Electromagnetic Acoustic Transducer and Method for Residual Stress Detection in Conducting Materials

Abstract

Residual stress present in rail, pipes or metallic structures used in industrial application is crucial to measure. Continuous use of these parts with residual stress may result in deterioration with time. If stress level crosses a certain threshold, then the structure may get damaged and sometimes result in fatal accident. Existing methods of residual stress measurement such as hole drilling and strain gauge are destructive in nature. Whereas, X-ray and neutron diffraction methods have their own inherent drawbacks. Acoustic birefringence has been proved as a powerful technique for the residual stress measurement. The technique, uses ultrasonic propagation velocity of two orthogonal polarized shear waves. Presently, two separate electromagnetic acoustic transducer (EMAT) coils are alternately used for the generation and measurement of ultrasonic propagation velocities in orthogonal polarizations or same EMAT is used for the measurement at 0° and then by rotating 90°. In the present work, the design of single coil based EMAT is proposed which is capable of generating simultaneously two orthogonally polarized ultrasonic shear waves in the object. The received signal generated by EMAT is the resultant superimposition of two waves traveling at different velocities due to residual stress present. The change in velocity and thereby present residual stress is estimated by simply measuring the echo duration. The developed EMAT provides various advantages over conventional two EMAT approach such as fast and direct estimation of change in velocity and thus residual stress in single acquisition.