scholarly journals Magnetic field gradient as the most useful signal for detection of flaws using MFL technique

2018 ◽  
Vol 69 (6) ◽  
pp. 422-425 ◽  
Author(s):  
Zbigniew Usarek ◽  
Marek Chmielewski ◽  
Leszek Piotrowski
Keyword(s):  
Magnetic Flux ◽  
Steel Plate ◽  
Magnetic Field Gradient ◽  
Normal Component ◽  
Magnetic Flux Leakage ◽  
Magnetic Flux Density ◽  
Scanning Velocity ◽  
Useful Signal ◽  
Flux Leakage ◽  
Proper Analysis

Abstract The magnetic flux leakage (MFL) technique is extensively used for detection of flaws as well as for evaluation of their dimensions in ferromagnetic materials. However, proper analysis of the MFL signal is hindered by the MFL sensor velocity causing distortions of this signal. Traditionally measured components of the MFL signal are particularly sensitive to the scanning velocity. In this paper, an another signal – the gradient of the normal component of magnetic flux density – was proposed as it is less sensitive to the scanning velocity. Results obtained for scans of the steel plate with artificially manufactured flaws confirm this statement.

Practical evaluation of velocity effects on the magnetic flux leakage technique for storage tank inspection

2020 ◽  
Vol 62 (2) ◽  
pp. 73-80
Author(s):  
A L Pullen ◽  
P C Charlton ◽  
N R Pearson ◽  
N J Whitehead
Keyword(s):  
Magnetic Flux ◽  
Storage Tank ◽  
Eddy Currents ◽  
Surface Defects ◽  
Signal Amplitude ◽  
Magnetic Flux Leakage ◽  
Scanning Velocity ◽  
Near Surface ◽  
Practical Evaluation ◽  
Flux Leakage

Magnetic flux leakage (MFL) is a technique commonly used to inspect storage tank floors. This paper describes a practical evaluation of the effect of scanning velocity on defect detection in mild steel plates with thicknesses of 6 mm, 12 mm and 16 mm using a fixed permanent magnetic yoke. Each plate includes four semi-spherical defects ranging from 20% to 80% through-wall thickness. It was found that scanning velocity has a direct effect on defect characterisation due to the distorted magnetic field resulting from induced eddy currents that affect the MFL signal amplitude. This occurs when the inspection velocity is increased and a reduction in the MFL signal amplitudes is observed for far-surface defects. The opposite applies for the top surface, where an increase is seen for near-surface MFL amplitudes when there is insufficient flux saturating the inspection material due to the concentration of induced flux near the top surface. These findings suggest that procedures should be altered to minimise these effects based on inspection requirements. For thicker plates and when far-surface defects are of interest, inspection speeds should be reduced. If only near-surface defects are being considered then increased speeds can be used, provided that the sensor range is sufficient to cope with the increased signal amplitudes so that signal clipping does not become an issue.

Magnetic Flux Leakage Sensing-Based Steel Cable NDE Technique Incorporated on a Cable Climbing Robot for Bridge Structures

2012 ◽  
Vol 83 ◽  
pp. 217-222 ◽  
Author(s):  
Seung Hee Park ◽  
Ju Won Kim ◽  
Min Jun Nam ◽  
Jong Jae Lee
Keyword(s):  
Magnetic Flux ◽  
Magnetic Flux Leakage ◽  
Magnetic Flux Density ◽  
Inspection System ◽  
Climbing Robot ◽  
Steel Cable ◽  
Monitoring Method ◽  
Sensor Head ◽  
Bridge Structures ◽  
Flux Leakage

In this study, an automated cable monitoring system using a NDE technique and a cable climbing robot is proposed. MFL (Magnetic Flux Leakage- based inspection system was applied to monitor the condition of cables. This inspection system measures magnetic flux to detect the local faults (LF) of steel cable. To verify the feasibility of the proposed damage detection technique, an 8-channel MFL sensor head prototype was designed and fabricated. A steel cable bunch specimen with several types of damage was fabricated and scanned by the MFL sensor head to measure the magnetic flux density of the specimen. To interpret the condition of the steel cable, magnetic flux signals were used to determine the locations of the flaws and the level of damage. Measured signals from the damaged specimen were compared with thresholds set for objective decision making. In addition, the measured magnetic flux signal was visualized into a 3D MFL map for convenient cable monitoring. Finally, the results were compared with information on actual inflicted damages to confirm the accuracy and effectiveness of the proposed cable monitoring method.

