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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
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)
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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