Nonlinear ultrasonic characterization of fatigue microstructures

Dislocation dipole substructures formed during metal fatigue are shown to produce a substantial distortion of ultrasonic waves propagating through the fatigued material. A model of ultrasonic wave–dislocation dipole interactions is developed that quantifies the wave distortion by means of a material nonlinearity parameter β. Application of the model to AA2024-T4 predicts a value of β approximately 300% larger in material cyclically loaded for 100 kcycles in stress-control at 276 MPa and R=0 than that measured for virgin material. Experimental measurements show a monotonic increase in β as a function of the number of fatigue cycles that closely approaches the predicted increase. The experiments also suggest that the relevant dislocation substructures are localized in the material.