Phased array ultrasonic testing

This is a series of articles put together by the Practitioner Committee to describe the principles of different NDT methods and techniques as an introduction to practitioners...

Although I’ve never been qualified in phased array ultrasonic testing (PAUT), I thought this would be a great opportunity for me to dig a bit deeper on a methodology I’m aware of but have no deep understanding of. My research enlightened me on the method and capabilities of PAUT and this article is intended to provide readers with a basic understanding of the methodology as well as outlining the requirements needed enter into the advanced NDT world of phased array.

What is phased array ultrasonic testing? Advances in ultrasonic inspection to phased array are often ‘born’ out of the medical industry, where arrays have been used for decades, such as in the ultrasound scanning of a developing foetus. The medical sector tends to drive advances in this area.

Phased array ultrasonic testing (PAUT) is an ultrasonic non-destructive testing (NDT) technique that uses the principles of wave interference and technology to inspect different materials for defects or damage during fabrication or while in service. It is a reliable and accurate testing method that can detect surface and subsurface flaws at higher speeds than conventional ultrasonic testing (UT), making it a popular choice for industries such as aerospace, construction and oil & gas.

The method of PAUT begins with the production of high-frequency (higher than the human audible range, hence the term ultrasonic) sound waves that are sent through the material being inspected. The sound waves are generated by a probe made up of multiple small elements, typically 64, each delivering separate sound waves at a different predetermined angle. By controlling the timing of the pulses, the element’s beam angle of each sound wave is generated; therefore, the PAUT system can produce a focused beam of sound that can detect flaws at specific depths and at a range of swept angles in the test material.

In Figure 2(a), the PAUT scan detects a number of side-drilled holes at once, which are shown on a screen (and recorded for analysis and review), whereas conventional UT (Figure 2(b)) requires multiple scans and multiple angles to achieve the same information (but it is not recorded).

What are the advantages of phased array? One of the key advantages of PAUT is its ability to inspect materials from a wide range of angles, providing better coverage of a test area. Unlike traditional or conventional NDT methods that rely on a single probe angle to detect subsurface defects, PAUT testing allows technicians to inspect materials from many different angles during the same scan, increasing the probability of identifying flaws that would not be captured by a typical 45°, 60° and 70° scan. This makes the probability of detection (POD) of defects across the test area far greater (see Figures 3 and 4).

Phased array testing can detect various types of defect (see Figure 5), including:

Cracks and fissures in materials such as metals or composites (hydrogen-induced cracking (HIC), stress-oriented hydrogen-induced cracking (SOHIC) and stress corrosion cracking (SCC);
Corrosion mapping, such as material loss or corrosion in metallic objects;
Weld imperfections, such as lack of fusion or porosity;
Inclusions, voids and other types of material anomaly that may be present in cast or forged materials; and
Thickness variations, delaminations or disbonds in composite materials.

Another advantage of phased array testing is its speed and accuracy. The system can scan large areas of material quickly and provide detailed images of any flaws detected. These images can be stored in the memory of the equipment and can be reviewed in more detail away from the test site. This can save time and money compared to traditional UT methods, where data on most sets cannot be captured and reviewed with the same level of detail.

Phased array testing is also highly customisable, with technicians able to adjust the frequency, angle and power of the sound waves to suit the material being inspected. This makes it an ideal method for inspecting a wide range of materials, from metals to plastics and composites.

