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WHAT IS PHASED ARRAY ULTRASONIC TESTING?

Advancement in modern technology has expedited at an exponential rate. Humanity has gone from the invention of the first telephone in 1876 by Alexander Graham Bell to the first personal phones in 1973 to now having new technology being released every year with multiple variations of high-tech personal devices.

This also means that structural technology and newer mechanisms have grown at a similar, if not more accelerated rate, to keep up with consumers’ demands. Better technology uses complex materials and aggregates that cannot be analyzed with conventional NDT techniques. Such mechanisms and materials require Advanced Non-Destructive techniques with digital data processing for modern-day digital workflow.

Corrosion and usage in a variety of environments deteriorate even the most advanced materials. This may form cracks, voids, and other such defects. Such defects cannot be detected with the single probe detection method of regular Ultrasonic Testing. A larger imaging area with adjustable angled beams is required because more minor deformities behave differently compared to larger defects in rebounding input signals.

Phased Array Ultrasonic Testing probe (Image credits: https://sunflagsteel.com/)

For such situations, Phased Array Ultrasonic Testing (PAUT) is used. Phased Array Ultrasonic Testing, as the name suggests, uses multiple transducers in an array inside the testing probe to manipulate the input ultrasonic beam by exciting them in various sequences that can be controlled digitally. These transducers are piezoelectric materials that convert one form of energy to another to fulfill the probes’ purpose.

The sequenced actuation of the transducers facilitates sectorial, depth, and linear scanning. It can also be used in situations where conventional Ultrasonic Testing (UT) is used. It can be used for checking wall thickness profiles and weld inspections, as well as inspections of Heat Affected Zones (HAZ), which makes it versatile and viable across industries like manufacturing, construction, petrochemicals, and aerospace industries.

The input ultrasonic beam of the Phased Array Ultrasonic Testing apparatus is made to sweep across the testing subject and hence picks up on deformities that would have been missed by a conventional single-beam Ultrasonic Testing device that produces a singular directed beam.

Also Read, What is Ultrasonic Testing (UT)

TERMINOLOGY OF PAUT

Phased Array Ultrasonic Testing uses electronic time delays to create beams of varying interferences (i.e., constructive and destructive). Various terms are used during and to describe the operation of this non-destructive testing method. A few of those terms are simplified as follows:

• Transducer:  A transducer is a device that converts one form of energy to another. The Phased Array Ultrasonic Testing unit converts the input electric signals to ultrasonic beams to impinge the test surface using multiple phased transducers inside the apparatus in an algorithmically planned sequence.
• Dynamic Depth Focusing (DDF): Dynamic Depth Focusing is an algorithm that uses only a single focused transmitted pulse. The focusing depth here is variably programmed at the reception point of the signal to inspect deeper within the component with a singular ultrasonic pulse. This can be used to inspect thicker components and eliminates the complexities of manual calculation as it’s based on a pre-programmed system.

Depth Focusing Technique (Image credits: https://inis.iaea.org/collection/NCLCollectionStore/_Public/47/111/47111878.pdf)

•  Angular Scanning (also called Sectorial or Azimuthal scanning): In this technique, the focal depth of the beam stays constant, and the probe is held at a constant position. The beam is capable of scanning an entire sector of the volume to be inspected and can hence be used in tight spaces and to test subjects with complex and irregular geometries.   

Sectorial scan of a shell weld (Image credits: www.twi-global.com)

•  Linear Scanning: This scanning method also involves no movement of the probe on the apparatus. Instead, the scanning occurs along an axis. The angle in this technique is kept constant and focal delay is actuated across a set of active elements. This is almost like a raster scan in the conventional Ultrasonic Testing (UT) method.

Linear Scanning Method (Image credits: www.tcradvanced.com)

• Beam Steering: The Phased Array Ultrasonic Testing apparatus allows for the adjustment of the input angles of the beams by adjusting the refracted angle. This allows for versatile usage of the same apparatus.
•  Beam Forming: The apparatus can also be used as a conventional Ultrasonic Testing (UT)machine if no time delays are added to the input signal.

Also Read, WHY PHASED ARRAY UT (PAUT) BEATS RADIOGRAPHIC TESTING (RT)?

PROCEDURE OF PHASED ARRAY WORK

One of the most prevalent uses of Phased Array Ultrasonic Testing is to inspect welded regions for potential defects and discontinuities. The procedure for a weld inspection would generally go as follows:

• The welded region should be thoroughly inspected, and a frequency should be chosen by a skilled operator based on the thickness of the region.
•  Positioning should be carried out with careful consideration of the heat-affected zone (HAZ) and the start of the weld with the start angle of impedance at the former and end angle at the latter region.
•  In cases wherein the end angle at the weld zone exceeds 60°, further scanning should be conducted (preferably line scanning).
• The probe is affixed on an angular wedge, generating a shear wave in the testing material for analysis. The distance between the center of the weld zone and this wedge should be calculated mathematically or ray traced.
• The testing zone should be covered by the angles covered by the probe.
•  A test sample of the same dimensions and material should be used to generate a time-corrected gain graph that contains defects created artificially at known distances from the surface.
• A steady line guide for probe movement should be drawn on the surface to ensure steady movement along the test zone.
•  The probe can hence be moved along the line drawn, and testing can be carried out.

