Detecting acoustic events during thermal and mechanical loading

We examined Acoustic Emission (AE) events during combined heat and tensile test carried out in different steels (S235JRG2, TRIP and TWIP steels) on Gleeble simulator. The simulator enabled us to control parameters for fast heating and cooling parallel with pressing and tensile the sample until its real break. The aim was to investigate the structural change of the material, phase transformation in the steel at different temperatures, and connect them to signatures measured by acoustic emission sensors. During testing we noticed characteristics of Barkhausen noise. We demonstrate and prove definitely that we were facing Acoustic Barkhausen Noise (ABN) due to AC current used to heating and to maintaining the temperature in the cylindrical ferritic sample. It was observed, that the magnitude of the ABN dropped suddenly to the half when the tensile test started after preheating, and it was growing back when the tensile test went to plastic deformation with elongation of the tested sample. Localization of the ABN sources has been done showing the distribution of the sources along the whole material. ABN sources were observed all along the sample with interesting density growth in the section where the diameter was smaller, thus the tension was higher. Nevertheless, this was not the only observation, since the place of the densest sources was displaced from one position to another position until the break occurred near to the densest place of ANB and AE source. Off-line examination of the structure of material afterward using destructive test proved that we could register those cooling periods, where phase transition took place in the material. Ferrite-bainite and magnetite-bainite transitions were connected to some higher distribution of ANB and AE signals during the test. Rate of hits and sum of hit were connected to material transition during cooling. The first results of AE measurements during tensile test in TWIP materials showed that AE events are connected with the well known sequences of hardening rate.