The Magnetic memory detection probe is fabricated by a three-axis linear Hall sensor MLX90393.A type of metal magnetic memory testing system is developed,based on the Arduino hardware development platform. The metal magnetic memory test of 30CrMo material is carried out under the static load,analyzes the variation of the magnetic memory signal,and analyzes the variation between the magnetic memory signal gradient and the tensile load. The re-sults show that,the maximum gradient of the tangential component of the magnetic memory signal on the surface of the test specimen changes markedly at the tensile load of 16 kN,and sharply increases to 0.0425 after 23 kN;The maxi-mum value of the normal component gradient increases abruptly to 0.55 after the tensile load reach 23 kN. This system provides an effective method for the determination and measurement of stress concentration in ferromagnetic materials.

Metal magnetic memory testing technology can detect the danger zone characterized by the stress concentration in a ferromagnetic metal member.It can thus implement an early diagnosis of the damage for the ferromagnetic member and prevent disaster accidents.This article outlines the metal magnetic memory testing principles and characteristics,analyzes the research status of magnetic memory testing,introduces a quantitative problem of stress concentration on several computational algorithms and several new types of magnetic memory testing instruments and systems currently emerging,and presents the current research in magnetic memory testing key issues to be studied and the magnetic memory testing technology in the future development direction.

利用涡流仪对带涂层碳纤维复合材料的涂层厚度进行测量,分析检测频率及涂层厚度对涡流检测信号的影响,然后利用扫描电子显微镜测量涂层厚度,对比涡流法和电镜法测量所得的涂层厚度值.试验结果表明,两者数值很接近,误差较小.涡流法可望实现碳纤维复合材料涂层厚度的高精度测量.

High quality alloy steel 40Cr is widely used in mechanical manufacturing industry and is selected for the high-cycle fatigue experiments in the paper.The relationship between the characteristic value of magnetic signals during the process of fatigue testing and the stress concentration position and the degree of stress concentration has been researched.The normal component and the tangential component are together detected.The method of Lissajous figure is used to draw two-dimensional curve,the relationship between area of Lissajous figure and the fatigue damage state of the specimen is discussed.It turns out that the mutation characteristics of differential gradient value can be used to accurately determine the stress concentration area caused by fatigue loading;very big change appears in magnetic signals under the beginning of the loading condition,while with the increase of fatigue cycles,the magnetic signal changes slowly.When the crack propagation stage in the specimen is aroused,magnetic signals increase significantly;and there is a good corresponding relationship between area of Lissajous figure and the fatigue damage state of the specimen.It is expected to provide test analysis basis for the quantitative analysis of twodimensional detection of magnetic memory effect.

Acording to material attributes and organization characteristics of carbon fiber composite material,three-dimensional simulation of eddy current test method for measuring the coating thickness of the composite material was carried out using ANSYS software,analysed the effect of changes in the coating thickness of the coil impedance under the conditions of non-uniform conductivity.The results show that selecting detection frequency(600 ~ 700 kHz),the coil impedance and coating thickness will have good linear correlation.The results of experiment and simulation calculation have good consistency.Indicates that the eddy current method to measure coating thickness special composite material has good resolution,which provides a reliable theory basis and technical support for realizing highaccuracy measurement of carbon fiber composite coating thickness using eddy current testing method.

依据磁记忆二维检测原理,研制设计了以微处理器为核心的磁记忆二维检测仪器,然后通过提取铁磁性构件疲劳区域的漏磁信号,分析磁记忆信号的法向分量与切向分量的梯度关系,实现对构件应力集中部位存在的残余应力的状态评估.并通过应力拉伸与实际传动连杆工件检测2个实验验证了磁记忆二维检测仪器工作的可行性,为进一步开展磁记忆二维检测定量分析的研究提供了有效的技术支撑.

The zero point in normal component and the maximum point in tangential component were together adopted to detect.The instrument was built with CPU and based on the principle of two-dimensional detection of magnetic memory.In addition,the test on the workpiece verified the feasibility of the instrument and showed that the instrument had good application prospects in the quantitative analysis of magnetic memory.

