Hysteretic Damping as an Energy Parameter in Gigacycle Fatigue

Abstract

The mechanical behaviour of materials subjected to dynamic loading is now a classical subject but is still a big challenge. The fatigue limit of the materials, defined as the maximum dynamic load amplitude level for which no fatigue failures occur, has been set for a number of load cycles up to 10E7. Extending this limit with the traditional hardware is a much more difficult technical task due to the length of time needed for the completion of tests, but there is a real industrial concern about this subject due to the growing need to extend the lifecycle of some structures beyond such limits. Therefore, the study of fatigue is being extended to more than 10E7 cycles, typically 10E9 and more, in a new area of studies known as gigacycle fatigue. It is within this framework that the study presented in this paper fits. The approach here presented is based on the fact that the fatigue problem can be understood in terms of the energy available for irreversible process triggering. This energy will be involved in micro-structural irreversible changes in the material before being dissipated as thermal energy. The energy needed to trigger such changes must come from the dynamic loading. 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 balance for each cycle.