Our research work on VHCF aims at predicting the fatigue behaviour of metallic alloys when submitted to cyclic loading beyond one billion cycles, based on accelerated mechanical tests. The scientific issue is related to the activated deformation mechanisms and the link with fatigue damage. The coupling of ultrasonic fatigue testing (at 20kHz), in situ thermographic analyses, and in situ time resolved X-ray diffraction under synchrotron radiation, constitutes an originality of our approach. It allows an analysis of the energy balance and the estimation of material damage for cyclic loading under stress amplitudes smaller than conventional fatigue limit.