Ultrasounds are sound waves with a frequency higher than the upper limit of the human hearing range. Sounds propagate in solids and liquids at much higher speed than air. Ultrasounds can be used to measure distances or find defects in different materials.
The most common use for ultrasounds in non-destructive testing is for flaw detection. Often called Ultrasonic Testing or UT, the technic involves pulsing sounds in a material to measure the thickness of the object or find flaws inside the material. In a homogenous material, the sound propagates fluidly and without anomaly. When there is a flaw, like a crack or an irregularity, the sound is reflected by the defect and the flaw can be detected. It is then possible to detect flaws that are inside of a material and consequently invisible to the human eye.
Inspection can be manual or automated. It is an essential step for many manufacturing processes, including inspection of metal structures, plastics and aerospace composites. The sound frequency used to inspect a material is dependent on the material properties the thickness of the object and the type of flaws that need to be detected. Frequency varies between 0.5 and 35 MHz for most dense material.
Ultrasonic Testing of welded joints is the alternative to radiography for non-destructive testing. It is much safer and convenient than the use of ionizing radiation. UT of a weld can detect the existence of flaws, measure their size, and define their location.
To emit sounds at high frequency, the sound need to be generated by a transducer. A transducer is a device that can convert electricity into sound waves by vibrating at a natural frequency. A transducer used to emit sounds is called a pulser. A transducer can also be used to convert sound waves back to an electricity signal. It is then called a receiver.
The same transducer can be used to pulse and receive. It is a very common use for thickness measurement. The transducer will emit a short pulse. The sound will travel from one side down to the other side of an object, and come back. The time between the initial pulse and reflected pulse is converted into a distance unit and the thickness can be measured. If a flaw exists in the trajectory of the sound beam, a sound wave will be reflected within a shorter time and the new signal will reveal the presence of the flaw and its depth location.