Abstract
The crack propagation behavior of hcp crystal has been simulated by molecular dynamics method using Lennard-Jones type potential. A crack was introduced to model crystals which have free surfaces. The size of the model was 13nm X 12nm X 4.6nm and 42000 atoms were included in the model. The crack was propagated by two type of loading method. In the case of applying mode I displacement to boundary atoms, cracks were propagated as brittle manner. It was found that there are two type of fracture mode existed at crack tip in mode I simulation and "Shear lip" is formed on the crack surface. Width of the shear lip was about 1.1 nm. The yield strength of the model crystal was 4.0GPa, which is estimated by tensile test of the model crystal without a notch. Size of the plastic zone at crack tip was estimated from above yield strength and the value of crack length by using Irwin's theory. The size is similar to the width of the shear lip. When model crystals with 8000 atoms was applied tensile strain, A definite dependence of crystallographic orientation on crack propagation behavior was obtained. In the case of model crystals with initial notch plane and direction were (101^^-0), [2^^-110], the crack propagated parallel to notch plane and two sets of prismatic slips were occurred at the crack tip in both crystals. Therefore, the crack in these crystals are deduced to extend by alternating shear on two intersecting {101^^-0}<12^^-10> prismatic slip systems. In a model crystal with (0001), [12^^-10] initial crack, {101^^-1} first order pyramidal slip and {101^^-2} twin occurred at crack tip. In the model crystal with (101^^-0)[0001] initial crack, the crack propagate parallel to initial crack plane with some basal slips in front of the crack.
