The flaw evaluation procedure in ASME Boiler and Pressure Vessel Code Section XI Appendix A is based on the linear elastic fracture mechanics, where the stress intensity factor for the given flaw geometry and applied stress is used to evaluate fatigue crack growth and unstable brittle fracture. The current original procedure for calculation of stress intensity factor for elliptical subsurface crack was based on a linearization method to have conservative estimation for nonlinear stress fields. To improve this current approach, the stress intensity factor solution for subsurface flaw was discussed to allow for the stress variation at the crack location to be represented by a forth order polynomial function. The use of a forth order polynomial fit to represent the actual stress distribution will greatly improve the accuracy of the method for nonlinear stress fields such as those caused by thermal transient loadings and residual stresses caused by weld. The coefficients of stress intensity factor for elliptical subsurface flaw for polynomial fit were estimated by the influence function method. These coefficients were verified by the comparison of stress intensity factor calculated by other similar solutions.

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