Research On Key Technology Of Residual Stress Detection Of Metal Components Based On Magnetic Measurement

Residual stress detection is an important basis for the condition assessment and life prediction of key components of the equipment,and it is also a prerequisite for stress regulation.Residual stress is generated during the entire life cycle of a product,and it is difficult to predict.Its size and state vary greatly with different processing and working conditions.Residual stress in a material can severely affect its strength,hardness,fatigue,and stability,and in extreme cases can cause brittle fracture and stress corrosion cracking.At present,the industry-recognized residual stress detection methods are mainly X-rays method and neutron diffraction method.However,X-rays are harmful to the human body,and can only measure the residual stress within 30 um of the surface layer.If deeper stress is to be measured,the test specimen needs to be electrochemically corroded and stripped,which will damage the test specimen.Neutron diffraction is expensive and complicated to operate.Aiming at the above problems,this paper proposes a "space-period excitation + magnetoresistance array" residual stress detection method.It conducts in-depth research from theoretical modeling,sensor design,inversion algorithm and experimental verification.The main work and innovations of the paper are as follows:1.The stress detection mechanism of different metal materials is expounded,and the relationship model between conductivity/permeability and stress is established.For non-ferromagnetic materials,the influence of stress on electrical conductivity is analyzed from a microscopic perspective.Based on the piezoresistive effect,a model of the relationship between stress and conductivity of non-ferromagnetic materials is established.For ferromagnetic materials,the influence of stress on magnetic domains and magnetic domain walls is analyzed based on the magnetic coupling theory.Based on the magnetoelastic effect(inverse magnetostrictive effect),a model of the relationship between stress and permeability of ferromagnetic materials is established.This study can provide theoretical guidance for the detection of residual stress.2.The theoretical model of planar array electromagnetic sensor is established,the mathematical relationship between the eddy current penetration depth and the sensor's structural parameters and material physical properties is derived.First,the planar array electromagnetic sensor on a layered medium is simplified into a two-dimensional model.Then,based on the time-harmonic electromagnetic field theory,by introducing the Magnetic vector,the magnetic field diffusion equation is obtained when the sensor is excited with a sinusoidal signal.Based on the magnetic field diffusion equation and the definition of the eddy current skin depth,the relationship between the eddy current penetration depth and the spatial wavelength of the sensor,the frequency of the excitation current,the magnetic permeability and electrical conductivity of the measured object can be obtained,which can provide theoretical guidance for the structural design of the sensor.3.Aiming at the problem that the internal stress of metal components is difficult to detect,a stress sensor design scheme based on a space periodic excitation coil and a magnetoresistive array is proposed,which solves the problem that the detection sensitivity decreases with increasing depth.In the eddy current detection,in order to increase the detection depth,the signal frequency needs to be reduced.Traditional detection coils obtain magnetic field information by measuring the rate of change of the magnetic field,and the induced voltage output at low frequencies will be very weak.Using magnetoresistance as the detection unit to directly measure the magnitude of the magnetic field can overcome the problem of weak sensor output signals.This overcomes the problem that the detection sensitivity decreases with increasing depth.Therefore,a stress sensor design scheme based on space periodic excitation coil and magnetoresistive array is proposed.Based on the COMSOL finite element simulation,the magnetic field distribution of the space periodic excitation coil was analyzed,and the parameters such as the spatial wavelength and line width of the excitation coil were optimized,and the optimal installation position of the magnetic resistance was determined to realize the optimal design of the sensor.The electromagnetic coupling between the sensor excitation coil and the detection unit is studied and an effective interference suppression method is proposed and verified by simulation.4.Aiming at the problem of inversion of internal residual stresses of metal members,a stress distribution inversion algorithm based on the material layered model and the equivalent impedance of the measured object was proposed,and the accurate inversion of stress values at different depths was achieved.First,it is assumed that the measured object consists of N thin metal layers of the same thickness,and the relationship between the measured object's equivalent impedance and the reduced impedance of the excitation coil is derived;Then,according to the skin effect,the working frequency is changed reasonably so that the penetration depth of the eddy current is increased by the thickness of the thin metal layer as a fixed step,and the impedance measurement of the material is completed by using the frequency sweeping technique.By analyzing the attenuation law of eddy current in the metal material,a layered conductivity inversion model is established,and the conductivity at different depths in the metal can be calculated based on the measured impedance characteristics of the material.Aiming at the ill-conditioned problem of the inversion model coefficient matrix,an estimation method using the stress empirical distribution as a constraint condition was proposed,and a more accurate solution was obtained.Finally,according to the corresponding relationship between the conductivity and the residual stress(approximately a linear relationship),the residual stress distribution in the thin metal can be obtained.5.A residual stress detection system was set up and a residual stress detection experiment was carried out to verify the effectiveness of the designed new stress sensor and stress distribution inversion method.A U-shaped variable stress test block was designed,and a stress fixed value test block was prepared by laser shock strengthening.A stress detection system was built based on a sensor,a preamplifier circuit and a digital lock-in amplifier,and a program-controlled tensile experiment was carried out.The experimental results show that the designed sensor can effectively detect the stress change within 2mm depth of the metal component,the spatial resolution in the depth direction is about 0.2 mm,and the detection resolution is about 10 MPa.The stress distribution obtained from the inversion is basically consistent with the X-ray diffraction measurement results,which verifies the effectiveness and correctness of the proposed method.