Surface oscillations and slow crack growth controlled by creep dynamics of necking instability in a glassy film.

We study experimentally the slow growth of a single crack in a glassy film of polycarbonate submitted to uniaxial and constant imposed load. Flame-shaped macroscopic zones of plastic deformation appear at the tips of the crack and the formation of these plastic zones involves a necking instability. In order to understand the crack growth dynamics, we study first the growth dynamics of the plastic zones alone, i.e. without crack, at constant imposed load. We find that the growth velocity of the neck can be very well described by the same Eyring's factor as the one describing the creep flow of polycarbonate. In addition, we discover that a surface oscillation with a very large wavelength-to-amplitude ratio occurs during the neck propagation, and that both wavelength and amplitude are proportional to the film thickness. Finally, we succeed in modelling analytically the dependence of the instantaneous crack velocity on experimental variables using Dugdale-Barenblatt static description of crack tip plastic zones associated to Eyring's law and an empirical dependence on the crack length that may come from a residual elastic field.