Ultrasonic underside inspection for fatigue cracks in the deck plate of a steel orthotropic bridge deck

摘要

Due to an unexpected increase of heavy traffic large fatigue cracks appeared through the deck plate of orthotropic steel bridge decks in the Netherlands. Visual inspection revealed that this particular type of crack initiates where a weld joins the deck plate, a rib and a girder. These critical points are commonly inspected from the roadside, which necessitates that the road is closed down for all traffic and that the wear layer is first removed. To overcome these costly drawbacks a new method is proposed that enables ultrasonic inspection of the deck plate from the underside of the bridge deck. The method requires a combination of two special measurement techniques, which are optimised for the bridge problem at hand. To detect the maximum crack depth an angled pitch-catch technique is employed. The crack length can be detected along the rib weld by employing the simpler pulse-echo technique. The crack depth and crack length are determined from the respective ultrasonic data sets by a calibration, which relates the number of detected waves to the actual crack size. The calibration is determined by ultrasonically monitoring the various crack stages during a fatigue test that is conducted on a bridge deck specimen. The original, uncracked state and the final state where the cracks can be visibly detected in the deck plate determine the extremes. The tests show that a reasonable accurate detection of crack size is quite possible, while the visual inspections prove to be useless until the crack has already grown completely through the deck plate. The new method provides a crack depth and crack length estimate with an accuracy of +-15%.