Effect of Plasticity Strain Rate on Low-Cycle Impact Fatigue Behaviour of Steels under Tension

Abstract

In this paper, to investigate the effect of plasticity strain rate on low-cycle impact fatigue bchaviour, tests are carried out on three kinds of steel i.e., SS41, S25C and S45C under repeated impact tensile loads. And effect of the specimen size on impact fatigue behaviour is also investigated. From experimental results, the conclusions are summarized as follows: (1) The process of fracture of the three kinds of materials is creep type and cyclic behaviour of permanent strain ε is almost similar to ordinary low-cycle fatigue behaviour. Its process is divided into three stages which are initial creep stage, constant creep strain rate (Δε_c=dε/dN) stage and accelerative creep stage. (2) The constant creep strain rate Δε_c in the 2nd stage occupying most of fatigue life is nearly equal to plasticity strain measured from the relation between stress and strain. The duration of plastic deformation is also same to the duration of maximum stress T^^-. Therefore, the relation between plasticity strain rate Δε_c and Δε_c is given by Δε_c=Δε_c/T^^- (a) where T^^- is the duration of maximum stress. (3) The relation between Δε_c and impact tensile stree σ^^- is expressed by σ^^-/σ_B=S(Δε_c/ε_f)^β (b) where σ_B is ultimate strength of the material,ε_f is ε at the time of fracture under static tensile test and β and S are experimental constants. (4) The relation between Δε_c and impact fatigue life N_f is expressed by (Δε_c/ E_f)N^m'_f=C' (c) where m' and C' are material constants. (5) The relation between N_f and the beginning life of the accelerative creep stage N_<us> is given by N_<us>=A・N^l_f (d) where l and A are experimental constants. (6) From above results considering the plasticity strain rate, the relation between the low-cycle impact tensile fatigue strength σ^^- and cumulative duration of maximum stress (N_f・T^^-) is approximately expressed by σ^^-(N_f・T^^-)^n'=D' where n' and D' are experimental constants. (7) Through impact fatigue tests for the S25C steel specimens whose lengths are systematically changed, it is found that the effect of the specimen lengths on impact fatigue behaviour is negligible.