In the last decade, the heat energy dissipated in a unit volume of material per cycle (the Q parameter) has been adopted by the authors as a fatigue damage indicator of metallic materials. The advantage of using such a parameter is that it can be readily and in-situ measured at a point or a component undergoing fatigue solicitations. Geometrical, mean stress and variable amplitude (limited to two stress-level tests) effects have been successfully analysed by using the Q parameter. Concerning geometrical effects, approximately 160 experimental results generated from constant amplitude, completely reversed, stress- or strain-controlled fatigue tests on plain or notched hot rolled as well as cold drawn stainless steel specimens have been rationalised. 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 theoretical background and the application of the energy-based approach are reviewed in order to analyse all previously mentioned effects, focusing mainly on the mean stress and the variable amplitude, two stress-level effects.

Introduction

Fatigue of metallic materials is an irreversible process, accompanied by microstructural changes, localised plastic strains and energy dissipation, which requires 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, Ep, which is responsible for fatigue damage accumulation and final fracture. The remaining part is dissipated as heat [1], 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 [2]. 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 [3]. 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. The Q parameter was initially adopted to rationalise geometrical effects in fatigue of metallic [3-7] as well as composite materials [8]. Afterwards, the heat-energy based approach was extended to include the mean stress effect [9], as well as to rationalise two stress level fatigue test results of steel materials [10].