Characterization of low cycle fatigue damage in 9Cr-1Mo ferritic steel using X-ray diffraction technique

X-ray diffraction (XRD) technique has been used to characterize the low cyc le fatigue (LCF) damage in 9Cr-1Mo ferritic steel. In this study, full-widt h at half-maximum (FWHM) of the XRD peak has been measured for assessing th e fatigue damage. Fully reversed total-axial-strain controlled fatigue test s have been performed at ambient temperature (300 K) at strain amplitudes o f +/- 0.25%, +/- 0.50% and +/- 0.75%. FWHM measurement of {3 1 0} plane has been carried out on specimens interrupted at different fatigue life fracti ons, which represent the various stages of deformation and fracture such as cyclic hardening, cyclic softening, saturation and surface crack initiatio n and propagation. The cyclic hardening, which occurred in the early stages of fatigue deformation, exhibited broadening of diffraction profile and a rapid increase in the FWHM at all strain amplitudes. Also, large oscillatio ns were observed in the d(2 theta) vs. sin(2)Psi curves. With further cycli ng, FWHM remained almost constant in the softening and saturation stages. F inally, at the onset of rapid stress drop in the cyclic stress response and cusp formation in the compression portions of stress-strain hysteresis loo ps, which indicate surface crack initiation and propagation, a rapid decrea se in FWHM was observed. This drop in FWHM is attributed to relieving of mi crostresses owing to surface crack initiation and propagation. Relieving of micro stresses has also been confirmed by a significant reduction in oscil lations in the d(2 theta) vs. sin(2)Psi curves. (C) 1999 Elsevier Science L td. All rights reserved.