United States Department of Transportation
i
Magnetic Flux Leakage (MFL) Method for Damage Detection in Internal Post-Tensioning Tendons
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2021-06-01
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[PDF-15.13 MB]
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Edition:Final Report Apr 2018-June 2021
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Abstract:The magnetic flux leakage (MFL) method has been identified as a promising non-destructive testing technique for detecting metal loss in external post-tensioned tendons and stay-cables. However, for internal tendons, the interfering signals from secondary ferromagnetic materials, such as mild reinforcement, can generate complicated signals and may lead to inaccurate prediction of damage location and severity. The objective of the study was to identify the improvements to be made on existing MFL technology, both in terms of hardware and testing protocols, which can be implemented for internal tendons in field conditions. A systematic study was conducted to improve the existing MFL system, and a fully functional measurement and magnetizing unit was developed. The developed system demonstrated that effective magnetization of steel strands, located at a depth of 8 inches, can be carried out. Also, a suite of user interfaces was developed in a programming language to improve data processing and visualization. As a proof of concept, the modified MFL system was used to conduct testing on laboratory mockup specimens and segments of a decommissioned bridge located at Florida International University. The results showed that the presence of transverse reinforcement and depth of magnetization have a significant effect on magnetic flux signals. The effect of transverse reinforcement was reduced using a novel testing protocol developed in this study. In this protocol, near-surface magnetization is initially carried out, followed by far-surface measurement, which allows identification of defects in embedded tendons. Additional laboratory testing was carried out on specimens which replicated conditions of an in-service bridge, and the developed system and testing protocol showed promising results in identifying the locations of the defects. To verify the merits of the equipment, a series of tests was conducted on an in-service bridge. The inspection methods showed that the MFL method is highly repeatable, and magnetization on the post-tensioned rods is retained over time. Area scans on the bridge spans showed anomalous patches which may be representative of a defect, coupler bar, or a construction deficiency.
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