Study On Magneto-Mechanical Relationship Based On Improved J-A Model

The transportation method of international oil and natural gas is mainly pipeline transportation.The key issue of pipeline operation is the safe maintenance of pipelines.Pipeline stress internal detection technology is a leading topic in the world’s pipeline detection field.Weak magnetic method has a huge development space in the research of stress internal detection technology.Under the long-term effect of alternating load,even if the maximum working strength of the pipeline is lower than its own strength limit or even below the yield limit,it will often break out vicious accidents such as pipeline burst without significant plastic deformation.During continuous strengthening,plastic deformation will occur locally in the pipeline,which will pose a serious threat to the normal operation of the pipeline.In order to prevent the outbreak of malignant accidents,it is necessary to carry out pre-judgmental detection on the pipeline in order to carry out reasonable maintenance.Conventional non-destructive testing methods mainly include ultrasonic testing,radiographic testing,magnetic particle testing,penetrant testing and eddy current testing,which play an important role in the detection of pipeline defects and accident prevention.However,the application of these methods in the detection range of pipeline stress has certain limitations.For example,ultrasonic testing is susceptible to environmental influences and requires a medium for detection;the X-ray radiography method has a low detection level for pipes with thick walls,long evaluation time,low detection efficiency,and the equipment cannot dynamically detect pipelines online.In recent years,the pipeline weak magnetic stress internal detection technology is commonly used.This method has the characteristics of fast detection speed,light equipment,simple operation,good reliability and does not require special magnetization equipment,and can perform non-contact online detection.It has broad development prospects in the detection range of pipeline stress.However,the existing theoretical models are all based on the magneto-mechanical analysis of the unidirectional stress state,which cannot meet the requirements of the detection signal identification of the oil and gas pipelines in service,and effective system research is needed.In addition,the weak magnetic signals of the elastic and plastic phases of the pipeline under load are different.Existing studies lack a comprehensive analysis of the magneto-mechanical relationship.Therefore,the establishment of a complete magneto-mechanical relationship system is of crucial significance for the application of the theory and engineering practice of weak magnetic detection technology.In this topic,based on the J-A theory,a numerical model of pipeline magnetics under complex stress conditions was established.The change rule of the weak magnetic signal with stress in the stress concentration area and the influence of the external magnetic field on the magneto-mechanical relationship under non-magnetic saturation conditions are calculated and analyzed.In addition,the elastoplastic strain magnetization model is established,the influencing factors of the strain magnetization model are analyzed,and the force-magnetic characteristics of the elastic stage and the plastic stage are calculated and described respectively.Experiments were conducted systematically on large-diameter oil and gas pipelines.The results show that the weak magnetic signal in the stress concentration area of the pipeline has a good correspondence with stress and strain.The error rate of the complex stress state magneto-mechanical model established in this subject is 0.5%,which is more suitable for the data analysis of pipeline stress internal detection.In the non-magnetic saturation state,as the strength of the external magnetic field increases,the strength of the weak magnetic signal in the stress concentration area increases,and the force-magnetic change rate of the weak magnetic signal gradually increases.In the elastic stage,the weak magnetic signal in the stress concentration zone of the pipeline reverses once.When the strain reaches 1%,the pipe body plastically deforms,and the weak magnetic signal again shows an inflection point.The weak magnetic signal eventually stabilizes with increasing strain.