Bk. Choudhary et al., An assessment of low cycle fatigue damage in 9Cr-1Mo ferritic steel at 300K using X-ray diffraction technique, T I INST ME, 53(3), 2000, pp. 353-359
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.