What are the limitations of phased array? Despite its many advantages, PAUT does have some limitations and it may not be as reliable as other techniques or able to detect certain types of defect, such as:

Surface-breaking cracks or defects: PAUT is not as effective at detecting defects that are very close to the surface; surface techniques such as magnetic testing (MT), penetrant testing (PT), eddy current testing (ECT) and alternating current field measurement (ACFM) are preferred;
Metal fatigue and defects in bolt holes and tubing: these inspections are often found to be more challenging, with ECT proving a more reliable method for these applications;
Certain defects in thinner materials: the system is highly sensitive to variations in material thickness and composition, which can affect the accuracy of the results. With thinner materials, radiography provides superior results subject to defect orientation;
Defects in rough castings made from coarse-grained material: the coarse surface scatters sound energy from the probe and wears away expensive array wedges quickly when large areas are scanned. However, specialist probes, for example the FlexoFORM and HydroFORM, provide solutions to these issues in larger areas; and
Defects in complex geometries: PAUT may have difficulty detecting defects in materials that have complex shapes or geometries, for example compound curves distort the sound paths, producing measurement errors, and areas with limited access for the probe cause difficulties. Again, specialist equipment is combating these issues. Axial outside diameter (AOD) and circumferential outside diameter (COD) wedges created for pipe curvature relative to the pipe diameters inspected and roller probes with rubber tyres facilitate these inspections.

Additional drawbacks include the following:

Interpreting the images produced by the system requires a high level of expertise and training, as flaws can be difficult to distinguish from other features of the material;
Far fewer qualified and experienced technicians are available compared to conventional UT; and
The equipment and software are more expensive than conventional UT.

Overall, PAUT is a highly advanced NDT method that can detect a wide range of defects, but it is important to understand its capabilities and limitations to ensure that it is used effectively for different materials and applications.

The case for PAUT versus radiography One area in which phased array testing is becoming more widely adopted is its use in lieu of radiography. Using PAUT instead of radiography for the inspection of defects in welded joints is often preferable for clients as it allows production to continue around the test area. PAUT negates the necessity for exposure to a radiographic source, providing a far safer approach to testing with no requirement for the disposal of process chemicals. PAUT provides a higher POD, especially for cracks and lack of fusion. In most studies, ultrasound tends to detect planar flaws better than radiography.

Additionally, this method is especially useful when radiography is not practical, such as when the weldment is too large, when access for film placement is not possible or when there are limitations on the use of ionising radiation due to plant monitoring sensors in the area.

It is also possible to encode scans and provide a permanent record, allowing for the future assessment of scanned areas. Additionally, the off-site review of scanned areas (as is possible with radiographs) gives the customer added value in their inspection results.

Overall, using phased array testing to supplement or replace radiography is becoming increasingly popular in many industries due to its POD in evaluating the internal structure of materials and welds without the need for harmful radiation and following as low as reasonably practicable (ALARP) procedures.

What does it take to become qualified? Phased array testing is commonly referred to as an advanced NDT method in industry and this is partly due to the entry requirements for attending a training course. Pre-existing NDT qualifications are needed, as detailed below. The course duration is 15 days, including the examination. This consists of ten days of phased array theory and a data acquisition practical, three days of analysis practical and two days of examination. Typical costs range from £3500-£4000.

The minimum prerequisite qualifications are:

PCN and CSWIP candidates must be qualified to a minimum of BS EN ISO 9712 Level 2 in ultrasonic testing.

The minimum required duration of training, including both theoretical and practical elements, is:

Level 1: 80 h
Level 2: 40 h (direct Level 2: 104 h for CSWIP and 120 h for PCN).

The minimum duration for experience prior to, or following success in, the qualification examination is:

Level 1: three months for CSWIP and one month for PCN
Level 2: three months (direct Level 2: six months for CSWIP and four months for PCN).

Conclusions Phased array ultrasonic testing is a highly effective NDT technique that offers a range of benefits over both conventional ultrasonic inspection and radiographic inspection, where industry requires reliable and accurate testing of materials. Its ability to customise the testing parameters for different materials and provide detailed, recordable high-speed scans makes it an ideal choice for industries such as aerospace, construction and oil & gas. However, as with any testing method, it is important to have an understanding of its limitations and where alternative methodologies can be more suitable.

Other advances in PAUT are already taking place, with techniques such as FlexoFORM now being used on elbows in pipework, adapting to the contours or diameter of the bend.

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