This project uses ultrasonic non-destructive testing (NDT) technique to detect any faults on the surface of any object. If a fault is detected, it lights up an LED and beeps a buzzer. The project uses HC-SR04 ultrasonic sensor module, which produces 40kHz ultrasound. The wavelength of ultrasound in air is ~0.85cm and since detection requires fault's size to be comparable with the wavelength, such a low frequency wave will not be able to detect a micron-level fault.
YouTube Link: https://www.youtube.com/watch?v=dQkqs5RaGr8