Research On Key Technologies Of Damage Of Ferromagnetic Materials Based On Magnetic Nondestructive Testing

Ferromagnetic materials,such as steel and its alloys,are widely applied to aerospace,bridge,railway,vehicle and petrochemical industry.The structural parts or equipments are usually subjected to high load,high speed,high temperature and high pressure in sevice,leading to material damage.As the damage accumulation,a sudden fracture occure,even catastrophic consequence.Therefore,it is important to test and evaluate the damage of ferromagnetic materials efficiently and accurately.Nondestructive testing(NDT)technology is an effective means to ensure the equipment work safely and reliably,while the magnetic nondestructive testing is one of the most widely used NDT technology at present.Based on analysis of metal magnetic memory(MMM)and magnetic flux leakage(MFL),the strengthening magnetic memory,micro-cracks quantitative recognition using MFL and the intergrated nondestructive testing technology are carried out.The main contents are as follows:(1)Aiming at the problem that magnetic memoy signals are susceptible to interference factors,an external steady weak excitation is applied to strengthen the magnetic signal,and to improve the sensitivity of the detection system and the detection effect.Qualitative analysis of the feasibility of the strengthening magnetic memory is carried out on the basis of the non-linear characteristics of permeability curve.Based on the stress-permeability model,the sensitivity in different weak magnetic environments,geomagnetic field(40 A/m)and weak magnetic field(100 A/m),can be obtained with ANSYS universal software.The results show that the weak magnetic excitation can improve the sensitivity of the detection system significantly,which is verified with the static tensile tests of the 20# steel plate specimens.(2)The distribution and variation of the normal magnetic field Hp(y)of smooth ferromagnetic specimen surface are studied under static tension and cyclic loading,and the results show that magnetic anomaly signal occur in stress concentration area,and the phenomenon of stress-induced magnetization reveral and the law of approach emerge.On the basis,a set of magnetic memory device is developed,the distribution and variation of tangential magnetic field Hp(x)along with cycles are studied.The scanning imaging experiments are carried out,realizing imaging display of magnetic signal in the process of fatigue.(3)According to the requirement of real-time monitoring of the crack damage around the bolt hole of bolt connection structure,a set of monitoring device is developed,which can real time on-line monitoring the state around the bolt hole,the results show that the deviece can predict the position of crack initiation and fracture effectively,and the magnetic anomaly signal is more sensitive in early damage stage.(4)Aiming at the problem that micro-cracks are difficult to quantify,a quantitative identification method is proposed.The MFL signal can be acquired on the basis of the magnetic dipole model,the eigenvalue extraction and the dimension reduction of the feature matrix are carried out using principal component analysis method,and the samples library is constructed.The artificial intelligence algorithms are employed to quantitatively identify the width and depth of micro-cracks,and the results demonstrate the method is feasible.On the basis of above-mentioned,quantitative identification of artificial micro-cracks and natural cracks are carried out,and the morphological filtering is proposed to preprocess the MFL signal.The results show that these two different kinds of micro-cracks can be identified quantitatively based on the PSO-LSSVM algorithm,and the three-dimensional profile reconstruction is explored at last.(5)Magnetic memory is sensitive to eary damage,MFL is sensitive to crack,and magnetic barkhausen noise(MBN)can identify residual stress quantitatively.According to these characteristics,the necessity and feasibility of magnetic integrated NDT technology is proposed on the basis of analyzing the universal of the detection platform,and develop a set of magnetic integrated NDT device which can provide varibable magnetic field excitation.The test results show that the device can detect residual stress,identify micro-cracks with different angles,and carry out the quantitative evaluation of the residual stress.In short,the nondestructive evaluation(NDE)of ferromagnetic materials is realized.