Magnetic Memory Signal Research Articles

Magnetic memory signals variation induced by applied magnetic field and static tensile stress in ferromagnetic steel

Stress can induce a spontaneous magnetic field in ferromagnetic steel under the excitation of geomagnetic field. In order to investigate the impact of applied magnetic field and tensile stress on variation of the residual magnetic signals on the surface of ferromagnetic materials, static tensile tests of Q235 structural steel were carried out, with the normal component of the residual magnetic signals, Hp(y), induced by applied magnetic fields with different intensities measured through the tensile tests. The Hp(y), its slope coefficient KS and maximum gradient Kmax changing with the applied magnetic field H and tensile stress were observed. Results show that the magnitude of Hp(y) and its slope coefficient KS increase linearly with the increase of stress in the elastic deformation stage. Under yield stress, Hp(y) and KS reach its maximum, and then decrease slightly with further increase of stress. Applied magnetic field affects the magnitude of Hp(y) instead of changing the signal curve′s profile; and the magnitude of Hp(y), KS, Kmax and the change rate of KS increase with the increase of applied magnetic field. The phenomenon is also discussed from the viewpoint of magnetic charge in ferromagnetic materials.

A general nonlinear magnetomechanical model for ferromagnetic materials under a constant weak magnetic field

Weak magnetic nondestructive testing (e.g., metal magnetic memory method) concerns the magnetization variation of ferromagnetic materials due to its applied load and a weak magnetic surrounding them. One key issue on these nondestructive technologies is the magnetomechanical effect for quantitative evaluation of magnetization state from stress–strain condition. A representative phenomenological model has been proposed to explain the magnetomechanical effect by Jiles in 1995. However, the Jiles' model has some deficiencies in quantification, for instance, there is a visible difference between theoretical prediction and experimental measurements on stress–magnetization curve, especially in the compression case. Based on the thermodynamic relations and the approach law of irreversible magnetization, a nonlinear coupled model is proposed to improve the quantitative evaluation of the magnetomechanical effect. Excellent agreement has been achieved between the predictions from the present model and previous experimental results. In comparison with Jiles' model, the prediction accuracy is improved greatly by the present model, particularly for the compression case. A detailed study has also been performed to reveal the effects of initial magnetization status, cyclic loading, and demagnetization factor on the magnetomechanical effect. Our theoretical model reveals that the stable weak magnetic signals of nondestructive testing after multiple cyclic loads are attributed to the first few cycles eliminating most of the irreversible magnetization. Remarkably, the existence of demagnetization field can weaken magnetomechanical effect, therefore, significantly reduces the testing capability. This theoretical model can be adopted to quantitatively analyze magnetic memory signals, and then can be applied in weak magnetic nondestructive testing.

Magnetic memory signals of ferromagnetic weldment induced by dynamic bending load

Metal magnetic memory testing is a novel method for evaluating the fatigue damage of ferromagnetic weldments. To promote a further understanding of the impact of fatigue loads on magnetic memory signals of weldments, the normal component of magnetic signals, Hp(y), on the centre-notched welding specimens were measured during dynamic bending fatigue tests. The variation of peaks–troughs differential value ΔHp(y) of magnetic signals and its maximum gradient Kmax at the welded joint were also investigated. Results indicated that the ΔHp(y) on scanning line near the notch increased sharply when stress concentration and crack initiation appeared in the specimens. The microstructures on scanning lines were also examined. It shows that there was an increase in intragranular slips with the increase in loading cycles, and the change in crystallographic textures and movement of dislocations contributed to the magnetic signals varying significantly. The maximum gradient Kmax was found to increase exponentially with the increase in crack length, which is well coincident with the analytic result of the magnetic dipole model. The research provides a potential possibility of quantitative evaluation on stress concentration and crack in weldments under dynamic bending loads.

The mechanics-magnetic properties analysis and experiment of ferromagnetic material specimen in tensile process

Metal magnetic memory testing based on mechanics-magnetic properties has been considered as an important non-destructive testing method in spite of the need for further research in the aspect of quantification. In order to facilitate the study of quantitative relationship between magnetic signal and structure damage, the mechanics-magnetic properties were described from the viewpoint of reversible rotation magnetisation under stress by taking Q235 plate specimen as an example. Magnetic memory signals and the corresponding gradients of the specimens under different loads were acquired, and their change trends were also analysed. Then the sample entropy characteristics of magnetic memory signals and gradients were extracted and a damage-associated model based on sample entropy was established, whose application was verified through testing a metal bolster in the engineering structure.

