Fatigue Crack Detection Using Unmanned Aerial Systems in Fracture Critical Inspection of Steel Bridges

Many state agencies are investigating the use of unmanned aerial systems (UASs) for bridge inspections. Some agencies are receiving pressure from consultants and their own administrations to implement UAS inspections with limited knowledge of their efficacy. This paper studies the feasibility of using UASs for fatigue crack detection in bridges with fracture critical members (FCMs) through real-time and postflight visual inspection. The effects of surface illumination on the maximum crack-to-camera (MCC) distance at which a fatigue crack can be detected were investigated in the laboratory. Mock field inspections evaluated the achievable crack-to-platform (ACP) distance in Global Positioning System (GPS)-denied and windy environments to determine whether known cracks can be identified at achievable standoff distances. Finally, two FCM inspections demonstrated the field performance of UASs in identifying fatigue cracks. Results highlight the importance of camera specifications and surface illumination in determining the required standoff distance of crack detection. Furthermore, the results demonstrate the difficulties in obtaining clear images with unstable UASs in GPS-denied or windy environments. Nevertheless, the best performing platform tested in this study exhibited a performance comparable to an average of 30 human inspectors at a fatigue crack identification training structure. The limited results presented here proved the feasibility of using UASs for fatigue crack detection in FCM inspections of steel bridges and highlighted the shortcomings of UASs for this type of hands-on inspection.