# Interpolation Method for the Calculation of API-579-1/ASME FFS-1 Stress Intensity Factor for a Longitudinal Internal Surface Crack in Pipeline under Internal Pressure

## Authors

• Patchanida Seenuan Department of Mechanical Engineering, Faculty of Engineering, Thammasat University
• Jirapong Kasivitamnuay Department of Mechanical Engineering, Faculty of Engineering, Chulalongkorn University, 10330, Thailand.
• Nitikorn Noraphaiphipaksa Department of Mechanical Engineering, Faculty of Engineering, Thammasat University, 12120, Thailand.
• Chaosuan Kanchanomai Department of Mechanical Engineering, Faculty of Engineering, Thammasat University, 12120, Thailand.

## Keywords:

pipeline, crack, stress intensity factor, API-579-1/ASME FFS-1 standard, interpolation

## Abstract

Pipelines are responsible for transporting liquid in various industries, e.g., petroleum, chemical and food industries. After a long period of service, the cracks are possible. Based on the linear-elastic fracture mechanics, the complete fracture of crack can occur if the stress intensity factor (K) at crack tip is equal or greater than the critical stress intensity factor (Kc) of material. At present, the calculation of K proposed by the fitness-for-service assessment standard (API-579-1/ASME FFS-1) is wildly applied for the cracked pipelines. However, the precision of K depends on the interpolation method of influence coefficients (G0, G1, G2, G3, G4) within the K solution. Therefore, 38 case studies of the longitudinal internal surface crack in a pipeline under internal pressure were selected to analyze the effect of interpolation method on the precision of K. The methodology covered both linear and nonlinear interpolations. The precisions of K from various interpolation methods were determined by comparing the results with K calculated by finite element analysis. It was found that the Ks from the linear interpolation were significantly different from those from the finite element analysis, i.e., the maximum difference of 25% for very small thickness pipe, and the maximum difference of 16% for small thickness pipe. While, the differences between the Ks from the cubic spline interpolation using 5 data of t/Ri and the finite element analysis were lower, i.e., the maximum difference of 7.3%. Therefore, the cubic spline interpolation using 5 data of t/Ri was the most appropriate interpolation method for the K calculation.

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2022-05-01

## Section

Research Articles