The video added to this post shows a part of the Philae spacecraft that descended onto a comet earlier this week. At around 2:30 into the video, it shows us the important part for this post: the SESAME research part.
Sesame is a great example of the practical applications for acoustic emission as a non destructive test method.
Each landing foot of Philae has acoustic emitters and receivers. Each of the legs will take turns transmitting acoustic waves (100 Hertz to KiloHertz range) into the comet which the sensors of the other legs will measure. How that wave is attenuated, that is, weakened and transformed, by the cometary material it passes through, can be used along with other cometary properties gained from Philae instruments, to determine daily and seasonal variations in the comet’s structure to a depth of about 2 meters.
To clarify this and explain acoustic emission: an emitter produces sound waves which get reflected or absorbed by the object we’re testing. The reflected waves are captured by the receivers and based on these patterns, it’s possible to reconstruct the object, determine the material and properties of the object or even detect small cracks and other imperfections in the object.
This last application, detection of imperfections, is often used for rods or ball bearings to trace fatigue before the component fails and could cause damage to other components of the machine.
Do you see other useful applications for this technique? Leave it in the comments.