Effect of Ageing on Fatigue Life of Low Carbon Steel under Two-Level Stress Loading

In the case of two-level cyclic stress loading, it is generally recognized in steels that the cumulative cycle ratio (ΣN/N_f) is greater than unity for a low to high stress sequence (σ_H→σ_L). This kind of deviation from the PalmgrenMiner\u27s 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 temperrature quenching, and specimen B, in which the ageing process in specimen A was completed by leaving them at room temperrature. In specimen B, the deviation from Palmgren-Miner\u27s rule was a little for both σ_L→σ_H and σ_H→σ_L sequences. On the other hand, in specimen A which had age-hardening ability, the deviation from Palmgren-Miner\u27s rule for σ_L→σ_H sequence was greater than that in specimen B. The change of internal friction energy and propagation fatigue crack were observed during cycle 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 depens on stress amplitude and increases with progress of ageing. Based on the avobe assumptions, the cumulative cycle ratio was calculated, and the result showed a similar tendency as the experrimental one