The dissipated heat energy as a fatigue damage index for experimental fatigue life estimations

Fatigue of metallic materials is a dissipative phenomenon involving plastic deformations that require a certain amount of mechanical energy in a unit volume of material, W. Only part of this energy is accumulated in the form of internal energy, which is responsible for fatigue damage accumulation and final fracture. The remaining part is dissipated as heat (Ellyin, 1997), thus translating into some temperature increase during fatigue testing. The thermal energy dissipated in a unit volume of material per cycle (the Q parameter) has been adopted as a fatigue damage indicator during fatigue tests of stainless steel specimens and a relatively simple experimental technique has also been proposed to estimate Q from in-situ measurements of the temperature at the surface of a specimen or a component (Meneghetti, 2007). Similar to W, Q is thought of as a material property, i.e. it is independent, within certain limits, of the thermal, mechanical and geometrical boundary conditions of the laboratory fatigue tests. Then, the specific heat loss per cycle Q at a given point of a component (similar to the plastic hysteresis energy) depends only on the applied load cycle, defined by amplitude, mean value and stress state. Originally, the Q parameter was adopted to rationalize in a single scatter band approximately 150 experimental results generated from constant amplitude, completely reversed, stress- or strain-controlled fatigue tests on plain or notched hot rolled stainless steel specimens. Afterwards, the heat-energy based approach was extended to include the mean stress effect, by using a thermodynamic fatigue damage variable that combines two parameters, i.e. Q and the thermoelastic temperature achieved by the material at the maximum stress of the load cycle. Finally, Q was used to rationalise two- stress level fatigue test results, by using the Q-based fatigue curve combined with Miner’s rule. In this paper, the Q-based approach is discussed, focusing mainly on its use including the mean stress as well as the two-stress level effects in fatigue of stainless steel specimens.