The Study Of Fracture Assessment Methods For High-Grade Girth Welded Pipes

It is impossible to totally avoid the defects in the welding process of high steel grade pipes with girth weld.The characteristics of non-continuous geometric transition and nonhomogeneous material in the local area of girth weld may easily result in the formation of a strain or stress concentration area.The failure of girth weld has become one of the prominent challenges encountered during the service of high-grade steel pipes.To further enhance the security and provide theory and methods for the safety assessment of highgrade girth-welded pipes,this paper presents the methodology research on material property-based evaluation,strain-based assessment,strain capacity estimation and inversion analysis of fracture for girth-welded pipes.The primary outcomes are as follows.The primary outcomes are as follows.(1)The failure assessment diagram(FAD)curves based on standard requirements of material properties have been constructed to improve the accuracy of failure assessment for different grades steel pipes.The local mechanical properties of weld metal,base metal and HAZ of X80 pipe have been obtained based on the DIC testing technique,and the FAD assessment models based on the measured mechanical properties of girth welded pipes were developed.The conservativeness of the FAD curves based on R6 Option 1,BS7910 Option 1M and BS7910 Option 2 for defect assessment have been quantitatively evaluated.The procedure of assessing the safety of girth-welded pipes based on material properties has been optimized.(2)A fracture analysis model for the X80 girth-weld pipes has been established.The strain-based failure assessment diagram(SBFAD)method and its influencing factors have been illustrated,and the critical strain bearing capacity of X80 girth-weld pipes has been assessed based on the measured mechanical properties.A model for fracture analysis of girth-welded pipe with unequal wall thickness containing a fusion line crack has been developed,the impact of wall-thickness ratio,strength matching factor,and internal pressure on the strain capacity of girth-welded pipe with unequal wall thickness has been quantitatively evaluated based on the apparent fracture toughness criteria.(3)A ductile fracture extension analysis model for X80 girth-welded pipe has been established.The influencing factors of the local strain concentration range in girth-weld and the strain concentration coefficient of the crack tip has been quantitatively evaluated.The axial remote strain corresponding to the onset of cracking was defined as the strain capacity of girth-welded pipe,the factors such as strength matching coefficient,HAZ softening degree and defect size on strain capacity have been quantitatively determined.An estimation formula for the strain capacity of X80 girth-welded pipe has been introduced.(4)The ductile fracture analysis model for notched specimen has been constructed based on the complete Gurson model.A method for identifying the initial void volume fraction of materials applicable to notched specimen has been presented,which is applicable not only to homogeneous materials but also usable for weldment.Nine uniaxial tensile tests of notched specimens have been performed,including two homogeneous materials(X80 pipe steel and 15 Cr Mo)and one inhomogeneous material(girth weldment of X80 pipes).The initial void volume fractions of three materials have been successfully and accurately identified with the proposed initial void volume fraction prediction method.The developed method for predicting the initial void volume fraction of materials will greatly facilitate the identification and determination of material damage parameters,and the identified initial void volume fraction can be further applied to the classification management of material ductile fracture behavior.