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.