Ultrasonic Testing: A Guide

Ultrasonic Testing is the use of sound waves to inspect the thickness of a material.

Ultrasonic Testing (UT) is also often called Ultrasonic Thickness Measurement (UTM). It is most commonly used on metal, because metal conducts sound waves in a manner that supports this kind of measurement.

This method is commonly used by inspectors as one of many Non-Destructive Testing (NDT) testing methods, allowing them to collect information about the condition of an asset without having to damage it.

[Ultrasonic testing is just one of the non-destructive testing (NDT) methods that inspectors use. Learn more about NDT in this in-depth guide.]


Ultrasonic testing was first developed after the Titanic sank in 1912. Researchers wanted to identify ways that ships could find icebergs before they could see them, and they began testing sound waves for this purpose.

After these tests, UT was developed further during World War I as a way to find submarines.

It wasn’t until 1928 that scientists started testing UT for industrial purposes, when a Soviet researcher named Sergei Sokolov found that soundwaves could be used to identify defects in metal materials. Over the next several decades the technique gained wide adoption, and since then it has become one of the more common NDT methods.

Here is a menu for this guide to UTM, in case you’d like to jump around:

What Is Ultrasonic Testing?

To get more specific, ultrasonic testing uses high-frequency sound waves (typically 500kHz-20Mhz) to find defects in materials by measuring their thickness.

In ultrasonic testing, an inspector will use a probe or some other kind of transducer to send sound waves through the material they want to test. If there are no defects in the material, the sound waves will pass through it, but if the sound waves hit a defect they will bounce off of it, indicating its presence.

Inspectors can use the signal from the sound waves to create a 3-dimensional visualization of the material and determine the distances between different defects found within it.

Here are the primary benefits of using ultrasonic thickness measurements in inspections:

  • Results are immediate
  • It doesn’t require very much preparation
  • It can detect defects both on the surface and beneath the surface of a material
  • It can be done on a single side of a material (i.e., it can be performed even if you can’t access the other side of the material)
  • It can be automated
  • Testing results can be shared fairly simply

Ultrasonic Testing Methods

There are three common testing methods used in ultrasonic thickness measurement:

  • Through transmission.Through transmission employs two transducers, each placed on opposite sides of the material being tested. One of the transducers creates a pulse and the other receives it. If there is a disruption in the pulse, inspectors will know a defect is present in the path between the two transducers.
  • Pulse echo. Pulse echo is more sensitive than through transmission. It is used to identify defects by measuring the time it takes amplitude signals to travel between different points or surfaces in a material.
  • Resonance. Resonance is similar to pulse echo, except that with resonance testing the regularity of transmission can be changed. Resonance testing is primarily used when only one side of a material can be accessed.

The Elios 2 is a drone created just for monitoring and inspections in confined spaces. Want to learn more about how it works? Watch a demo now.


Where Is Ultrasonic Testing Used?

The basic principle of ultrasonic testing is the use of sound to inspect a material’s thickness at different points. 

Ultrasonic thickness measurements can help inspectors find defects such as tiny cracks, gaps, corrosion, or other flaws in materials that are too minute to be seen by other NDT methods. It can also be used to find corrosion—if one area is thinner than another, that could be a sign that the area has been corroded, and may require maintenance.

In addition to metal, UT can also be used to test plastics, composites, and ceramics. It can also be used to test concrete but the findings may not be as reliable.

Here are some assets commonly tested using ultrasonic thickness measurement:

  • Flare stacks
  • Wind turbines
  • Large storage tanks

Her are the industries that commonly use ultrasonic testing as part of their inspection procedures:

  • Aerospace
  • Automotive
  • Electronics & Battery
  • Metals & Casting
  • Oil and Gas
  • Power Generation
  • Railroad


Ultrasonic Testing Equipment

Inspectors use several different types of ultrasonic testing equipment.

Some of this equipment is highly specialized, and may require the use of a technician trained in its use. Some companies hire third parties who are experts in the use of certain types of UT equipment, either to train internal team members on its use or to use the instruments and analyze their findings themselves.

That being said, even the most sophisticated UT equipment is usually easy to care and use, and highly reliable.

Here are the most common types of ultrasonic equipment that inspectors use:

  • Ultrasonic transducers and probes. Transducers are used in several types of ultrasonic thickness measurement, including weld testing and gauging thickness. Types include phased array, immersion, and contact transducers.
  • Flaw detectors. Field-tested portable ultrasonic testing solutions for fast, accurate inspections for internal product integrity, searching for defects, cracks, and other discontinuities. Flaw detectors are portable, powerful, and sensitive, allowing inspectors to penetrate materials at a considerable depth.
  • Thickness gauges. Ultrasonic thickness gauges are commonly used to inspect the thickness of various metals, including brass, steel, nickel, and lead, among others. Thickness gauges can be especially helpful for identifying corrosion.
  • Automated UT systems. Automated ultrasonic thickness systems are systems that can be put in place and collect ultrasonic readings without an inspector physically present, allowing data collection that can be useful for the longevity of an asset even when someone isn’t there. These systems are commonly used to monitor pipes in the Oil and Gas industry.

Drones and Ultrasonic Testing

Drones are typically used for visual inspections—that is, to collect visual data reflecting the condition of an asset—but they’re also starting to be used for UT.

Two big benefits to using a drone to collect ultrasonic thickness measurements instead of a person are safety and savings.

For safety, a drone can be used in scenarios that would require a person to work at height using a lift, catwalk, or scaffolding. Using the drone to collect UT data removes the need for a person to endanger themselves with this kind of work.

For savings, a drone can potentially collect UT data more quickly than a person, and also helps avoid the need for scaffolding, which can be costly and time-consuming to erect and take down, driving up costs through prolonged downtimes.

Ultrasonic testing by drone is still in its infancy. Right now, one of the only drone companies that offers it is Appelix.



Photo credit: Appelix

To perform UT by drone, the drone must apply a couplant to the surface that will be tested in order to optimize it for propagating soundwaves.

As drone technology progresses, we are sure to see more inspection drones offering ultrasonic testing.

Learn more about how the Elios 2 helps provide high quality visual data for indoor inspections.

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