frequency

Hi 

I have few questions:

1- why most the experimental fatigue studies on metal or metal-composites in the literature were adopted a frequency less than 35Hz (the majority of these test have conducted using 20Hz).

I ran into some articles mentioned that the frequency (0-100Hz) has a marginal effect on the fatigue life of metal. is that the only answer

I am trying to create a model of a travelling wave ultrasonic motor and have found it difficult to find out how to apply the natural frequency of a steel ring to it at two seperate points, seperated by an angle of about 11 degrees. Any ideas or help would be appreciated.