Evaluation of a Digital Image Correlation Bridge Inspection Methodology on Complex Distortion-Induced Fatigue Cracking

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Distortion-induced fatigue cracking in aging steel bridges is a primary concern for many bridge owners and stakeholders, accounting for the majority of fatigue cracks in bridges in the United States. Currently Departments of Transportation primarily use visual inspections to locate and characterize fatigue cracks. However, this approach has drawbacks, as recent studies have shown that visual inspections are not able to consistently identify realistically-sized fatigue cracks in highway infrastructure. Additionally, it requires significant time and cost to perform visual inspections, and both inspectors and the traveling public are placed at risk during the inspection. Bridge owners and stakeholders would benefit from the development of inspection techniques that do not rely on hands-on human visual inspection – to reduce the risks of harm to the inspectors and traveling public, increase reliability, and decrease the time and cost of performing inspections.Vision-based technologies are an active area of research in the field of structural health monitoring, aimed at developing alternatives to manual inspection for identifying damage to transportation infrastructure. The majority of these studies have focused on macro-indicators of damage, including identifying excessive corrosion, concrete deck deterioration, and large displacements due to substructure movement. Digital image correlation (DIC) is one such vision-based technology that shows promise for detecting and characterizing fatigue cracks, but investigations and applications have thus far been limited to simple, in-plane cracking.Three-dimensional DIC measurements have the ability to capture full-field displacements and surface strains, allowing for the potential of identifying and characterizing out-of-plane fatigue cracks, such as those occurring on steel bridges exposed to loading through differential girder displacement. Since the majority of fatigue cracks arise from out-of-plane loading, investigating the efficacy of DIC for detecting distortion-induced fatigue cracks is of clear value.This paper describes an experimental study in which a scaled steel girder-to-cross-frame specimen was cyclically loaded to produce geometrically complex distortion-induced fatigue cracking. To examine the potential usefulness of DIC for future automated bridge inspections the resulting fatigue cracks were characterized using DIC and visual inspection. The methodology applied proved successful at quantifying certain aspects of the complex, bifurcating cracks, but had difficulty characterizing crack segments that are no longer actively loaded. Additional work is needed to improve the accuracy and reliability of the crack detection results for this technology to potentially be used as an automated inspection tool in the future.