DISLOCATION EMISSION FROM SHARP AND BLUNT CRACKS IN
F.C.C. METALS
Center for Atomic Scale Materials Physics (CAMP) and
Department of Physics, Building 307, Technical University of Denmark,
DK-2800 Lyngby, Denmark
and
Department of Physics, Washington University, St. Louis,
MO 63130-4899
Abstract
We use computer simulations to study the behavior of
atomically sharp and blunted cracks in various f.c.c.\ metals. The
simulations use effective medium potentials which contain many-body
interactions. We find that when using potentials repre\-sen\-ting
platinum and gold a sharp crack is stable with respect to the
emission of a dislocation from the crack tip, whereas for all other
metals studied the sharp crack is unstable. This result cannot be
explained by existing criteria for the intrinsic ductile/brittle
behavior of crack tips, but is probably caused by surface stresses.
When the crack is no longer atomically sharp dislocation emission
becomes easier in all the studied metals. The effect is relatively
strong; the critical stress intensity factor for emission to occur
is reduced by up to 20\%. This behavior appears to be caused by the
surface stress near the crack tip. The surface stress is a
consequence of the many-body nature of the interatomic interactions.
The enhanced dislocation emission can cause an order-of-magnitude
increase in the fracture toughness of certain materials, in which a
sharp crack would propagate by cleavage. Collisions with already
existing dislocations will blunt the crack, if this prevents further
propagation of the crack the toughness of the material is
dramatically increased.
To be published in Philos. Mag. A (accepted 9 Feb 1999).
A preprint is available as compressed
PostScript (17 pages, 166 kB compressed (gzip), 519 kB uncompressed).
Last modified: 16 February, 1999.
Jakob Schiøtz,
schiotz@fysik.dtu.dk