Magneto-optical Imaging Method Research Articles

Research on rail crack detection technology based on magneto-optical imaging principle

The detection of rail top crack is of great significance in ensuring the safe operation of railway. Based on the principle of magnetic flux leakage detection and Faraday magneto-optical effect, a high-resolution and non-destructive magneto-optical imaging detection method is proposed in this paper. This paper analyses the principle of magneto-optical imaging, establishes a rail magnetic flux leakage detection experimental system based on magneto-optical effect, and detects the rail specimen with crack through experiment. The experimental results show that the change of crack width and depth will affect the magneto-optical imaging results, and the minimum crack size that can be detected by this experimental system is 0.4mm wide and 0.5mm deep. This paper realizes the detection of rail crack defects by using magneto-optical imaging method, and provides the basis for building an efficient, portable and high-resolution rail non-destructive testing equipment.

Research on Microstructure Characteristics of Welded Joint by Magneto-Optical Imaging Method

This paper proposes an approach for analyzing the microstructure evolution of laser welding seam by magneto-optical imaging (MOI). The Faraday magneto-optical effect and magnetic domain theory are used to account for the MOI mechanism. The influence of laser welding on the welded joint was inspected by the analysis of color, grayscale and brightness of magneto-optical images. The relation between the brightness of magneto-optical image and grain size of microstructure is discussed as well, and the characteristics of magneto-optical images of weld microstructure were compared with those of scanning electron microscope (SEM) images. Experimental results show that three regions, including the weld zone (WZ), the heat-affected zone (HAZ), and the base metal (BM) are distributed in the magneto-optical image of the welded joint. The MOI method can investigate the microstructure evolution of welded joints and provide a theory and experimental basis for detecting weld defects.

Magneto-optical imaging of coherent spin dynamics in ferrites.

The explosive development of quantum magnonics is associated with the possibility of its use as macroscopic quantum systems. In particular, they can find an application for quantum computing processors and other devices. The recently discovered phenomenon of magnon Bose-Einstein condensation and coherent precession of magnetization can be used for these purposes. Our letter describes a method for the optical observation of the coherently precessing magnetization in conditions when the concentration of magnons reaches the value necessary for their quantum condensation. The investigations were conducted in the out-of-plane magnetized yttrium iron garnet films. The required magnon density was achieved by magnetic resonance technique. The magneto-optical imaging method provides such important parameters of the coherent spin dynamics as the amplitude and phase distributed all over the sample. It should become an indispensable read-out tool for the upcoming quantum technologies based on the magnon Bose-Einstein condensation.

Increasing reliability of forensic analysis while research on destroyed relief marking with magneto-optical devices

While research on destroyed relief marking of metal objects using the magneto-optical method, visualization of (invisible) fields of internal stress in the VIN plate area is performed and then a forensic analysis of obtained instrumental data is carried out (indirect organoleptic observation of the visualization results); thus, forensic analysis reliability of results directly depends on sensitivity of instruments and informativeness of instrumental data.
 The main quantitative characteristic in this case is probability of correct signal recognition (contours of marking signs) against the background of noise (structural noise of investigated surface and the noise of the visualization method itself) determined by the signal-to-noise ratio.
 This article presents results of a comparative experimental assessment of signal-to-noise ratio and probability of correct signal recognition while restoringthe destroyed relief markings for two complexes of magneto-optical imaging – models of 2006 and 2018.
 This article purpose is a quantitative and qualitative comparative assessment of results of visualization of internal stresses in areas of completely removed relief marking of metal objects. The results of successful practical research obtained by forensic experts from different countries make it possible to assess effectiveness and prospects of using the magneto-optical imaging method.
 In a new modification of the magneto-optical complex:
 
 signal level is 4.35 dB higher (contrast of reconstructed marking signs);
 2.71 dB lower noise level (surface relief/texture and magnetic copying noise);• probability of correct character recognition is P> 0.995 (increased by 14.9%).
 
