Ultrasonic Testing (UT)

The inspection technique known as ultrasonic testing (UT) has a long and varied history. Much of the current understanding of wave motion came from the study of acoustics. Ancient Greek philosophers hypothesised that there was a connection between waves and sound and that vibrations, or disturbances, must be responsible for sound. Pythagoras observed in 550 BCE that vibrating strings produced sound and he worked to determine the mathematical relationships between the lengths of string that made harmonious tones.

Scientific theories relating to wave propagation became more prominent in the 17th century, when Galileo Galilei (1564-1642) published a clear statement regarding the connection between vibrating bodies and the sound they generate. In his work Principia (1686), Isaac Newton published a mathematical description regarding how sound travels. In the 18th century, French mathematician and scientist Jean Le Rond d’Alembert derived the wave equation, a thorough and general mathematical description of waves that laid the foundation for generations of scientists to study and describe wave 

It was during the 1930s and 1940s that our understanding of sound propagation was translated into a useful inspection method known as UT, with the need to test materials volumetrically. The dangers of radiography were understood and the need for protection from this invisible enemy was recognised. Ultrasound did not appear to pose any danger and could be transmitted into steel and other materials to look for flaws, both at the manufacturing stage and in service. Nowadays, manual UT, phased array UT, time-of-flight diffraction (TOFD) and long-range UT (LRUT) are in daily use in 

On the railways, wheel tappers once listened to that dull sound that comes from a defective wheel. This technology is still in use in concrete structures and aircraft using the ‘toffee hammer tester’. Other basic tests include the tapping of tanks to see if they have any fluid inside. I hope that the certification bodies do not 
read this, or we will all have to undertake annual hearing tests along with annual vision tests!

Where will sound take us next with inspection and condition monitoring techniques? One thing is for sure: it will be on a wave with its peaks and 


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