Traditional magnetic flux leakage method includes measuring the vertical component of magnetic flux leakage of specimen surface after magnetization,and researching the picked signal characteristics.However,the method,which determines the defects just through a single vertical component characteristic,is prone to undetected and misjudgment.In view of the deficiency,a new two-dimensional detection method which measures the vertical component of leakage magnetic field at the same time picks and analyses the horizontal component characteristic was used,since the horizontal component is easily affected by the jamming signal,a vector synthesis method was used to extract it,the vertical component of the leakage magnetic field existing zero crossing and the horizontal component having the characteristics of the maximum were comprehensively considered,extensive experiments and researches were done on various standard and nature sample.The results showed that the two-dimensional detection of magnetic flux leakage can detect defects effectively and improve the detection reliability,and it is expected to have good prospects for practical engineering applications.

People of domestic and foreign non-destructive testing have paid widely attention to the magnetic memory technology which is as the early diagnostic model technology. But the existing magnetic memory technology mainly depends on leakage magnetic field signal in a single direction (normal component) to determine, doesn't make full use of the entire information of magnetic memory signals, prone to undetected miscarriage of justice, etc. Aimed at the deficiency of the detecting technology, two-dimensional testing method which joint conducts characteristic curves of magnetic memory signals of the zero point in normal component and the maximum point in tangential component is introduced. Using this method, the normal component signals and tangential component signals of magnetic memory are collected at the same time, vector resultant method is adopted to extract the maximum value of tangential component signals, then the lissajous figure of synthesis between the gradient of normal component and the gradient of tangential component is obtained, the area of lissajous figure is led in for analyzing the stress state. And the self-developed two-dimensional magnetic memory apparatus are applied to engineering practice. Practice has proved that the two-dimensional system and detection method can efficiently be determined on the stress concentration, and it is expected to be a new method which can achieve the quantitative analysis of magnetic memory testing.

Metal magnetic memory testing technology is an effective way of the non-destructive testing techniques for early damage diagnosis of ferromagnetic materials.To further explore the mechanism of metal magnetic memory testing,the magnetic domain structure of Q235 tablet samples are observed by bitter pattern,the characteristics changes of magnetic domain structure under the different stress states in earth magnetic are researched,and the high-precision weak magnetic detection devices are used to obtain the magnetic flux leakage signal of the specimen surface.The results show that the magnetic domain types and domain wall morphology change significantly with the change of external loads;and different regions have different change speed of magnetic domain,but they have a similar trend;the same time,the magnetic flux leakage signal of specimen surface increases with the external loads increases.The magnetic domain of ferromagnetic materials move by stress and preserve such changes after the withdrawal of the stress in earth magnetic are verified.Thus it provides reliable and effective experimental basis for further study of the microscopic mechanism of ferromagnetic materials magnetic memory testing.

为了研究受载铁磁材料应力集中部位的磁记忆信号和磁畴组织的变化规律,对含中心圆孔的Q235平板试样进行静拉伸试验和磁畴组织观察.结果表明:随着载荷的增加,磁记忆信号变化幅度增大,局部变形断裂阶段的磁记忆信号幅值小于强化阶段的信号幅值;应力集中区和非应力集中区确定点处的磁记忆信号变化趋势相似,在强化阶段磁记忆信号幅值达到最大值;同一载荷下,应力集中区磁畴分布较非应力集中区更密集,大多数为90°畴壁,磁畴长边较长;同一点处随着载荷变化,从弹性到强化阶段,磁畴分布逐渐密集,180°畴壁逐渐变为90°畴壁,磁畴宽度变小,数量增多,局部变形断裂阶段出现一定量的180°畴壁,磁畴数量减小,宽边的宽度增大.

In the paper, the theory of two-dimensional magnetic memory testing is employed. The zero point in the normal component and the maximum point in the tangential component are together adopted for detection. Lissajous figure analysis is introduced. The Lissajous figure of synthesis for the normal and tangential components and the Lissajous figure of synthesis for the differential value of the normal component and the tangential component are used for analysis. It turns out that the area of the Lissajous figure of synthesis for the differential value of the normal and the tangential components can be used to determine the degree of stress concentration, and it is concluded that in different stress concentration degrees, the order of magnitude of the Lissajous figure area can be used as a threshold. This study can provide a test analysis basis for further research on the quantitative analysis of two-dimensional detection of magnetic memory by using the Lissajous figure analysis method.

在对铁磁平板试样进行不同载荷的静拉伸试验基础上,利用粉纹法观察该试样的磁畴形貌变化,对比分析相同部位在不同应力作用下磁畴组织的变化特征.试验结果表明:随着拉应力的增大,被观察部位的磁畴结构由180°片状畴逐渐转变为迷宫畴;且两组对比试验结果反映出的变化趋势相同.试验可为进一步进行磁记忆检测微观机理的研究提供直观的依据.

