Phased array ultrasonic testing: how does it differ from conventional UT?

Ultrasonic testing is firmly established in materials testing. Alongside X-ray testing, it is one of the most important methods for obtaining information about the interior of components and materials without damaging them during testing. Whether shrinkage cavities in castings, laminations in sheet metal, corrosion-related material loss, lack of fusion in welds or cracks in forgings, ultrasonic testing is a versatile non-destructive testing (NDT) method.

Technical progress naturally affects this field too. Phased array testing instruments entered materials testing some years ago, using technology that had already been applied in medical engineering. But what exactly is the difference between conventional ultrasonic testing and phased array ultrasonic testing?

Conventional ultrasonic testing (UT)

Conventional ultrasonic testing generally uses a single transducer element. A block of piezoelectric ceramic is excited electrically and begins to vibrate. This generates ultrasonic waves that are coupled into the component being tested. The sound waves travel through the material and encounter back walls, discontinuities, interfaces or areas without reflections. The nature of the reflected signal, or its absence, enables testing personnel to draw conclusions about the condition of the material.

The result is usually displayed as an A-scan: a graph of signal amplitude against sound path. Put simply, this display shows how long an echo took to travel and how strong the received signal is. Experienced technicians can use it to identify discontinuities, determine wall thicknesses or locate cracks. Position transducers (encoders) also allow conventional UT to produce overview images of larger inspection areas.

Limitations of conventional UT

Conventional ultrasonic testing usually employs a single probe. It has a fixed geometry, a defined size and a fixed beam angle. If testing is to be performed at a different angle or with a different probe size, for example, the probe must be changed or the test repeated.

Phased array ultrasonic testing (PAUT)

This is where phased array ultrasonic testing comes in. Instead of a single transducer element, the probe contains an array of many small individual elements, for example 16, 32 or 64. These elements are electrically and acoustically isolated from each other and can be activated individually or in groups, simultaneously or with time delays (phase shifts). This controlled activation enables different inspection strategies:

• Linear scan: in a linear scan, the active area of the probe is shifted step by step. Elements 1–8 are activated, followed by 2–9, then 3–10, and so on. This creates the effect of a virtually moving probe.
• Sector scan: in a sector scan, the elements are excited with time delays. This generates different beam angles that sweep through the component in a fan shape. An inspection area can therefore be examined at different angles in a single pass.

Advantages of the phased array method

A single phased array probe can thus perform the functions of several conventional probes with different angles and sizes. Combined with an encoder, it can capture almost all relevant information about a component in a single inspection pass and produce clear inspection images.

Another advantage is that all test data are stored in full. Every point in a graphical representation, such as a sector scan, corresponds to its own A-scan, which can be reviewed and evaluated in detail later. Comprehensive analyses are therefore possible even after testing has been completed.

Limitations and challenges of PAUT

As is often the case, technical advantages also have a downside. Phased array systems are considerably more expensive than conventional ultrasonic instruments, more complex to set up, and require extensive training and practical experience. Operating them therefore requires sound expertise in ultrasonic testing.

Conclusion: phased array versus conventional ultrasonic testing

Conventional ultrasonic testing (UT) and phased array ultrasonic testing (PAUT) will continue to coexist. While conventional UT remains a robust, proven and economical solution for many inspection tasks, phased array offers clear advantages in certain applications.

Particularly for complex geometries, demanding weld evaluation or extensive documentation requirements, PAUT already enables more information to be captured in less time, allowing us, quite literally, to see more in many cases.