Suggestions to the cohesive traction-separation law from atomistic simulations
H Krull and HA Yuan, ENGINEERING FRACTURE MECHANICS, 78, 525-533 (2011).
The cohesive model becomes popular in crack analysis for its clear physical background and flexible implementation. The cohesive traction- separation law, however, is a critical point and will generally be empirically assumed. In the present paper the cohesive traction- separation law is investigated based on constrained three-dimensional atomistic simulations. The computations under mode I conditions show that crack growth even in the nano-scale single-crystal aluminum is in the form of void nucleation, growth and coalescence, which is similar to ductile fracture at meso-scale. The concentrations of the atomic tensile stress and the atomic hydrostatic stress at a certain distance from the crack tip characterize void nucleation and final crack growth. The Mises stress does not play a role in the material failure in the nano-scale. This implies that ductile failure under mode I loading condition is dominated by the normal traction, which agrees with the assumption of the cohesive zone model. Variations of the atomic stresses near the crack tip provide the theoretical background for the cohesive zone model and can be used to identify the cohesive traction-separation law. The traction curve is very sensitive to the distance to the crack tip, which is related with the stress triaxiality. The atomistic simulations show tendentious agreement with the known cohesive traction-separation laws, whereas the scattering of the atomic stress versus separation implies effects of the hydrostatic stress in the traction-separation law. The computation provides important information for constructing the cohesive zone model. (C) 2009 Elsevier Ltd. All rights reserved.
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