 Technical improvements in implementation of magneto-optical visualization method made it possible to expand the range of materials for research objects(magnetic and electrically conductive materials were investigated).
 The high efficiency of method for restoring marking is illustrated by results of forensic examinations for materials with a low level of residual stresses (aluminum alloy, low-carbon steel) which chemical etching method did not give results for.
 The use of new modification allows examining the rust layer, up to cases of corrosion to the entire depth of marks.
 Considering non-destructive nature of magneto-optical researches, possibility of their repeated repetition without losing object properties, this method (in accordance with the order of application of types of studies) deserves more attention for application.

Influence of Sampling Frequency on Magneto-Optical Imaging Under Alternating Magnetic Field Excitation

This paper studies the magneto-optical image (obtained by different sampling frequencies) features of weld defects under alternating magnetic field excitation. Weld cracks with different gaps—0.03 mm, 0.05 mm, 0.07 mm, and 0.1 mm—were detected through the magneto-optical imaging method. In order to study the effect of sampling frequency on magneto-optical imaging, different sampling frequencies, such as 5 fps, 15 fps, 25 fps, 35 fps, 45 fps, 55 fps, 65 fps, and 75 fps, were used under the excitation of an alternating magnetic field. The distribution range of the gray-scale value for the magneto-optical image obtained at each sampling frequency was analyzed. The effect of the sampling frequency on the magneto-optical imaging was analyzed by comparing the magneto-optical images of weld defects and their gray-value curves at different sampling frequencies. Experimental results show that the sampling frequency of the magneto-optic sensor affects the distribution range of the gray-scale value of the magneto-optic image. Under the same experimental conditions, as the sampling frequency increases, the whole magneto-optical image becomes dark, and the span of the gray value becomes larger.

Identification of weld defects using magneto-optical imaging

The weld cracks of the high-strength steel are identified by magneto-optical imaging. The background and basic principle of micro-crack inspection after welding by magneto-optical imaging (MOI) are discussed. The key point is to adopt continuous fuzzy enhancement on the basis of fuzzy set theory, to improve the degree of separation of welding crack and weld and solve the problem of uneven magnetic surface of high-strength steel. The experiment of restoring the magneto-optical image is carried out by using the algorithm of unevenness of crack magneto-optical imaging of high strength steel. After restoration, the PSNR data of magneto-optical image is large, indicating that image quality is greatly improved. According to the characteristics of magneto-optical imaging method, an array crack identification model of laser welding is established by using principal component analysis (PCA) method and support vector machine (SVM). The test results validate that our proposed method can efficiently extract the features of welding cracks and improve the precision of detecting welding cracks.

Magneto-optical imaging characteristics of weld defects under alternating magnetic field excitation

This paper examines the characteristics of magneto-optical images of weld defects under alternating magnetic field excitation. Weld defects such as non-penetration, surface cracks and sub-surface cracks were detected by a magneto-optical imaging method. Magneto-optical imaging nondestructive testing experiments under alternating magnetic field excitation were carried out to detect the weld defects. Image processing methods which include contrast enhancement of original image, fused image, contrast enhancement of fused image were applied to extract the defect information of the magneto-optical images. What's more, the difference among the magneto-optical images of weld defects was obtained by contrast analysis. Experimental results show that non-penetration welding images possess significant differences in brightness and darkness, and this difference in cracks is smaller than non-penetrating ones. Under the same excitation conditions, the leakage flux of welds with non-penetration is stronger than that of weld cracks.

Features of microstructure and magnetic flux dynamics in superconducting Nb-Ti tapes with strong anisotropic pinning

Results of micro-structural, phase and texture analysis of Nb-Ti tapes and results of magnetic flux dynamics fixed by Magneto-Optical Imaging (MOI) method are presented in order to investigate origin and consequences of the anisotropic pinning model proposed, and implemented earlier experimental results including Guided Vortex Motion (GVM) phenomenon.

