Abstract
Abstract Flutter of a crack tip occurs when the relatively flexible tip collides with density fluctuations in a solid. The crack velocity must exceed a critical value. The crack tip behaves like a self-excited oscillator. Its fundamental frequency can be estimated using Friedel's dislocation model of a crack. Surface energy, γ, provides the restoring force for displacements, x of the leading dislocation line. The force constant is, −4γx/b, where b is the Burgers displacement. The oscillator frequency is then about: (v L/b)/π where v L is the velocity of longitudinal acoustic waves. Thus it is proposed that self-excited oscillations of crack tips cause the hackle markings on fracture surfaces after high speed crack propagation.
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