Numerical Simulation and Analysis on Magnetizing Exciter for Magnetic Flux Leakage

2011 ◽  
Vol 474-476 ◽  
pp. 1187-1190
Author(s):  
Qiang Song
Keyword(s):  
Magnetic Flux ◽  
Permanent Magnet ◽  
Flux Density ◽  
Pipe Wall ◽  
Three Dimensional ◽  
Magnetic Flux Leakage ◽  
Magnetic Flux Density ◽  
Outer Diameter ◽  
Element Analysis ◽  
Flux Leakage

Magnetic flux leakage (MFL) is a non-destructive testing method used to inspect the pipe and magnetization of the pipe wall to saturation is essential for anomalies to be reliably and accurately detected and characterized. Axial components of magnetic flux density obtained during the MFL inspection have been simulated using three-dimensional finite element analysis and the effects of magnetizing exciter parameters on magnetic flux density are investigated. The pipe modeled in this paper has an outer diameter of 127mm (5 in.) with a wall thickness of 9 mm (0.354 in.). According to numerical simulations, an increase in the magnetic flux density of pipe wall is observed with an increase in the permanent magnet length and height. It clearly illustrates that Nd-Fe-B permanent magnet assembly with 70 mm length and 40 mm height may magnetize pipe wall to near saturation.

Magnetizing Method for High Precision MFL Inspection of Longitudinal Defects of Thick Wall Steel Pipe

2013 ◽  
Vol 718-720 ◽  
pp. 898-902 ◽  
Author(s):  
Bo Feng ◽  
Jian Bo Wu ◽  
Yun Yang ◽  
Yi Hua Kang
Keyword(s):  
Magnetic Flux ◽  
High Precision ◽  
Magnetic Sensors ◽  
Magnetic Flux Leakage ◽  
Steel Pipe ◽  
Thick Wall ◽  
Magnetic Flux Density ◽  
Detection Zone ◽  
Flux Leakage ◽  
Magnetic Flux Leakage Testing

Circumferential magnetization of the thick wall steel pipe is carried out for the inspection of longitudinal defects in magnetic flux leakage testing. For high precision magnetic flux leakage testing, magnetic field distribution along the axial direction must be uniform, thus a new type of pole shoe structure is proposed in this paper to enlarge the uniformly magnetized area. Thick wall steel pipes are hard to be magnetized to near saturation, especially when the new pole shoe is used, which makes difficult for the magnetic sensors to pick up the magnetic leakage field. Therefore, the relationship between the magnetic flux density of the detection zone and the dimensions of the magnetizer has been studied by finite element simulation. These results are helpful in the design of circumferential magnetizer.

scholarly journals Magnetic Flux Leakage Sensing-Based Steel Cable NDE Technique

2014 ◽  
Vol 2014 ◽  
pp. 1-8 ◽  
Author(s):  
Seunghee Park ◽  
Ju-Won Kim ◽  
Changgil Lee ◽  
Jong-Jae Lee
Keyword(s):  
Magnetic Flux ◽  
Magnetic Flux Leakage ◽  
Magnetic Flux Density ◽  
Inspection System ◽  
Steel Cable ◽  
Monitoring Method ◽  
Sensor Head ◽  
Long Span Bridges ◽  
Long Span ◽  
Flux Leakage

