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DOI:

2015. . No. 2(68) - Jun 2015, pp. 74-77

Application of Correlation Analysis for Weld Joint Inspection by Magnetic Flux Leakage Method

A. A. Markov, A. G. Antipov

Introduction. Magnetic Flux Leakage (MFL) is a known method of rail NDT inspection. It implies the inspection vehicle is equipped with a magnetizing system to form a strong longitudinal magnetic flux in the rail. The distribution of magnetic field in the air in immediate proximity to the rail contains information on the internal structure of it.
Method. Besides the detecting of rail flaws, the method can be used to localize rail weld joints. A specific magnetic state of ferromagnetic material near the weld results in a distinctive signal recorded by a magnetic sensor. The signal of weld joint is very stable, that is confirmed by presented data. The stability means the signal form does not change significantly from inspection to inspection which are usually carried out several times a year. But if the defect develops in the weld joint the signal changes considerably. So, the changes in weld MFL signal are the evidence of defect presence in weld. 
Results. To detect signal changes the correlation analysis of the signal is used. The research shows if the cross-correlation coefficient between signals of two successive inspections is less then 0.75 then the probability of defect presence is high and the additional investigation of this weld is needed. The automated correlation processing increases the efficiency of NDT data analysis significantly.
Conclusion. The other observed property of MFL weld signal is the uniqueness. Due to specific conditions during a welding process the magnetic state of the rail near the weld differs one from another. In conjunction with the signal stability the uniqueness makes cross-correlation matrix between signals of the same set of welds obtained in two successive inspections strongly diagonal. This means the probability of correct identification of certain weld on MFL data records using the correlation technique is rather high. Later facilitates the automated process of record matching.
 
Keywords: magnetic flux leakage, MFL, rail inspection, weld joint, correlation analysis
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