How are fractures in airplane structures detected by ultrasound?

[pgirl1729]

TL;DR Summary

How does fractures in airplanes detected by ultrasound?

For applications of ultrasound, detecting fractures using ultrasound is listed in one of my textbooks. According to that, ultrasound cannot travel through air right after it travelled through solid parts of the plane. Therefore if there was a fracture, the sound was supposed to be reflected back. How does it happen exactly? I don't have much knowledge about it. Is it because solid has the ability to cope with the high frequency of ultrasound, but air doesn't? apparently making it go back?

 

[Baluncore]

Welcome to PF.

When you strike a good bell, it will ring. If it has a crack, it will sound dead. Energy is absorbed or reflected from where there is a difference in the mechanical properties of the material.
https://en.wikipedia.org/wiki/Ultrasonic_testing
https://en.wikipedia.org/wiki/Impedance_matching#Acoustics
https://en.wikipedia.org/wiki/Acoustic_impedance

 

[Ibix]

I think the key reference in Baluncore's post is the one on acoustic impedance. Different materials have different impedances, and where there's a change you get some reflection.

So I think the OP's book is over-stating it when it says "ultrasound cannot travel through air right after it travelled through solid" (my bold). Actually, it's only difficult for sound to exit from the solid, but that does mean that very little energy travels into the air gap. The rest reflects, revealing the presence of the crack.