TESTING & MEASUREMENT
A Layman’s Guide to Ultrasonic Inspection Non-destructive testing (NDT) is a large and increasingly multifaceted discipline, encompassing a wide range of techniques such as magnetic particle inspection, radiography, ultrasonic testing, eddy current testing and more. As of the end of 2020, Ultrasonic Testing (UT) was the most commonly used method, accounting for 27%1 of the total NDT equipment market share. In this article you’ll find a brief overview of the history, fundamentals, and current capabilities of UT as a technique. A Short History of Ultrasonic Testing Ultrasonic testing’s roots can be traced back to the classic ‘tap test’ of historical blacksmiths and metalworkers. In these tap tests, an experienced craftsman would give test pieces a firm whack with a hard object, such as a hammer or coin, and then listen for the resulting ring to tell whether or not the object
was ‘sound’ from the sharpness, tone and other sonic qualities of the produced noise. While this is a long way practically from the scientific approach of modern UT, the concept of utilising sound to test materials was the driving constant throughout its subsequent development. The development of modern ultrasonics really got underway in the 1940s and 50s, with a marked acceleration in sophistication following the electronic revolution at the tail end of WW2. In 1945 Floyd Firestone, an American physicist specialising in acoustics, patented a device built from modified RADAR instruments called ‘The Supersonic Reflectoscope’2 which he described as ‘a means of inspecting the interior of solid parts for flaws’. The Reflectoscope operated in a ‘pulse-echo’ configuration using a single quartz crystal search unit which both sent and received, which was notable in comparison to his contemporaries who made use of separate sending and receiving units. The search unit emitted a 2.25MHz ultrasonic pulse into materials and then plotted the amplitude of returning echoes against time on an oscilloscope screen, making the Reflectoscope arguably the world’s first A-Scan flaw detector. Firestone’s invention was a breakthrough moment for the field of ultrasonic testing, but it would take a number of years and a lot of innovative industrial design before UT developed into the mature and reliable inspection technique it’s considered to be today. For this to happen, certain technologies needed to be pioneered first, such as the discovery of high-power composite crystals to replace quartz or digital electronics to replace the analogue valve based oscilloscope systems used in the 40s.
Floyd Firestone with his A-Type Supersonic Reflectoscope
What is Ultrasound? Fundamentally, sound is just the name for a type of mechanical wave that travels through solids, liquids and gases by making their particles oscillate, or vibrate, between 20 and 20,000 times a second (Hz). Because this frequency region is what the human ear evolved to detect, it is also known as audible sound. Ultrasound, therefore, refers to the region of frequencies that are too high, i.e. that vibrate too fast, to be detected by the human ear. Conversely, Infrasound is the range of frequencies that are too low to be detected by the human ear. The waves used for non-destructive testing are extremely high pitched when compared to the audible range. The lowest commonly used frequency is around 1 MHz (1 million Hz), which is fifty times higher than the highest sound you’ll ever hear. For specialist applications like scanning acoustic microscopy the frequencies employed can go all the way up to a staggering 2,000 MHz3! The important trade-off engineers consider when it comes to their choice of ultrasonic frequency is penetration vs. resolution; as ultrasonic waves increase in frequency they can be used to make out much smaller features within a material, but will lose much of their ability to penetrate deep into them. Conversely, lower frequency ultrasound will have a much easier time penetrating material, but have much lower resolving power. The most commonly used frequencies within industrial testing sit within the 0.5MHz to 20MHz range. The bottom end of the range is employed when testing composites and highly attenuative polymers while the top end is more useful for very thin metals and high-precision applications like bolt tension measuring. CONTINUED ON PAGE 28
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MAY 2022
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