Set-up of various scans done in PAUT (Image credit: Anandamurugan, S. (2009). Manual phased array ultrasonic technique for weld application. In Proceedings of the national seminar & exhibition on non-destructive evaluation, NDE.)

PHASED ARRAY ULTRASONIC TESTING STANDARDS

Industries that use non-destructive testing techniques, such as Phased Array Ultrasonic Testing, require uniformity to gauge the status of structures and materials tested correctly. Some standards used for PAUT studies include:

• American Society for Testing and Materials-“Standard Practice for Contact Ultrasonic Testing of Welds Using Phased Arrays”- E2700-09
• American Society for Testing and Materials-“Standard guide for evaluating characteristics of phased array ultrasonic examination instruments     and Systems”-E2491-06
• American Society of Mechanical Engineers - “Use of Ultrasonic Examination in Lieu of Radiography”-2235-9
• American Society of Mechanical Engineers- “Case of ASME Boiler Pressure Vessel Code. Use of Manual Phased Array Escan Ultrasonic Examination per Article 4 Section V”- 2558

PARAMETERS OF PHASED ARRAY WORK

With complex apparatus like that of the Phased Array Ultrasonic Testing Machine, a number of variables are present within the mechanisms of the testing unit. These parameters affect the manner of analysis to be conducted on the test subjects, and a standard operating procedure can be created so every inspection performed can be conducted under similar test conditions and/or device settings.

The parameters that can be adjusted are as follows:

• Probe adjustments can be made where the element width, number of elements, kerf, element length, pitch, and nominal frequency can be adjusted.
• Wedge material, wedge material velocity, incident angle, and height of reference element can be inputted.
• The type of scan, the number of focal elements in the incident beam, and the number of steps can be varied as per testing needs.
• The scan pattern, test environment (immersed or physical contact), and test mechanics can also be inputted to adjust test results accordingly.
• The couplant material, test material geometry, and velocities, along with the blocks used for calibration, are also important parameters that need to be considered before planning the test design.

ADVANTAGES OF PHASE ARRAY WORK

Conventional single probe Ultrasonic Testing devices can be delicate and complex to use. It also has many limitations when it comes to the size, geometry, and accessibility of test material. For more complex inspections like welding, it can often prove to be detrimental and inaccurate. The probability of false readings is reduced in Phased Array Ultrasonic Testing as it can detect multiple signals simultaneously.

Phased Array Ultrasonic Testing also provides the following advantages:

• It uncomplicates testing procedures by automating the beam adjustments and digitizing the results. It also eliminated the need for multiple calibrations of the individual probes as it uses a phased array of signals.
•  Small, complex, and irregularly shaped test surfaces can often provide incorrect readings due to irregular reflection of the result beams. Due to the controlled, adjustable input of multiple ultrasonic waves, these issues are 
• The electronic scanning feature of the Phased Array Ultrasonic Testing apparatus also eradicates the need for repeated manual reconfiguration and drastically reduces the probability of human errors. This increases testing speed and saves labor hours.
• Phased Array Ultrasonic Testing avoids the loss of coupling or potential changes to it by avoiding probe movement. The results hence obtained are reliable and consistent.

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DISADVANTAGES OF PHASE ARRAY WORK

Phased Array Ultrasonic Testing provides numerous benefits as an advanced non-destructive testing methodology. However, it also poses disadvantages, such as:

• Phased Array Ultrasonic Testing cannot be used for a variety of materials and operations, such as surface defects, defects caused due to cyclic tensile loadings like bolts and tubes, etc.
• Sectorial scanning also poses the risk of providing an unsatisfactory angle of incidence of the beam.
• Skilled labor is required to operate this apparatus as it comprises a complex array of transducers and multiple data management and manipulation features that need skill and experience to utilize the PAUT efficiently.
• Being an advanced device, the Phased Array Ultrasonic Testing apparatus is expensive to purchase compared to the conventional Ultrasonic Testing method.
• As per the American Welding society's bridge-welding cost (AWS D1.5), Phased Array Ultrasonic Testing is not recognized as an official inspection technique. Hence it is not used frequently in the bridge fabrication industry.

APPLICATIONS OF PHASED ARRAY WORK

Phased Array Ultrasonic Testing is used in the offshore oil and gas industry, Offshore Wind power generation industry, Shipping, Ship Building and repair, Mega Yachts and Luxury Boating industry, Civil engineering, Steel construction and machine building industry, Process Building, and many more such fields. This makes it incredibly flexible across industries and modern-day technology.

