Introduction
Excellent quality non-destructive testing (NDT) data is required to ensure the structural integrity of bridges, piping systems, pressure vessels and other pressure and load bearing structures. The data quality must be assured before leaving the inspection site to assure sound engineering conclusions are made on the future performance of the asset. The cost to repeat non-destructive testing extends beyond the actual inspection costs and may include traffic control, equipment to access the inspection location, and other safety measures. The coupling channel used for ultrasonic testing (UT) and phased array ultrasonic testing (PAUT) is a reliable method to ensure that data is acquired at the desired signal-to-noise (SNR) ratio. The UT and PAUT coupling channel are used across many industries to ensure data integrity including vehicle based rail inspection [1], automated steel and aluminum cylinder inspection [2], AWS D1.1 and D1.5 code weld inspections [3], and ASME BPVC code inspections [4]. This article explores the value of the coupling channel and how to implement it using the Olympus Omniscan inspection platform.
Figure 1: Phased array ultrasonic testing multi-group weld inspection with dedicated PAUT group for monitoring the backwall reflection and one group for shear wave inspection.
Automated Ultrasonic Testing Data Integrity
Automated ultrasonic testing (AUT), conventional or phased array, often required inspection speeds of at least 6 inches per second and up to 20-30 miles per hour for vehicle based rail inspection. At these test speeds, it is impossible for an inspector to assure data integrity as data rates and volumes exceed the observation capability of human inspector.
Vehicle based ultrasonic testing requires detection of transverse defects, axial defects, and bolt hole cracks but the running surface can present challenges for coupling ultrasonic waves generated in the wheel probe into the rail. A straight beam channel is dedicated to ensuring that the desired shear and longitudinal waves are coupled into the rail. If the straight beam channel drop significantly in amplitude, the section of rail must be reinspected at a slower speed, manually, or reported as not inspected.
Automated inspection of steel and aluminum cylinders present a similar coupling challenge due to tolerances on cylinder straightness and the contour presented by diameters in the 4” to 9” range. Cylinders are typically inspected with a minimum of 5 transducers: 2 dedicated to axial shear waves, 2 dedicated to circumferential shear waves, and 1 dedicated to thickness testing, generalized corrosion, and pitting corrosion. The longitudinal wave is dual purposed as a coupling channel.
Ultrasonic and phased array weld inspections to AWS D1.1, D1.5 and ASME codes also benefit from integrating the coupling channel for automated encoded scans. The weld inspection surface is often non-flat, contoured and may be inhibited with weld spatter and tool marks. Most AWS and ASME compliant phased array instrumentation have a built-in coupling channel with a dedicated PAUT group that monitors the backwall reflection of the component tested.
Coupling Channel for Phased Array Testing
The integrity of shear wave data, compared to longitudinal wave date, is difficult to assess since there is not a known reflector to reference during the inspection. In standard L-wave inspection, the backwall or component end can be monitored to confirm that ultrasound has been effectively inserted into the component. As noted above, a reflection from the rail base or cylinder inside-diameter can monitored for variation. If the straight beam signal amplitude is within an allowable +/- % full screen height (FSH) tolerance, it can be concluded that ultrasound has been coupled into the tested component.
Shear wave testing presents a different problem since there is no reliable perpendicular reflector to reference. While there may be a root or weld reinforcement present, these features vary in geometry and are not a reliable reference reflector. A more robust reference reflector is required and the default is to dedicate a separate group in PAUT testing, or separate channel in UT testing, to a coupling channel focused on the backwall reflection. The concept is illustrated in Figure 3. In this case a 64-element PAUT transducer is used to perform a compound S-can of the weld. A separate group of 8 elements is dedicated to the L-wave coupling channel. Note that the objective of this channel is confirm the SNR of the L-wave backwall, to infer coupling, rather than accurate thickness testing.
Example phased array weld inspection data with the A-scan coupling channel is shown below. The A-scan is shown vertically on the left. The red gate is positioned over the first backwall. In this scan, the amplitude of the first backwall reflection is relatively stable at 80% FSH indicating excellent and consistent acoustic coupling. An acoustic coupling C-scan is displayed in a narrow strip below the phased array C-scan. In this example the A-scan coupling channel %FSH amplitudes are color coded from light to dark blue in the 50% to 100% range. Notice the light blue area in the coupling channel C-scan where surface conditions affected the ultrasonic wave insertion. Over this 6-7 inch scan range, the amplitude dropped from 80% to 55 %FSH.
Figure 4: Phased array ultrasonic acoustic coupling channel A-scan showing excellent signal-to-noise ration over entire scan axis.
Conclusion
The cost associated with acquiring poor quality non-destructive data is significant. This includes the cost to re-inspect which can include traffic control, equipment downtime, and safe access equipment. The acoustic coupling channel is an effective tool that can be implemented to overcome ultrasonic testing couplant challenges for many types of load bearing and pressure bearing applications.
References
1. 2024 Manual for Railway Engineering, Chapter 4 Rail
2. ISO 18119:2018 Gas cylinders — Seamless steel and seamless aluminium-alloy gas cylinders and tubes — Periodic inspection and testing
3. American Welding Society (AWS) Bridge Welding Code D1.5M/D1.5:2015
4. American Welding Society (AWS) Structural Welding Code D1.1/D1.1M 24th Edition, 2020
5. ASME Boiler and Pressure Vessel Code, Section V Non-destructive Examination, 2023