The design of mechanical structures that are subject to repeated loads relies upon the knowledge of the fatigue limit of the constitutive materials. Conventional methods for the fatigue limit evaluation are lengthy and therefore expensive. We propose a new approach for this problem. It is based on the detection of a modification of the thermomechanical couplings occurring together with the damage onset. A specific synchronous demodulation thermography approach was devised which provides a map of the two first harmonics of temperature and a map of the temperature mean rise. Experiments performed on steel XC48, 316L and on Al 7010, Al 2024 show the high potential of this method.