Electromagnetic excitation technique for nonlinear resonant ultrasound spectroscopy

Abstract

Nonlinear resonant ultrasound spectroscopy (NRUS) is a method that can be used to determine the amount of microcracking in structures. The nonlinear parameter extracted in NRUS is based on the dependence of the resonance frequency on vibration amplitude in the presence of damage. NRUS measurements typically are excited with a piezoelectric transducer, but here the application of an electromagnetic transducer is explored as an alternative. The electromagnetic transducer, unlike a single piezoelectric, allows selective excitation of longitudinal, torsional, and bending vibrations in a rod-shaped sample. Measurement of the nonlinear properties of the sample for each type of vibration is therefore possible. This electromagnetic technique involves gluing a coil of wire onto the end of a rod sample and placing it in a magnetic field. By controlling which part of the coil is inside the strongest region of the magnetic field, the principal direction of the driven oscillations in the rod can be controlled. Both piezoelectric and electromagnetic excitation techniques are tested by measuring the parameter of nonlinearity for longitudinal motion, αE, in a sample of Berea sandstone. The electromagnetic technique was shown to measure a 30% higher mean value for αE than the piezoelectric technique.