基于面磁荷密度的金属磁记忆检测正演模型

Forward model of metal magnetic memory testing based on equivalent surface magnetic charge theory

磁偶极子理论在金属磁记忆检测正演分析中得到了广泛应用,但是现有模型中的磁荷密度多为假设值,且假设磁荷在缺陷断面或应力集中区域呈均匀分布或线性分布,无法实现磁记忆信号的准确定量分析.针对上述问题,本文在结合力磁耦合理论、磁荷理论和有限元方法基础上,将试件离散为有限多个微小单元,假设单元内部磁特征参数分布均匀,建立了单元应力、磁化强度与磁荷密度之间的关系,构建了能够考虑应力不均匀分布对磁荷密度分布影响的金属磁记忆检测正演模型.与实验结果对比发现,本文建立模型无论是定性还是定量上都能很好预测磁记忆信号变化规律;在此基础上,对比了应力集中和宏观裂纹表面磁场分布的差异,并以裂纹缺陷为例,详细讨论了裂纹缺陷尺寸参数对磁记忆信号及其特征参量的影响规律.理论分析表明,本文模型可为定量分析磁记忆检测过程中缺陷附近磁场变化规律提供新的理论模型.

The magnetic dipole theory has been widely and successfully used to qualitatively analyze the testing signals of metal magnetic memory(MMM) testing.However,the magnetic charge density of the existing models is always an assumed value distributed uniformly or linearly along the defect section or in a stress concentration area,as a result the existing models are unsuitable for quantitatively analyzing the metal magnetic memory signal.In this work,a new forward model of MMM testing is established by considering the influence of uneven stress on magnetic charge density distribution,discretizing the specimen into numbers of micro elements firstly,assuming that the magnetic characteristic parameters of each element are evenly distributed and the magnetic charge density changes with stress in each element which can be determined by combining the modified magneto-mechanical model and the classical theory of magnetic charges.Compared with the experimental results of hole defect and crack defect specimen,the theoretical results calculated by the proposed model prove to be in good agreement with the testing results both qualitatively and quantitatively.Consequently,the proposed model is a new theoretical and quantitative model for analyzing the experimental change rule of metal magnetic memory testing.Then,the effects of stress concentration and macroscopic defects on the distribution of magnetic field are analyzed,showing that when there is only a stress concentration in the specimen,the horizontal component is negative valley,and the normal component changes from negative to positive valley peak in the stress concentration area;when there is a crack defect in the specimen,the distribution of magnetic field is just opposite to that when there is only a stress concentration.The distribution characteristics of the magnetic field can be used to judge the damage type in the specimen.Moreover,taking crack defect for example,the horizontal and normal component of magnetic field and their characteristic parameters changing with the size parameters of crack defect,such as width,length,depth and buried depth of crack defect,are analyzed in detail.The results show that the W?Hx and W?Hz increase lineally with the increase of the width of crack,?Hx and ?Hz increase with the increase of the length and depth of crack,but gradually decrease with the increase of defect buried depth.