PHASED ARRAY TECHNOLOGY

PHASED ARRAY TECHNOLOGY

The laws of physics apply to phased array as well as to classical ultrasonic testing.

Element Size: Maximum Sweep Range of Phased Arrays is determined by the element size. This is done using the classical UT formula for calculating beam spread (γ). Smaller the element size, higher is the sweep range. This is what gives Phased Arrays the ability to sweep large angles. Once the desired beam sweep is achieved there is little point using smaller elements. A 5 MHz, 1mm element size will give an approximate sweep of 70 degrees for S-wave in steel and 140 degrees for L-waves in steel. Reducing the element size and thereby increasing the number of elements is of no significant benefit thereafter.

Focal limit is based on the wavelength and overall aperture of the probe. Same formulas apply as that of classical UT probes. No focusing is possible beyond the near field. The near field of a 5 MHz, 12 mm aperture probe using shear waves in steel is 60 mm.

Focal Spot Size The focal spot size is determined using the classical beam spread (γ) formulas. The focal spot size depends on the wavelength and probe aperture. The spot size becomes sharper with reduced focal length (F), increased probe aperture and increased frequency. Distortion of focal spot can occur from refraction and reduced elements.

                       Sin γ_-6 = 0.51λ/D  γ_-6 is the half angle at the 6 dB drop points of the echo field
                       Sin γ_-20 = 0.87λ/D  γ_-20 is the half angle at the 20 dB drop points of the echo field

A hole in a calibration block will show as an arc on the PAUT image and not as a hole (see the picture below). This is because of the beam spread. Even if the beam is focused at the hole location, the beam finite focal spot size controls the size measured by phased arrays.

PHASED ARRAY UT vs. MANUAL UT

1. Manual UT produces a single A-scan at a specific angle. Manual UT evaluation requires plotting the indication using the refracted angle, metal path and surface distance. PAUT displays images in real time showing the depth and location of indication relative to the probe.
2. Manual UT is limited to a single refracted angle. PAUT simultaneously takes data from a range of angles, eg 40 to 75 degrees and reconstructs an image in real time
3. PAUT image is easy to comprehend as it gives a display of the ultrasound superimposed on the test piece
4. Using an encoder with the PAUT probe, all raw A-scan data can be stored. Once stored, the data can be replayed. This is most important to retain a complete record of the inspection. There is no data storage capability in manual UT.

PHASED ARRAY UT vs. Automated UT (AUT)

1. AUT reconstructs the test piece cross-section (B-scan) after taking data using a single refracted angle and scanning it back and forth on the test piece. PAUT reconstructs such image from a single probe location with no scanning.
2. In many cases, especially for thin plates, a single line scan will perform the inspection. AUT always requires either a 2-axis scan or multiple probes to reconstruct the image. Line scan done with PAUT scanning is much simpler than raster scanning.
3. On applications that require16 to 32 probes with AUT, PAUT can be done with significantly lesser number of probes, eg. one array on either side of the weld.
4. PAUT requires significantly less inspection space for scanning compared to AUT.
5. Both PAUT and AUT store raw A-scan data that can be replayed for analysis
 

Calibration on 1.6 mm dia side drilled holes using the Omniscan

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PHASED ARRAYS FOR WELD INSPECTION

The concept of weld inspection is shown below. A probe is selected that illuminates the weld as shown below. In the first leg, only the bottom half of the weld is illuminated. However, with reflection from the ID surface the entire weld is illuminated and the complete weld volume can be inspected. For example, the indication ‘a’ in the weld is detected by the reflected sound and displayed as the mirror image ‘a-’ in the second leg. Similarly other flaws are displayed in the image.

Concept of phased arrays for Weld Inspection. Indication ‘a’ detected by the reflected beam is displayed as a mirror image ‘a-‘ in second leg

Examples of defects images detected in welds are shown below.

Root detected in first leg.	Image shows a strong ID crack signal

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What are Ultrasonic Phased Arrays?

What are Ultrasonic Phased Arrays?

Ultrasonic phased arrays use a multiple element probe whereby the output pulse from each element is time delayed in such a way  so as produce constructive interference at a specific angle and a specific depth. These time delays can be incremented over a range of angles to sweep the beam over the desired angular range. For example, 40 to 75 degree beam sweep would be produced by calculating the time delays to produce constructive interference at 40, 41, 42 ...75 degrees. This NDT technology is also referred as Swept Beam Ultrasonic testing.

