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Phased Array Ultrasonic Testing of Compressor Bolts
Release Date:
2022/06/15
Bolts are extensively used to connect critical mechanical components in large compressor units, and their quality directly affects the operational safety of the entire unit. The loads borne by these bolts are highly complex, arising from intricate stress states, corrosion, temperature variations, and other factors. Consequently, the bolted joints are prone to fatigue cracking, which can lead to bolt fracture or failure. During the operation of compressor equipment, bolt failure can result in production interruptions and shutdowns; therefore, workshops must conduct scheduled bolt inspections in accordance with the production plan and task requirements.
In the past, in-service bolts were typically inspected by first removing them and then performing fluorescent magnetic particle inspection. This approach involved a heavy workload, long inspection times, low efficiency, and disruption to production operations. To address these challenges, inspectors have adopted phased-array ultrasonic testing to detect fatigue cracks in the bolt threads without dismantling the bolts, achieving excellent results.
Routine Ultrasound Examination
Bolts are slender, cylindrical components. Conventional ultrasonic testing typically employs longitudinal wave straight-beam probes for their inspection. The advantage is that this method can effectively detect crack defects with a groove-depth equivalent of 1 mm or greater; however, the following drawbacks exist:
1. The A-scan waveform exhibits a complex shape, making interpretation challenging; in particular, interference signals predominantly originate from the bolt body structure or the thread flanks, and cannot be effectively distinguished or identified, thereby posing certain difficulties for inspection work.
2. The threaded portions at both ends of the bolt are prone to fatigue cracking; however, the single-angle scanning provided by a longitudinal-wave straight-probe inspection can easily result in missed defects, thereby significantly reducing the reliability of the inspection.
3. Test data cannot be recorded, or defects lack traceability.
4. The detection sensitivity and signal-to-noise ratio are relatively low.
Phased Array Ultrasonic Testing
Compared with conventional ultrasonic testing techniques, phased-array ultrasonic testing offers the following advantages:
1. The defect detection results can be presented in multiple views, effectively distinguishing defects from structural echoes and resolving the challenge of accurately identifying defect echoes in bolt inspections.
2. Can be applied to the inspection of parts with complex geometric structures.
3. Phased-array ultrasonic technology features multi-angle, steerable focused beams, enabling comprehensive coverage of defect locations, enhancing detection sensitivity and the signal-to-noise ratio of the images, and improving the ability to distinguish between different types of defects.
I. Bolt Style Machining
A bolt in service is selected as the test specimen. As shown in Figure 1, grooves of varying depths are machined on the threaded portion of the outer surface at end A of the bolt, with the grooves aligned along a single line and oriented perpendicular to the bolt’s axis to simulate cracks. The groove depth, length, and opening width are all 1.0 mm, 10.0 mm, and 0.25 mm, respectively. The bolt material is 35CrMoA, and the unknown positional parameters of the artificially created grooves are listed in Table 1.
II. Sector Scanning
An OmniScan MX2 phased-array ultrasonic testing instrument was used, equipped with a 32-element 5 MHz probe having a element pitch of 0.5 mm, an element height of 10 mm, an angular step of 0.5°, and a sector angle range of –30° to +30°.
To perform internal inspection of the bolt, simply place the probe on end faces A and B. During sector scanning, four artificial defects were detected in the imaging of the threaded region at the end face near side A, as shown in Figure 2(a). When inspecting end face A, radial back-and-forth movement combined with rotational scanning was employed, and the images consistently revealed the defects with high clarity. This demonstrates that the probe’s inspection coverage extends to the entire threaded portion.
When the ultrasonic beam is incident on a defect-free bolt, the thread echoes are clear and stable, with uniform spacing, as shown in Figure 2(b). As the inspection distance increases, the amplitude of the thread echoes gradually decreases. Crack echoes appear between the thread echoes, and their color differs significantly from that of the adjacent thread echoes; the degree of this color difference is related to the amplitude of the crack echo.
III. Influence of the Depth Direction on the Detected Image
Adjust the reflected echo from the artificially machined groove located 110 mm from the A-side end face of the bolt to 80% of full screen, and use this setting as the baseline sensitivity; then increase the gain by 6 dB to establish the inspection sensitivity. This parameter enables normal-incidence detection of cracks in the threaded region, enhances the reflectivity of defect echoes, and significantly improves the signal-to-noise ratio.
The fan-shaped images of the bolt at different inspection depths (focusing depths) are shown in Figure 3. As can be seen, the greater the inspection depth, the weaker the energy of the groove reflection, and the signal-to-noise ratio gradually increases with increasing inspection depth. The reason for this is that the image of the bolt threads exhibits high contrast, making it easier for inspectors to make judgments.
IV. Influence of Detection Position on the Image
The probe positioning schematic and the fan-shaped ultrasonic beam pattern for the bolt are shown in Figure 4. When the probe is positioned at Location 1, the artificial notch at a depth of 50 mm lies outside the probe’s beam sweep area, so the inspection image displays only part of the defect. In contrast, when the probe is positioned at Location 2, both the artificial notches at 50 mm and 70 mm depths fall within the probe’s beam sweep area. From the above analysis, it is evident that the inspection position is critical: only by placing the probe in the appropriate location can all target areas to be inspected be fully covered.
V. Applications
The 20 bolts removed on-site were sequentially subjected to phased-array ultrasonic testing. The probe was positioned on the A and B end faces of each bolt for inspection, with an initial sweep performed first: the probe was moved radially back and forth while simultaneously rotating. Following the initial sweep, the focal depth was set to the region most prone to cracking; when defect echo signals were detected at a particular location, the focal depth was then adjusted to precisely coincide with the defect’s position.
A crack was detected at a depth of 85 mm from the A side of a bolt, as shown in Figure 5. The defect morphology is clearly visible, demonstrating excellent inspection performance.
Conclusion:
1. Phased-array ultrasonic testing employs wide-angle scanning, producing intuitive inspection images that facilitate clear defect identification.
2. Optimizing the inspection parameters can enhance detection sensitivity and improve the image signal-to-noise ratio, thereby significantly increasing the crack detection rate.
3. The signal-to-noise ratio of the images obtained by manual groove-cutting inspection gradually increases with increasing inspection depth. This is because the reflected echoes from the bolt threads diminish as the inspection distance increases, while the contrast between the crack defect image and the thread image remains distinct.
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