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

Magnetic Barkhausen emission (MBE) technique has been used to assess the lo w cycle fatigue (LCF) damage in 9Cr-1Mo ferritic steel. LCF tests have been carried out at ambient temperature at total strain amplitudes of +/-0.25, +/-0.50 and +/-0.75%. MBE measurements have been performed on LCF tested sp ecimens interrupted at various life fractions. The various stages of LCF su ch as cyclic hardening. cyclic softening, saturation and crack initiation a nd propagation have been examined and identified using peak height Value of the root mean square (RMS) voltage of MBE. The cyclic hardening, which occ urred in the early stage of LCF cycling, decreases the MBE peak height valu e. The progressive cyclic softening stage displayed reversal in MBE respons e i.e., the MBE peak high value increased following softening due to rearra ngement of dislocations into cells. Further, the saturation stage, where th e stress value remained constant for a large number of cycles due to format ion of stable dislocation substructure, results in a constant value of MBE. Finally, the onset of rapid stress drop and cusp formation in the stress-s train hysteresis loop, which indicate surface crack initiation and propagat ion, exhibited a rapid increase in the MBE peak values. This is also confir med 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 re verse domains produced at the crack surfaces.