The main advantages of phased array in NDE are:
1. Ability to sweep a range of angles
2. Ability to display the image in real time for the swept angles
3. Ability to focus

The above figure displays the concept of phased arrays. Time delays to the eight elements control focusing and beam sweep. Focal spot size (shown by the shaded orange area) is controlled by beam spread. Checking Phased Array Probe Resolution. Resolution determines flaw definition and sizing accuracy. Instrument: Phasor

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A Brief History of Phased Array Testing

During their first couple decades, commercial ultrasonic instruments relied entirely on single-element transducers that used one piezoelectric crystal to generate and receive sound waves, dual element transducers that had separate transmitting and receiving crystals, and pitch/catch or through transmission systems that used a pair of single-element transducers in tandem. These approaches are still used by the majority of current commercial ultrasonic instruments designed for industrial flaw detection and thickness gaging, however instruments using phased arrays are steadily becoming more important in the ultrasonic NDT field.

The principle of constructive and destructive interaction of waves was demonstrated by English scientist Thomas Young in 1801 in a notable experiment that utilized two point sources of light to create interference patterns. Waves that combine in phase reinforce each other, while waves that combine out-of-phase will cancel each other.

Phase shifting, or phasing, is in turn a way of controlling these interactions by time-shifting wave fronts that originate from two or more sources. It can be used to bend, steer, or focus the energy of a wave front. In the 1960s, researchers began developing ultrasonic phased array systems that utilized multiple point source transducers that were pulsed so as to direct sound beams by means of these controlled interference patterns. In the early 1970s, commercial phased array systems for medical diagnostic use first appeared, using steered beams to create cross-sectional images of the human body.

Initially, the use of ultrasonic phased array systems was largely confined to the medical field, aided by the fact that the predictable composition and structure of the human body make instrument design and image interpretation relatively straightforward. Industrial applications, on the other hand, represent a much greater challenge because of the widely varying acoustic properties of metals, composites, ceramics, plastics, and fiberglass, as well as the enormous variety of thicknesses and geometries encountered across the scope of industrial testing. The first industrial phased array system, introduced in the 1980s, was extremely large, and required data transfer to a computer in order to do the processing and image presentation. These systems were most typically used for in-service power generation inspections. In large part, this technology was pushed heavily in the nuclear market, where critical assessment more greatly allows use of cutting edge technology for improving probability of detection. Other early applications involved large forged shafts and low pressure turbine components.

Portable, battery-powered phased array instruments for industrial use appeared in the 1990s. Analog designs had required power and space to create the multi-channel configurations necessary for beam steering, but the transition into the digital world and the rapid development of inexpensive embedded microprocessors enabled more rapid development of the next generation phased array equipment. In addition, the availability of low power electronic components, better power-saving architectures, and industry-wide use surface mount board design led to miniaturization of this advanced technology. This resulted in phased array tools which allowed electronic setup, data processing, display and analysis all within a portable device, and so the doors were opened to more widespread use across the industrial sector. This in turn drove the ability to specify standard phased array probes for common applications.

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Introduction to Ultrasonic Testing

Ultrasonic test instruments have been used in industrial applications for more than sixty years. Since the 1940s, the laws of physics that govern the propagation of high frequency sound waves through solid materials have been used to detect hidden cracks, voids, porosity, and other internal discontinuities in metals, composites, plastics, and ceramics, as well as to measure thickness and analyse material properties. Ultrasonic testing is completely nondestructive and safe, and it is a well-established test method in many basic manufacturing, process, and service industries, especially in applications involving welds and structural metals.

The growth of ultrasonic testing largely parallels developments in electronics, and later in computers. Early work in Europe and the United States in the 1930s demonstrated that high frequency sound waves would reflect from hidden flaws or material boundaries in predictable ways, producing distinctive echo patterns that could be displayed on oscilloscope screens. Sonar development during the Second World War provided further impetus for research in ultrasonics. In 1945, US researcher Floyd Firestone patented an instrument he called the Supersonic Reflectoscope, which is generally regarding as the first practical commercial ultrasonic flaw detector that used the pulse/echo technique commonly employed today. It would lead to the many commercial instruments that were introduced in the years that followed. Among the companies that were leaders in the development of ultrasonic flaw detectors, gages, and transducers in the 1960s and 1970s were Panametrics, Staveley, and Harisonic, all of which are now part of Olympus NDT.

In the late 1940s, researchers in Japan pioneered the use of ultrasonic testing in medical diagnostics using early B-scan equipment that provided a two-dimensional profile image of tissue layers. By the 1960s, early versions of medical scanners were being used to detect and outline tumors, gallstones, and similar conditions. In the 1970s, the introduction of precision thickness gages brought ultrasonic testing to a wide variety of manufacturing operations that required thickness measurement of parts in situations where there was access to only one side, and corrosion gages came into wide use for measurement of remaining wall thickness in metal pipes and tanks.

The latest advances in ultrasonic instruments have been based on the digital signal processing techniques and the inexpensive microprocessors that became available from the 1980s onward. This has led to the latest generation of miniaturized, highly reliable portable instruments and on-line inspection systems for flaw detection, thickness gaging, and acoustic imaging.

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