A damage parameter for HCF and VHCF based on hysteretic damping

The fatigue limit of materials, due to the improvement of fatigue life of structures and mechanical components should be extended from the traditional 106-107 cycles up to 109 and more, but with traditional testing hardware this is a difficult technical task due to the length of time needed for the completion of tests. Ultrasonic fatigue testing machines seem to be adequate for very high cycle fatigue (VHCF) tests. We propose here to evaluate the behavior of the hysteretic damping in an attempt to associate that with damage parameter. The approach here presented is based on the fact that the fatigue issue can be understood in terms of the energy available for irreversible process triggering. This nonconservative energy will be involved in micro-structural changes in the material before being dissipated as thermal energy. In fact, the balance between the energy supplied to and returned by the material is positive and the hysteretic damping factor represents the inelastic fraction of energy in each cycle. Aiming at building a model to correlate the hysteretic cycle parameters and the fatigue process, both energy loss and material response of the specimens are measured during the fatigue test. The fatigue tests are carried out with an ultrasonic machine test, operated at 20 kHz with amplitude or temperature control, under HCF and VHCF for copper specimens. The results show the behavior of hysteretic damping parameter during fatigue life, the equivalent dissipated energy per cycle and a good correlation between the hysteretic damping factor parameter and the fatigue process S-N curve, suggesting that factor as a promising fatigue life parameter useful for some cases of fatigue life prediction.