No abstract

Distortion-induced fatigue cracking is a concern for many aging steel bridges in the United States. Human visual inspections to characterize fatigue cracks have many drawbacks including inconsistencies in identification, significant time and monetary costs, and safety risks posed to the lives of both inspectors and the traveling public due to lane closures. Digital image correlation (DIC) is a vision-based technology which has shown promise for identifying and characterizing fatigue cracks. Three-dimensional DIC measurements can capture full-field displacements and strains, allowing for detection and characterization of both in-plane and distortion-induced fatigue cracks. This paper describes an experimental study in which a half-scale steel girder to crossframe subassembly was subjected to distortion-induced fatigue loading to produce multiple geometrically complex cracks. A DIC-based crack characterization methodology was applied to quantify the cracks, which was successful at characterizing cracks propagating in the girder web but struggled to characterize horizontal cracks. Additional work is needed to improve the accuracy of the DIC-based crack characterization methodology to use as an automated bridge inspection tool.

Distortion-induced fatigue accounts for the majority of fatigue cracks in aging steel bridges in the United States. Departments of Transportation primarily use visual inspections to locate and characterize fatigue cracks, but recent studies have shown that visual inspections fail to consistently identify realistically sized fatigue cracks in highway bridges. Developing an inspection technique that does not rely on human visual inspection has the potential to reduce the risk of injury for inspectors and the traveling public, decrease the time and cost of performing inspections, and increase reliability, benefitting both bridges owners and stakeholders, as well as the traveling public. Digital image correlation (DIC) has the potential for detecting and characterizing fatigue cracks. The authors developed a methodology for crack characterization on fatigue specimens loaded in-plane which was then applied to out-of-plane distortion-induced fatigue cracks. The work was performed in the laboratory with conditions ideally suited for DIC data collection. Field implementation will require a robust system able to function in spite of numerous physical obstacles. These obstacles include non-ideal lighting, variable levels of camera focus, and inconsistent material surface quality. To understand how the DIC crack characterization methodology performs under non-idealized conditions, the effects of each of these physical obstacles must be examined. This paper describes an experimental study in which the developed DIC crack characterization methodology was applied to data obtained under nonidealized conditions. Of particular interest are the influence of lighting and camera focus on DIC data quality and its corresponding applicability to fatigue crack characterization. Preliminary results indicate that DIC may be feasible under non-idealized conditions, allowing for potential field application to in-service structures.