Study on the Nonlinear Electromagnetic Acoustic Resonance Method for the Evaluation of Hidden Damage in a Metallic Material

Recently, much attention has been paid to nonlinear ultrasonic technology as a potential tool to assess hidden damages that cannot be detected by conventional ultrasonic testing. One nonlinear ultrasonic technique is measurement of the resonance frequency shift, which is based on the hysteresis of the material elasticity. Sophisticated measurement of resonance frequency is required, because the change in resonance frequency is usually quite small. In this investigation, the nonlinear electromagnetic acoustic resonance (NEMAR) method was employed. The NEMAR method uses noncontact electromagnetic acoustic transducers (EMATs) in order to minimize the effect of the transducer on the frequency response of the object. Aluminum plate specimens that underwent three point bending fatigue were tested with a shear wave EMAT. The hysteretic nonlinear parameter , a key indicator of damage, was calculated from the resonance frequency shift at several levels of input voltage. The hysteretic nonlinear parameter of a damaged sample was compared to that of an intact one, showing a difference in the values.