Development of 4th Order Polynomial Influence Coefficients for API 579-1 / ASME FFS-1 Stress Intensity Factor Solutions for Through-Wall Cracks in Plates

Abstract The API 579-1/ASME FFS-1 2021 (API 579) Fitness-For-Service stress intensity factor solutions for a through-wall crack in plates is currently limited to linear through-thickness stress distributions. Under-prediction of the stress intensity factor solution may result when the standard solutions are applied to non-linear stress distributions. In addition, stress distributions symmetrical about the midplane thickness result in loss of bending stress contribution, more significantly underpredicting the stress intensity factor. This paper investigates the limitations of the current approach, and develops new influence coefficients for through-wall cracks in plates. A parametric study was performed, considering both plate thickness to crack length ratios and crack length to plate width ratios. Finite element models were developed utilizing explicit crack modelling to extract influence coefficients based on 4th order polynomial stress distributions. Linear elastic analysis was performed, using the contour integral approach and a focused mesh in Abaqus 2024 software. A new table of influence coefficients is provided, accompanied by an updated stress intensity factor solution suitable for 4th order polynomial stress distributions. This update extends the applicability of the code formulas and is consistent with the approach applied throughout API 579 Annex 9B. Further research is planned for investigating and possibly refining the nonlinear stress influence coefficients that are near zero.