Noncontact fatigue crack visualization using nonlinear ultrasonic modulation

• A complete noncontact fatigue crack visualization technique based on nonlinear ultrasonic wave modulation is developed.
• Micro and macro-cracks are successfully visualized by scanning.
• The main source of nonlinear modulation is discussed based the visualization results and the microscopic images.

This paper presents a complete noncontact fatigue crack visualization technique based on nonlinear ultrasonic wave modulation and investigates the main source of nonlinear modulation generation. Two distinctive frequency input signals are created by two air-coupled transducers and the corresponding ultrasonic responses are scanned using a 3D laser Doppler vibrometer. The effectiveness of the proposed technique is tested using aluminum plates with different stages of fatigue crack formation such as micro and macro-cracks. Furthermore, the main source of nonlinear modulation is discussed based on the visualization results and the microscopic images.