Characterisation of Acoustic Emission Signatures of Fatigue Crack Growth in Steel Structures in Nigeria

Fatigue crack growth in structural steel is a critical failure mechanism in bridge decks, oil platform jackets, and industrial buildings across Nigeria. Acoustic emission monitoring offers real-time detection of active crack growth, enabling condition-based maintenance before structural failure. This study characterises acoustic emission signatures associated with fatigue crack growth in mild steel CT specimens under constant amplitude cyclic loading. Specimens are cut from steel plates sourced from rolling mills in Lagos to represent materials actually used in Nigerian construction. Fatigue loading is applied at R-ratios of 0.1 and 0.5 and two stress intensity factor ranges using a servo-hydraulic testing machine at the University of Lagos. A four-sensor AE acquisition system records waveform data continuously. Hit-based parameters including cumulative energy, ring-down count, peak frequency, and centre frequency are extracted and correlated with crack length measured by optical microscopy at loading intervals. K-means clustering analysis identifies two distinct AE signal populations associated with plastic zone formation and crack tip propagation. Results demonstrate that cumulative AE energy increases monotonically with crack advance at a rate highly sensitive to stress intensity range. A threshold-based crack growth alarm criterion is validated against optical crack length data with 94 percent detection reliability. Keywords: acoustic emission, fatigue crack, steel, structural monitoring, Nigeria