Abstract
Among the energy-based approaches for fatigue life estimation of steel specimens, the experimental method based on the specific heat loss per cycle Q proved effective. The parameter Q is readily evaluable using temperature measurements and was validated through a considerable number of experimental results. It constitutes an effective fatigue damage index able to synthesize in a single scatter band results generated from fatigue tests in different conditions for a given material. More in detail, testing conditions differed for (i) specimens’ geometries, i.e. plain and notched, (ii) loading conditions, consisting of pure axial or torsion, and multiaxial loadings and (iii) mean stress, the latter requiring the use of a properly defined temperature-corrected parameter Q¯ . Therefore, the Q-based approach has been employed in the present investigation for fatigue strength assessment of plain and bluntly notched specimens, extracted from a 42CrMo4 Q&T connecting rod big end of a marine engine. Stress- and strain-controlled constant amplitude fatigue tests were carried out on an axial servo-hydraulic MTS machine, adopting R=-1 and R=0.5. For each specimen, the Q parameter evolution was monitored by suddenly stop the fatigue test several times during the same test and measuring the cooling gradient of material temperature. As result, two types of fatigue curves were obtained: 1) the traditional stress-life (S-N) curves, each of them fitted on results derived from the same geometry and R-ratio, 2) the single Q¯- N curve, fitted on all results independently of the geometry and R-ratio, assuming the Q¯ representative value at 50% of total life. Session