Identification of mining steel rope broken wires based on improved EEMD

Metal magnetic memory (3M), which is used to evaluate the stress concentration and fatigue damage of ferromagnetic materials, belongs to a weak magnetic detection. When 3M technology is used to detect working steel rope, the detected signal contains lots of interfering signals to distort real signal. To improve signal to noise ratio (SNR), ensemble empirical mode decomposition (EEMD) is used to denoise the original signal. A new method of confirming white noise standard deviation (WNSD) added to EEMD has been proposed, which has a higher accuracy. Besides, a self-adaptive method of selecting intrinsic mode functions (IMFs) has been proposed. The method is used to denoise magnetic memory signal of steel rope metal and the result is better than other common denoising methods. After denoising the tested signal of steel rope, the broken wires field of the rope can be recognised by the normal component of 3M crossing zero point.

Bending experimental study on metal magnetic memory signal based on von Mises yield criterion

Metal magnetic memory (MMM) is an effective magnetic method in evaluating stress concentration and early dam- age. To get the normal component of surface spontaneous magnetic signal of the specimen under different loads, three-point flexural test and four-point flexural test of Q235B steel specimens were carried out. Von Mises yield criterion was employed to calculate equivalent stress. The results show that the magnetic signal curve has a strong regularity during loading stage. For a three-point bending specimen, the magnetic signal curve presents triangle distribution. For a four-point bending specimen, the magnetic signal curve of the flange and web present trapezoid and saddle distribution, respectively. The symmetry of magnetic signal curve can be used to determine whether a specimen is under eccentric compression. The main tendency and local extrema of equivalent stress are coinciding with that of magnetic signal. A reasonable qualitative correlation is found between the distribution of equivalent stress and magnetic signal.

Experimental investigation of stress and damage characterization of steel beam buckling using magnetic memory signals

Summary Metal magnetic memory is a new passive nondestructive method for assessing stress conditions that can be used in the detection of early damage in ferromagnetic materials. To obtain the signal characteristics of steel beams, the normal components of the surface-spontaneous magnetic signals of a flange and web of Q235B I-steel beams were measured using four-point flexural tests under different loads. The variation in the normal component and its gradient were analyzed. The results show that the normal component can reflect the stress distribution of the flange, a significant decrease in the normal component of the flange, and a reversal of the normal component of the web can predict the instability of a beam. The location of a concentrated load on a steel beam can be determined by the highest value of the magnetic signal and the peak–peak value of the magnetic gradient. Finite element software was employed to calculate the von Mises stress, and a qualitative relationship between the distribution of the normal component and the distribution of the von Mises stress was obtained. A quantitative correlation between the applied stress and the average value of the normal component of the flange was developed. Quantitative analyses of the applied stress should consider the dimensional effect of different specimens. A new method is proposed to estimate the location of the stress concentration and to evaluate the degree of damage using a gradient curve, and several parameters are defined to characterize the metal magnetic memory signal. These characteristic parameters may reflect the degree of damage and the range of loads. Copyright © 2015 John Wiley & Sons, Ltd.

Diagnosis Model of Pipeline Cracks According to Metal Magnetic Memory Signals Based on Adaptive Genetic Algorithm and Support Vector Machine

Metal magnetic memory (MMM) signals can reflect stress concentration and cracks on the surface of ferromagnetic components, but the traditional criteria used to distinguish the locations of these stress concentrations and cracks are not sufficiently accurate. In this study, 22 indices were extracted from the original MMM signals, and the diagnosis results of 4 kernel functions of support vector machine (SVM) were compared. Of these 4, the radial basis function (RBF) kernel performed the best in the simulations, with a diagnostic accuracy of 94.03%. Using the principles of adaptive genetic algorithms (AGA), a combined AGA-SVM diagnosis model was created, resulting in an improvement in accuracy to 95.52%, using the same training and test sets as those used in the simulation of SVM with an RBF kernel. The results show that AGA-SVM can accurately distinguish stress concentrations and cracks from normal points, enabling them to be located more accurately.

Non-destructive evaluation of the damage of ferromagnetic steel using metal magnetic memory and nonlinear ultrasonic method

Based on the measurement of residual magnetic spontaneous stray field, metal magnetic memory (MMM) technique is an effective method in evaluating the degree of early damage for ferromagnetic materials due to the existence of stress concentration. In this paper, the normal and tangential components of the stress induced self-magnetic leakage field (SMLF) intensity on the surface of specimens were measured at predetermined tensile forces throughout the tensile process. The results show that the stress induced normal and tangential components of the self-magnetic leakage field are effective in characterizing different stress levels in elastic deformation stages; and the tangential component is more effective in characterizing different plastic deformation stages. Furthermore, experimental measurements using nonlinear ultrasonic technique (NUT) were also conducted at the same stress level when the magnetic memory signals were collected. It shows that MMM technique is ef- fective in charactering the stress-strain state in elastic deformation stage for ferromagnetic materials; and NUT is effective in charactering the degree of plastic deformation.

