Resonant acoustic nonlinearity of defects: Non-classical manifestations and applications for diagnostic imaging

Abstract

Acoustic nonlinearity of non-bonded interface (Contact Acoustic Nonlinearity (CAN)) is a type of non-classical nonlinearity perspective for applications in non-destructive evaluation of cracked defects. The presence of such defects also leads to a local decrease in rigidity and manifests in a characteristic frequency of the defect. A frequency match between the driving acoustic wave and this frequency provides a Local Defect Resonance (LDR) and an efficient energy delivery from the wave into the defect. A combination of CAN and LDR, therefore, manifests a profound nonlinearity even at moderate acoustic excitation level. Under resonance conditions, a strong enhancement of the higher harmonic (HH) amplitudes is observed. A high quality factor of LDR is also used as a filter/amplifier/resonator for efficient frequency mixing. These “conventional” nonlinear effects are not the only dynamic scenario of nonlinear phenomena for resonant defects. A combined effect of LDR and CAN also results in qualitatively new features characteristic of nonlinear and parametric resonances. Due to parametric instability, nearly total input energy at fundamental frequency can be converted into HH or subharmonic vibrations of the defects. Both super- and subharmonic LDR are strongly localized in the defect area that provides a background for highly-sensitive defect-selective imaging.