EFFECTS OF CRACK TIP GEOMETRY ON DISLOCATION EMISSION AND CLEAVAGE: A PATH TO ENHANCED DUCTILITY?
Department of Physics, Washington University, St. Louis,
MO 63130-4899
Abstract
We present a systematic study of the effect of crack blunting on
subsequent crack propagation and dislocation emission. We show that
the stress intensity factor required to propagate the crack is
increased as the crack is blunted by up to thirteen atomic layers,
but only by a relatively modest amount for a crack with a sharp
60 degree corner. The effect of the blunting is far less than
would be expected from a smoothly blunted crack; the sharp corners
preserve the stress concentration, reducing the effect of the
blunting. However, for some material parameters blunting changes
the preferred deformation mode from brittle cleavage to dislocation
emission. In such materials, the absorption of preexisting
dislocations by the crack tip can cause the crack tip to be locally
arrested, causing a significant increase in the microscopic
toughness of the crack tip. Continuum plasticity models have shown
that even a moderate increase in the microscopic toughness can lead
to an increase in the macroscopic fracture toughness of the material
by several orders of magnitude. We thus propose an atomic-scale
mechanism at the crack tip, that ultimately may lead to a high
fracture toughness in some materials where a sharp crack would seem
to be able to propagate in a brittle manner.
When the crack is loaded in mode II, the load required to emit a
dislocation is affected to a much higher degree by the blunting, in
agreement with the estimates from continuum elasticity. In mode II
the emission process is aided by a reduction of the free surface
area during the emission process. This leads to emission at crack
loadings which are lower than predicted from the continuum analysis
of Rice.
Published in Physical Review B, 55, 6211-6221 (1997)
A preprint is available as compressed
PostScript (12 pages, 152 kB compressed, 395 kB uncompressed).
Also available from the Preprint Server as document mtrl-th/96006004.
Last modified: May 21, 1997.
Jakob Schiøtz,
schiotz@howdy.wustl.edu