"Ultrasound" is more commonly used in medical terminology than in industry. In our segment of the industry it actually is more commonly termed "Ultrasonic" and is one of several disciplines in the field known as Nondestructive Testing (NDT esting, NDI nspection or NDE valuation). Using high frequency sound waves that are sent into or (propagated) into a material of unknown quality, one can determine indirectly the internal quality of the object being inspected without damaging it. Ultrasonic waves which are send into a part are either reflected back like a sonar would off of a ship or measured in time which is converted into material thickess, or the lack of sound being transmitted in the case of "through transmission" ultrasonic inspection. There are many manners in which ultrasonic inspection is used to inspect critical materials.
Any complex machine that could cause damage (explode, crash) to people or property is normally inspected by a variety of disciplines. Ultrasonics, along with X-Rays for example are used to inspect airplanes, nuclear power plants, gasoline refineries, etc. Potentially dangerous situations are avoided by nondestructively examining the materials that are assembled together prior to being put into service. They are also inspected at given time intervals after being in service to ensure they do not present a danger to anyone.
One of the earliest industry specifications came about in Hartford, CT, USA to use Ultrasonic inspection to determine the thickness of a boiler tube. A boiler explosion had occured and did some major damage. Materials become corroded or damaged while in service leading to a catastrophic failure that such NDT inspections are made to avoid.
Newly manufactured parts of an airplane (Engine, Airframe, Control surfaces) for example will be inspected ultrasonically to determine the quality of material and insure that it is without defects that of a certain engineering specified size that could lead to failure. Historically aluminum has been used but now composite materials are bonded together and used alot more in Aircraft designs now. They are tested to determine bond strength using ultrasonic waves.
Most all critical parts that are found in industrial machines and systems which receive high temperatures or stresses are Ultrasonically tested to engineering specifications and certified to meet tolerances dictated by previous engineering data and destructive testing. Turbine Engines on a Jet are one of the most complex machines in the world. They contain hundreds of parts made of high nickel content alloyed materials, titanium, etc. to withstand high temperatures, corrosion and stress/abuse that are inspected with ultrasonic waves before being assembled. Once they are in service they normally cannot be inspected unless the engine is disassembled and at great cost in labor to do so.