【作者】 徐明秀；

【导师】 徐敏强；

【作者基本信息】 哈尔滨工业大学 ， 一般力学与力学基础， 2012， 博士

【摘要】 疲劳破坏是工程中常见的材料失效方式，材料的疲劳损伤评估研究具有重要意义。金属磁记忆技术在铁磁材料的疲劳损伤评估上极具潜力，但由于缺乏足够的理论支撑，定量评估困难，影响该技术的应用。因此，研究疲劳过程中的磁记忆产生机制研究是推进该技术应用的关键任务。磁记忆现象是材料在地磁场、应力和材料微观结构等共同作用下产生的磁性能改变在材料表面的体现，力-磁效应、位错磁化效应和漏磁效应是与疲劳过程中与磁记忆现象相关的主要的三种磁效应。本文针对这三种效应，设计一系列疲劳试验，以理论与试验相结合的方式研究疲劳过程中磁记忆现象的产生机制和变化规律。考虑恒定磁场和循环应力的共同作用，建立了适合描述疲劳过程中材料磁化特性的力-磁效应模型。首先，分析了循环应力下的磁化特性，构造了磁化稳定状态M₀；然后，借鉴J-A理论的思想，构造了适合描述疲劳过程中材料磁化特性的J-A-F模型。结果表明，在地磁场和循环应力的作用下，材料磁化逐渐到达稳定状态M₀，与试验结果符合；M₀在一个载荷循环内沿一个环线变化，并且曲线形状随应力范围、外加磁场强度、钉扎参数k1的变化呈规律性变化；J-A-F模型比原模型更能描述拉、压应力产生的不同磁化特征，能更合理地解释实验中观察到的磁化强度随应力变化的速率改变符号的现象；J-A-F模型可以通过钉扎参数k1的变化来描述疲劳过程中的磁化改变。考虑疲劳损伤过程中位错对畴壁运动的影响，建立了M₀与位错之间的关系，用M₀描述位错对磁化的影响。首先，对不同疲劳阶段的塑性变形、位错、磁记忆信号变化进行同步实验研究，并分析三者的关系。然后，考虑局部平衡状态M₀受位错密度、位错结构和塑性应变的影响来表征位错磁化效应。结果表明，高应力疲劳过程分为快速塑性变形、平稳塑性变形和快速断裂3个阶段；在第一阶段位错密度和结构同时变化，使得磁记忆信号快速变化，在第二阶段只有位错结构的变化，磁记忆信号缓慢变化。位错密度及位错平均位移影响应力范围、钉扎参数k1；随损伤发展而变大，使得磁记忆信号增加。因为位错变化在宏观上表现为塑性变形，位错对磁化的作用可用塑性应变表示。因为磁记忆现象是材料损伤造成的磁性能改变的外在反映，建立了损伤区域的漏磁模型，描述磁记忆信号变化。首先，从磁荷观点分析磁记忆信号的产生和组成；然后，考虑疲劳过程中材料损伤集中在一个区域、裂纹在这个区域萌生并发展，建立漏磁模型；接着，通过磁荷密度表达式把M₀和漏磁场联系起来。结果表明，磁记忆信号的变化由损伤区的漏磁场变化引起；损伤区漏磁场切向分量分布呈小山峰形状、法向分量分布呈斜坡状，切向信号数值及法向信号分布梯度随损伤发展而变大，裂纹产生使得裂纹位置的磁场波动。损伤区的磁荷密度与M₀变化趋势相同，所以M₀能用于表征裂纹出现前的磁记忆信号变化；裂纹的磁荷密度比损伤区的大很多，使得裂纹产生的漏磁信息能在总漏磁场上反映出来，所以疲劳后期磁记忆信号突变。对比疲劳过程中的塑性应变、硬度与磁记忆信号的变化特点，得出磁记忆信号比塑性应变、硬度对材料损伤敏感，磁记忆信号更适合用于疲劳损伤检测。结合三种磁效应，建立了材料表面磁记忆信号与应力、塑性应变和裂纹影响因子之间的关系，以及卸载状态下的磁信号Hp与塑性应变p及裂纹影响因子g的关系。结果表明，表面磁记忆信号受应力、损伤情况、材料特性、加载情况、环境磁场等影响，并随塑性应变及裂纹尺寸变化；Hp与及g之间的量化关系式能描述磁信号在疲劳过程中的变化，并能解释疲劳后期磁信号变化趋势不同的现象。更多还原

