Ultrasonic is a term used to describe the range of sound waves that possess frequencies higher than that of audible human hearing. Ultrasonic testing (UT), therefore, involves using these sound waves to inspect the structural integrity of various materials.
This technique, also known as ultrasonic nondestructive testing (NDT), has the ability to detect the presence of discontinuities or anomalies in test specimens by emitting high-frequency sound waves through the specimen. Most ultrasonic tests use sound waves ranging from 500 kHz up to 20 MHz.
To fully understand how this technology accurately detects corrosion, it is essential to have a basic understanding of the mechanisms involved in the testing method.
High-frequency sound waves are considered to be very directional, propagating uniformly through almost any material (e.g., steel, concrete or plastic) until it encounters another medium, also known as a boundary.
When the traveling sound wave impacts this boundary, a portion of the wave is reflected back to the source, while the remaining waves continue to be transmitted through the object. This is due to the acoustic impedances between the two mediums.
A typical UT inspection system consists of three main components: the pulser/receiver, the ultrasonic transducer and the display. The pulser/receiver is an electronic device capable of producing high-voltage electrical pulses.
This voltage powers the transducer, which generates high-frequency ultrasonic waves. When these waves impact a boundary (usually in the form of a material change), it gets reflected and is converted to an electrical signal by the transducer to be displayed on a screen.
The time for the sound wave to travel back to the transducer is directly proportional to the distance traveled by the signal. This information can then be used to deduce the thickness of the object, as well as the location, orientation and dimensions of pipeline defects.
The calculation of the thickness is a direct measurement compared to other non-destructive techniques such as magnetic flux. (Another nondestructive inspection method is discussed in Using 3D Laser Analysis for Nondestructive Testing and Evaluation of Pipeline Corrosion.)
Corrosion results in the thinning of metal due to material loss, thus making ultrasonic testing ideal for detecting the presence of corrosion in pipeline systems. During pipeline corrosion detection, the ultrasonic beam travels perpendicular to the pipe wall (straight beam testing).
Compromised areas of the pipeline where thinning occurs due to uniform corrosion or pitting corrosion are easily identified and displayed on ultrasonic thickness gauge instruments.
Other types of defects, such as cracks due to fatigue and stress corrosion cracking, can be detected using another type of ultrasonic test, known as angle beam testing.