Published in Physical Review Letters 73, 272 (1994).
(C) 1994 The American Physical Society
INSTABILITY
DYNAMICS OF FRACTURE: A COMPUTER SIMULATION INVESTIGATION
Farid F. Abraham, D. Brodbeck, R. A. Rafey, W. E. Rudge
IBM
Research Division
Almaden Research Center
650 Harry Road
San Jose, CA 95120-6099
Abstract:
Implementing molecular dynamics on the IBM SP1 and PVS parallel
computers, we have studied the fracture of two-dimensional notched
solids under tension using million atom systems. Many recent
laboratory findings occur in our simulation experiments, one of the
most intriguing being the dynamic instability of the crack tip as it
approaches a fraction of the sound speed. A detailed comparison
between laboratory and computer experiments is presented, and
microscopic processes are identified. In particular, an explanation
for the limiting velocity of the crack being significantly less than
the theoretical limit is provided.
Continuum fracture theory typically assumes that cracks are smooth
and predicts that they accelerate to a limiting velocity equal to the
Rayleigh speed of the material (1,2). In contrast, experiment tells us
that, in a common fracture sequence, an initially smooth and
mirrorlike fracture surface begins to appear misty and then evolves
into a rough, hackled region with a limiting velocity of about
six-tenths the Rayleigh speed. In some brittle materials, the crack
pattern can also exhibit a wiggle of a characteristic wavelength.
Recent experiments have clearly shown that violent crack velocity
oscillations occur beyond a speed of about one-third the Rayleigh
speed and are correlated with the roughness of the crack surface (3-5). Two different amorphous brittle
materials, PMMA (3,4) and soda-lime glass (5) were used. Unlike the PMMA, soda-lime
glass has nearly crystalline order at small length scales. Since both
materials showed similar fracture behavior, Gross et al. concluded
that the fracture dynamics may be universal (5), or materials structure independent,
and that a dynamical instability of the crack tip governs the crack
velocity behavior and the morphology sequence of `mirror, mist and
hackle (3-5).' All of these features
are unexplained using continuum theory, though recent theoretical
advances (e.g., by Langer (6) and Marder (7)), are providing very important
insights into this difficult problem. This suggests that a fundamental
understanding may require a microscopic picture of the fracturing
process. Pioneering atomistic simulations of crack propagation by Ashurst and Hoover (8) and the brittle to
ductile transition by Cheung and Yip
(9) were too small in size to study the crack stability issue.
