A two-parameter, heat energy-based approach to analyse the mean stress influence on axial fatigue behaviour of plain steel specimens

Recently, fatigue data generated from fully reversed stress- and strain-controlled tests on plain and notched stainless steel specimens were rationalised in a single scatter band by using the specific heat energy per cycle as fatigue damage index. In this paper, the energy approach is extended to analyse the mean stress influence on the axial fatigue behaviour of un-notched bars made of cold drawn AISI 304L stainless steel or hot rolled quenched and tempered C45 steel. In view of this, stress controlled fatigue tests at different load ratios R were carried out. A new two-parameter, energy-based approach is defined to account for the R-ratio effects, which combines the specific heat loss and the thermoelastic temperature corresponding to the maximum stress of the load cycle. Such parameters can be readily evaluated at a point of a specimen or a component undergoing a fatigue test by means of temperature measurements, while controlling or monitoring the thermal boundary conditions of the tests is unnecessary. The new two-parameter approach was able to rationalise the stress ratio effect observed experimentally.