Hysteretic damping as an energy parameter in gigacycle fatigue

The fatigue limit of the materials, in rotating bending, defined as a load amplitude level such as below that level no fatigue failures occur, has been set for a limited number of cycles of loading in the range of 10E6 to 10E7 cycles. With the development of new materials, such as high strength aluminium alloys with a different microstructure from steels, materials do not have a classical fatigue limit and conventionally it is accepted that the fatigue limit is the stress level such that there is no fractures up to 10E7 cycles. Extending the range of fatigue to more than 10E7 cycles is a much more difficult technical task due to the length of time needed for the completion of tests. Indeed, rotating bending machines hardly have a frequency of more than 30 Hz, servo-hydraulic machines go up to the range of 50 Hz and sonic resonant fatigue machines work at around 150 Hz. With these test frequencies on classical machines, fatigue tests take too much time in order to be practical. This problem is now being solved with the development of ultrasonic fatigue machines with frequencies up to more than 20 kHz. Several difficulties are still present in the development of ultrasonic fatigue machines and in the standardization of tests. Therefore, the concept of fatigue is also being extended to ranges larger than 10E7 cycles, typically 10E9 and more, in a new area of studies known as gigacycle fatigue.