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 specime ns for different constitutive phases, ferrite and pearlite, are measured un der different stress amplitudes during the fatigue damage process and their distribution characteristics are further examined. The near-surface disloc ation structures in individual phases are observed at the different stages of the fatigue damage process. The variation in Vickers microhardness durin g cycling is then associated with the evolution of fatigue damage by introd ucing the concept of continuum damage mechanics (CDM) and a probability exp ression for a fatigue damage variable is proposed and applied in the discus sion of the surface damage evolution of annealed 16Mn steel during fatigue. The results show that the damage variable defined on the basis of the vari ation in microhardness during cycling can reveal some characteristics of fa tigue damage evolution and distribution particularly in the stages prior to the nucleation of fatigue cracks. Thus the Vickers microhardness test pote ntially offers a new non-destructive inspection technique of pre-nucleation fatigue damage for fatigued components under service conditions. (C) 2001 Elsevier Science Ltd. All rights reserved.