FATIGUE HARDENING SOFTENING BEHAVIOR INVESTIGATED THROUGH VICKERS MICROHARDNESS MEASUREMENT DURING HIGH-CYCLE FATIGUE

The physical grounds of the detection of fatigue hardening/softening b ased on the Vickers hardness measurement is first discussed. The value s of Vickers microhardness on the specimen surface for annealed 16Mn a nd normalized 45# steels are measured under different stress amplitude s during the process of high-cycle fatigue. The main experimental resu lts obtained are as follows: for annealed 16Mn steel the mean value of the Vickers microhardness increases in the initial stages, reaches a maximum after a certain number of cycles and then decreases asymptotic ally with increasing number of cycles; for normalized 45# steel the me an value of the microhardness is characterized by a slight decrease in the initial stages followed by an increasing process, and a decrease again at the later stages of cycling. The scatter of the microhardness for the two materials is larger in the individual measurements within the whole fatigue process. The above variation of microhardness and t he corresponding fatigue hardening/softening behaviour of the two mate rials are further analysed on the basis of the character of Vickers ha rdness and the dislocation structure evolution near the surface of the specimen during cyclic deformation. (C) 1998 Elsevier Science S.A. Al l rights reserved.