An approach to investigate pre-nucleation fatigue damage of cyclically loaded metals using Vickers microhardness tests

In this paper, a method for measuring microhardness is used to investigate the surface damage evolution of cyclically loaded annealed 16Mn steel. For this purpose, the values of Vickers microhardness on the surface of specimens for different constitutive phases, ferrite and pearlite, are measured under different stress amplitudes during the fatigue damage process and their distribution characteristics are further examined. The near-surface dislocation structures in individual phases are observed at the different stages of the fatigue damage process. The variation in Vickers microhardness during cycling is then associated with the evolution of fatigue damage by introducing the concept of continuum damage mechanics (CDM) and a probability expression for a fatigue damage variable is proposed and applied in the discussion of the surface damage evolution of annealed 16Mn steel during fatigue. The results show that the damage variable defined on the basis of the variation in microhardness during cycling can reveal some characteristics of fatigue damage evolution and distribution particularly in the stages prior to the nucleation of fatigue cracks. Thus the Vickers microhardness test potentially offers a new non-destructive inspection technique of pre-nucleation fatigue damage for fatigued components under service conditions.