AE Criteria for the Loss of Load‐Bearing Capacity of Prestressing Bars After Thermal Exposure–Microstructural Validation

The paper presents the results of experimental studies on the effect of thermal exposure (500°C and 700°C) on the microstructure, mechanical properties and acoustic emission (AE) signatures of cold‐drawn prestressing bars. Specimens in the as‐received state (RT) and after annealing at 500°C and 700°C were subjected to uniaxial tensile tests to failure, with simultaneous AE signal acquisition. The analysis was based on a three‐parameter diagnostic approach employing the Ib‐value index, the number of counts and the amplitude–duration relationship, enabling quantitative detection of the moment of load‐bearing capacity loss. It was shown that for RT specimens, the failure process is ductile with late crack localisation; for S_500 specimens, it is abrupt, while for S_700 specimens, it is dispersed and microplastic. Changes in AE parameters exhibit a clear correlation with microstructural evolution: from lamellar pearlite in the RT state, through partial spheroidisation and lamella fragmentation at 500°C, to complete spheroidisation and cementite coarsening at 700°C. A decision criterion was developed, based on a permanent exceedance of the stability threshold in at least two out of the three analysed AE parameters, which enabled consistent identification of the failure moment within the investigated specimens and constitutes a potential diagnostic tool for assessing the integrity of steel components under and after fire exposure.