What is Ultrasonic testing?
The ultrasonic or ultrasound testing using the echo impulse method is described as a sort of material radar. An instrument introduces short impulses of very high frequency, mechanical oscillations into the material. Although the oscillations are similar to sound waves, they cannot be heard by the human ear. The similarity to sound waves is why it is called ultrasound.
The impulses spreads out in the material as a relatively short beam, until they reach and end surface, for example the bottom or end of the material or more importantly, an internal flaw or crack. From here the impulse is reflected fully or partly back to the transducer which now acts as a receiver.
The time interval between sending and receiving the impulse is depending on the distance traveled inside the material, as the velocity of which it spreads is constant. Changes in the echoes can then be used to indicate and locate internal cracks and flaws.
If you send an impulse in a straight angle into the material and measure the time interval, you can calculate the thickness of the material. This is the principle used by Ultrasound thickness gauges.
When locating internal flaws in a component, besides knowing the distance traveled, it is imperative to know where the impulse originates from and in which direction it has moved. Bigger flaws can be located by moving the transducer (or probe) across the material surface.
History of Ultrasound testing:
Testing with ultrasound impulses has been used for many years in the shape of listening to the tone an object like a coin, cup or wheelbandage would give after being hit to make it oscillate, and thereby "ring out" for the human ear to hear.
During the second world war, the development of electronics was greatly accelerated and among other things, the radar-system was invented.
After discovering that certain pieces of crystals, quartz in particular, could be set into mechanical oscillations by introducing an electrical alternating current to the surface of the material, it enabled the production of the first ultrasound devices which applied the impulse-echo method.
Fully funtional devices was introduced to the industrial market in the early 1950s. Subsequently they have been upgraded and improved in many ways but the basic principle still remains the same; Introduce mechanical impulse oscillations into the material and listen for the echo to determine changes and flaws within or simply calculate distance covered by the echo.
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