The Study Of Local Constitutive Relation And Fracture Toughness Test Of Girth Weld For High Grade Pipeline Steels

Oil and gas pipeline is a typical welding structure.There are various kinds of welding defects in weld-seam and Heat Affected Zone(HAZ).In recent years,there have been many accidents of high grade steel pipeline in China,most of which show that the strain capacity of girth weld is insufficient.The strain capacity of girth weld is closely related to its mechanical properties.Due to the heterogeneity of girth weld,it is a difficult problem to test and characterize the mechanical properties of girth weld joints.Therefore,the research on the mechanical properties of pipeline girth weld must be attached great importance,which is of great significance to ensure the safe operation of pipeline.The mechanical properties of girth welds are divided into two parts:local constitutive relation identification and fracture toughness testing.It is respectively studied:the difficulty to accurately identify the local constitutive relationship of pipe girth weld,the insufficient research on J-integral plastic factor equation of base metal and girth weld metal in fracture toughness test of Single Edge Notch Tensile specimen(SENT),and inaccurate normalization method in J-R resistance curve test of SENT specimen,The main research contents and achievements are as follows:(1)A method of measuring the local constitutive relationship of welded joints by Digital Image Correlation(DIC)was proposed.The tensile test platform based on DIC technology was built.The local mechanical properties of X80 and Q235 pipeline welded joints were tested,and the local true stress-strain curves of base metal and weld metal specimens at different positions were obtained.The true stress-strain curves at different positions of the base metal were analyzed,the results show that DIC method is good toaccurately test the material properties and are able to distinguish the mechanical properties of heterogeneous materials,and can be used to test the local constitutive relationship of welded joints.The microstructure and hardness distribution of X80 and Q235 pipeline welded joints were tested and analyzed.The hardness distribution is consistent with the yield strength distribution obtained by DIC method,which identifies the accuracy of the test results.The parameters of Ramberg Osgood model of base metal,weld and HAZ of X80 and Q235 specimens were studied,and the matching strength of specimens was determined.The average matching coefficients of X80 and Q235 welded joints were 0.95 and 1.21,respectively.(2)The three-dimensional finite element model of SENT specimens of pipe base metal were established to research the influence of crack length(a/W=0.2～0.7,increment 0.1),specimen thickness(B/W=0.5,1,2),hardening index(n=5,7,10,15,20),and side groove of specimen on J-integral plasticity factor.Through the numerical analysis results,the J-integral plastic factor equations without side groove and with side groove of 10%B depth were proposed by fitting method,which considers the influence of crack length,specimen thickness,material hardening properties and other factors.(3)The three-dimensional finite element model of SENT specimens for pipeline girth weld were established.For the girth weld of pipeline,the J-integral plastic factor of SENT specimen of weld metal under different crack length(a/W=0.2～0.7),specimen thickness(B/W=0.5,1,2),weld width(2h/W=0.5,0.75,1)and matching coefficient(M_y=1.1～1.4)were simulated and calculated.The influence of different factors on J-integral plastic factor of SENT specimen of pipeline girth weld was studied.The J-integral plastic factor equations of SENT specimens of weld metal without side groove and with side groove depth of 10%B were proposed by fitting method.(4)A modified normalization method for J-integral resistance curve test of SENT specimen was proposed and applied to fracture toughness test of X80 pipeline steel base metal and girth weld.Compared with the original normalization method,the deviation between the modified normalization method and the unloading compliance method is smaller,and the accuracy of the modified method is higher than that of the originial normalization method.The determination of crack initiation toughness in BS 8571,ASTM E1820 and GB/T 21143 were compared.The influence of crack length,final crack propagation and side groove depth on fracture toughness was analyzed.The optimal side groove depth and specimen manufacture for SENT specimens were proposed.Based on the P-CMOD curve and J-R resistance curve of SENT specimen,the parameters of GTN model were checked and modified.The results show that the model parameters can better predict the ductile tearing behavior of materials,which provides a certain basis for the engineering application of pipeline steel.