Determination of Increase in Brittle Transition Temperature of Steel Due to Fatigue by Hardness Test

Abstract

In order to prevent brittle fracture of a structure, it is important to know how much the brittle transition temperature of a steel structure increases with cyclic loading. In a past, a number of investigations were carried out, for example, by means of the Charpy impact test. However, quantitative evaluation of the influence of fatigue stressing and cycling on the brittle transition temperature of steel have not yet been made clear. In this paper, the brittle transition temperature of a structural steel subjected to various degrees of fatigue was measured by the hardness testing method in which the brittle transition temperature can be defined as the intersection of the two straight lines in the plot of the logarithmic Vickers hardness vs. the reciprocal of absolute temperature.
The results obtained are summarized as follows:
(1) When a constant fatigue stress S is repeated, the transition temperature of hardness ΔT_* increases with increasing number of stress cycles N and saturates to a certain value.
(2) When a constant number of stress cycles N is repeated, ΔT_* increases with increasing repeating stress S.
(3) The maximum increase of brittle transition temperature ΔT_*max appears in the fatigue limit.
(4) An experimental formula relating ΔT_* with S and N is found to satisfy fairly well the measured data.