Correlation between Magnetic Memory Signals and Mechanical Properties of 35CrMo Tempered and Quenched Steel

Magnetic memory signals and hardness of 35CrMo tempered and quenched steel were acquired during tensile testing. The magnetic signals of 35CrMo steel increased monotonously with the increase of tensile stress before yielding, followed by sudden decrease with further increase of stress after yielding. The zero-cross point ofHp(y) varied during tensile testing, indicating varied position of stress concentration zones with the process of tensile testing. Vickers hardness of the stress concentration zones of 35CrMo steel was lower than that around it, which may be attributed to the residual stress. The relationship between zero-cross point ofHp(y) and Vickers hardness was considered.

Analysis of stress-magnetic coupling effect in weak magnetic environment

Metal magnetic memory (MMM) signals are difficult to be analyzed due to noise interference, which limits its practical engineering application. A method of improving the magnetic signals is proposed in this paper by placing the excitation device which generates a weak external magnetic field about 100 A/m. The effect of the external magnetic field on the magnetic signals is studied using both finite element method (FEM) and uniaxial tensile tests. Comparison of the test data with the simulation ones of stress-magnetic coupling shows that the magnetic signals are strengthened and the measurement sensitivity of the detection system is greatly improved through the external magnetic excitation. Moreover, the FEM result has a good agreement with the testing results of No.20 steel plate. The proposed method has laid a foundation for further practical engineering application.

Research on the relationship between metal magnetic memory signal distortion and yield strain under static tension test

Research on the relationship between metal magnetic memory signal distortion and yield strain under static tension test

Research on the Attenuation Law of Metal Magnetic Memory Testing Signal

In order to clear the timeliness issues of metal magnetic memory signal value after unloading, we conduct some static tension experiments on some specimens in materials of KMN, which is commonly used in centrifugal compressors? Furthermore, we analyze the specimen, which is prefabricated artificial defects, on the surface normal magnetic field strength decay law with EMS-2003, the intelligent magnetic memory tester. The results show that: the surface magnetic field signal value is reduced by time after unloading, in other words it has certain timeliness.

Stress concentration impact on the magnetic memory signal of ferromagnetic structural steel

A novel method for quantitatively evaluating the impact of stress concentration on the magnetic memory signal of ferromagnetic structural steels was proposed. A theoretical model was established to illustrate the impact of stress concentration and microdefects on the normal component of surface magnetic signals, Hp(y), and its gradient K. The Hp(y) signals of the notched sheet specimens with different stress concentration factors were measured throughout the tension–tension fatigue tests, and the variation in measured Hp(y) and K was studied. It shows that the Hp(y) varied intensively and changed its polarity when crack initiated in the stress concentration area. The maximum gradient, Kmax, was used to indicate the stress concentration degree, which was found to be theoretically exponential increasing with an increase in the crack length. The research provides the potential possibility of quantitative inspection on stress concentration and microdefects for ferromagnetic structural steels.

Simulation Study of Magnetic Memory Signal Correlation Effect of Dual Defects by ANSYS

To study the correlation effect of multiple defects of metal magnetic memory (MMM) testing technology, the stress-magnetization effect on 20# steel specimen with dual defects under exercise of the geomagnetic field and tensile load is simulated by using the finite element analysis (FEA) software ANSYS. With the stimulation, the magnetic flux leakage (MFL) distribution with different measured paths and different lift-off values of the surface specimen is given. The results showed that the correlation effect of dual defects produce more severe stress concentration phenomenon on the associated region of ferromagnetic specimen, and had no obvious effect on the edge of ferromagnetic materials; Gradient value K of the normal component H(z) of leakage magnetic field in the specimen, as a parameter evaluation of stress concentration in the specimen, has high reliability in areas without correlation effect, but it has no credibility in areas under dual defect correlation effect. Moreover, this article also discusses the influence of measured path and the lift-off value on magnetic memory signals.

