Local defect resonance (LDR): A route to highly efficient thermosonic and nonlinear ultrasonic NDT

Abstract

The concept of LDR is based on the fact that inclusion of a defect leads to a local drop of rigidity for a certain mass of the material that should manifest in a particular characteristic frequency of the defect. A frequency match between the driving ultrasonic wave and this characteristic frequency provides an efficient energy pumping from the wave directly into the defect. For simulated and realistic defects in various materials the LDR-induced local resonance increase in the vibration amplitude averages up to ∼ (20–40 dB). Due to a strong resonance amplification of the local vibrations, the LDR-driven defects manifest a profound nonlinearity even at moderate ultrasonic excitation level. The nonlinearity combined with resonance results in efficient generation of the higher harmonics and is also used as a filter/amplifier in the frequency mixing mode of nonlinear NDT. The LDR high-Q thermal response enables to realize a frequency-selective imaging with an opportunity to distinguish between different defects by changing the driving frequency. The LDR-thermosonics requires much lower acoustic power to activate defects that makes it possible to avoid high-power ultrasonic instrumentation and proceed to a noncontact ultrasonic thermography by using air-coupled ultrasonic excitation.