How are fractures in airplane structures detected by ultrasound?

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SUMMARY

Ultrasound is effectively used to detect fractures in airplane structures by leveraging the principles of acoustic impedance. When ultrasound waves travel through solid materials and encounter a fracture, the difference in mechanical properties causes a significant reflection of sound waves. This reflection occurs because solid materials can transmit high-frequency ultrasound, while air cannot, leading to minimal energy transfer into the air gap. The detection process relies on the principle that a cracked structure will absorb or reflect sound differently compared to an intact one.

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pgirl1729
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TL;DR
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?
 
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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.
 
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