The effects of fatigue on acoustic nonlinearity in aluminum alloys

Experimental evidence is presented which suggests a strong nonlinear interaction of acoustic waves with dislocation dipoles in fatigued metals. A recently formulated model of the interaction of an acoustic wave with dislocation dipoles and dipole-array approximations to veins and persistent slip bands (substructures) formed during metal fatigue is examined. The model predicts the generation of a substantial acoustic second harmonic that depends on the distance between the glide planes of the dipole pair, on the dipole density, and on the particular arrangement and volume fraction of dipoles in a given substructure of the fatigued solid. Experimental evidence which strongly supports the essential features of the theoretical model is presented for fatigued aluminium alloy 2024-T4. The effects of artificial aging on acoustic harmonic generation and on fatigue-induced microstructural features in precipitation-hardened Al 2024 are discussed