Study On Strain Characteristics And Fracture Behavior Of Girth Weld Of High-grade Steel Pipeline

There are many factors affecting the safe operation of the pipeline,among which the girth weld has become one of the most important factors.The material heterogeneity and possible defects of the girth weld make the pipeline more prone to failure at the girth weld.Especially for the high-grade steel pipeline,the fracture control of the girth weld is more difficult.This paper studied the strain characteristics and the fracture behavior of the pipeline girth weld,which can provide a basis for ensuring the safety of the girth weld of the high-grade steel pipeline.The main results are as follows:(1)The strain evolution characteristics and the strain concentration mechanism of the whole section of the girth weld were revealed.The DIC technique was used to conduct the tensile tests for the full-wall thickness specimens of the pipeline girth weld,and the strain distribution states of the whole section of the girth weld during the failure process was obtained.The strain concentration degree and the evolution characteristics of the strain concentration factor in different regions of the girth weld were clarified.The microstructure characteristics and the hardness distribution of the whole section of the girth weld were determined by the microscopic observation and the hardness measurement.(2)A method for determining the local constitutive relationship of the pipeline girth weld based on the combination of the DIC tests and the finite element correction was proposed.Considering the effect of the material constraint on the deformation of the girth weld,the stress-strain curves obtained from the tests were corrected by the finite element analysis method.The accurate constitutive parameters of the girth weld were obtained.Based on the obtained constitutive parameters,a finite element model of the pipeline girth weld was established.The strain distribution states,the strain concentration regions and the strain concentration degree of the girth weld under different internal pressures were determined.The strain capacity of the pipeline under different internal pressures was obtained.(3)A constraint-based fracture failure criterion considering material parameters was proposed(applicable to X65～X100 pipeline steels).The SENB models with different geometric dimensions were established to study the effects of the yield strength,the hardening exponent and the initial void volume fraction on the crack tip constraint and the fracture toughness of the pipeline steels.The main factors affecting the fracture of the pipeline steels were determined,and the quantitative relationships between the crack tip constraint and the fracture toughness under different material parameters were obtained.Furthermore,a constraint-based fracture failure criterion that can reflect the hardening exponent,the yield strength and the initial void volume fraction was proposed.The criterion is applicable to X65～X100 pipeline steels.The verification results show that the absolute value of the relative error of the criterion prediction results is less than 7%.(4)A constraint-based fracture failure criterion for the pipeline with the Lüders plateau was proposed(applicable to X52～X80 pipeline steels with the Lüders plateau).The SENB models with different geometric dimensions were established to study the effect of the Lüders plateau on the relationship between the crack tip constraint and the fracture toughness.The interaction between the Lüders plateau and other material parameters(the yield strength,the hardening exponent and the initial void volume fraction)in affecting material fracture were discussed in detail.The quantitative relationships between the constraint and the fracture toughness of the steels under different material parameters were obtained.A constraint-based fracture failure criterion for the pipeline with the Lüders plateau was proposed.The criterion can reflect the effects of the Lüders plateau length,the hardening exponent,the yield strength and the initial void volume fraction on the relationship between the fracture toughness and the constraint.The criterion is applicable to X52～X80 pipeline steels with the Lüders plateau.The verification results show that the absolute value of the relative error of the criterion prediction results is less than 11.3%.(5)The effects of the weld geometric dimensions and the matching conditions on the weld crack tip constraint and the fracture toughness were revealed.The SENB models with the weld were established,and the effect of the weld width on the crack tip constraint and the fracture toughness was studied.The critical weld width,which can make the base metal affect the crack tip constraint and the fracture toughness of the weld,is obtained.The effect of the material matching characteristics on the weld fracture was further studied.The crack tip constraint level and the fracture toughness under different weld mismatch degrees were determined.(6)The characteristics of the crack growth behavior,the resistance and the driving force of the pipeline girth weld with the center crack and the interface crack were revealed.The strain capacity of the pipeline girth weld under different conditions was obtained.The full-scale models of the pipeline girth weld with the center crack and the interface crack were established respectively.For the pipeline girth weld with the center crack,the distribution states of the equivalent plastic strain at the crack tip during crack growth were revealed.The effects of the weld matching characteristics and the internal pressure load on the crack growth resistance and driving force were determined.For the pipeline girth weld with the interface crack,the effects of the material matching characteristics on the equivalent plastic strain distribution,the stress triaxiality distribution and the interface crack growth path during crack growth process were revealed.The effects of the internal pressure and the matching characteristics on the interface crack driving force of the girth weld were determined.The comparative analysis results shows that the strain capacity of the girth weld with the interface crack is smaller than that of the girth weld with the center crack.In the case of overmatching,the difference between the strain capacity of the two types of girth welds is more significant.