The alternative current magnetic flux leakage (AC-MFL) method using a U-shape inducer is one of the advanced MFL methods. Because sine wave excitation signal causes eddy current, so the MFL field and disturbance eddy current field in the defect area of a conductor can lead to the confusion in the detection mechanism. Aiming at this problem, a 3D finite element model of DC MFL testing is developed. A judgment criterion of detection principle is presented according to simulation results. On this basis, simulation and test are carried out for U-shape inducer. The results show that horizontal component (B y) of magnetic leakage flux density at the crack center is smaller than that in the case without defect when the crack direction is in parallel with the magnetic field direction and probe moving direction. Moreover, the horizontal component (B y) is independent of signal frequency and excitation intensity. Finite element analysis (FEA) and experiment results indicate that according to the judgment criterion, the mechanism using a U-shape inducer to detect longitudinal crack is not primarily originated from AC-MFL effect.

High-cycle fatigue tests of notched 40Cr steel specimens are carried out under three different fatigue stresses and two-dimensional metal magnetic memory testing(2-D MMMT).Two-dimensional signals are detected and analyzed by the method of Lissajous figure.The effects of stress concentration,fatigue damage and fatigue stress on two-dimensional magnetic signals are investigated.The relationship of the area of Lissajous figure and the degree of fatigue damage is analyzed.The results show that zero-crossing of the magnetic memory normal component and peak values of the magnetic memory tangential component appear in areas of stress concentration,and that the position of these phenomena drifts gradually closer to the notch root with the increase of fatigue damage.Location of the specimen damage can be characterized by the mutation peak characteristic of K curve of the normal component gradient and the tangential component gradient of magnetic memory signals.The maximum values of the normal component gradient and tangential component gradient of the magnetic memory signals increase gradually with the augmentation of the degree of fatigue damage in the whole trend.Therefore the maximum value is regarded as a characteristic quantity that reflects the degree of specimen damage,and the area of Lissajous figure has a good correlation with the degree of fatigue damage.

The metal magnetic memory testing(MMMT) can detect the stress concentration zone of ferromagnetic metal components.The technology researches were hopeful that solving the key of the components early diagnosis and life-time prediction.This paper introduces the metal magnetic memory testing principles and applications,analyzes the magnetic memory testing research situation in domestic and foreign,proposes the inadequacies on MMMT researches and the trend of MMMT development in the future.

The acoustic impact testing method is presented with two different excitation methods: the tap testing method and the acoustic impedance testing method.The tap testing method includes the whole vibration testing(wheel-tap test) method and the local vibration testing method(coin-tap test),and the acoustic impedance method has the mechanical impedance method and the local resonance testing method.Though the acoustic impact testing method has a long history in testing activities,most concerns of this method are only restricted in its experiential applications.Here,the principals and their developments of several branches of the acoustic impact testing method are covered,with the main purpose to improve this old but effective method with the help of the people who concern its development and applications.

The method of resultant vector was adopted to extract tangential component signals of metal magnetic memory due to the problem that the tangential component signals of metal magnetic memory is very hard to be extracted, and its validity was verified by the use of ANSYS simulation.The relationship between tangential component signals of magnetic memory with stress concentration position and stress concentration degrees was investigated untler static tension conditions testing,and the K curve of tangential component signals of magnetic memory was analyzed.The result shows that the tangential component signals of metal magnetic memory can be effectively extracted by the method of resultant vector,and the stress concentration positions and stress concentration degrees can he reflected intuitively and accurately by tangential component signals of metal magnetic memory.

In the eddy current testing for steel tubes,the problem that the phases of defect signals could not be distinguished existed when the magnetization intensity of the tubes was oversaturated.Theoretical analysis and experimental results showed that there were two testing mechanisms in the eddy current testing for steel tubes: eddy current effect and magnetic flux leakage effect.When the magnetization intensity of the tubes was oversaturated,the magnetic flux leakage effect was stronger than eddy current effect and was dominant,which made the phases of defect signals could not be distinguished.

The change of surface magnetic field vertical component of ferromagnetic materials under different loading conditions for a 20# steel plate sample with a round hole is studied. Experiments show that the zero-crossing point of magnetic field strength vertical component can not completely characterize the stress concentration position, and the variation of magnetic field intensity gradient is related to the degree of stress concentration. As the load increases, the magnetic field strength along the axial direction of the specimen changes in certain way; and when the concentration factor m=K max/K au ≥ 2.6, the specimen is in critical condition.