Nondestructive evaluation (NDE) of steel cables in long span bridges is necessary to prevent structural failure. Thus, an automated cable monitoring system is proposed that uses a suitable NDE technique and a cable-climbing robot. A magnetic flux leakage- (MFL-) based inspection system was applied to monitor the condition of cables. This inspection system measures magnetic flux to detect the local faults (LF) of steel cable. To verify the feasibility of the proposed damage detection technique, an 8-channel MFL sensor head prototype was designed and fabricated. A steel cable bunch specimen with several types of damage was fabricated and scanned by the MFL sensor head to measure the magnetic flux density of the specimen. To interpret the condition of the steel cable, magnetic flux signals were used to determine the locations of the flaws and the levels of damage. Measured signals from the damaged specimen were compared with thresholds that were set for objective decision-making. In addition, the measured magnetic flux signals were visualized as a 3D MFL map for intuitive cable monitoring. Finally, the results were compared with information on actual inflicted damages, to confirm the accuracy and effectiveness of the proposed cable monitoring method.

Magnetic flux leakage–based local damage detection and quantification for steel wire rope non-destructive evaluation

2017 ◽  
Vol 29 (17) ◽  
pp. 3396-3410 ◽  
Author(s):  
Ju-Won Kim ◽  
Seunghee Park
Keyword(s):  
Magnetic Flux ◽  
Damage Detection ◽  
Permanent Magnets ◽  
Steel Wire ◽  
Threshold Value ◽  
Wire Rope ◽  
Magnetic Flux Leakage ◽  
Magnetic Flux Density ◽  
Sensor Head ◽  
Flux Leakage

A magnetic flux leakage method was applied to detect damage when inspecting steel wire rope. A multi-channel magnetic flux leakage sensor head was fabricated using Hall sensors and permanent magnets to adapt to the wire rope. Three types of artificial damage were created on a wire rope specimen. The magnetic flux leakage sensor head scanned the damaged specimen to measure the magnetic flux density while the damage was expanding in three steps. Signal processing processes including the enveloping process based on Hilbert transform were performed to clarify the flux leakage signal due to the damage. The enveloped signals were then analyzed for objective damage detection by comparing with the threshold value. For improvement of quantitative analysis, three types of new damage indexes that utilize the relationship between the enveloped magnetic flux leakage signal and the threshold value were additionally proposed. By using the proposed damage indexes and the general damage indexes for the magnetic flux leakage method, the detected magnetic flux leakage signals from each damage type were quantified. The trends of the extracted damage indexes according to damage levels were analyzed to examine the applicability and reliability of the proposed damage indexes for the magnetic flux leakage based wire rope inspection.

Metal Magnetic Memory Testing of Welded Joints under Fatigue Load

2017 ◽  
Vol 898 ◽  
pp. 1069-1078
Author(s):  
Ning Qiao ◽  
Mu Xiao Shan ◽  
Ye Zheng Li
Keyword(s):  
Stress Concentration ◽  
Magnetic Flux ◽  
Crack Propagation ◽  
Normal Component ◽  
Fatigue Loading ◽  
Fracture Morphology ◽  
Magnetic Flux Leakage ◽  
Magnetic Memory ◽  
Metal Magnetic Memory ◽  
Flux Leakage

To investigate the influence of stress concentration, crack propagation and types of fatigue loading on metal magnetic memory signals, two groups of fatigue experiments with different types of fatigue loading were carried out on Q235B steel welded joint. The normal components of magnetic flux leakage were measured by metal magnetic memory tester in the course of fatigue test, and the fracture surfaces of specimens were observed by scanning electron microscopy after fatigue tests. The experimental results showed that the normal components of magnetic flux leakage filed, as well as the metal magnetic memory signal, changed polarity and their gradients have peak values at stress concentration zones. The zero position of the normal component of magnetic flux leakage changed gradually with increasing cycle numbers. In addition, the metal magnetic memory signal feature of fatigue crack propagation was affected by the loading type clearly. Moreover, a combination of brittle rupture and ductile rupture was obtained in the fracture morphology figure.

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