Comparative Study On Method Of Handling Secondary Stress In Fracture Assessment Of Cracked Pipelines

Industrial pipelines are often accompanied by secondary stresses.In order to make the project assessment simple and easy,it is obviously not appropriate for the project appraisal personnel to make accurate finite element calculations for each assessment.Therefore,most specifications use plasticity correction factors to correct the fracture ratio in failure assessment.Effects of different in different standards are not clarified for the assessment results.Thus,investigating the accuracy of the plasticity correction factors is significant to the evaluation of fractures of pipelines with defects under the secondary stress.Based on the combination of finite element numerical calculation and theoretical analysis,this paper focuses on the above issues and studies the influence of fracture evaluation on cracked industrial pipelines under secondary stress.The main research contents and conclusions are as follows:（1）Compared with R6-rev4 and GB/T 19624-2004,API 579-2016 is found to provide a detailed method for the selection of basic parameters,the calculation of stress intensity factor and reference stress.For the assessment of crack-like flaws,failure assessment charts are used.In the treatment of secondary stresses,the plastic correction factors used are different.（2）Using finite element software ABAQUS to create three-dimensional pipe models with axial semi-elliptical cracks,respectively by changing the crack size and 0.4),material strain hardening coefficients and primary load and the ratio of secondary loads,accurately calculate stress intensity factors with different loads:the primary load,secondary load as well as the combination load with the primary load and secondary load.The calculation results indicate thatK_J is no relations with the strain hardening coefficient under the secondary load.The variation forK_J is similar under the primary load and the combination load with the primary load and secondary load.（3）Mutual influence factorg_FE is calculated by using the theoretical equations and the finite element calculation results.The results indicate that effects of strain hardening coefficients for different materials are investigated,which reveal that all curves ofg_FE have same characteristics with R6 option 2 failure assessment curves;Based on the above conclusions,an inter-influence factor based on the finite element results was proposed and used to calculate the stress intensity factor with secondary stress.（4）Compare the results of theoretical calculations and the finite element calculations,the accuracy of V factor method in R6,（37）factor method in API579,?factor method in GB/T 19624 and new g factor method are verified.（37）factor method and V factor method have the similar accuracy.At the same time,the results show that the factors obtained in this paper are more reasonable than the standard calculation results in all load cases considered in this paper,and are verified by the actual case data.