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 crack signal

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
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 elements.
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 points).

Q4. Is it possible to focus past the near field ?
A4. No.

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 email

 

See also
FAQ on Phased Arrays
PA Applications



Phased Array Equipment Manufacturers

1. AGR, UK
2. GE Inspection Service, USA
3. Harfang, Canada
4. Krautkramer, Japan
5. OlympusNDT, USA
6. Sonovation, UK


 

NDE Associates, Inc.
515 Tristar Drive
Webster, TX 77598
Phone: 281-488-8944    Fax: 281-488-8485