Ultrasonic testing of steel and aluminum U.S. DOT cylinders is required to recertify the cylinders every 5 or 10 years depending on the cylinder type. Ultrasonic testing of DOT cylinders is currently performed with automated ultrasonic testing systems that uses wheel probes, squirter systems, or water jet systems. The ultrasonic immersion systems are configured with two circumferential shear wave channel, two axial shear wave channels, and one longitudinal straight beam probe. The automated conventional ultrasonic testing systems are very robust and operate at very high throughput. The article discusses the possibility of performing phased array testing of steel and aluminum DOT cylinders. The objective is to potentially reduce the amount of custom design and instrumentation required to perform ultrasonic testing of the DOT cylinders.
Figure 1: Phased array squirter system for DOT steel and aluminum cylinders.
Guidelines for Testing of Steel and Aluminum DOT Cylinders
Candidate ultrasonic testing systems must demonstrate sensitivity simulated fatigue cracks in the circumferential and axial directions. The fatigue cracks are most commonly simulated with ID and OD EDM notches. In addition to fatigue cracks, it is also of interest to detect ID wall loss in the form of an isolated cluster of pits and/or generalized corrosion. These phenomena are usually simulated using a flat bottom hole and EDM pad. Internal surface wall loss is detected with a 5–10 MHz linear array longitudinal wave setup.
Feasibility of Phased Array Testing DOT Cylinders
Conventional ultrasonic testing of DOT cylinders is very efficient, and while PAUT is often cited for its inspection advantage over conventional UT, this advantage may not be achieved for the DOT cylinder application. The potential advantage may like is decreasing the number of sensors and channels used to perform the inspections. Additional automation and/or motion control may be required if interfacing with conventional commercially available phased array units versus due to limited channels.
Three conventional ultrasonic transducers may be replaced by a single 64-element or higher phased array probe. The phased array probe is interfaced with multiple PAUT groups to create low angle shear waves in the plus and minus directions and a L-wave group approximately centered in the probe for straight beam inspection. The multi-group concept is shown below in the Olympus X3 phased array platform. In the top left of the display the low angle shear wave in the plus and minus directions are shown. A fixed angle linear PAUT scan is used since the objective is to generate 2D C-scan mapping the cylinder flaw locations. In the top right, the linear PAUT L-wave scan is shown. The bottom half of the display, from top to bottom, is dedicated to the PAUT C-scans of the positive direction shear wave, negative direction shear wave, and L-wave. On the right, the active A-scan if the current PAUT focal law is displayed. In this multi-group approach, the conventional ultrasonic testing axial shear wave transducers and straight beam transducer are replaced by a single phased array high element probe.
Figure 2: Multi-group PAUT shear wave linear scan, longitudinal wave scan, and shear/longitudinal wave C-scans.
Phased Array L-wave Linear Scan Performance
The performance of the L-wave group using the middle elements of a 64-element transducer are shown below for generalized corrosion via an EDM pad. The EDM pad was machined to 0.50 x 0.50”. Using the amplitude drop sizing techniques and sizing the simulated flaw in the index and scan axes directions produces almost identical dimensions. Example PAUT L-scan group data is shown in Figure 3. The top right image is the top view C-scan showing accurate PAUT sizing if the EDM pad. Sizing is performed using the scan and index axis measurement cursors. The image in the top right is the l-scan across the length of the PAUT probe, essentially a B-scan along a vertical slice of the C-scan. The image in the bottom left is the l-scan along the length of the scan axis, essentially a B-scan along a horizontal slice of the C-scan. The A-scan is displayed is the active PAUT focal law.
Figure 3: L-wave scan of generalized corrosion via and EDM pad
Phased Array Shear Wave Linear Scan Performance
Phased array linear shear wave scans differ from PAUT sectoral scans in that they used a fixed angle and a dynamic aperture. In contrast, PAUT S-scans required a fixed aperture and sweep through different angles. The former is required if the objective the phased array scan is to generate a 2-D map of the steel or aluminum cylinder flaws. The performance of the shear wave group using the lower elements of a 64-element transducer are shown below for an ID simulated fatigue crack via an EDM notch. The EDM notch was machined to a length of 1”.
Using the amplitude drop sizing techniques and sizing the simulated fatigue crack in the index axis, or circumferential direction, the flaw is reproduced with very accurate length and depth directions. Example PAUT shear wave group data is shown in Figure 4. The top right image is the top view C-scan showing accurate PAUT length sizing of the 1” EDM notch. Depth sizing is also very accurate and may be performed in the shear wave linear scan or B-scan produced. The image in the top right is the shear wave linear scan across the length of the PAUT probe, essentially a B-scan along a vertical slice of the C-scan. The image in the bottom left is the shear wave fixed angle linear scan along the length of the scan axis, essentially a B-scan along a horizontal slice of the C-scan. The A-scan is displayed is the active PAUT focal law.
Figure 4: PAUT shear wave scan of ID EDM notch.
Summary
Further testing is required to firmly establish multi-group phased array ultrasonic testing (PAUT) of steel and aluminum U.S. DOT cylinders as a viable solution. Conventional ultrasonic testing of DOT cylinders is currently performed with automated ultrasonic testing systems that uses wheel probes, squirter systems, or water jet systems at high throughput. The article demonstrated that multiple conventional UT channel / probes may be replaced by a single 64 element or higher phased array probe potentially simplifying the design of an automated solution.
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