S45C鋼のねじり塑性疲労寿命に及ぼす平均ひずみの影響

概要

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Low-cycle torsional fatigue tests were carried out on annealed carbon steel of S 45 C and the results were examined by taking the mean strain as a parameter. The main conclusions obtained are summarized as follows : (1) The hysteresis energy per cycle is influenced by the mean strain at the beginning of strain cycling, but it saturates approximately to a constant value due to the rapid relaxation of mean stress after progressive strain cycling. (2) On the basis of hysteresis energy, an equation for estimating the effect of mean strain on the crack initiation life is proposed as follows; D=(K_0)/(K^^-_0)=1/K^^-_0(W^e_<1/2>+λ∫^<N'>_<1/2>W^edn) where N' is the number of cycles to crack initiation when mean strain is applied, N_0' is that when mean strain is zero, e and K^^-_0 are material constant and the average value of the constant K_0 obtained from the equation 1/(K_0)∫_0^<N_0'> Wdn=1 when mean strain is zero, W_<1/2> and W are the hysteresis energies for the first half cycle and the subsequent cycles, respectively, when mean strain is applied, and dn is the infinitesimal increment of hysteresis energy cycle n. The test results confirms that this equation is well applicable to the estimation of crack initiation life. (3) Crack growth life is not influenced by mean strain, because mean stress is relaxed in a short period after the initial stage of strain cycling.
1978-08-15