Analysis of Intrinsic Dissipations and Fatigue Behaviour ofSteelsby Measuring Thermal and Mechanical Signals DuringFatigueTests

The fatigue behaviour and intrinsic dissipations are studied in the present research by using different approaches: a thermography-based one leading to the use of a second harmonic amplitude temperature component as a damage parameter and the one using the mechanical energy input related to the energy dissipated during fatigue processes. Moreover, the relation between second amplitude harmonics of the heat-converted energy and of instantaneous power density is investigated for C45 steel undergoing fatigue tests by using stepwise loading sequences at two stress ratios. The analysis allows to assess the mathematical relation between second amplitude harmonics of instantaneous power density and temperature. Once these calibration coefficients are assessed the area under the hysteresis loop can be finally determined. It will be shown that the relation between second amplitude harmonics of the heat converted energy and instantaneous power density is independent from the damage level and cycles run, but depends on material only. The approach leads to a local analysis and does not require specific loading conditions because no hypothesis is made on the percentage of energy converted in heating. Furthermore, the analysis does not require material stabilization. The proposed procedure involves the estimation of the area under the hysteresis loop at any loading condition by simply assessing second harmonic temperature variations. The verification of the procedure is presented on extra samples.