Research On Electromagnetic Acoustic Transducer Oriented To Rail Foot Flaw Detection

With the fast development of high speed railways, the rail defects increase day by day. The commonly used technique for rail detection is piezoelectric ultrasonic technique, which could only detect the rail head, the rail web and the rail foot center. The two sides of the rail foot could not be detected because the complex structure of the rail. Electromagnetic acoustic transducers(EMATs) are non-contact transducers. They are capable of the online detection of the rail foot. However, the previous models of EMAT operating on ferromagnetic materials are not completed, the ultrasonic waves generated by meander-line coil EMAT propagate in two directions and the propagation characteristics of the guided wave in the rail foot are not clear, which hinder the application of electromagnetic acoustic technique in the rail flaw detection field.The previous models have not considered the influence of the magnetostriction and magnetization in arbitrary magnetic field, so the EMAT model operating on ferromagnetic media was not completed. A two dimensional frequency domain finite element model, based on Maxwell equation, elastodynamic equation and magnetostrictive constitutive equation, was proposed including Lorentz force, magnetization force and magnetostrictive strain, which establishes the foundation for the design of EMAT used in rail foot flaw detection. The performance of EMAT was analyzed under various magnetic field conditions. The simulation and experimental results indicated that the magnetostriction is the main mechanism under horizontal magnetic field. The Lorentz force is the main transduction mechanism under normal bias magnetic field. The optimal magnetic field is determined for EMAT operating on rail accordingly.A unidirectional line-focusing shear vertical wave EMAT(ULFSV-EMAT) is proposed to suppress the clutter signal in the non-focusing side of the EMAT for rail foot center flaw detection. The unidirectional line-focusing shear vertical wave is generated using two coils based on the line-focusing shear vertical wave EMAT and acoustic wave superimpose principle. The two coils are driven by two burst currents with proper phase and magnitude. The directivity patterns of the four shear vertical wave EMATs were analyzed; the received signal amplitude ratio of the enhanced side to the weakened side(EWR) of ULFSV-EMAT could reach 6, which showed that the acoustic wave could be focused unidirectionally. The ULFSV-EMAT can be used for rail foot center flaw detection.A unidirectional guided wave EMAT(UG-EMAT) is proposed to overcome the deficiency of piezoelectric techniques for the rail foot side detection. Firstly, the structure of the UG-EMAT is determined due to acoustic wave superimpose principle. Then the guided wave dispersion curves and wave structures of the rail foot were calculated using the eigenfrequency method. A three dimensional frequency domain model for guided wave EMAT was built to analyze the acoustic wave energy distribution of electromagnetic ultrasonic guided wave in the rail foot. The optimal working frequency of the guided wave EMAT was determined. The main parameters of the UG-EMAT was fixed considering the requirement of rail foot detection. The guided wave modes and acoustic wave energy region were analyzed through two dimensional Fourier Transform for the guided wave generated by EMAT in the rail foot detection, which demonstrated the potential for the rail foot side flaw detection using UG-EMAT.To testify the effectiveness of the designed EMAT for rail foot flaw detection, a finite element model for EMAT was established for rail foot with defects. The influence of detection distance and defect size on the performance of the EMAT were analyzed. The rail foot detection system based on EMAT was designed according to the requirement of rail detection. The ULFSV-EMAT and UG-EMAT are used for rail foot center flaw detection and the rail foot side flaw detection. The experimental results showed that the designed EMAT could be used for rail foot flaw detection. The minimum detectable radius of the semi-circle slot defect was 7.5 mm and the effective detect distance was 120~600 mm. The research makes a good foundation for the further research on the online rail foot inspection using EMAT.