Fatigue damage assessment using x-ray diffraction and life prediction methodology

X-ray diffraction line broadening was used to monitor surface damage due to deformation (distortion) that was induced by low cycle fatigue. The integral breadth of selected diffraction peaks was identified as a useful parameter with which to evaluate cumulative fatigue damage. Torsional fatigue tests were conducted on nickel-based Waspaloy material which exhibited planar slip at 1200° F (649°C). X-ray diffraction measurements were taken at 22, 41, 60, and 90% of the life. The data disclosed an increase in breadth with each increment of cycling. The results obtained from line broadening analysis were carefully correlated with observations made on the specimen surface using scanning electron microscopy which showed the progressive distortion occurring in the cycled specimen. The integral breadth, β, was successfully correlated with the applied shear strain to predict the expended fraction of life and hence the remaining cyclic life.