Effects of Service-Induced Degradation on Mechanical Properties and Acoustic Emission Response of Steel from a 40-Year-Old Bridge

Abstract Aging of steel bridge components under mechanical stress and environmental exposure modifies the essential material properties, particularly strength, toughness, and ductility. Acoustic emission (AE) monitoring offers a nondestructive means to assess damage initiation, location, and progression in such structures. However, AE response is influenced by loading history, preexisting defects, and age-related changes in mechanical properties because AE signals originate from localized strain energy release. In this study, a methodology to correlate hardness, corrosion mapping, and AE amplitude characteristics is developed. The developed methodology was tested on an approximately 40-year-old decommissioned steel bridge. The mechanical properties of web and flange sections were characterized through tensile testing, hardness measurements, and optical microscopy to evaluate corrosion depth. Their behaviors are compared with pristine Grade 50 steel. Results show that aged steel exhibits lower toughness and higher hardness and yield strength than pristine material, which indicates strain aging, local work hardening, and loss of ductility. Corrosion damage was more severe in flanges than in webs. AE mechanisms were also found to depend on toughness: high-toughness steel produced lower-amplitude, continuous AE associated with micro-yielding, dislocation motion, and void nucleation, while low-toughness steel generated burst-type, high-amplitude AE dominated by crack initiation and unstable propagation. Tensile test results confirm that AE amplitude distribution, duration, and frequency content during deformation strongly depend on the aged steel properties. The frequency spectra of pristine waveforms consistently contain wider frequency spectra, whereas the naturally aged specimens have more fluctuations depending on their location. These findings highlight the need to incorporate current mechanical property data into the interpretation of AE characteristics to ensure reliable evaluation of aging steel bridges.