What’s the difference between UT and radiography in NDT?

Ultrasonic testing (UT) and Radiographic testing (RT) are both important methods of non-destructive testing. Although they’re both used to detect flaws in components, they have different strengths and weaknesses. Let’s look at the main difference between UT and radiography first, before delving deeper.

The difference between radiography and ultrasonic testing

The difference between radiography and ultrasonic testing is that radiography provides a direct image of internal defects using radiation, whereas ultrasonic testing uses sound waves to infer the presence of defects based on wave reflections. Both are types of non-destructive tests and both require the NDT technician to interrupt the result.

So, now let’s unpick the two types of tests.

Ultrasonic testing

Ultrasonic testing is a non-destructive testing technique that uses high-frequency sound waves to detect flaws in materials. The waves are generated by a transducer, which is placed against the surface of the material being tested. The transducer converts the electrical energy from the generator into sound waves, which travel through the material.

If there are any flaws in the material, they will reflect some of the sound waves back to the transducer. The transducer converts these reflected waves back into electrical signals, which are then displayed on an oscilloscope. If there are any flaws within the material, the sound waves bounce back at different times and frequencies, meaning the oscilloscope can build up a picture of the interior structure of the material. 

There are a variety of ways to apply the principles of UT testing. Different technologies that use the UT method have been developed that allow technicians to test different for flaws in different circumstances. These technologies include Phased Array, Time of Flight Diffraction and standard Ultrasonic Testing

1. What is standard UT testing?

Regular UT testing involves using a single hand-held probe. The sound waves are released from the probe and return to the same place. This type of UT testing can be used to determine material thickness (for example when testing storage tank shells), but can also be used to test welds and smaller components. As the probe is manual, it needs to be manipulated by the technician to ensure that the whole component has been thoroughly checked. 

2. What is Phased Array ultrasonic testing?

Phased Array differs from standard UT testing in that it uses multiple transducers rather than one to direct a series of waves through the material being tested. This allows the technician to get a more accurate picture of the internal structure of the material as the transducers can be made to release sound waves at certain times, sweeping the material more thoroughly. 

Phased Array transducers can be controlled by a computer, so the probe head can be mounted on robotic crawlers, allowing technicians to map large areas that are difficult to reach.

3. What is Time of Flight Diffraction ultrasonic testing?

Time of Flight Diffraction (TOFD) differs from standard UT testing in that it only needs to be moved along the weld seam being tested. In comparison, standard UT probes need to be moved perpendicular to the weld as well as along the weld to get a better impression of its integrity. TOFD uses a transducer head that includes a transmitter and a receiver. The sound wave is released from the transmitter and then bounces back from the flaw (or back surface of the material) into the receiver. The time it takes to make these bounces is used by the computer to build up a picture of the internal integrity of the material. 

Radiographic testing

Radiographic testing is a non-destructive testing (NDT) method that uses x-rays or gamma-rays to examine an object for flaws. The technique is similar to an X-ray taken at the doctor’s office, but instead of looking at bones, radiographic testing looks for defects in materials.

Radiographic testing is often used to inspect welds, castings, and other metal parts for cracks or voids. The test can be performed on both ferrous and non-ferrous materials. To conduct the test, a radioactive source is placed on one side of the material being tested and an x-ray film is placed on the other side. The film is then developed and examined for any defects.

The difference between UT and radiography

1. Material thickness

Although both tests can be used on a variety of materials and in a variety of situations, one benefit of radiography over ultrasound as a nondestructive test is that radiography is not limited to any minimum material thickness or material type. An ultrasound test would be a better option for really thick materials, as the sound waves travel instantaneously, whereas thicker materials would require longer exposure times using Radiography. Ultrasound can be limited in use depending on the material grade.

2. Consumables

Until recently, Radiographic testing has relied on consumable items such as films and chemicals to develop the image. These items have to be replaced once used and create waste. Ultrasonic testing does not use consumables, although the probes will wear down after time and need replacing.

Recently though, digital technology has improved, allowing firms to develop Radiographic technology that does not require the use of consumables. Instead, images are captured on a phosphor imaging plate and then scanned into a computer. This means that the plate can be reused and the image stored and accessed digitally. 

Find out more about how we use digital imaging for our radiography tests in our recent case study.

3. Safety

One of the biggest challenges in Radiographic testing is the need to manage the health risk associated with using a radioactive source. Testing has to happen within large “bays” that contain the radioactive source and protect anyone outside from x-ray and gamma-ray exposure. If radiographic testing has to take place onsite, whole areas need to be shut down and evacuated before testing can take place. 

In comparison, UT testing is completely safe for humans to use, and requires no protective equipment or safety measures. 

Once again, though, recent developments in Radiographic testing technology have improved the challenges involved in using the technique. For example, new portable radiographic systems are designed to reduce the containment areas required, meaning only small sections of the site need to be evacuated at any one time. Some allow for containment areas as small as 2 metres, meaning that operational disruption is reduced. 

Find out more about how we used our new radiographic system to reduce operation disruption. 

Both are vital for safety

Both UT testing and Radiographic testing have their uses and are vital non destructive testing services. New technology has allowed Radiographic testing to be almost as portable and safe to use as Ultrasonic testing, meaning both techniques can be applied with less cost and disruption to the client. 

Responsive testing that delivers assurance that your equipment is safe.

Contact us for a quality testing service with a fast report turnaround. We can respond to any challenge.


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