The PAUT can also be used for:

• Time of flight diffraction (TOFD) calibration blocks to rectify and confirm the required test setup.
• Phased Array Ultrasonic Testing calibration blocks to characterize input beams.
• To assess the structural integrity of pressured vessels to ensure safety during operation.
• PAUT can also be used to measure the thickness of test subjects.
• It can be used to assess composite materials.
• To inspect for corrosion on a sub-surface.
• The Phased Array Ultrasonic apparatus is also used in railways to inspect the rolling stock wheelset.

PAUT on railway rolling stock axels (Image credit: Phased Array Ultrasonic Technology-Eclipse scientific)

WHAT IS THE DIFFERENCE BETWEEN UT AND PAUT

The Ultrasonic Testing beam may well be considered a predecessor to the Phased Array Ultrasonic Testing machine, but there are minor differences that separate them as entirely different testing techniques, such as:

• PAUT provides the option to vary between single-angle or variable-angle testing, hence providing a range of impeding angles, whereas UT provides only single-angle inspection.
• In the Active usage direction, PAUT provides a variable aperture and a fixed aperture in the passive direction. Ultrasonic Testing only provides a single aperture size.
• Phased Array Ultrasonic Testing provides the option to carry out A-scans that represent data, B-scans that show data trends, and S-scans that are sectorial scans, whereas UT only provides A-scans.
• PAUT also provides variable focal depths and planes, increasing the testing range and potential, whereas UT only provides a two-dimensional testing range, focusing along a line or singular point.
• PAUT automatically stores the complete waveform testing data, whereas UT relies on the operators’ records of the inferred results.

Read Acceptance Criteria for Ultrasonic Testing

WHAT IS THE DIFFERENCE BETWEEN PAUT AND AUT

AUT, or Automated Ultrasonic Testing, is used for applications wherein Phased Array Ultrasonic Testing is required over large testing materials. Manual operation of PAUT in such situations is cumbersome and reduces the probability of detecting flaws due to the sheer size of the testing subject.

Automated Ultrasonic Testing utilizes a robot on magnetic wheels to travel across the surface of the structure to be tested and performs an automatic scan of the entire area using the technology of Phased Array Ultrasonic Testing. It covers approximately 60 cm of an area in a scan, a significant difference from the area covered by Phased Array Ultrasonic Testing.

The advantage that Automated Ultrasonic Testing provides over Phased Array Ultrasonic testing, despite both being advanced non-destructive techniques, is as follows:

• The automated robot further eliminates the errors caused due to human interference and increases the speed of acquiring data from the testing process.
• The measurable data points in AUT have a higher density as compared to PAUT.
• The probability of detection of defects in the structure to be tested is increased as the area tested simultaneously is larger.
• The magnetic wheels and robotic testing apparatus make it easier to test larger curved surfaces accurately without missing potential defects.

PHASED ARRAY ULTRASONIC TESTING VS RADIOGRAPHY

Radiography as a non-destructive technique is gradually becoming obsolete, only to be replaced by Phased Array Ultrasonic Testing.

• Radiography Testing has a major downfall as it causes radiation exposure-related problems and concerns. Extra measures related to radiation protection like lead suits, Geiger counters, and Dosimeters (to measure absorbed ionized radiation).
• Evacuation of work zones is required before testing due to radiation, which in turn halts production and/or operation.
• An experienced technician is required in Radiography testing, much like the Phased Array Ultrasonic Testing process. However, the impeding angle of the testing beam in Radiography must be perfectly normal to the tested surface. The thickness of the material also affects the limits of the area that can be tested, hence further limiting the process.
• The resulting image obtained from radiography testing produces a negative that needs to be skilfully developed by a trained operator, further consuming time and risking exposure.
• The image, once developed, is monochromatic and needs years of skill and experience to be appropriately inferred. PAUT, on the other hand, provides a colored image with sufficient contrast and representations of defects.
• The discontinuity length observed in the Phased Array Ultrasonic Testing method is slightly greater than that of radiography, as the latter fails to detect edges of defects.

 CONCLUSION

Phased Array Ultrasonic Testing is a technique that truly showcases the benefits of Non-destructive testing as a field. It provides accurate scientific and statistical data, is time efficient, and avoids the wastage of operating hours during the testing process.

 The apparatus can cost anywhere between  $2,000 to  $50,000, depending on the features available. The devices available in the market come with sublime features that include remote access to data, data correction for testing curved surfaces, upgradability, dedicated data recording software, imaging software, and instant visual models.

 Choosing the right testing model can prove to be cost-efficient in the long run as it provides easy access to regular monitoring of the integrity of materials and machinery, hence improving longevity.