A Consideration on Fatigue Strength of Notched Steel Plates Based on Crack Initiation Life

Abstract

In this study, the repeated tensile loading fatigue tests for SS41 and SM50A steel plates are carried out on smooth specimens and the two kinds of notched specimens with similar form and different sizes (scale is 1/8 and plate thickness is constant), i.e., stress concentration factor K_t=2.9 and 6.2. And from the results on the fatigue strength based on crack initiation life N_c in intermediate cycle range, the relation between the fatigue strength and K_t and the elastic stress gradient G, is discussed. At the same time, the size effect of notched plates on tensile fatigue strength is also discussed from a viewpoint of the stress gradient G. The results obtained in this study are summarized as follows: (1) Results of above tests show that the fatigue strength of notched plates depends on not only K_t but also G for each material and that there exists the size effect. (2) The ratio of the fatigue strength of notched specimens at notch root K_tσ_<max> for that of smooth specimens σ_<max0>, K_tσ_<max>/σ_<max0>=K_t/K_f (K_f=the fatigue strength reduction factor), is obtained as a function of G independent of K_t in the both materials. (3) From above results, the empirical formula giving the fatigue strength reduction factor of N_c-basis, K_f, is obtained in the following form: K_f=(K_t/(1+a_1G^<b1>))N_c^a_2G^b_2(K_f≧1) where a_1, a_2=constants, b_1,b_2=exponents (see Eq.(7)). This formula shows good agreement with the results of other researchers in comparatively high cycle range. (4) The size effect factor showing degree of the size effect of notched plates, K_s, is also obtained as a function of G, i.e., K_s=K_t/K_f(K_s≦K_t).