This contribution presents a recently developed infrared measurement and data processing methodology that allows for early detection of localized metal fatigue damage. As an extension of the classic Thermoelastic Stress Analysis (TSA) the use of second harmonic temperature amplitudes for spatially resolved damage detection due to cyclic plasticity or clapping of crack faces in specimens made of low-alloy carbon steel is demonstrated. It is found that cyclic plastic material behavior significantly contributes to the nonlinear thermal response with respect to loading in case of mildly notched specimens. Magnitude and area size of the nonlinear temperature amplitude are correlated with the actual material damage on the micro scale. In contrast, early damaging of welded samples can be identified more easily by detection of a local drop of the elastic temperature amplitude. This is verified by making use of ultrasound excited thermography, an alternative method for crack detection. The exploitation of thermo mechanical coupling effects exhibits great potential for damage detection and quantification.