Magneto-optical imaging reveals impacts of niobium preparation for superconducting performance

Researchers developed a magneto-optical imaging method to analyze magnetic flux properties of superconducting niobium, revealing the effects of heat treatment on its trapped flux distribution.

Magneto-optical imaging deviation model of micro-gap weld joint

Seam tracking is important for butt joint laser welding. A magneto-optical imaging approach is proposed to detect the micro-gap weld whose width is less than 0.2mm. The symmetry of the magnetic field above the weld joint is an important precondition to ensure the detection accuracy of the magneto-optical imaging method. However, in actual complex industrial scene, it is difficult to guarantee complete symmetry of the magnetic field. This paper proposes an effective method for solving the problem of magneto-optical imaging deviation under an asymmetric magnetic field. Two possible factors causing the asymmetry of magnetic field above weld joint are firstly investigated using finite element analysis. By analyzing the characteristics of the magneto-optic medium used in the sensor and measuring the magnetic field distribution of weld joint at different lift-off height and different excitation voltage, the prediction model of deviation between the weld position detected by magneto-optical imaging and the actual weld position is built by back propagation (BP) neural network. The experimental result of weld seam tracking based on magneto-optical imaging shows that the change of the lift-off height will affect the detection accuracy of the weld position, and sufficient accuracy can be ensured after correcting the deviation according to the prediction model of magneto-optical imaging deviation.

Magnetic domains and magnetic stability of cohenite from the Morasko iron meteorite

Magnetic properties, texture and microstructure of cohenite grains from Morasko iron meteorite have been investigated using electron backscattered diffraction, Bitter pattern technique, magneto-optical imaging method and magnetic force microscopy. Cohenite shows much stronger magnetic contrast compared to kamacite because it is magnetically harder than the Fe-Ni alloy, and thus causes higher stray fields. A surprising result is the high stability and reversibility of the global stripe-like magnetic domain structure in cohenite when applying high magnetic fields up to 1.5T, and exposing it to high temperatures above the Curie temperature of about 220°C. Heating up to 700°C under atmosphere conditions has shown that cohenite remains stable and that the global magnetic domain structures mainly recover to its preheating state. This observation suggests that magnetic domains are strongly controlled by the crystal anisotropy of cohenite. Branching magnetic domain structures at the grain boundary to kamacite can be annealed, which indicates that they are very sensitive to record deformation. EBSD observations clearly demonstrate that increasing deviation from the easy [010] crystallographic axis and stress localization are the main factors controlling the distortion of Bitter patterns, and suggest a high sensitivity of the cohenite magnetic domain structure to local microstructural heterogeneities. The results of this study substantiate the theory that cohenite can be a good recorder of magnetic fields in planetary core material.

Anisotropic flux penetration into amorphous Mo80Ge20 square networks

We have investigated anisotropic flux penetrations into superconducting amorphous Mo80Ge20 square networks by using a magneto-optical imaging method. Pinning force dependence of the anisotropy is investigated by changing the fabrication process of the Mo80Ge20 thin films. Diagonal flux penetration becomes more dominant with less pinning sites, which is consistent with the earlier prediction with the time-dependent Ginzburg–Landau simulation with an assumption of pinning-force free in superconducting region. These results strongly indicate that the diagonal flux penetration in the square superconducting networks with weak pinning is intrinsic.

Anisotropic flux penetration into Nb square superconducting networks

Anisotropic flux penetration into square superconducting networks of Nb are studied by a magneto-optical imaging method. Their dependences on the lattice constant, the line width, the thickness of the networks, and temperature are systematically investigated. An anomalous diagonal flux penetration is observed (1) when the saturation number of vortices in a hole is equal to one possibly due to minimization of the Ginzburg–Landau free energy or (2) when the fan-shaped vortex penetration (vortex jet) from the corner of the square hole reaches the next-nearest-neighbor hole due to the enhanced current density with a high current–voltage exponent.

