Contactless evaluation of stress in anisotropic metallic plates using nonlinear electromagnetic acoustic resonance technique

In this paper, a contactless nonlinear acoustic method is developed for the evaluation of stress states in anisotropic metallic plates by combination of electromagnetic acoustic resonance (EMAR) technique and higher harmonic generation. Electromagnetic transducers (EMATs) designed and applied for exciting and receiving ultrasonic signals can maintain the coupling condition consistently on the measure of higher harmonics generated. EMAR provides sufficient magnitude of signals for higher harmonics generated. In addition, conventional EMAR technique based on the measurement of shear wave velocity and attenuation within a certain frequency range, is also carried out in the specimens. Effect of stress on the higher harmonic generation is explored and discussed. It is found that nonlinear parameters measured by nonlinear EMAR method change significantly versus the increase of external tension stress loadings, whereas the variations of linear acoustic parameters measured are negligible. In addition, the obtained results clearly indicate that the variation of the measured acoustic nonlinear parameters versus external stresses is direction-depended in anisotropic materials. The contactless nonlinear acoustic technique combines the feature of EMAR with the merit of higher harmonic generation, providing an effective means for stress evaluation in weakly anisotropic materials with improved reliability and sensitivity over linear ones.