United States Department of Transportation
i
Design, Develop and Fabricate a Prototype Nondestructive Inspection and Monitoring System for Structural Cables and Strands of Suspension Bridges, Volume I: Final Report
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1989-05-01
[PDF-197.41 MB]
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Abstract:This report describes the development of a prototype magnetic perturbation NDE system for inspecting and monitoring structural cables and strands of suspension bridges. Although this method had been applied successfully to a variety of NDE problems, including small diameter cables, the application to larger diameter cables was an extremely difficult endeavor. A limited parametric laboratory investigation was conducted using a rudimentary system consisting of a 200-lb (91 kg) electromagnet with an attached Hall probe to measure the magnetic perturbations as a simulated cable was scanned. The first specimen consisted of 370 steel rods (0.25-in. (6.4-mm) diameter by 20-ft (6.1-m) long) enclosed by a 6.0-in. (15.2-cm) I.D. by 0.28-in. (7.1-mm) wall thickness steel pipe 20-ft (6.1-m) long that supported the rods and also simulated the 0.2-in. (5.1-mm) diameter soft steel spiral protective wrap used on some main cables. Magnetic anomalies in the steel pipe obscured flaw signals except for very large flaws. After many changes in the experimental arrangement which included replacing the steel pipe with an aluminum pipe, detection of a 0.27% cross-sectional area flaw at a depth of 1.9 in. (4.8 cm) was achieved. Subsequently, a prototype Magnetic Perturbation Cable (MPC) NDE system was constructed and field demonstrations were successfully conducted on the 6.3-in. (16-cm) diameter Luling Bridge stay cables (12-15 December 1988). Approximately 420 data scans (each 100 in. (2.5 m) long) were recorded. Comprehensive analyses disclosed the possibility that grouting was so thin in several regions on top of a cable that segments of wires were moved by the magnetic attractive forces at each poleface. The MPC incorporates the high sensitivity, excellent resolution, long-term stability, etc., of the magnetic perturbation method with modern microprocessor control, digital communication, computer archival storage, digital signal analyses routines, etc., into an integrated, state-of-the-art, automated NDE system that now makes it possible to apply modern quantitative structural lifetime assurance strategies to suspension bridge cables.
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Content Notes:FHWA Contract Manager: Charles McGogney, HNR-10.
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