【Abstract】 Fatigue failure is a common failure pattern of material in engineering practice,and fatigue damage evaluation of material has important significance. The metalmagnetic memory （MMM） technique has great potential in the fatigue damageevaluation of ferromagnetic material. However, the quantitative evaluation isdifficult for lack of enough theoretical bases, which makes the application of thistechnique hampered. Therefore, mechanism study of this method in the process offatigue is the key task for propelling this method in application.The MMM phenomenon is the embodiment of material magnetization, on thesurface of material, and the magnetization is combined action of geomagnetic field,stress and material’s microstructure. Magnetomechanical effect, dislocationmagnetization effect and magnetic leakage effect are mainly three effects connectedwith MMM phenomena. In this dissertation, a series of fatigue experiments aredesigned to research the three magnetic effects, and the MMM phenomenon in theprocess of fatigue is researched using theoretical and experimental method.A magnetomechanical model that is suitable for MMM phenomenon isestablished considering the combined action of constant field and cyclic stress. First,the magnetization characteristic caused by cyclic stress is analyzed, and themagnetization stable state M₀is constructed; and then, the J-A-F model suitable forMMM phenomena is obtained based on the J-A theory. The results show that, themagnetization of material reaches stable state M₀gradually under geomagnetic fieldand cyclic stress, which is accord with experimental results; M₀changes regularlywith stress, external field and pinning parameter k1; the J-A-F model can describemagnetization features in tension-release processes better and explain the signchange of dB/d that has been observed in experiments more reasonably. And theJ-A-F model can describe the magnetization change in the process of fatigue by thevariation of pinning parameter k1.The relationship between M₀and dislocations is established to describe themagnetization caused by dislocations, considering dislocations’ influence to domainwall motion in the process of fatigue. First, the change characters and correlationsof plastic deformation, dislocations of material and the MMM signal in the processof fatigue are analyzed synchronously in experiments. And then, the dislocationmagnetization effect is formulated using M₀, considering the influence ofdislocation density, dislocation structure and plastic strain. The results show that,rapid plastic deformation, stable plastic deformation and fast fracture composethree stages of high stress fatigue; at the first stage, dislocation density and dislocation structure both change, which makes the MMM signal change quickly; atthe second stage, only dislocation structure changes, which makes the MMM signalchange slowly. Dislocation density ρ and the average displacement of dislocationaffect stress range and pinning parameter k1. increases as fatigue damagedevelops, which makes the MMM signal increase. Because that dislocation changemanifests as plastic deformation at the macro level, dislocations’ influence tomagnetization can be expressed using plastic strain.A magnetic flux leakage model is constructed to describe the variation ofMMM signal, since the MMM phenomenon is the external reflection ofmagnetization caused by material damage. First, the generation and compose ofMMM signal is analyzed from the view of magnetic charge; after that, the magneticflux leakage model is constructed to formulate the leakage field considering thatmaterial damage happens in a certain area and cracks generate and develop in thisarea; and then, connect the leakage field with M₀via the expression of magneticcharge density. The results show that, the MMM signal variation is caused byleakage magnetic field from damage area; the tangential component distribution ofthe leakage magnetic field has peak value, and the normal component distributionof the leakage magnetic field is slope shape; the leakage magnetic field increases asfatigue damage develops, and its distribution fluctuates at the position of crack. Themagnetic charge density of damage area has the same variation trend with M₀, soM₀can be used to represent MMM signal variation before cracks emerge. Themagnetic charge density of crack is much bigger than that of damage area, so theleakage message reflects in the surface leakage field, and makes the MMM signalabnormal at the late stage of fatigue. Contrast the variation laws of plastic strain,hardness and magnetic signal, the conclusion is obtained that MMM signal is moresensitive to fatigue damage than plastic strain and hardness, and the MMM signal ismore suitable for fatigue damage detection.Combine the three magnetic effects, the relation between MMM signal, stress,plastic strain and crack impact factor is obtained; the relation between MMM signalHp, plastic strainpand crack impact factorgfor a certain point in unloadcondition is obtained. The results show that, the MMM signal is influenced bystress, damage status, material, load condition and environment field etc., andchanges with plastic strain and cracks size; the formula between Hp,pandgcandescribe the Hpvariation in the process of fatigue, and explain why the MMMsignal has different variation trends at late fatigue stage.更多还原