低炭素鋼の二段二重々複応力疲れ寿命に及ぼす時効の影響

Abstract

In the case of two-level cyclic stress loading, it is generally recognized in steels that the cumulative cycle ratio (ΣN/Nf) is greater than unity for a low to high stress sequence (σL→σH) but less than unity for a high to low stress sequence (σH→σL). This kind of deviation from the Palmgren-Miner's rule (linear damage rule) is expected to be related to the fatigue damage accumulation process and age-hardening ability.Two kinds of low carbon steels were prepared; specimen A, in which the age-hardening ability was attained through low temperature quenching, and specimen B, in which the ageing process in specimen A was completed by leaving them at room temperature. In specimen B, the deviation from Palmgren-Miner's rule was a little for both σL→σH and σH→σL sequences. On the other hand, in specimen A which had age-hardening ability, the deviation from Palmgren-Miner's rule for σL→σH sequence was greater than that in specimen B.The change of internal friction energy and propagation of fatigue crack were observed during cyclic loading. Based on the experimental results of accumulation process of internal friction energy, the following has been assumed.(1) A part of the internal friction energy is dissipated for the accumulation of fatigue damage.(2) Crack growth corresponds to fatigue damage.(3) The accmulation process of fatigue damage is expressed by the Marco-Starkey and New-mark models in which an exponent depends on stress amplitude and increases with progress of ageing.Based on the above assumptions, the cumulative cycle ratio was calculated, and the result showed a similar tendency as the experimental one.