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3D atomistic simulations via molecular dynamics (MD) at temperature of 0 K and 295 K (22°C) with a high quasi-static loading rate dP/dt of 2.92 kN/s show that cleavage fracture is supported by surface emission of oblique dislocations < 111 >{011} and by their subsequent cross slip to {112} planes, which increases separation of the (001) cleavage planes inside the crystal. Under the slower loading rate by a factor 5, the crack growth is hindered by twin generation on oblique planes {112} and the fracture is ductile. The MD results explain the contribution of the crack itself to the ductile-brittle transition observed in our fracture experiments on Fe-3wt%Si single crystals of the same orientation and geometry, loaded at the same rates dP/dt as in MD. The loading rates are equivalent to the cross head speed of 5 mm/min and 1 mm/min used in the experiment. The MD results also agree with the stress analysis performed by the anisotropic LFM and comply with experimental observations.

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