Large-scale strain sensing approach for detecting fatigue cracks in steel bridges

Fatigue cracking in steel highway bridges presents major safety and maintenance concerns for bridge owners: if bridges are not regularly inspected and maintained, fatigue cracks can lead to structural failure. It is of key importance to detect and assess fatigue crack activity in steel bridges in a timely and holistic manner. This study presents a novel strain sensing technology for fatigue crack monitoring based on soft elastomeric capacitors (SECs). SECs are inexpensive, easy to fabricate, highly stretchable, and mechanically robust capacitance-based strain gauges. Additionally, SECs are a highly-scalable technology, capable of monitoring deformations over a much larger area than can be achieved with traditional metal foil strain gauges. This characteristic enables SECs to be applied over and around entire fatigue details in steel bridges to detect, localize, and monitor fatigue cracks. Here, we first review previous experimental work using the SEC as a crack monitoring device for in-plane fatigue crack detection on small-scale compact, C(T), specimens. Then, we extend the SEC technology for sensing fatigue damage in a large-scale structural surface through dense SEC arrays. An effective data visualization method is proposed to convert a fatigue damage feature, termed crack growth index (CGI), from individual SECs to a 2-dimensional CGI map, which depicts the fatigue damage over a large-scale area. It is possible to monitor fatigue damage evolution by observing the change in intensity in CGI maps during the fatigue test. The effectiveness of this approach was validated through an experimental test on a bridge girder-to-cross-frame connection subjected to out-of-plane fatigue loading. Overall, the results of this study indicate that the SEC technology has great potential for detecting and monitoring fatigue crack growth in practical applications.