by Haley Cowans

While many of our readers are well-versed in NDT, some might be just learning about our industry. This series will serve as a primer to the major NDT methods and techniques covered in ASNT’s publications, educational programming, and events. The following is an excerpt from the second edition of Ultrasonic Testing Classroom Training Book, published by ASNT in 2018, which has been adapted for this blog post.

The book covers Level I and II knowledge of the ultrasonic testing (UT) method and is designed for classroom training, but could also be used for self-instruction and as a reference. It is available from ASNT as both a print book and an ebook.

Introduction

Ultrasonic phased array (PA) testing uses multiple ultrasonic elements and electronic time delays to generate and receive ultrasound, creating beams by constructive and destructive interference. As such, PA offers significant technical advantages over conventional single-probe UT: the phased array beams can be steered, scanned, swept, and focused electronically.

Terminology

Electronic scanning permits very rapid coverage of the components, typically an order of magnitude faster than a single-probe mechanical system.

Beam forming permits the selected beam angles to be optimized ultrasonically by orienting them perpendicular to the discontinuities of interest — for example, lack of fusion in welds.

Beam steering (usually called a sectorial or azimuthal scan) can be used for inspecting components with a range of appropriate angles to optimize probability of detection.

Sectorial scanning is also useful for inspections where restricted access to the test object limits conventional scanning. A sectorial scan sweeps through a selected range of angles, such as 40° to 70° shear, using the same group of elements to generate all angles.

Electronic focusing permits optimizing the beam shape and size at the expected discontinuity location, as well as probability of detection. Focusing significantly improves the signal-to-noise ratio, which also permits operating at lower pulser voltages. Overall, PA optimizes discontinuity detection while minimizing test time.

Background

Phased arrays are widely used in medical ultrasonic imaging but had limited use in NDT in the 20th century, mainly because of the complexity and cost of the systems. As the cost of production decreased and portable computing power increased, the use of phased arrays for NDT has become more practical.

Operation

Ultrasonic phased arrays are similar in principle to phased array radar, sonar, and other wave applications. Phased arrays use an array of elements, all individually wired, pulsed, and time shifted. These elements can be a:

• linear array,
• two-dimensional matrix array,
• circular array, or
• more complex form (see Figure 1).

Most applications use linear arrays. These are the easiest to program and are significantly cheaper than more complex arrays because of fewer elements. As costs decline and experience increases, greater use of the more complex arrays is predicted.

The elements are ultrasonically isolated from each other and packaged in normal probe housings. The cabling usually consists of a bundle of well-shielded micro-coaxial cables. Wireless systems have also been on the increase. Commercial multiple-channel connectors are used with the instrument cabling.

Elements are typically pulsed in groups from 4 to 32, typically 16 elements for welds. With a user-friendly system, the computer and software calculate the time delays for a setup by using either operator input on interrogation angle, focal distance, scan pattern, and other test circumstances, or by using a predefined file (see Figure 2). The time delays are back calculated using time of flight from the focal spot and the scan assembled from individual focal laws. Time-delay circuits must be accurate to around 2 ns to provide the phasing accuracy required.

Per ASTM E-1316, a focal law is “the entire set of hardware and software parameters affecting the acoustic sensitivity field of a phased array search unit, whether a pulse-echo or a pitch-catch configuration.” Focal laws include delay mechanisms in both the transmitter and receiver.

Each element generates a beam when pulsed; these beams constructively and destructively interfere to form a wave front. The summed waveform is effectively identical to a single-channel ultrasonic instrument or discontinuity detector (referred to as a flaw detector in industry) using a probe with the same angle, frequency, focusing, aperture, and other settings. Figure 3 shows typical time delays for a focused normal (perpendicular) beam and reflected wave. Another sample time delay setup is shown in Figure 4.

Implementation

From a practical viewpoint, ultrasonic phased arrays are merely a means of generating and receiving ultrasound; once the ultrasound is in the material, it is independent of generation method, whether generated by piezoelectric, electromagnetic, laser, or phased arrays. Consequently, many of the details of UT remain unchanged; for example, if 5 MHz is the optimum test frequency with conventional UT, then phased arrays would typically start by using the same frequency, aperture size, focal length, and incident angle.

While phased arrays require well-developed instrumentation, one of the key requirements is good, user-friendly software. Besides calculating the focal laws and calibrating each element in the array, the software saves and displays the results, so good data manipulation is essential. As phased arrays offer considerable application flexibility, software versatility is highly desirable. PA inspections can be manual, semiautomated (that is, encoded), or fully automated, depending on the application, speed, budget, and other considerations. Encoder capability and full data storage are usually required.

Although it can be time consuming to prepare the first setup, the information is recorded in a file and only takes seconds to reload. Also, modifying a prepared setup is quick in comparison with physically adjusting conventional instruments and probes.

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The second edition of Ultrasonic Testing Classroom Training Book contains chapters on phased array, time of flight diffraction, and guided wave method, and follows the body of knowledge outlined in CP-105 (2011). It includes up-to-date information on ultrasonic principles, equipment, specialized techniques, and test applications. It also covers qualification and certification of personnel, ultrasonic displays, transducer operation and calibration, and reference standards.

For additional information on UT and PA, check out these ASNT resources:

Books available from the ASNT Store:

• Nondestructive Testing Handbook, Vol. 7: Ultrasonic Testing, third edition
• ASNT Level III Study Guide: Ultrasonic Testing
• ASNT Level II Study Guide: Ultrasonic Testing
• Ultrasonic Testing Programmed Instruction Book

Papers available on the NDT Library:

• “Phased Array Beam Steering for Composites” Barry Fetzer, presented at NDT of Composites 2021 https://ndtlibrary.asnt.org/2021/PhasedArrayBeamSteeringforComposites
• “Advantages of Compound S-scan over Sectorial Scan or E-scan: A Case Study,” Stephen Sundarraj, Materials Evaluation, Vol. 79, No. 12, December 2021, DOI: https://doi.org/10.32548/2021.me-04219
• “Optimizing Probe Active Aperture for Phased Array Weld Inspections,” Anmol Birring, Materials Evaluation, Vol. 79, No. 8, August 2021, DOI: https://doi.org/10.32548/2021.me-04220

ASNT Webinars and Refreshers

• “Crack Sizing of Welds Using TFM and PA” Webinar Recording, Parrish Furr and William Betz, https://www.pathlms.com/asnt/events/2035
• Refresher/Certification Prep Courses https://asnt.org/MajorSiteSections/Learn/Refresher_Courses.aspx