Abstract:

The effect of load-shedding sequences, stress ratio, specimen thickness and crack length on the fatigue threshold behaviour in a high-strength steel, 30CrMnSiNi2A, is investigated. Three load-shedding sequences are adopted, i. e. conventional manual load-shedding technique, computer-based k-gradient methods with constant load control and constant stress intensity factor control. The fracture surfaces in near threshold regime are examined by TEM and SEM. The examination demonstrates that there is little difference in the loadshedding sequences when a/w is not too large, but as a/w enlarges, the k-gradient method with constant stress intensity factor control is superior to the other two sequences in economy of test time and reliability of experimental data. No appreciable influence of the crack length or the specimen thickness on the fatigue threshold values is found in the investigation. The dependence of the fatigue threshold value on the stress ratio is also weakened in this high-strength steel. This fact is explained in terms of crack closure effect and hydrogen embrittlement enhanced by fretting oxidation process.