Analysis of the fatigue strength of a stainless steel based on the energy dissipation

Determination of fatigue limit under uniaxial tests 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 for a given material, loading and mechanical boundary conditions the former parameter 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 thermal boundary conditions that determine the rate of heat transfer 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 is to define a theoretical model in order to derive the specific heat loss per cycle from temperature measurements performed during the fatigue test. The model has been applied to analyse the fatigue strength of smooth and notched specimens made of AISI 304 L stainless steel. Then, it has been verified to which extent the proposed approach holds true while varying the notch tip radius. Finally, it has been analysed the material response in terms of energy released as heat in two-level fatigue tests.