Effeds of Static Stress and Prestrain on Low-cycle Impact Fatigue of Steels under Axial Load

Abstract

In this paper, as a basic study on the impact fatigue of ship and offshore structual members under wave induced loads, effects of static stress due to still-water loads and prestrain due to extreme-wave loads on impact fatigue strength of steel materials are investigated experimentally. Low-cycle impact fatigue tests for specimens systematically varying constant static stress and prestrain are carried out on three kinds of steel materials (SS41, S25C and S45C). From experimental results and theoretical considerations, conclusions are summarized as follows: (1) From the results of tests systematically varying static stress, it is found that the relations between the constant creep strain rate Δε_c (or plasticity strain rate Δε_c=Δε_c/T^^-), maximum impact stress σ^^-_<max> and fatigue life N_f are expressed by σ^^-_<max>/σ_B=S(Δε_c/ε_f)^β …(a) (Δε_c/ε_f)N_f^^m'=C' …(b) σ^^__<max>(N_f・T^^-)^n'=D' …(c) where m' and C' are material constants and S, β, n' and D' are experimental constants depending on σ_s and materials (see Tables 3 and 4). σ_B and ε_f are the ultimate stregth and the fracture ductility of each material under the static tensile test. T^^- is the duration of maximum stress under the impact fatigue test. (2) From the above results, it is also found that the relation between stress amplitude σ^^-_a and mean stress σ^^-_m for each material is approximately given in the following form: σ^^-_a=(σ^^-_0)/(1-σ^^-_a/σ_B)(1-σ^^-_m/σ_B) …(d) where σ^^-_0 is stress amplitude for the case of σ_s=0. This approximation results in an underestimation of the strength for the case of (σ^^__s>0,σ^^__m>σ_a), on the other hand, in an overestimation for the case of σ_s<O. (3) From the results of impact fatigue tests for specimens systematically varying prestrain ε_<pre>, it is found that the process of fracture is creep type but cyclic behaviour of permanent strain ε is different from that for the case ofε_<pre>=0. It is also difficult to obtain expressions (a) and (b) for the case of ε_<pre>≠0 by using the constant creep rate Δε_c and the plasticity strain rate Δε_c. (4) Therefore, the expression (c) is directly applied to the estimation of the relation between the fatigue strength σ_<max> and the fracture life N_f. The strength parameter D' in eq. (c) is also estimated by using the ultimate strength σ'_B and the reduction in area ψ' in the following form: D'=(0.80+0.0016ψ')σ'_B …(e) And n' is assumed to be the same value with that for the case of ε_<pre>=0. This estimation shows good agreement with the experimental results except the case of SS41 steel specimens having precompressive strain and in the comparatively high cycle range.