A scan type magnetic camera is developed to measure the residual stress generated in the welded part of a tubular-type power transmission tower. The change in strain of the ferromagnetic structure changes the magnetic domain distribution and therefore induces spontaneous magnetization. Thus, the magnetic field intensity distribution measured along the welds correlates with the change in stress generated in the structure. Residual stresses in welds occur more frequently in defects, and consequently, anomalies in the magnetic field strength distribution indirectly indicate the presence of weld failures. Conversely, the scanner is fabricated to be in a chevron shape with an angle of 150 degrees considering the cross section of the twelve-squared shape. Two cylinder-type wheels are arranged on each side of the chevron to prevent slipping. Linearly arrayed giant magnetoresistance (GMR) sensors are arranged on the weld line, and the magnetic field distribution can be measured along the weld. The correlation between the magnetic field intensity and defects is verified by comparing the magnetic field intensity distribution and the ultrasonic signal at the weld.
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