Rapid Surface Inspection of Steel Bridges Using ACFM

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Many bridges feature bolted or welded steel sections as part of their structure. These are susceptible to fatigue cracking due to the cyclic action of traffic and wind forces on the bridge. Since the structure is often painted to reduce corrosion, detection of surface cracks at early stages by conventional means can be slow and costly as the paint must be removed and then reapplied in order to conduct magnetic particle testing (MT). Alternating Current Field Measurement (ACFM) is an electromagnetic inspection technique capable of reliably detecting and sizing surface-breaking fatigue cracks in metals. Importantly, it can do so without the need to remove most paints and coatings. Its common use for the inspection of offshore structures, above and below sea level, have demonstrated the technique’s suitability to inspect large structural welds in difficult conditions, both environmentally and with less than ideal surface conditions. ACFM provides information on the length and depth of detected cracks, allowing repair efforts to be concentrated on the most serious defects, without wasting time on insignificant ones. Recent improvements to ACFM equipment portability, speed of acquisition and sensitivity, combined with the introduction of advanced multi-element array probes, have improved the speed and ease of inspections. Use of the ACFM technique can therefore greatly speed up inspection, reduce waste and save time compared to MT inspection. Civil structures are sometimes corrosion protected using conductive coatings which are known to pose a challenge to electromagnetic inspection techniques. In this article, simulations, experimental results and recommendations related to the application of ACFM on galvanized and thermal sprayed aluminum structures are presented. Examples of various bridges inspections are presented; from a historic span over the river Thames, originally built in 1867, to more recent road and rail crossings.