| Elbows are important components in industrial pipeline equipment and are also weak links in pipelines.The Stress Intensification Factor(SIF)of elbows is an important parameter for calculating elbow stress.During the service life of a pipeline,elbows are prone to local deformation and stress concentration problems under the combined action of structural and fluid loads,which can even lead to pipeline failure.Therefore,accurate stress calculations and fatigue performance studies of elbows have good scientific research and engineering application value for improving the safety performance of existing pipelines.In this paper,a combination of theoretical analysis,numerical simulation,and experimental methods is used to study the calculation method of the Stress Intensification Factor and fatigue characteristics of pipeline elbows.The specific work and main conclusions are as follows:(1)This study focuses on the U-shaped continuous elbow structure commonly used in engineering.The pipe element model based on the SIF calculation method in the specification and the traditional shell element model were established to analyze the influence of the two models on the stress calculation results of pipe elbows.An experimental platform for elbow stress testing was constructed,and the variation law of elbow stress with external load was measured and compared with the results of finite element analysis and ASME B31.3specifications.The results show that the stress results of pipe elbows calculated based on the SIF calculation method in the specification are underestimated,with large errors compared to experimental values,and the error gradually increases with the increase of external force.The shell element model analysis results are closer to the experimental values,with a smaller average error.(2)A study was conducted on the stress amplification factor of elbow components in pipeline systems.The calculation errors of SIF in the current ASME pipeline specification were analyzed.Based on this,modified SIF calculation methods were proposed for both force and displacement loads.The accuracy of the modified calculation method was verified based on finite element simulations and experimental results.The results show that the stress calculation results of the modified formula are closer to the actual stress values of the elbow than those of the specification formula.The calculation accuracy is higher,and the error is smaller.The modified formula also agrees well with the stress values obtained from finite element analysis.The calculation accuracy has been improved by 16.77% and 22.65%,respectively.(3)This study focuses on the fatigue life of elbow components in pipeline systems.By analyzing the defects in the Miner criterion in the fatigue damage accumulation theory,a modified Miner criterion based on the damaged S-N curve is proposed.The rationality of the modified theory is verified through numerical analysis,and the effects of alternating loads,thickness ratio,and internal pressure on the fatigue life of pipeline elbows are discussed.The results show that the modified Miner criterion based on the damaged S-N curve reduces the calculation error by 17.62% compared to the original criterion.The main influence of in-plane alternating loads on the fatigue life of elbows is the occurrence of stress concentration and crack initiation at the neutral layer of the inner wall of the elbow.The effective fatigue life of elbows is proportional to the thickness ratio.The effective fatigue life of elbows increases with the increase of internal pressure,and the alternating stress at the centerline of the elbow decreases with the increase of internal pressure. |
