رشد ترك خستگي در يك لوله انتقال گاز تحت تاثير تغييرات فشار داخلي و تنش پسماند جوشكاري

چكيده انگليسي :

Fatigue Life of a Pipeline Subjected to Cyclic Internal Pressure Mahdi Adineh m adineh@me iut ac ir Date of Submission 2011 09 19 Department of Mechanical Engineering Isfahan University of Technology Isfahan 84156 83111 Iran Degree M Sc Language FarsiSupervisor Mohammad Mashayekhi mashayekhi@cc iut ac irAbstractInternal pressure in gas pipes are fluctuating during their service lifes and it can start fatigue in these pipes In this thesis fatigue crack growth rate in a gas pipeline of API 5L X70 with the weld regions arecalculated Two regions in the gas pipe are welded First region is a longitudinal welding across thecenterline of pipe witch has made when the pipe was producing second region is a circumferential weldingregion between two pipes witch merges two pipes in a pipeline The residual stresses produced in theprocess of welding strongly affect the crack growth rate If residual stresses are tensile the crack growthrate increases and fatigue life will be decrease The design formulations and methods for fatigue of this structures are based on S N curves determinedexperimentally This analysis method determinate the total fatigue life of a pipe with any defect Butvarious events such as accident earthquake and explosion may cause a crack on the body of pipes So inthis thesis a semi elliptical crack with a predefined dimensions are applied to the welded regions of gaspipe and the crack growth are determined by Linear Elastic Fracture Mechanics method Previousresearches shows that stress intensity factor is the most important parameter to describe fatigue crackgrowth rate in Linear Elastic Fracture Mechanics In this research ABAQUS software is used to simulatethe crack in the pipe and calculate stress intensity factor in the crack front Crack surfaces impelimented tothe FE model of pipe are in radius plane of pipe and one of elliptic diameters is in the internal surfaces ofpipe Calculated SIF s are compared to an analytical method results Paris and Forman equations withsuitable constant material parameters are used to calculate fatigue crack growth rate due to residual stressesand internal cyclic pressures in the pipe These equations strongly depends on stress ratio and range ofstress intensity factors a step by step method is used to simulate fatigue crack growth and calculate fatiguelife In this research a 2 D thermal elastic plastic analysis in ABAQUS has been done to calculate the residualstresses magnitude and distribution along circumferential weld region Goldak function is used to applyheat flux to the FE model A DEFLUX subroutine has performed for applying heat flux to the finite elementmodel and a FILM subroutine has been used to simulate thermal boundary conditions This analysis showsthat residual stresses along weld direction near the center line of weld are comparable with yield stress This tensile stresses can strongly increase crack growth rate in this region Fatigue life of two welded regions are compared The results of this compariation shows thatcircumferential welded region are the most critical region in the pipe in fatigue crack growth the number ofload cycle are showed versus crack dimensions in suitable charts and fatigue life of the pipes with a crackcan be estimated by using these charts The result of simulating fatigue crack growth shows that 2 D crackmodeling for investigating fatigue life may causes an error in the results and it suggests to model 3 D crackfor assessment of fatigue life Keywords fatigue life welding residual stress stress intensity factor pipelines

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رشد ترك خستگي در يك لوله انتقال گاز تحت تاثير تغييرات فشار داخلي و تنش پسماند جوشكاري

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