Exploratory Stage-Resolved Acoustic Emission Characterization During Tensile Testing of Furnace-Exposed S235 Structural Steel

Thermal exposure changes the residual mechanical response of structural steel and may modify acoustic emission (AE) recorded during subsequent loading. This exploratory study examined individual AE hits recorded during tensile testing of an S235 steel product after exposure at nominal furnace temperatures of 700, 900, or 1100 °C for 40, 80, or 120 min, followed by still-air cooling. The mechanical programme comprised 30 specimens across ten conditions, whereas archived AE records suitable for stage-resolved analysis were available for 20 specimens. Hits were assigned to specimen-specific pre-yield, yielding, plastic-deformation, and post-ultimate-tensile-strength stages. Amplitude, duration, rise time, and absolute energy were summarized using the specimen as the unit of analysis. The largest residual-strength reductions occurred after the 1100 °C exposures. In the available AE records, median amplitude was generally highest during Stage I, which included both early material-related activity and possible contributions from load introduction and specimen–grip interaction. Temporal and energetic descriptors showed greater inter-specimen and channel-dependent variation than amplitude. The results provide an exploratory description of stage-dependent AE characteristics under the investigated nominal furnace-exposure conditions; they do not establish statistically confirmed effects among all ten conditions.