Comparison of Stress Intensity Factor Calculation for Surface Flaws Using Polynomial Stress Fit Versus Using Universal Weight Functions for Highly Nonlinear Through-wall Stress Distributions

摘要

In order to evaluate flaws detected during in-service inspection of nuclear power plant components, linear elastic fracture mechanics flaw evaluations based on the ASME Section XI flaw evaluation procedure and acceptance criteria are performed. The magnitude of stresses in the flawed component is needed to determine the crack tip stress intensity factors, along with the geometry of the component, flaw dimensions and orientation. The stress intensity factors can then be used in further analyses of the detected flaw such as crack growth calculations as well as determining the maximum acceptable flaw sizes for continued operation. For some of the pressurized water reactor (PWR) power plant weldments, the through-wall residual stress distributions can be highly nonlinear. Appendix A of ASME Section Ⅺ and API-579 provide the methodology for calculating crack tip stress intensity factor based on 3rd and 4th order polynomial fits of the through-wall stress distribution, respectively. For highly nonlinear through-wall stress distributions where 3rd or 4th order polynomial fits may not adequately represent these stress distributions, closed-form equations for calculating crack tip stress intensity factors based on the Universal Weight Function Method may be more accurate, In this method, the through-wall stress distribution is divided into multiple piece-wise segments and the stress distribution in each segment is curve fitted by either a linear equation or a polynomial. This method has shown to provide higher accuracy in the stress intensity factors calculated for complicated stress distributions. In this paper, stress intensity factors calculated based a 4th order polynomial stress fit are compared with that based on the Universal Weight Function Method for several highly nonlinear through-wall stress distribution profiles. The resulting crack tip stress intensity factors are then used in stress corrosion crack growth calculations. The goal of the study is to assess the impact on the crack growth results for highly nonlinear through-wall stress distributions using the crack tip stress intensity factors calculated based on a 4th order polynomial stress fit and the Universal Weight Function Method.