Imaging of contact acoustic nonlinearity using synthetic aperture technique

• A technique for imaging of contact acoustic nonlinearity at closed interfaces was developed.
• Variation in the second harmonic magnitude at the contact interfaces as the contact pressure was visualized.
• Variation in the acoustic nonlinear parameter at the contact interfaces as the contact pressure was visualized.
• This technique will be useful to detect the closed cracks and improve the sizing accuracy.

The angle beam incidence and reflection technique for the evaluation of contact acoustic nonlinearity (CAN) at solid–solid contact interfaces (e.g., closed cracks) has recently been developed to overcome the disadvantage of accessing both the inner and outer surfaces of structures for attaching pulsing and receiving transducers in the through-transmission of normal incidence technique. This paper proposes a technique for B-mode imaging of CAN based on the above reflection technique, which uses the synthetic aperture focusing technique (SAFT) and short-time Fourier transform (STFT) to visualize the distribution of the CAN-induced second harmonic magnitude as well as the nonlinear parameter. In order to verify the usefulness of the proposed method, a solid–solid contact interface was tested and the change of the contact acoustic nonlinearity according to the increasing contact pressure was visualized in images of the second harmonic magnitude and the relative nonlinear parameter. The experimental results showed good agreement with the previously developed theory identifying the dependence of the scattered second harmonics on the contact pressure. This technique can be used for the detection and improvement of the sizing accuracy of closed cracks that are difficult to detect using the conventional linear ultrasonic technique.