Research On The Effect Of Defect Depth On Piezomagnetic Signals Of X70 Pipeline Steel

The marine pipeline is the lifeline of the transportation of oil and gas,and plays an important role in the development of oil and gas resources.However,in the long-term services,due to the corrosion induced by the seawater and inside conveying mediums,the pipelines are prone to generating various corrosion defects,which may lead to the leakage of the oil and gas,and even cause the failure of pipeline,which will cause huge economic losses.As a result,detecting corrosion defects in pipelines,is of great engineering significance for evaluating the residual life pipelines and timely repairing.Traditional nondestructive testing methods,such as ultrasonic testing and X-ray testing,have some limitations in the detection of corrosion defects in marine pipelines due to their high cost and long-time consumption.However,the magnetic detection technology based on the piezomagnetic effect can effectively identify the defects by detecting the abnormal magnetic signals induced by the corrosion pits.At present,scholars at home and abroad have carried out a lot of researches on the magnetic field distortion caused by the defect length and width.They found the magnetic parameters such as the amplitude of tangential magnetic fields and the normal magnetic gradients,are closely related to the defect size.However,the relationship between the defect depth and magnetic signals was seldom studied.Moreover,the research of the influence of defect depths mainly focused on numerical simulation and lacked the experimental verification.Therefore,in this paper,the combined research methods of experiments and numerical simulations are applied to investigate the influence of defect depths on magnetic signals.The objective of this research is demonstrating the potential possibility of quantitative defect depth identification of ferromagnetic steels using piezomagnetic magnetic,and promoting the application of this technology in engineering.The main work can be summarized as follows:(1)The spatial magnetic field distribution around the X70 specimen under static tensile loads.Cyclic loads were applied to the X70 specimen by a tensile testing machine.Piezomagnetic fields near the specimen were measured by two magnetic probes.The spatial magnetic field distributions were investigated.Additionally,the influence of the distance between the probe and the specimen was studied by changing the position of the probe,which provides the support for the subsequent studies(2)The relationship between the stress and the piezomagnetic field on the surface of the X70 defect specimen under static tensile loads.Predetermined tensile loads were applied to the X70 specimens with different defect depths by a tensile testing machine Piezomagnetic fields on the surface of the specimen were measured by magnetic probes after reaching the predetermined load and with the load being maintained.The relationship between the load and the piezomagnetic field was investigated,and the evolution of piezomagnetic fields in the elastic stage and plastic stage was analyzed.The magnetic field distributions on the surface of specimens were obtained by the program of ORIGIN,and the stress distributions on the surface of specimens were obtained by the finite element software ABAQUS.The correlation of the magnetic field and the stress was studied by comparing these two distributions.(3)Dependence analysis on the defect depth and the piezomagnetic parameters The correlation of the defect depth and the piezomagnetic field was analyzed by comparing the experimental data and the theorical model.Magnetic parameters were proposed by analyzing the experimental data.The functional relationship between the magnetic parameters and the defect depth was established,which proved that it is possible to identify the defect depth using the piezomagnetic field.