Mechanically Referenced Early Acoustic Emission Indicators of Thermally Modified Deformation in S235 Structural Steel

Post-fire assessment of structural steel typically relies on residual mechanical properties, although thermally induced changes in deformation behaviour may emerge before conventional strength parameters are clearly affected. This study examines whether mechanically referenced acoustic emission (AE) descriptors can reveal such changes in S235 steel after exposure to 700, 900, and 1100 °C for 40, 80, and 120 min, followed by air cooling. Thirty specimens were subjected to tensile testing with two-channel AE monitoring. The two channels were recorded independently and were not combined. Channel 2 was used consistently for quantitative comparisons, whereas channel 1 served as a control of signal transmission and sensor coupling. Median cumulative AE counts were evaluated at 75% of the lower yield load, the lower yield point, ultimate tensile strength, and fracture. Thermal exposure altered both the magnitude and distribution of AE activity, with the greatest differences occurring before or near yielding. After exposure to 900 °C for 120 min, the mean lower yield strength decreased by approximately 14%, while pre-yield AE counts increased by approximately 210% relative to the reference condition. Specimens exposed to 1100 °C for 40 min showed similar final AE counts to the reference specimens but markedly different distributions across the loading stages. Stage-resolved AE analysis therefore provides information complementary to residual mechanical properties and final AE counts. Although the proposed pre-yield descriptor is retrospective and cannot be applied directly as a field-diagnostic threshold, it provides a sensitive experimental basis for comparing thermally modified deformation histories and for future development of independently defined AE-based assessment criteria.