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MM: Fachverband Metall- und Materialphysik

MM 31: Topical session (Symposium MM): Fundamentals of Fracture

MM 31.4: Vortrag

Mittwoch, 14. März 2018, 11:15–11:30, TC 006

New theory for crack-tip dislocation emission and twinning in fcc metals — •Predrag Andric and William A. Curtin — Ecole Polytechnique Fédérale de Lausanne, Switzerland

Dislocation emission from a crack tip is a necessary precursor to crack blunting and toughening. Intrinsically ductile fcc metals under Mode I loading first emit a partial dislocation followed either by a trailing partial ("ductile" behavior) or a twinning partial ("quasi-brittle" behavior). The critical stress intensity factor K_Ie at which these processes occur is usually estimated by the Rice and Tadmor/Hai theories. Atomistic simulations show these models to be reasonable but not highly accurate, for predicting K_Ie. Analysis of the energy changes during nucleation reveals that the first and trailing partial emission are always accompanied by creation of a surface step, while twinning partial emission is not. Here, we present a new analysis in which first and trailing emission are controlled by a crack-tip instability due to the necessity of step formation. The absence of the step during twinning motivates another new model that accounts for the fact that twin nucleation does not occur directly at the crack tip. Both theories are quantitatively validated against molecular statics simulations across a wide set of fcc metals described with EAM potentials and excellent agreement is obtained. A twinning mode is also reported wherein the crack first advances by cleavage and then emits the twinning partial at the new crack tip.