Research of magnetic memory signal characteristics corresponding to early fatigue damage

A newly arisen technique named metal magnetic memory is deemed to have potentials in detecting early damage, whereas it is lack of systematic micro-mechanism research in early fatigue damage of ferromagnetic components. The microscopic fatigue experimental system including XH-500 metallograhpic microscope, DIG300 special digital camera and magnetic memory testing device, combined with the magnetic memory microscopic fatigue observation method is adopted, and the tension-tension fatigue experiment with specimens made of steel 45 are conducted. The evolution process of fatigue shortcrack initiation and growth is observed dynamically from the microscopic scale, and the variation regularity of microscopic Vickers hardness during the fatigue procedure is also given. At the same time, the variation of the magnetic memory signal on the surface of the V-shaped notch with cyclic numbers is analyzed, and the magnetic signal curve distribution regularity corresponding to different crack lengths is investigated. The results show that the magnetic memory signals exhibit an abnormal nonlinear wave when the local plastic deformation or micro-damage occurs, providing a feasible detection method for early fatigue damage of ferromagnetic materials.

Experimental and theoretical analysis of metal magnetic memory signals in the stress concentration area of 45# steel under tensile testing

Tension tests were conducted on 45# steel to investigate metal magnetic memory in stress concentration areas. The normal and tangential components of metal magnetic memory signals at seven test points in each specimen were simultaneously measured with probe lift-off values of 1 mm, 3 mm, and 5 mm. Axial tensile load was applied until visible necking occurred, and magnetic flux density was compared with the fracture location predicted according to Doubov’s theory. Experimental results showed that the final fracture locations were inconsistent with the locations predicted by metal magnetic memory testing theory. A theoretical analysis of the determination criterion in Doubov’s theory also revealed that no perfect correlation between the two areas existed.

Micro-mechanism of metal magnetic memory signal variation during fatigue

Tensile fatigue tests were designed to study the relation between the tangential magnetic memory signal and dislocations. According to experimental results, in the early stage of fatigue, the magnetic signal and the dislocation density rapidly increase; while in the middle stage, the magnetic signal gradually increases, the dislocation density remains steady, and only the dislocation structure develops. On the other hand, in the later stage, the magnetic signal once again increases rapidly, the dislocation structure continues to develop, and microscopic cracks are formed. Analysis reveals that the dislocations block the movement of the domain wall, and the area of dislocation accumulation thus becomes an internal magnetic source and scatters a field outward. In addition, the magnetic memory field strengthens with increasing dislocation density and complexity of the dislocation structure. Accordingly, the dislocation pinning factor related with the dislocation density and the dislocation structure has been proposed to characterize the effect of dislocations on the magnetic memory signal. The magnetic signal strengthens with an increase in the dislocation pinning factor.

Investigation of Magnetic Memory Signals Induced by Dynamic Bending Load in Fatigue Crack Propagation Process of Structural Steel

Metal magnetic memory effect, induced by applied stress under the excitation of the geomagnetic field, has attracted a lot of attentions due to its unique advantages of stress concentration identification and early damage detection for ferromagnetic materials. To further investigate the regularity of magnetic memory signals in the fatigue crack propagation process under the dynamic bending load, the surface magnetic field intensity \(H_{p}(y)\) of ferromagnetic structural steel was measured throughout the dynamic three-point bending fatigue tests; variation of \(H_{p}(y)\) and its maximum gradient \(K_{max}\) were studied; meanwhile the possibility of using \(K_{max}\) to predict the fatigue crack propagation was discussed. The results showed that \(H_{p}(y)\) was relatively stable at different loading cycles and its maximum value appeared at the fatigue crack area before the specimen fractured; instead the \(K_{max}\) increased exponentially with the increase of loading cycles, and an approximate linear relationship was found between \(K_{max}\) and crack length 2a. The cause for this phenomenon was also discussed.

Residual magnetic field variation induced by applied magnetic field and cyclic tensile stress

Magnetic memory testing (MMT) method is a novel non-destructive testing technique due to its unique advantages of stress concentration identification and early damage detection for ferromagnetic materials. However, a thorough understanding of the impact of exciting magnetic source and cyclic stress on the residual magnetic field variation has not been clearly addressed. The surface magnetic memory signal Hp(y) induced by applied magnetic field and cyclic tensile stress was measured throughout the fatigue process. The correlation of Hp(y) and its gradient K changes with loading cycles and applied magnetic field intensity H reported. The results show that applied magnetic field can only change the magnitude of MMT signal instead of changing the Hp(y) curve׳s profile. The Hp(y) value increases with the increase of the H, and the K value is approximately linear to the H. The maximum gradient Kmax indicating the degree of stress concentration increases with the increase of either stress cycles or H. The phenomenon was also discussed from the view of the magnetic dipole in a ferromagnet.