The Effect of Strain Aging on Threshold Value for Surface Crack : Propagation of S45C Round Bar in Rotatory Bending

Abstract

Both specimens fatigue-stressed at room temperature to introduce a surface crack in the range of length 50 μm to 1.5 mm and ones annealed to remove the prestress history were fatigued in rotatory bending at elevated temperatures. Precracked specimens fatigue-stressed at elevated temperatures were fatigued at room temperature also. The threshold values for fatigue crack propagation were examined. The following results were obtained. (1) The valleys and the peaks in the threshold-temperature relations for both the prestressed and annealed specimens appear at about 200°C and 375°C, respectively. These peaks are due to a rapid strain aging. (2) While the threshold values at the peaking temperature is increased by the room temperature stress history, those at room temperature are decreased by the same stress history. (3) The threshold values at room temperature are increased by the peaking temperature stress history.