hello ,everyone

I have doubt about m in the Fatigue Calculation

I see many paper about fatigue,values of m are 3-5 if the matrial is steel ,and alues of m are 8-12 if the matrial is composite.I don’t understand the dfference between the m=3 and m=4,Are they different steel matrial ?As we know if the m is different, the lifetime we calculated is different.Can you tell me how to distinguish the m,and how we choose the appropriate m .

Best Regard

Ruiliang.Wang