Design of passive acoustic hyperbolic-shaped filter for nonlinear ultrasonic inspection method optimisation

Nonlinear ultrasonic methods for non-destructive evaluation and damage detection rely on the measurement of nonlinear elastic effects, such as amplitude of the second harmonic in the frequency response of the sample, to reveal the presence of surface and internal cracks of various scale and nature. These methods require an amplification system and a high sensitivity ultrasonic transducer to measure nonlinear features, since harmonics are typically an order of magnitude lower than the fundamental frequency. In this work, we investigated various geometrical filters to amplify nonlinear signals and improve nonlinear air-coupled inspections: Hyperbolic, cylindrical and conical duct. A 40 kHz ultrasonic speaker and standard Air Coupled ultrasound system arranged with 88 transmitting elements and 1 receiving element were used to conduct the experimental test. The results show that the passive hyperbolic-shaped filter was able to increase the second harmonic response of the damage region of 5db, compared to standard nonlinear inspections, and increases the signal to noise ratio of the measured signal of 11 db. Results shows that higher harmonics generated from instrumentation highly decrease as the wave propagates through the converging horn. Air-coupled inspection confirms the increase at the damage location of the second harmonic of the wave propagating though the diverging horn. The proposed setup could allow more accurate nonlinear air-coupled inspection of complex materials.