Study On Propagation Characteristics Of Metal Magnetic Memory Signal

Pipeline transportation is one of the major ways of international oil and gas transportation,and pipeline security is a core issue of pipeline operations.Accidents of occur frequently in new pipelines without macro defects as known from the analysis of international pipeline accidents in recent years.Conventional non-destructive testing techniques such as magnetic powder,magnetic flux leakage,eddy current and penetration have played an important role in defect detection and accident prevention of pipelines,however,only formed macroscopic volume defects are found,the stress concentration regions where macroscopic volume defects are unformed due to construction,welding,foundation settlement,medium internal pressure,thermal expansion,etc.cannot be effectively evaluated and thus it is impossible to avoid sudden accidents caused by stress damage.In addition,the pipeline is plastically deformed under stress and initiates corrosion cracking.And the residual stress or strain causes the crack tip to rapidly expand into the interior of the metal material.Since the circumferential stress of the pipeline is about twice the axial stress,the axial crack is more harmful than the circumferential crack.The axial excitation of the traditional magnetic flux leakage pipeline detection technology makes the magnetic force line parallel to the crack,which causes the axial crack to be missed and poses an important hidden danger for the safe operation of the pipeline.As an emerging technology in the field of non-destructive testing,magnetic memory detection technology has great application potential in the field of stress internal detection of long distance oil and gas pipeline for the advantage of supporting non-contact,dynamic online detection compared with current stress detection techniques(such as ultrasound,ray,coercivity,etc.).Therefore,to study the stress concentration damage and crack of pipe characterized by magnetic signal,propagation characteristics and its influencing factors of magnetic memory signal during the detection process provides a scientific basis for the setting of the lift value of the stress detector in the pipeline and a theoretical basis for the scientific and reliability of the detection to the crack of the pipeline,meanwhile,is of great significance for analyzing the detection signal of pipeline stress,and pushes the magnetic scientific application of memory detection technology in other non-contact online detection fields.In this paper,a three-dimensional magnetic equilibrium magnetic charge model is established based on the magnetic charge theory,and the distribution characteristics of magnetic memory signals in the stress concentration region are described.The propagation characteristics of different components of the magnetic memory signal in the stress concentration region are quantitatively analyzed,and the influence of different media on the magnetic memory signal propagation law is compared.In addition,the numerical model of magnetic memory signal of crack in pipeline is established,and the influence of crack size and orientation on magnetic memory signal characteristics and magnetic memory detection signal transmission law is calculated.The experimental study is carried out systematically.The results show that the magnetic memory signal decreases exponentially with the propagation distance.During the propagation process,the one-to-one correspondence between force and magnetism is stable.Specifically,the tangential signal is attenuated quickly,and the normal signal is relatively stable,which is suitable for positioning the stress concentration region.Magnetic memory signals in ferromagnetic media decay faster than in non-ferromagnetic media.At each tip of the crack,the axial component of the magnetic signal has a peak and a valley,and the radial component has an extreme value.The amplitude of the two components increases linearly with the depth and length of the crack.The signal attenuation is also consistent with the falling exponential function.The smaller the crack depth,the faster the signal decays and eventually stabilizes.The magnetic signal of different channels around the crack at an angle to the axial direction are arranged at a same displacement pitch.