Comparison Guide: UT v.s. PAUT

How often do you feel handicapped trying to differentiate between ultrasonic testing and phased array ultrasonic testing? Believe it or not, these two testing methods are different.

Most individuals in the non-destructive testing industry, including some experienced technicians, often make the mistake of misrepresenting either of the two quality assessment methods as the other. The confusion often stems from the common theme of using ultrasounds in the test. However, regardless of this similarity, the difference in the exact processes involved in both testing methods makes each method unique and very different from the other.

This guide explains all you need to know about the individual uniqueness of the testing methods and clears all your doubts right from the get-go.

So, if you are ready to know more, let’s dive into it.

What is Ultrasonic Testing?

To begin with, ultrasonic testing is a non-destructive testing method that involves the projection of high-frequency sound waves onto an object to detect defects and other anomalies in the object. Additionally, these high-frequency sound waves are reflected on touching the object’s surface. This leaves the ultrasonic device to pick up the signal and convert them into electrical forms that are easy to analyze. 

The primary component of the ultrasonic testing device that ensures its accuracy is the transducer, an element for converting back and forth between sound signals to electrical signals. Consequently, the ability to acquire more of the reflected sound waves significantly affects the efficiency of the ultrasonic testing device. Likewise, the conversion ability of the transducer to convert sound to electrical signal and back will also significantly impact the accuracy of the ultrasonic testing equipment.

Ultimately, the flaw detection ability of an ultrasonic testing device relies on the quality of sound waves acquired and the conversion strength of the device for a proper analysis of the signal. 

Features of Ultrasonic Testing

To state the fact, all the features of ultrasonic testing primarily revolve around the transducer as it performs a significant part of the testing. The main objective of the ultrasonic test is to pick up sound signals and be able to convert them into electrical signals that are analyzable for accurate detection of flaws. By and large, the transducer carries out this objective by sending out high-frequency signals, picking them up when the object reflects it, and afterward converting them back into electric signals.

Also, ultrasonic testing features high-frequency sound wave generation. Sound frequencies that ultrasonic testing devices can detect range from 0.1 MHz to 15 MHZ and sometimes even higher frequencies. In addition, ultrasonic testing features short waves that a single piezoelectric device can detect in a transducer, making it unique from other types of ultrasonic testing. 

Lastly, ultrasonic testing features a monolithic construction, which means all the signal-generating and receiving elements are housed in the same container. 

Calibration Method of UT

Generally, calibration is required to standardize the measurement and the flaw-detection ability of an ultrasonic testing device to ensure that its accuracy meets set requirements. Hence, the importance of calibration. Usually, ultrasonic testing calibration involves the time-based methods of ultrasonic velocity and zero offset, which compares the time taken to by the ultrasonic sound waves to reach the object inspected and the time of its return to the receiver. 

For ultrasonic velocity as a time-based calibration method, a constant value is set relative to the sample material, and the operator sets this value. However, this value varies with temperature change and, as such, needs to meet certain conditions to be an effective calibration method. These conditions include an assumption that it’s an elastic and nondispersive material and the temperature and pressure of the atmosphere are constant. Also, it is assumed that a high enough frequency exists with as much lesser wavelength than the material.

The zero offset calibration method, on the other hand, considers the travel time of the wave from production to contact with the material layer. 

What is Phased Array Ultrasonic Testing?

Unlike the conventional ultrasonic testing method, which naturally has one single-element transducer, the phased array ultrasonic testing is usually associated with multiple or an array of transducers. Hence the name. The number of elements in the transducer for a PAUT is often a minimum of 16 elements and can go as high as 128. Consequently, these transducers can be programmed or sequenced in a particular pulsing pattern to achieve a high result accuracy depending on the type of flaw anticipated or material properties. Still, these elements have more sensitivity than conventional ultrasonic testing, making PAUT preferable to conventional ultrasonic testing. 

Features of Phased Array Ultrasonic Testing

The features of a phased array ultrasonic testing often include a multi-element transducer. In general, the number of these elements varies depending on certain factors. These factors include the beam angle required, beam spot size, and focal distance. Usually, the number of elements ranges from between 16 to 128. Nonetheless, a phased array transducer with 256 elements is also possible. 

Another feature is the ability to steer and shape the beam, which allows for a lot of flexibility and accuracy that are not possible with single-element transducers. Also, phased array ultrasonic testing often features angle beams. These angle beams can come with either a straight plastic shoe, plastic wedge, or a delay line. 

Similarly, ultrasonic testing features also include beam steering, which often increases as the elements’ width reduces and vice versa. Furthermore, the cost increases as the number of elements increases, especially when the coverage area is large. 

➤ Related Article: Difference: PAUT v.s. TOFD

Calibration Method of PAUT

For phased array ultrasonic testing (PAUT) calibration, a similar time-based method as obtainable with conventional ultrasonic testing is used. However, the material and transducer requirements for carrying out the calibration are different for PAUT. These calibration requirements include material requirements like material velocity, part thickness, and radius of curvature. Also, transducer requirements for the calibration include frequency, zero offsets, amplitude response, number of elements, pitch, and angle of the beam. The last requirement for calibration is the wedge requirements, including incident angle and sound velocity through the wedge, beam index point, and the height offset for the first element are all critical to the accuracy of the calibration process.  

UT vs. PAUT: the Showdown

Here is the thing, comparing ultrasonic testing with phased array ultrasonic testing comes with many similarities and differences. However, it is the differences that make them unique from one another. 

A few of these similarities include: 

• Both are non-destructive testing methods.
• They both utilize ultrasonic sound and signals in identifying flaws 
• Both use the same calibration methods 
• They can identify tiny or hidden flaws that are often difficult for other testing methods like the visual inspection to identify. 

However, there are still differences between these two test methods, including 

• The difference in the number of piezoelectric elements in the transducers where PAUT has more elements
• Phased Array Ultrasonic Testing supports beam steering, which increases the ability to control the beams for higher accuracy. This feature does not apply to conventional ultrasonic testing.
• For PAUT, the angle and signal patterns can be programmed, unlike the UT, which can only be operated manually.
• The transmitter and receiver are often housed in the same container as conventional ultrasonic testing. But for the phased array ultrasonic testing.
• PAUT has a higher inspection speed and accuracy than UT.

Conclusion

To wrap up, ultrasonic testing and phased array ultrasonic testing have similarities in their primary working principle. However, the similarities end as factors like incident angles, focal points, and the number of transducer elements are very different for both testing methods. 

In any case, the phased array ultrasonic testing method has become the preferred testing procedure for many professionals as it offers many advantages over UT. Still, an understanding of the working principles, their features, and calibration methods for both testing methods can help you identify the suitable testing method for your peculiar quality assessment situation.