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Inspection of Welding Discontinuities in Tubular-Type Transmission Towers Using Metal Magnetic Memory

Welded joints in a tubular-type transmission tower (TTT) are potential locations for stress concentration zones (SCZs) owing to the thermal effects caused by welding and external loads during operation. The SCZs degrade the quality of welded joints and need to be inspected at an early stage by nondestructive testing methods. In this study, a metal magnetic memory (MMM) technique is presented for the inspection of SCZs in the welded joints of a TTT. The MMM system uses four pairs of fluxgate sensors to measure the weak residual magnetic field on the welded surface without an external magnetic source; each fluxgate sensor pair can measure the normal and tangential magnetic field components on the welding surface. The SCZs can be areas of welding discontinuities or of early stages of discontinuity initiation (before the existence of defects); thus, not all SCZs inspected by the MMM technique will have defects. For further evaluation of the state of the welds at the SCZs, the A-scan ultrasonic testing technique is used to determine the existence of discontinuities at the SCZs that were detected by the MMM technique. Thus, a fast and reliable assessment result of the welded joints can be obtained. A failed 154 kV TTT welding specimen was used in the experiment, and it was found that defects existed in some of the SCZs.


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