The rise of metal additive manufacturing demands faster fatigue assessment, as conventional testing remains time-consuming and costly. This study applies a combination of infrared thermography-based techniques: Heat Source Reconstruction (HSR) and Second-Harmonic Thermoelastic Stress Analysis (SHTSA), under constant and continuously varying stress amplitudes, for rapid fatigue characterisation. HSR quantifies mechanical dissipation, while SHTSA assesses nonlinear spectral responses beyond thermoelastic coupling. Tests on 5052-H32 aluminium and SS304 steel specimens validate the integrated approach.