Cumulative fatigue damage characterisation of AISI 304L steel based on the energy dissipation

Determination of fatigue limit based on the experimental measurement of material thermal increments (typically by means of infrared cameras) is well documented in the literature. Anyway the energy dissipated in a unit volume of material as heat seems to be a more promising parameter for fatigue characterisation rather than the surface temperature. In fact the former depends only on the applied stress amplitude and load ratio in a constant amplitude fatigue test, while the latter depends also on the specimen geometry, test frequency and the boundary conditions that define the heat transfer conditions from the material to the surroundings. Then it is expected that the fatigue strength of both smooth and notched specimens can be rationalised in terms of the thermal energy dissipated in a unit volume of material per cycle. The first aim of this paper was to derive the specific heat loss per cycle from temperature measurements. Then it has been verified to which extent the proposed approach holds true while varying the notch tip radius. Finally, experimental tests have been performed also in order to study the material damage accumulation process in variable amplitude fatigue tests on the basis of the energy released as heat.