Flux Avalanches in Multi-layer Superconducting Strip Arrays

We have fabricated three-dimensional double- and triple-layer strip arrays of Nb and observed flux penetration into these strip arrays with an applied field perpendicular to the plane by using the magneto-optical imaging method. In the triple-layer strip arrays with large overlaps between the strips, we observed flux avalanches and one-dimensional penetrations perpendicular to the strip similar to the case of double-layer strip arrays. In thicker triple-layer samples, we also observed flux avalanches parallel to the strip. Enhanced demagnetization effect, thermal conductivity between the neighboring layers, and the driving force parallel to the strip are believed to cause these avalanches.

Flux avalanches in Nb superconducting shifted strip arrays

Flux penetrations into three-dimensional Nb superconducting strip arrays, where two layers of strip arrays are stacked by shifting a half period, are studied using a magneto-optical imaging method. Flux avalanches are observed when the overlap between the top and bottom layers is large even if the width of each strip is well below the threshold value. In addition, anomalous linear avalanches perpendicular to the strip are observed in the shifted strip array when the overlap is very large and the thickness of the superconductor is greater than the penetration depth. We discuss possible origins for the flux avalanches, including linear ones, by considering flux penetration calculated by the Campbell method assuming the Bean model.

Flux penetration into three-dimensional superconducting strip array

We have fabricated three-dimensional double-layer strip arrays of Nb and observed flux penetrations into these strip arrays with applied field perpendicular to the plane by using the magneto-optical imaging method. In strip arrays with large overlaps between strips we observed flux avalanches even at 1.5K below the critical temperature. Novel flux avalanches showed up as one-dimensional penetrations perpendicular to the strip arrays in samples with the maximum overlap. Enhanced demagnetization effect and thermal conductivity between the top and bottom layers are believed to cause these avalanches.

Origin of Diagonal Flux Penetration into Square Superconducting Networks

The origin of the anomalous diagonal flux penetration is investigated in Nb square superconducting networks. The magnetic field dependence and heat transfer coefficient dependence of this phenomenon are observed by using the magneto-optical imaging method. The anomalous diagonal flux penetration occurs independent of the applied magnetic field. Flux avalanches at low temperatures are suppressed by depositing a thick gold layer due to the enhancement of the thermal conductivity. However, the diagonal flux penetration appears even in the area covered by the gold layer. Combining these observations, we conclude that flux avalanches do not promote the diagonal flux penetration.

Thickness and hole-shape dependence of flux penetration into square superconducting networks

We have observed flux penetrations into square superconducting networks of Nb with various thicknesses and hole-shapes by using magneto-optical imaging method. The penetrated pattern does not depend on the thickness and extends along diagonal direction when the samples have smaller area in the intersection of networks. We have also observed similar superconducting networks with and without small holes at the intersection. The small holes at the intersection do not work as a promoter for the diagonal flux penetration though the parallel flux penetration becomes weak. We discuss possible origins of these anomalous anisotropic flux penetrations.

Development of surface magneto-optical imaging method

We have observed single vortices in a NbTiN thin film by using magneto-optical (MO) imaging method, in which the light reflected from the surface of the sample produces MO contrasts. The gap between the sample and the indicator film is estimated by the Newton ring and curve fitting to the flux density profile of single vortices.

Inter-granular current in iron-oxypnictide superconductors

Inter- and intragranular currents, Jcinter and Jcintra, in LaFePO0.94F0.06 polycrystalline sample are evaluated by measuring the remanent-state field profile using magneto-optical imaging method. Obtained images show the absence of magnetic field modulation associated with the weak-link nature of grain boundaries, indicating that Jcinter and Jcintra are comparable in magnitude in contrast to other iron-arsenide superconductors. Jcinter is estimated to be 2×104A/cm2 in the limit of T=0K, which is larger than the values in other iron-arsenide superconductors. Implication of these Jc behavior is discussed in relation with possible pairing symmetries.