An assessment of low cycle fatigue damage in 9Cr-1Mo ferritic steel at 300 K using x-ray diffraction technique

Abstract

Low cycle fatigue (LCF) deformation and fracture behaviour of 9Cr-J.Mo ferritic steel has been examined using X-ray diffraction (XRD) technique. LCF tests have been performed at room temperature at strain amplitudes of ± 0.25. ± 0.50 and ± 0.75 %. The LCF deformation is characterized by an initial cyclic hardening followed by progressive cyclic softening and saturation stages. The onset of crack initiation and propagation is indicated by a rapid stress drop in the stress response and cusp formation in the compression portions of stress-strain hysteresis loops. In this study. full width at half maximum (FWHM) of XRD profile has been measured for assessing LCF damage of LCF tested specimens interrupted at various fatigue life fractions. Cyclic hardening observed in the early stage of fatigue deformation exhibited broadening of diffraction profiles (i.e., a rapid increase in FWHM) at all strain amplitudes. Further fatigue cycling to cyclic softening and saturation stages results in a constant value of FWHM. However, a marginal increase in FWHM is observed in the saturation stage at high strain amplitudes of ± 0.50 and ± 0.75 %. A rapid decrease in FWHM is observed following crack initiation and propagation. This drop in FWHM is anributed to relieving of microstresses due to crack initiation and propagation. Relaxation of microstresses is confirmed by a significant reduction in oscillations in d vs sin²Ψ curves measured on the crack compared to that observed in case of away from the crack at all strain amplitudes.