Fatigue Life Evolution of and Surface Magnetic Flux Correlation for ASTM A572 Gr 50 W Steel Shapes Subjected to Pure Bending

Six fatigue tests were performed on W6×15 steel beams fabricated from A572 Grade 50 steel, each 4 m in length and subjected to sinusoidal bending with stress amplitudes ranging from 0.10 Fy to 0.70 Fy at 4 Hz. In five of the six specimens, a Charpy V-notch-type defect was introduced at mid-span on the lower flange to initiate localized damage. Cyclic loading was applied until fatigue failure occurred. Throughout testing, two primary parameters were continuously monitored: (i) strain and (ii) surface magnetic flux density. Analysis of the magnetic flux evolution revealed distinctive signal patterns that emerged as fatigue damage progressed, particularly near the point of failure. These magnetic variations correlate with the accumulation of microstructural damage and enable the estimation of a safe-life prediction for each specimen under cyclic loading. Furthermore, a qualitative relationship between the fractographic features and the corresponding magnetic response was identified. The results demonstrate that monitoring surface magnetic flux provides a reliable early-warning indicator of fatigue damage in full-scale steel members, offering a promising tool for structural health monitoring and public safety in elements of steel infrastructure such as bridges.