Abstract
A new remote nondestructive inspection technique, based on thermoelastic temperature measurement by infrared thermography was developed for evaluation of fatigue cracks propagated from welded joints in steel bridges. Fatigue cracks were detected from localized high thermoelastic temperature change observed at crack tips due to stress singularity under variable loading by traffics on the bridge. Self-reference lock-in data processing technique was developed for improvement of signal/noise ratio of the thermal images in the crack detection process. The technique makes it possible to perform correlation processing without an external reference signal under the random loading. In this paper, remote and nondestructive crack detections by the self-reference lock-in thermography were performed for fatigue cracks in actual steel bridge in service. Accuracy improvement of self-reference lock-in thermography method was made by motion compensation technique based on the two-dimensional SSD (Square Sum of Differences) parabola fitting method.
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