In recent years, fatigue crack propagations in aged steel bridge which may lead to catastrophic structural failures have
become a serious problem. For large-scale steel structures such as orthotropic steel decks in highway bridges, nondestructive
inspection of deteriorations and fatigue damages are indispensable for securing their safety and for estimating
their remaining strength. As conventional NDT techniques for steel bridges, visual testing, magnetic particle testing and
ultrasonic testing have been commonly employed. However, these techniques are time- and labor- consuming
techniques, because special equipment is required for inspection, such as scaffolding or a truck mount aerial work
platform. In this paper, a new thermography NDT technique, which is based on temperature gap appeared on the surface
of structural members due to thermal insulation effect of the crack, is developed for detection of fatigue cracks. The
practicability of the developed technique is demonstrated by the field experiments for highway steel bridges in service.
Detectable crack size and factors such as measurement time, season or spatial resolution which influence crack
detectability are investigated.
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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