NDT Overview: An Introduction to Phased Array Ultrasonic Testing

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).

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