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

Low cycle fatigue (LCF) deformation and fracture behaviour of 9Cr-1Mo ferri tic steel has been examined using X-ray diffraction (XRD) technique. LCF te sts have been performed at room temperature at strain amplitudes of +/- 0.2 5, +/- 0.50 and +/- 0.75 %. The LCF deformation is characterized by an init ial cyclic hardening followed by progressive cyclic softening and saturatio n stages. The onset of crack initiation and propagation is indicated by a r apid stress drop in the stress response and cusp formation in the compressi on portions of stress-strain hysteresis loops. In this study, full width at half maximum (FWHM) of XRD profile has been measured for assessing LCF dam age of LCF tested specimens interrupted at various fatigue life tractions. Cyclic hardening observed in the early stage of fatigue deformation exhibit ed broadening of diffraction profiles (i.e., a rapid increase in FWHM) at a ll strain amplitudes. Further fatigue cycling to cyclic softening and satur ation stages results in a constant value of FWHM. However, a marginal incre ase in FWHM is observed in the saturation stage at high strain amplitudes o f +/- 0.50 and +/- 0.75 %. A rapid decrease in FWHM is observed following c rack initiation and propagation. This drop in FWHM is attributed to relievi ng of microstresses due to crack initiation and propagation. Relaxation of microstresses is confirmed by a significant reduction in oscillations in d vs sin(2)Psi curves measured on the crack compared to that observed in case of away from the crack at all strain amplitudes.