Identification of different stages of low cycle fatigue damage using magnetic Barkhausen emission in 9Cr-1Mo ferritic steel

Abstract

Magnetic Barkhausen emission (MBE) technique has been used to assess the low cycle fatigue (LCF) damage in 9Cr-1Mo ferritic steel. LCF tests have been carried out at ambient temperature at total strain amplitudes of plus/minus 0.25, plus/minus0.50 and plus /minus 0.75%. MBE measurements have been performed on LCF tested specimens interrupted at various life fractions. The various stages of LCF such as cyclic hardening, cyclic softening, saturation and crack initiation and propagation have been examined and identified using peak height value of the root mean square (RMS) voltage of MBE. The cyclic hardening, which occurred in the early stage of LCF cycling, decreases the MBE peak height value. The progressive cyclic softening stage displayed reversal in MBE response i.e., the MBE peak high value increased following softening due to rearrangement of dislocations into cells. Further, the saturation stage, where the stress value remained constant for a large number of cycles due to formation of stable dislocation substructure, results in a constant value of MBE. Finally, the onset of rapid stress drop and cusp formation in the stress-strain hysteresis loop, which indicate surface crack initiation and propagation, exhibited a rapid increase in the MBE peak values. This is also confirmed by MBE measurements on the crack and away from the crack using surface MBE probe. The increase in MBE is ascribed to the movement of additional reverse domains produced at the crack surfaces.