Frequency effect on fatigue life of metal-composite

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

2- Is the fatigue loading's frequency has the effect on the adhesive strength in the adhesively bonded metal components, debonding failure, maybe by generating or rising the adhesive temperature in somehow?

3- apart from the frequency, what is the loading ratio (max.stress/min.stress) that most applicable to structures in service? again in many researchers use the 0.1 ratio

Thanks