Notch Effect on Strain-Controlled Low Cycle Fatigue Strength of 80kg/mm² High Tensile Steel

Abstract

This paper deals with the results of the fatigue test on hourglass type and round notched specimens of 80 kg/mm² high strength steel. Strain controlled low cycle fatigue tests were carried out to investigate the notch effect on the fatigue strength reduction factor and the life ratio in the low cycle range. Two kinds of hourglass type and four kinds of round notched ones were tested. The theoretical stress concentration factor were varied in five conditions.
The following conclusions were obtained :
(1) The natural fracture strain in static tension, ε_f, is decreased as the theoretical stress concentration factor, K_t, increased. The degree of decrease of ε_f is large in the range of 1 to 1. 6 of K_t.
(2) The relation between total true strain amplitude and number of cycles to the initiation of visible crack on the surface of specimen, N_c, was obtained as ε_ta=C_eN_c^-k_e=C_pN_p^-k_p.
(3) The values of C and k can be determined in the K_t-ε_ta and K_t-C curves in the range of 1 to 4 of K_t value. For the condition that K_t is equal to 1. 00 the ε_ta-N_c relation was obtained as ε_ta=0. 36 N_c^-0.50+0. 0054 N_c^-0.087.
(4) The relation between propagation life and failure life is represented as, N_f-N_c=0. 607 N_f^0.89.
(5) The fatigue strength reduction factor, K_{f, ε}, was defined as the ratio of total true strain amplitude of smooth specimen (K=1.00) to that of notched specimen. K_{f, ε} is always higher than K in the low cycle range.
(6) The life ratio was defined as the ratio of failure life of smooth specimen (K=1. 00) to that of notched specimen. Life ratio increase logarithmicly with increasing of K_t value.