A threshold formula for fatigue crack growth with mean stress intensity factors

Abstract From the fatigue limit condition of crack front, a threshold formula has been derived with respect to mean stress intensity factors. Examinations with experimental results show that this formula can express threshold quantitatively and uniformly. It is found that threshold varies with mean stress intensity factor just as that fatigue limit varies with mean stress if the mean stress intensity factor is smaller than a critical value, while if the mean stress intensity factor is larger than the critical value, plastic deformation at crack front would affect threshold greatly. It has also been investigated that why there would be such a critical mean stress intensity factor behavior. The reason lies on that whether damage induced by plastic deformation at crack front could be neglected or not. A formula to estimate the critical mean stress intensity factor has also been derived.