Mechanical and magnetic hysteresis as indicators of the origin and inception of fatigue damage in steel

Abstract

N₂ and N₃ are known as the transition points of the three principal stages of fatigue: initial accommodation, accretion of damage and terminal fatigue. Many experiments show that the ratios of N₂/N_f and N₃/N_f tend to be stable even though the specific N₂ and N₃ values may fluctuate widely. The primary goal of this research is to study the piezomagnetic field surrounding AISI 1018 steel specimen under repeated loads and to find the ratio values of N₂/N_f and N₃/N_f by analyzing 11 sets of low-cycle fatigue data. An MTS-810 testing system with a peak capacity of 222 kN was used to obtain the data which consisted of stress, strain, and piezomagnetic field. A computer program was constructed to track the evolution of the piezomagnetic field and regression analysis was carried out to determine N₂ and N₃ values. It was observed that there exists a consistent relationship between N₂ and N_f. The apparent invariance of the ratio N₂/N_f implies that N₂ may be identified as an index of performance in the early loading response of a specimen that forecasts its fatigue life, N_f. It has been demonstrated that measurements of the magnetic and mechanical hysteresis can yield significant insights into the various stages of the development of a fatigue critical microstructure which culminates in complete rupture of the material.