New Nondestructive Evaluation Methods For Damages Of Ferromagnetic Components Based On Magnetic Barkhausen Noise Effect

With the continuous advancement of China's Reform and Opening-up,the national economy has been rapidly developing,and the demand for industrial production capacity is also constantly improving.Steels and other ferromagnetic materials,as the core engineering materials of industrial construction,are widely used in railway,oil,building,aerospace,shipping,energy,and other industries.However,the ferromagnetic components in service in these industries,such as pipelines and rails,are generally subjected to a relatively harsh working environment.During the longterm service,they are inevitable prone to corrosion,cracks,pits,and other defects on the internal and external surfaces of the components.For one thing,the existing macroscopic corrosion defects will reduce the service performance of the component.For another,the early microcracks will cause local stress concentration and reduce the fatigue resistance of the component.If these damages are not detected in time,the defects would further deteriorate and eventually cause component failure,leading to catastrophic accidents.Therefore,regular in-service non-destructive testing and evaluation for ferromagnetic components is a necessary means to ensure safety in production.As an important branch of electromagnetic nondestructive testing technology,magnetic Barkhausen noise technology has not only compact and light structure,but also fast detection speed and no couplant required.After in-depth research by scholars at home and abroad,the magnetic Barkhausen noise technology has gradually matured and can be used to detect various damages and material properties of ferromagnetic materials,such as stress state,defect location,surface hardness,plastic deformation,and so on.However,the magnetic Barkhausen noise detection is still faced with three problems at present.First,in engineering applications,there is still a lack of research on on-site detection of complex magnetic field environments.Second,For the detection of macroscopic defects of ferromagnetic materials,the existing method is more suitable for detecting surface and near-surface defects,but not suitable for detecting internal surface defects.Third,the excitation efficiency of the traditional magnetic Barkhausen noise method is low,which is not conducive to the surface stress detection of large-size pipeline components.Based on the magnetic Barkhausen noise technology,this paper proposes two new methods to detect the internal surface defects and surface stress state of ferromagnetic components in service to meet the non-destructive detection requirements,respectively.First of all,based on the magnetic domain theory,the paper analyzes the main influence factors of the magnetic Barkhausen noise technology in the engineering application.Take the insufficient research of the magnetic Barkhausen noise detection technology used in a biased magnetic field environment into consideration,systematic studies of the effect of DC biased magnetic field on the magnetic Barkhausen noise signal characteristics are conducted.Experimental results show that the biased magnetic field will move the working point of hysteresis,in turn,influence the magnetic domain movement,and finally,affect the signal features such as peak value,root-mean-square value,root-mean-square envelop.The results provide a theoretical basis to explore the magnetic Barkhausen noise testing method to use in the complex magnetic field environment.Then,according to the research results of the effect of the biased magnetization on the magnetic Barkhausen noise,combined with the disturbing effect of internal defects on DC magnetic field distribution,a magnetic Barkhausen noise method based on the biased magnetization is proposed to detect the internal defects of ferromagnetic components.The feasibility of this method is theoretically analyzed,and the systematic experiments prove that this method can effectively detect the internal surface defects of 7mm plate specimens buried up to5 mm.Finally,the motion-induced magnetic Barkhausen noise method is proposed to solve the problem of the low efficiency of the traditional method for detecting the surface stress of the large-size pipeline.A circular array of permanent magnets with alternating magnetic poles is used to generate alternating magnetic fields in the pipeline,which is used to excite the pipeline surface to generate Barkhausen noise,and the peak value of the root mean square envelope of the signal is extracted to characterize the surface stress on the pipeline.The experimental results show that this method can effectively detect the level and distribution of the surface tensile stress on the pipeline when the array of permanent magnets keeps a constant rotating speed.