I do not undertand your question - Please clarify - are you asking about the difference in their fatigue life? Steel, under proper load levels, can be considered to have infinite life (more than 10E7 cycles). Composites however are generally made of fibers and resins. The resins have finite life and composites are usually designed with fiber orientation as a prime consideration for strength so they have vastly different properties that are a function of loading direction. The fibers themselves may have infinite life under proper loads but the resin systems and load orientation can significantly reduce this property. Composites have been used in applications where fatigue was a concern, drive shafts for automobiles, rotor blades for helicopters, aerostructures for aircraft and springs for aircraft doors are examples, but periodic inspection is needed to qualify for continued use.

I try to tell the correct question. Do you know any useful fatigue estimation for composite structural?
What kind of fatigue tests are necessary to define the fatigue properties for a composite material?
What kind of test speciments can we use? (I have found some test speciments' shape for steel. Is it also correct for composite material?)

Irwin