Trudy Odesskogo Politehničeskogo Universiteta (Nov 2018)
EFFECT OF PITTING CORROSION ON STRESS INTENSITY FACTORS OF SEMI-ELLIPTICAL SURFACE CRACKS ON THE OUTER WALL SURFACE OF A TUBULAR T-JOINT
Abstract
Fatigue and pitting corrosion are two important design considerations for strength assessment of steel structures of the offshore oil and gas industry. It is vital to evaluate the impact of corrosion on the fatigue life of corroded steel structures. Close examinations of the crack propagation life for fatigue-induced defects in the pipeline and tubular joint exposed to corrosion environment have posed serious challenges to engineering practice. The approach of linear elastic fracture mechanics (LEFM) can be used to analyze the growth of high-cycle fatigue crack, which typically occurs when the applied stresses are well below the yield stress. It is mostly accepted that the stress intensity factor (SIF) is calculated in the analysis and design of structures using LEFM. In the present study the effect of pitting corrosion on stress intensity factor along the crack front is investigated. Very few studies exist that employ the Finite element method (FEM) to estimate the stress intensity factor along the crack front in T-joint with and without pitting corrosion. In order to show the effect of pit on the fracture parameter, the pit-effect coefficient is defined as the difference between normalized stress intensity factors with and without pitting corrosion. A series of three-dimensional circular cone pit models together with a semi-elliptical surface crack in a tubular T-joint are simulated in ANSYS. In order to validate the model, the results of an un-corroded model are examined against the available results in the literature, and good agreements are observed. Three-dimensional finite element analyses are conducted in order to investigate the effects of depth, orientation, cross-sectional area and location of pitting corrosion. Thus the effects of the geometry and position of pitting corrosion, as well as the size of crack, are being analyzed. It is found that the location and geometry of pitting corrosion are the dominant parameters affecting the stress intensity factor. The pitting corrosion located in the direction of crack face and close to the surface crack has a significant influence on the stress intensity factor, whereas the pitting corrosion located in brace has a negative effect on the SIF.
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