to the actual stress distribution was proposed for implementation into Appendix A of Section XI. The alternate method provides closed-form solutions for the stress intensity factor. A numerical approximation is the assumed piece-wise linear variation of stress between discrete locations where stresses are known.
For highly nonlinear stress distributions, piece-wise cubic interpolation of stress over intervals between discrete locations where stresses are known is an improvement over piece-wise linear interpolation. Investigation of a cubic interpolation of stress between discrete locations where stresses are known has been conducted. Closed-form equations for calculation of the stress intensity factor for a surface flaw were developed using the Universal Weight Function Method and generic piece-wise cubic interpolation of stress over intervals. Example calculations are provided to compare the results of stress intensity factors using piece-wise cubic interpolation with piece-wise linear interpolation.