A Study on Low-Cycle Impact Fatigue of Mild Steels Under Tension

Abstract

In this paper, to obtain the basic data on impact fatigue behaviour, low-cycle impact fatigue tests are carried out on three kinds of the circular bar specimens of mild steels SS41 i.e., (A) smooth specimen, (B) circumferential U notched specimen (stress concentration factor K_t=1.8) and V notched specimen (K_t=5.7), under repeated impact tensile loads. From experimental results, the conclusion is summarized as follows : (1) The process of fracture of (A) and (B) specimens of which K_t-values are small is creep type and cyclic behaviour of permanent strain ε (=2・1n (d_0/d), d=diameter at the minimum cross section of each specimen under impact fatigue test and d_0=the initial value of d) is almost similar to ordinary low-cycle fatigue behaviour. Its process is divided into three stages and the 2nd. stage where permanent strain rate Δε_c is constant occupies most of fatigue life. (2) The relation between impact tensile stress σ^^- and Δε_c is expressed by σ^^-/σ_B=T^^-^α・S・(Δε_c/ε_f)^β …(a) where σ_B is ultimate strength of (A) specimen, T^^- is duration of maximum stress in one cycle, ε_f is ε at the time of fracture of each specimen under static tensile test and α, β and S are experimental constants. (3) The relation between Δε_c and impact fatigue life N_f is expressed by (Δε_c/ε_f)・N_f^m'=C' …(b) where m' and C' are material constants. (4) The process of fracture of (C) specimen of which K_t-value is large is crack type, but impact fatigue behaviour is formally expressed by Eqs. (a) and (b) as same as (A) and (B) specimens, when ε is estimated by using diameter d at the crack bottom as a matter of convenience. (5) The low-cycle impact tensile fatigue strength in this experiment is expressed by σ^^-(N_f・T^^-)^n'=D' …(c) where n' and D' are experimental constants. And n' decreases with increasing the value of K_t, while D' increases with increasing the value of K_t.