Zetec, Inc

Does Ultrasonic Testing Have Thickness Limitations?

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Courtesy of Zetec, Inc

Ultrasonic testing has become one of the most common and effective methods of nondestructive testing. Without the dangers posed by radiography, the cumbersome setup of liquid penetrant testing, or the surface-level limitations of visual inspection, ultrasonic testing allows fast, simple inspections of many materials. Ultrasonic inspection also achieves deeper material penetration than most other NDT methods can typically offer.

Ultrasonic testing does have thickness limitations, but these are difficult to reach. For this reason, one of the primary uses of ultrasonic testing is actually to gauge the thickness of materials. For example, a hull plate or storage tank wall that appears thinner than expected can indicate corrosion and may require replacement.

However, ultrasonic testing is not a panacea. Depending on the type of materials being scanned and their geometry, even surprisingly thin materials can sometimes pose challenges. To understand where and when ultrasonic scanning can be best applied, it’s important to know how the technology uses sounds waves to map the interior of objects. 

What Causes Thickness Limitations for Ultrasonic Testing 

Ultrasonic testing uses piezoelectric transducers to convert electrical impulses into mechanical vibrations. After the vibrations have passed through the object being inspected, these same piezoelectric elements translate the mechanical vibrations back into electricity, which is interpreted by software embedded in ultrasonic testing equipment to map the object’s interior. Vibrations can be detected by the same crystals that induced them using the pulse-echo technique, or by different crystals on the other side of the object using time-of-flight-diffraction (TOFD).

Piezo- is borrowed from the old greek, and roughly means “to squeeze or press.” Piezoelectric transducers are crystals that, when squeezed or pressed, produce electricity. Conversely, they can convert electricity into pressure. This pressure produces the mechanical vibrations that travel through the pipe weld or train track being scanned.

Mechanical vibrations? Isn’t ultrasonic made of high-frequency sound waves? Yes to both. Sound as we know it is a mechanical vibration of air. Our ears detect changes in air pressure, which they convert into electricity, which we use to make a map of our surroundings. That’s how people know a train is coming before they see it. Sound familiar? The high-frequency waves created by ultrasonic transducers travel through metals and composites the way lower-frequency sound carries to our ears. 

Obstacles to Ultrasonic Mapping

If you’ve ever tried talking to a friend on the other side of an open field, you’ll know that sound only carries so far. If you tried shouting at that friend, you’d know that increasing the power of a vibration increases it’s carrying distance. If you cupped your hands to your mouth or rolled up a newspaper into a cone, you’d know sound carries farther when focused. If there was a wall in the way, or you were shouting in the forest, it’d be obvious that different materials carry sound differently, or don’t carry it at all. If you heard an echo off of a wall you couldn’t see, you might get a good idea of how far away it was.

All of the above is true for ultrasonic NDT. More powerful pulses of ultrasound carry farther. Focused beams travel farther. Changes in materials change vibration frequencies. That includes large changes, like reaching the other side of a turbine blade. It also includes small changes, like the air in a crack or a void in a rotor shaft. When the piezoelectric crystals translate these back into electricity, the different vibrations become visible to the equipment’s processor, and the crack or void shows onscreen.

If an ultrasonic wave runs into a wall or a variety of coarse grains and complex geometry, the sound will be scattered and difficult to read. “Walls” can be layers of dissimilar materials, such as plastic, wood, or composite backing to a metallic object. Curved surfaces can likewise pose difficulties.

On the other side of the field, ultrasonic testing can also exhibit minimum thickness limitations. Materials whose thickness is measured in millimeters or decimal millimeters can be difficult for ultrasonic equipment to adequately map, as there is very little room for waves to travel through. With specially designed probes and effective software, most common materials are thick enough to measure, however. Thinner conductive materials can be measured by eddy current technology; nonconductive materials may require radiography.

Advanced UT Equipment Offers Superior Depth Penetration

So, while ultrasonic testing does have thickness (and thinness) limitations, these are difficult to reach in practice. Advanced ultrasonic testing equipment has been designed to emit waves with the frequency and velocity needed to travel through all common applications. With greater depth penetration than other NDT methods, ultrasonic will be the best option for thicker materials.

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