Estimating the intrinsic dissipation using the second‐harmonic temperature signal in the tension–tension fatigue

The intrinsic dissipation experienced by a metal under tension–tension fatigue is linked to the second-harmonic temperature signal obtained from Fourier analysis. According to the proposed model, self-heating due to intrinsic dissipation spreads out into all harmonics of the temperature signal, including that modulated at the same frequency as the applied load, which primarily captures the thermoelastic effect. Furthermore, the model is adapted to estimate the intrinsic dissipation starting from the second-harmonic temperature if the command signal generated by the closed-loop digital controller of the fatigue test machine is not a pure sine wave modulated at the desired load test frequency; instead, it contains high-order harmonics. Finally, the theoretical model is applied to measure the intrinsic dissipation during stepwise fatigue tests on plain specimens made of C45 normalized steel, and the results obtained are validated using a previously well-established approach.