An analytical model was presented of the microelastic-plastic nonlinearities resulting from the interactions of a stress perturbation with dislocation substructures and cracks that evolve during cyclic fatigue of planar slip metals. The interactions were quantified by a material nonlinearity parameter β extracted from acoustic (ultrasonic) harmonic generation measurements. The β parameter for a given fatigue state was highly sensitive to the volume fractions of active persistent Luders bands and persistent Luders band internal stresses, as well as to the densities, loop lengths, and dipole heights of the dislocation monopoles and dipoles that form the persistent Luders bands. The β parameter was predicted to increase monotonically with the increase in the hardness of the metal during cyclic loading, thus allowing an unambiguous assessment of the remaining life of the material. The model was applied to the calculation of β as a function of percent full fatigue life of IN100 nickel-base superalloy. The theoretical predictions were in good agreement with experimental measurements reported in the literature of IN100 samples fatigued in strain-controlled, low cycle, fully reversed loading.

Ultrasonic Harmonic Generation from Fatigue-Induced Dislocation Substructures in Planar Slip Metals and Assessment of Remaining Fatigue Life. J.H.Cantrell: Journal of Applied Physics, 2009, 106[9], 093516