For Phased Array inspections
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 degs. 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
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
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
PHASED ARRAY TECHNOLOGY
The laws of physics apply to phased array as well as to classical
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
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.
γ-6 = 0.51λ/D
is the half angle at the 6 dB drop points of the echo field
γ-20 = 0.87λ/D
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.
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
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
4. PAUT requires significantly less inspection space for scanning compared
5. Both PAUT and AUT
store raw A-scan data that can be replayed
Calibration on 1.6 mm dia side drilled holes
using the Omniscan
FAQ on Phased Arrays
Q1. Which probe has a sharper focus: 16 mm aperture or a 12 mm aperture
(both probes have same frequency and number of elements).
A1. Focusing depends directly on the aperture. So the 16 mm has a sharper
focus than the 12 mm probe.
Q2. Which 5 MHz Probe has a sharper focus: (a)16 mm aperture-16 element
or (b) 12 mm aperture -32 element.
A2. The 16 element probe (16 mm aperture) has a sharper focus than the 32
element (12 mm aperture) probe. The increase in elements from 16 to 32 has a
minimal effect on focusing. Sharper focusing is from the larger aperture 16
mm vs 12 mm
Q3. If the 16 mm aperture-16 element probe has a sharper focus than the
12 mm aperture -32 element, then is there any advantage of using a more
A3. The larger elements transforms to smaller element size. Smaller the
element size greater is the beam sweep. However, once the desired beam sweep
is achieved there is minimal advantage of increasing the 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 (based on 20 dB drop
Q4. Is it possible to focus past the near field ?
Q5. The software of my equipment allows me to enter a focal length that is
greater than the near field. Is that valid ?
A5. This entry will be meaningless.
Q6. Can the inspection be done past the focal length ?
A6. Yes inspection can be done; however there will be loss of resolution
past the focal length.
Any other questions, please send us a
FAQ on Phased Arrays
1. AGR, UK
GE Inspection Service, USA
4. Harfang, Canada
5. Imasonic, France
8. Sonatest, UK
9. Sonovation, UK
10. Zetec, USA