September 2020, Vol. 247, No. 9
Features
Using Phased Array UT in Lieu of Radiography for Weld Inspections
By Guy Maes, Zetec
For years, inspectors have used industrial radiography for nondestructive testing (NDT) of welds and heat-affected zones on large-diameter pipes. Like a medical X-ray machine uses radiation to highlight cracks in a bone, conventional radiographic testing (RT) uses X-rays or gamma rays in combination with film to indicate welding flaws and defects.
And, like a medical X-ray, conventional RT is sensitive to the relative orientation of the film and the material under test. The image quality and time required to complete the inspection depend on the skill and experience of the technician.
Furthermore, the danger of radiation creates regulatory and safety concerns for pipeline operators and their employees. The extra steps necessary to safeguard welders from exposure and to comply with codes add billable hours and complexity to the inspection, driving up costs and risk in the process.
Since its introduction in the early 2000s, phased array ultrasonic testing (UT) instruments have been embraced by NDT service providers for weld inspections in lieu of RT. Portable instruments such as the Zetec TOPAZ64 can collect and manage large amounts of inspection data, render detailed images in real time, and provide online and offline analyses using phased array UT and advanced algorithms like the total focusing method (TFM).
Pipeline operators and contractors have choices when it comes to inspection companies and techniques. In order to make a decision that ensures the highest level of quality, repeatability and productivity, they should arm themselves with at least a basic understanding of phased array vs. RT.
UT Explained
Ultrasonic testing (UT) encompasses several methods that use pulses of high-frequency sound energy to detect surface and subsurface cracks and other defects. These pulses come from a piezoelectric transducer or probe that the technician moves over the surface of the component under inspection.
The ultrasonic waves enter the material at precise intervals and a set angle. When a wave encounters a defect, some of that energy is reflected back and generates an echo.
The time it takes for that energy to reflect back to the probe is calculated and analyzed by the test instrument and presented instantaneously as a graphic on a screen for the technician to review. The inspector can validate completed welds and provide feedback to the welding crew as they go about their work.
Phased Array
There are several UT methods used in pipeline inspections today, notably conventional or standard UT, and phased array UT.
A conventional or standard UT probe is capable of generating and receiving a single ultrasonic beam. The focal point and angle of the beam are fixed, which can make it more difficult to locate and visualize flaws, especially in components with complex geometries. The use of a custom-shaped probe can help ensure the accuracy of such inspections.
Phased array UT uses multiple independent elements (typically from 16 to 64) in a single probe. This makes it possible to capture and store all possible time-domain signals (A-scans) from every transmitter-receiver pair of elements in the array, a technique called full matrix capture or FMC.
FMC allows this “full matrix” of raw A-scan signals to be processed in real time or saved for offline processing using different sets of reconstruction parameters for any given focal law or beam (aperture, angle or focus depth). Having access to the raw signal information opens the door to advanced algorithms like the TFM, which uses FMC data to produce high-resolution 2-D and 3-D images for defect characterization and sizing.
Signal Quality
Because FMC consists of consecutive single-element emissions, very weak acoustic signals are emitted during FMC data acquisition. Consequently, the energy levels of the received signals are also very low. Therefore, to make sure the reflected UT signal is not lost in the electronic noise of the acquisition system, high-quality pulser-receiver channels are absolutely necessary.
Most portable phased array UT instruments use a negative square pulse excitation between 75 and 100 volts to drive the phased array probe elements. Higher voltage would improve the signal-to-noise ratio of the individual A-scans.
But, sustaining this nominal voltage for the full duration of the pulse – and with a certain pulse-repetition frequency – to comply with ISO and EN requirements is difficult for most battery-powered instruments.
One solution found in TOPAZ64 UT instrument is a pulser design that can generate a bipolar pulse – a negative square at 75 volts immediately followed by a positive square at 75 volts, resulting in 150 volts peak-to-peak with 40% more acoustic energy and an improved signal-to-noise ratio for both standard phased array and TFM configurations.
TFM
The TFM algorithm sums the elementary A-scan signals from all elements in the array to generate a frame of pixels where each individual pixel is computed using a dedicated focal law. The TFM frames can be used for “live” interpretation or they can be stored for each position of the probe.
In theory, TFM should result in one perfectly rendered focal point per pixel. In practice, the near-field length of the considered probe defines the maximum depth at which a sound beam can be focused; a beam cannot be focused beyond the near field.
Choosing the proper TFM frame resolution will depend on the component thickness, the wave mode, L-waves or S-waves, and the probe frequency. The operator must calculate the ratio between the resulting pixel size and wavelength, and the amplitude fidelity, which is the maximum possible amplitude error with the given settings.
The more advanced portable phased array UT instruments like TOPAZ64 with its UltraVision software platform can make all necessary calculations automatically.
TOFD
Time-of-flight diffraction (TOFD) is another UT technique that is often combined with phased array UT to improve the detection probability and sizing accuracy of welding defects. TOFD has been around since the early 1980s but has become increasingly utilized due to improvements in computer processing power.
In a TOFD system, ultrasonic probes are situated on opposite sides of a weld. One probe acts as a transmitter, emitting an ultrasonic pulse into the material; the other is a receiver. Instead of recording only the high-amplitude sound waves generated by specular reflection (“mirror” effect) from well-oriented planar defects, TOFD also records low-amplitude waves diffracted from the tips of crack-like defects. The technique mostly relies on accurate time-of-flight measurements to characterize and size defects.
The simultaneous use of phased array UT and TOFD, and more recently TFM and TOFD, can detect all welding flaw types and provide reliable through-wall sizing capability in a single inspection sequence.
The operator can use a two-sided phased array UT examination with shear wave phased array probes to detect planar and surface-breaking flaws, while the TOFD technique can detect and locate embedded flaws while offering accurate through-wall sizing performance.
Using phased array UT and TOFD together can increase the productivity of the inspection crew simply by reducing the number of scans and manipulations.
Next Steps for FMC/TFM
FMC and TFM are becoming common applications of phased array UT technology for pipeline weld inspections, and efforts are ongoing to support these techniques in the ASME code. A working group established within ASME Section V has recently published a mandatory Appendix XI on full matrix capture and is now concentrating on issuing guidelines for operator training and certification.
In the meantime, portable phased array UT hardware and software continue to advance, allowing NDT technicians to make faster, more accurate inspections virtually anywhere the job takes them.
In lieu of RT, phased array UT can keep the balance of safety, efficiency, accuracy and manufacturing productivity in check – and reduce the costs and risks involved in weld inspections.
Author: Guy Maes is sales engineer director for UT solutions at Zetec, a nondestructive technology testing solutions company. He has more than 30 years of experience in advanced UT method development and implementation.
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