Research On Magnetic Domain Wall Dynamic Behaviors For Stress Characterization

Ferromagnetic materials are widely used in different industrial applications,e.g.transportation,petroleum pipeline and bridge construction.et.al.As the materials are exposed to high temperature,high pressure and high cyclic loading,stress concentration,defect initiation and material degradation is easily induced,leading to final failure of the material.There is a need in industry to supply safe,effective and reliable technique to characterize the degradation status of steel components and structures,both at the manufacturing stage and in service.Non-destructive testing and evaluation is one of the most effective non-invasive technique to ensure safe and reliable operation of material,which has brought outstanding social and economic benefits.Crack initiation is the root cause of a material failure,which is dubbed as“industrial cancer”.Cracking is generated from the accumulation of microcracking due to long-time cyclic loading.Residual stress and stress agglomeration are signs of crack initialization.Stress measurement enable to predict the formation of defect,which can be used as a fundament indicator of stress concentration.Magneto-optical imaging of magnetic domain wall dynamics has potential to evaluate the stress status,develop new magnetic sensors and non-destructive testing technique.There exist three main challenges for stress characterization by using magnetic domain dynamics.Firstly,the sample is needed to be carefully polished by using traditional magnetic domain observation technique.Secondly,magnetic domain wall dynamic behavior are confined to qualitative analysis.However,the quantitative analysis are essential for stress measurements.Thirdly,the time resolution of magnetic domain observation is too low to enquire the magnetic domain state under AC excitation.Time-resolved studies of changing magnetic domains with magnetic field excitations are needed.Based on these challenges,the main content and novelty of this thesis are as follows:1)To solve the issue of the sample preparation for domain observation,this work incorporates hybrid method of Magneto-optical Kerr Effect microscopy and Magneto-optical Indicator Film to image in-situ magnetic domain states of coated grain-oriented electrical steels under dynamic excitation.Utilizing the in-situ imaging,we investigated the effect of stress on magnetic domain dynamics.Threshold magnetic field feature to reflect 180°domain wall?DW?characteristics behaviors in individual grains is proposed for stress detection.The Pearson's coefficient between stress and proposed feature is higher than 0.95.It increases 89.3%as the stress increases from 0MPa to 90MPa at which the increase is linear with an average slope of 3.57 A m^-1 per MPa.It is verified that proposed feature is a threshold feature with better sensitivity and brings linear correlation for stress characterization in comparison to classical coercive field,remanent magnetization,hysteresis loss and permeability parameter measurements.2)To obtain the quantitative relationship between stress and magnetic domain wall dynamics characteristics,a series of domain images is obtained.We proposed determination of 180°domain wall velocity distribution for stress evaluation.For the evaluation peak values and peak positions are extracted from 180°domain wall velocity distribution.The difference of 180°domain wall behavior in different grains are investigated.The results show that peak position is insensitive to the domain pattern.It is verified that peak position is a robust feature for stress evaluation.From the viewpoint of macroscopic level,the in-plane magnetic field in the process of magnetic domain wall movement was measured.It is shown that the variation of magnetic stray fields is highly correlated with the formation,development and vanishing of lancet domains.From this,the interplay between domain wall dynamics,development of lancet domains and magnetic stray fields is extracted.It is revealed that the stray fields act as demagnetizing fields,which in turn affect the domain wall velocity.In addition,the local hysteresis loops are constructed from the local domain wall velocity.Pearson's coefficients between local hysteresis loops and macro B-H loops are analyzed,which reveal the interplay relationship between domain wall dynamics and macro magnetic property.3)In order to address time resolution of domain imaging,time resolved magneto-optical imaging is performed to investigate the magnetic domain state under AC excitation field.The influence of domain wall orientation on magnetic Barkhausen noise and magneto-mechanical behavior is investigated.Micro and macro magnetic properties are measured from individual crystallographic grains with different domain wall orientations under different tensile stresses.From this,the relationship between domain wall orientation,magneto-mechanical behavior and Barkhausen noise is established.The results show that magnetic Barkhausen noise?MBN?activity and magneto-mechanical behavior display completely different tendencies with stress in different grains and with different magnetization misalignment angles.This is attributed to the equilibrium between magneto-elastics and demagnetizing effects.The variation of supplementary domains under tensile stress is directly connected to the demagnetizing effect,further affecting the MBN activity.4)Finally,we investigate the influence of stress direction on magnetic domain dynamics and Barkhausen noise.With the actual excitation of ferromagnetic materials,the stresses may be generated from different directions.The angle between domain wall orientation and stress may range from 0°to 180°.We investigate the effect of applied tensile stress on the Barkhausen noise response and magnetic domain dynamics of grain-oriented transversely or parallel to the rolling direction of Fe-3%Si steel.By using time-resolved Kerr microscopy,the time evolution of magnetic domain construct under AC excitation field is elaborated for magnetic Barkhausen noise and magneto-mechanical behavior analysis.Further,quantitative Kerr microscopy is performed for investigating domain rotation and domain reorganization of electrical steels cut transversely to the rolling direction.From zero stress to mediate stress,the magnetic domain confugration firstly are changed from slab-like to I stress domain pattern.In higher stress,the magnetic domain structure are changed into II stress domain pattern.From the viewpoint of physics,the influence of stress-induced anisotropy on magnetic domain dynamics and stress character is analyzed.The above research extends the magnetic domain wall dynamics for stress evaluation on a microscopic level of the magnetic domain structure.Microscopic physical mechanisms of Barkhausen noise and magnetization curve response for stress measurements is investigated.The relationship between macro hysteresis loops,magnetic Barkhausen noise and magnetic domain wall dynamics are established.The results provide substantial microscopic insight for evaluating macroscopic magnetic properties.Analysis of the characteristics of magnetic domain wall dynamics is essential to develop a micro to macro magnetic technique for evaluation of stress status in key regions of a ferromagnetic component.